This article documents and discusses the results of several tests of various propellers on a Boston Whaler Revenge 22 Walk-Through Whaler Drive powered by a single 225-HP Evinrude outboard engine. By way of example, it provides a complete road map to assessing and choosing a proper propeller for your Boston Whaler boat.
The boat in these propeller tests is a 1990 Boston Whaler Revenge 22 Walk-Through Whaler Drive (W-T WD). This boat was originally designed as a notched transom boat, but after many years of production it was adapted to the Whaler Drive appendage. The Whaler Drive option added significantly to the boat's price, weight, and waterline. The Whaler Drive increased the boat's price by about $4,500 in c.1990. It also added several hundred pounds to the boat's weight. In addition to the drive appendage itself, the hull weight is increased from the addition of a full transom and a stern deck. The 1990 catalogue lists the increase as 250 pounds, raising the hull dry weight to 2,600 pounds, although there has been some speculation that all of the weights from this era are generally on the low side of actual hull weights. The drive increased the length overall by 23-inches, making the boat 24-feet 2-inches long, or just five inches shorter than the "25" model. In addition, in 1990 the fiberglass bow pulpit was made standard, and this adds another 2-feet to the length, making the boat over 26-feet long by today's measurement standards. In all, you might say it is a "large 22-footer".
The Whaler Drive appendage was initially intended to be used with twin engines with a shaft length of 25-inches. At the time, counter-rotating engines were often only available in 25-inch shaft length, and this may have played a role in the design of the Whaler Drive. The normal practice on the Boston Whaler hull with the standard notched transom was to use a single engine of 25-inch shaft, or, if rigging with twin engines, to use 20-inch shaft engines. The reason for the change in shaft length is the nature of the v-hull: in the center the hull has deeper draft and a longer engine must be used. Since counter-rotating engines were not often available in 20-inch shaft modes, to make the Whaler Drive option more attractive to owners who wanted twin counter rotating engines, the Whaler Drive was built to use 25-inch shaft twins.
The initial recommendation for boats with a Whaler Drive was to use them with only twin engine installations. Back then, the 30-inch shaft engine was not on the market. A 30-inch shaft engine might have made sense as a single-engine installation on a Whaler Drive, but because it was not available, Boston Whaler modified the Whaler Drive to permit it to be used with a single 25-inch-shaft engine. The change took place on the bottom of the drive, where the v-shape of the drive was flattened in the center over a width of 13-inches. The flat bottom of the Whaler Drive is about 3-inches (at the transom) to 2.25-inches (at the end of the Whaler Drive) above the line projecting aft from v-hull keel. As a result, when a single engine with 25-inch shaft is mounted in the center of the Whaler Drive in the lowest mounting position, it will be mounted about the equivalent of "2-holes" (1.5-inches) high on a normal transom configuration. It has been speculated that the change in design was driven by dealer complaints about the high cost of the boat when rigged with twin engines.
The Whaler Drive appendage extends the hull running surface, and, unlike a pure setback bracket such as an Armstrong bracket, some of bottom of the Whaler Drive will be in the water when the boat is on hydroplane. Thus, the bottom surface of the drive adds drag to the hull, and, in general, a hull with a Whaler Drive will not be as fast as the same hull without the drive. However, the Whaler Drive does make the boat ride significantly differently, giving it the characteristics of a longer hull and minimizing the tendency for bow rise as the boat transitions to hydroplane mode from displacement mode. The Whaler Drive has many benefits, and not the least is the closed transom which will keep water out of the boat's cockpit in all but the most extreme conditions. The added hull form buoyancy also resists the loading of heavier motors, particularly twin engines, and keeps the boat's static trim more level. This is especially welcome with modern low-emission engines that have grown in weight beyond the expectation of any boat designer of the 1980's. The drive has other benefits, too, including use as a swim platform.
The bottom of the Whaler Drive has been flattened across the center 13-inches to permit use of a single engine with 25-inch shaft. As you can see from the position of the engine's anti-ventilation plate, this results in a rather high mounting height for the engine. The plate is 2.5-inches above the hull keel.
Photo Credit: JWH
With only a single engine on the Whaler Drive, the boat sits in very level trim at rest. You can see the drains for the splash well are about two inches above the water line. The boat is loaded with cruising gear in this situation, and is sitting about as low as it gets. The location is the cold, tanin-stained, fresh water of Byng Inlet, Ontario, in Lake Huron's Georgian Bay.
Photo Credit: JWH
This close up of the engine shows the limited depth of immersion of the gear case at static trim. On a similar OUTRAGE 22 hull with this same engine the engine will be several inches deeper in the water.
Photo Credit: JWH
The outboard is an Evinrude 1992 225-HP Model E225TXENR, an OMC two-stroke with a displacement of 3.0-liters. The V-6 is a 90-degree block, and weighs about 455 pounds dry without propeller. The lower unit has a gear ratio of 14:26 (1:1.86). The engine has six carburetors and uses solid-state magneto powered capacitor discharge ignition with individual coils for each cylinder. The rated horsepower is at 5750 RPM, and the maximum engine speed is rated to 6000 RPM. At 6700 RPM the ignition controller ("power pack") enforces an over speed limit.
The engine seems in remarkable condition for its age (12 years old), and an hour meter which appears to be part of the original installation showed only 150-hours use at the beginning of the testing period.
Before beginning testing of propellers, there ought to be some target speed that is expected, otherwise it is difficult to assess if the boat is performing up to its potential. To calculate a target speed requires three parameters:
These parameters are related like this:
SQRT(Horsepower/Weight) X (Hull Factor) = MPH
Horsepower is the easiest to estimate, as one can take the manufacturer at his word and use the rated horsepower of the engine. In most of the boating world, outboard motors are rated according to the same standard, ICOMIA 28-83, and this requires reasonable accuracy. So in this case the figure of 225-HP is used in our calculations.
Weight can be more difficult to estimate. As mentioned previously, published weights for boats and motors often assume a lightest possible configuration, and in general they tend to be on the low side of the real weights. In addition, everything added to the boat, including fluids like fuel, oil, grease, and beer, as well as all rigging, controls, batteries, cushions, and other gear must be added to the total. It may be simpler to just weigh the boat and trailer, and then weight the trailer without the boat. In this case I used a combination of these methods to arrive at what I feel is a fairly accurate boat weight of 4400-4800 pounds depending on gear and fuel on board. See the addendum below for more details.
The Hull Factor is a constant in the performance equation that converts the answers into a convenient unit, Miles-Per-Hour. In the case of the Boston Whaler hull, it is known from many real-world examples that the hull factor is in the range of 180-190.
Using a Hull Factor of 180 and the heavier weight, the boat should reach:
SQRT (225/4800) X 180 = 39 MPH
Using a Hull Factor of 190 and the lighter weight, the boat should be faster and perhaps reach:
SQRT (225/4400) x 190 = 43 MPH
Based on these expectations, we can begin testing propellers to see if we can achieve speeds of 39-43 MPH from the boat, motor, propeller combination.
An additional target speed is the engine's speed. In order for the engine to deliver its full horsepower, the engine must be able to accelerate to its rated maximum crankcase speed. This value is taken from the manufacturer's specifications. The full throttle operating range is specified as 5000 to 6000 RPM, with 5750 as the speed at which the rated horsepower is produced.
To conduct experiments and obtain accurate results, you need accurate measuring equipment and good measurement technique. To record the engine speed, the boat's tachometer was used. Although the precise accuracy is not known, the error is anticipated to be small. All engine speed measurements are made with this instrument, so the error, if any, is consistent across all the tests. Boat speed was measured by a fixed-mount GPS with WAAS enhancement. This is considered to be an accurate measurement of speed-over-ground. To convert speed-over-ground to speed-through-water, the effects of current must be eliminated. Most of the testing reported here was done on a small inland lake which has no tide or current. Some measurements were made in water with current, and the effect of the current is readily seen on the results. The sea state also affects the boat's performance, and, again, most test were conducted under very similar conditions of no wind or very light wind, and only the smallest of surface ripples on the water.
The boat's weight and wind resistance was held as constant as possible. The amount of fuel onboard was kept as constant as possible. The crew was the same for most all tests, and the amount of additional gear on board was made as similar as possible. The wind resistance was held as constant as possible by always rigging the same amount of canvas. Some observations were collected outside of these more controlled conditions, but this variance is noted in the reporting.
Data from the tests were collected as follows: the engine was allowed a few minutes to warm up, then the engine speed was set to a series of bench mark speeds. At each engine speed the boat speed produced was recorded. In most cases several trials were done, and multiple results were recorded. These were later averaged. The values for engine speed and boat speed were then entered into the Propeller Calculator to obtain a value for propeller SLIP.
To understand the data presented, the desired result is to find a propeller that is well-matched to the boat and motor. This is indicated in the test results by two important numbers. The boat speed at wide-open-throttle (WOT) should be in the target range calculated for the weight, horsepower, and hull factor. And, simultaneously, the engine speed at WOT should be in the manufacturer's recommended range.
The first propeller to test is the one that is already on the boat: an OMC SST II stainless steel three-blade propeller with a conventional rubber hub and somewhat raked blades. The diameter is 14-1/2 inches; the pitch is rated at 19-inches. This is OMC Part Number 389924. According to one informed correspondent, "SST" stands for stainless steel Teflon coated, and indeed this propeller has a black coating, although somewhat worn away on the blade tips from cavitation or ventilation.
OMC SSTII Propeller
The original propeller on the boat was this OMC three-blade stainless steel model from their SST-II line. The black teflon coating has been worn away on the blade tips. The 19-inch pitch seemed to be too "tall" for the boat. Running with very minimal trim up produced high levels of SLIP. The wear pattern of the coating is revealing; it shows you what part of the blade is doing all the work!
Photo Credit: JWH
With the engine trimmed down and the propeller running in solid water, the engine speed was held down to 4500 RPM. To get the engine to spin up to maximum rated speed, 5500 RPM, the engine was trimmed up. This seemed to cause quite a bit of propeller slip, and although the engine speed increased 1,000 RPM, the boat speed only increased 4.8 MPH. With the engine trimmed up, the SLIP increased to over 27%, a rather high figure. The SLIP figures should, generally, decrease as the boat speed increases, and as the boat tops out in speed, the SLIP figure should be steadily declining. Here we see the reverse of that, the SLIP increasing as the boat speed increased.
This propeller does not seem to be able to run with good holding in any elevated trim setting of the engine. If the engine is trimmed up, not much bow lift is seen, and the propeller tends to lose its grip on the water, causing its SLIP numbers to climb. When operating the boat in any waves, the propeller tends to break loose if being run with any trim up setting at all. This can be seen in a sudden increase of 500 RPM in engine speed, with little or no increase in boat speed. If trimmed just at the edge of this setting, it is possible to get some lift, giving the bow a lighter feeling, but often the propeller will lose its grip. When this happens, the trim must be lowered to get the propeller to restore its better "bite".
In some seaway conditions it can be desirable to give the bow a light trim, and it is difficult to archive this. The problem is caused by a combination of factors, including the extra weight in the bow of the boat from the cabin structure of the REVENGE model, the elevated engine height relative to the waterline due to the configuration of the Whaler Drive transom which makes the propeller run near the surface, and what appears to be quite aerated or disturbed water coming off the bottom of the Whaler Drive.
The OMC 19-inch pitch three-blade propeller is frequently found on their 225-HP engines, but generally the boat configuration is somewhat lighter and without the Whaler Drive. For example, it performs quite well on a notched transom OUTRAGE 22, where it can produce boat speeds of 50-MPH. To do this, the SLIP would have to be below 10-percent. As can be seen in the table below, the propeller does not approach that range of SLIP with this boat and motor set up.
July 2, 2004, Orchard Lake OMC SST-II 3-blade SS 14-1/2 X 19 (PN 389924) Fuel Load: 1/4-tank or less Canvas: Flying top stowed in Up position Gear: Minimal RPM MPH SLIP % 1000 5.2 46.2 1500 7.5 48.3 2000 8.8 54.5 3000 21.4 26.2 3000 19.8 31.8 3500 26.0 23.2 3500 26.8 20.8 3500 26.2 22.6 4000 30.4 21.4 4500 35.0 19.6 4500 35.6 18.2 4500 32.3 25.8 4500 34.0 21.9 5000 5500 38.8 27.1 (trimmed up)
With the engine trimmed down to where the propeller still has good holding, the engine speed is limited to 4500-RPM, more than 1,000 below the target. This propeller appears to be too much pitch for this application.
The boat also has a spare propeller, also a conventional 3-blade of the same pitch, 19-inches. We tried this propeller only briefly, during the first few days we owned and operated the boat. Its performance was similar to the OMC SST, except that it was slower by several miles per hour. The data was recorded with a different GPS and without great accuracy, so it is not presented in detail. As with the original propeller, maximum engine speeds were only reached by trimming up, which led to very high propeller slip figures.
Dave Zammit at Lockeman's Hardware and Boats, a local OMC dealer who has been selling Evinrude engines at that location since the 1920's, loaned me a four-blade propeller for testing. We both thought the pitch was going to be too much, based on the way the current 3-blade was running, but to be sure, we put the propeller on the boat and went for a ride. The test conditions were not consistent with the other tests. We had two additional passengers, which added considerable weight. The boat had 3/4-tank of fuel, and the winds and waves were considerable. As expected, the propeller was too much pitch for these conditions, and we could only turn it to about 4700 RPM at wide open throttle. With a tail wind, some current in our favor, and just the right small waves to get the boat bouncing, we did see speeds of almost 40-MPH with this propeller. However, the general feeling was that it was lugging the engine, particularly with the load we had aboard. Again, the test conditions were inconsistent with most of the other tests, and details are not given here.
Next a Mercury Revolution4 propeller was tried. This is a stainless steel propeller recommended for engines above 200-HP. The size of this propeller was massive, as was its weight! The propeller had a long exhaust snout behind the blades that continued the thick stainless steel hub for several inches beyond the blade root. Also, the connecting walls in the hub were shaped in a spiral so that the exhaust passing through them would be twisted. Considering the speed at which the propeller was rotating, perhaps this was done more for structural reasons than to spin the exhaust. Giving this design some more thought, perhaps the spiral of these walls is to act as a rotary pump to pull the exhaust out of the hub? This might help reduce exhaust back pressure. This propeller used a Flo-Torq II hub with a plastic coupling to transmit the forces between the propeller shaft and the propeller hub. The plastic is also intended to shear if the propeller hits something, preventing damage to the drive shaft. When shifting in and out of gear, a very pronounced "clunk" was noted. This was likely an artifact of both the high mass of the propeller and the plastic coupling technique.
This large, heavy, four-blade stainless steel propeller had too much pitch for the boat/motor combination. Its holding power was impressive. Note how the trailing edges of the blades are ground to a flat surface. Also, the blade ear is slightly truncated by grinding, as is seen on the blade in the ten o'clock position in this view.
Photo Credit: JWH
The Revolution4 seemed like too much propeller in many respects. The engine could not spin this propeller up to its rated wide-open-throttle (WOT) speed, and the heavy weight seemed to be a burden on the gear case. The results were impressive, however, for the low slip numbers. In another test, on open water with some wind and current in our favor, we did hit 40-MPH with this propeller.
The initial testing with this propeller raised some questions about the engine's performance. It was felt that perhaps the engine was not producing rated horsepower as it had such trouble turning this 17-inch propeller to normal engine WOT speeds. Also, during the testing the first signs appeared of failure of the engine's voltage regulator. This was later replaced, but it did not appear to affect engine performance. Also, the spark plugs were changed, and a new fuel filter installed in an attempt to ensure proper engine output. None of these seems to have a marked effect on the engine output, and, short of testing on a dynamometer, the power output of the engine is assumed to be in the normal range for its rating.
We ran the propeller for a weekend in some open water. The most lasting impressions were of the loud noises it made when shifting in and out of gear, and the inability of the engine to turn this propeller to proper wide-open-throttle speeds. However, I think at this point we were still becoming familiar with the boat and its characteristics. A Whaler Drive changes the way the boat handles, and it is not possible to get as much bow lift from higher engine trim with my set up, in part due to the high engine mounting height. Therefore, the engine is not trimmed up very much in these tests. This difference in technique holds the engine speeds down to lower numbers, but, as the SLIP figures show, the propeller is making good contact with the water and there is very little loss of thrust due to the propeller losing its grip.
July 10, 2004, Orchard Lake Mercury REVOLUTION 4 4-blade SS 14-5/8 X 17 (PN 48-83 024-17) Fuel Load: less than 1/4 tank Canvas: Flying top stowed in Up position Gear: Minimal Wind: Calm RPM MPH SLIP % 1000 5.7 34.1 1500 7.9 39.0 2000 3000 19.7 24.1 3500 25.5 15.8 3750 27.4 15.6 4000 30.0 13.3 4000 30.7 11.3 4500 35.8 8.0 4600 36.8 7.6 4700 36.9 9.3
The low SLIP numbers are impressive, but unfortunately this propeller is too tall for the boat and motor. Too bad it is not available is a lower pitch. (Note: Mecury now has a 15-inch pitch REVOLTION4 propeller available.) An alternative may be to try the VENSURA four-blade propeller (formerly called OFFSHORE), which is now recommended for smaller horsepower engines, up to 175-HP. This propeller has given excellent performance on Steve Farnsworth's Boston Whaler 21-WALKAROUND powered with a 200-HP engine.
Again thanks to the generous resources of Lockeman's Hardware and Boats in Detroit, I was able to borrow an OMC 3-blade VIPER series propeller. Many OMC dealers have propellers available for testing, usually in a distinctive red paint finish. This is a stainless steel propeller with a conventional rubber hub. The VIPER propeller is designed for speed. It has long swept blades and relatively large diameter, and is claimed to have a good grip on the water. It is noted for fast acceleration and good top end speed.
The propeller did come through with the highest speed numbers, but seemed to lag at lower RPM. At 3500 it was several MPH slower than other propellers and had quite high SLIP. The 225-HP engine was able to turn this propeller to its rated WOT speed of 5700 RPM. This was encouraging, as we were beginning to think the big Evinrude did not have the true guts to push the boat to its potential.
July 11, 2004, Orchard Lake OMC VIPER 3-blade SS 14-3/4 X 17 Fuel: less than 1/4-tank Canvas: Flying top stowed in Up position Gear: Minimal Wind: 0-5 RPM MPH SLIP 1000 5.3 38.7 1500 7.3 43.7 2000 8.6 50.3 3000 21.4 26.2 3500 21.4 29.3 3500 21.7 28.3 3800 25.0 24.0 4000 27.3 21.1 4000 27.0 22.0 5000 35.3 18.4 5500 39.6 16.8 5600 39.8 17.8 5600 40.6 16.2 5700 40.3 18.3
Looking back at these numbers, the results look quite good. This propeller finally let the engine wind out to full speed yet still had decent SLIP numbers. The one drawback seems to be the lower boat speed in the 3000-3500 range, which would be the optimum cruise setting. The propeller seemed to have some trouble getting the large, heavy boat up on plane. Once the boat was up on a nice plane, the propeller ran quite well. Its SLIP numbers were quite low in comparison to the SST-II three-blade.
I also borrowed an older MIRAGEplus propeller from Gary at K & M Marine in Redford, Michigan, for testing. The MIRAGEplus series is designed for larger boats and engines. With 4400 pounds of weight and 225-HP, our situation seems to be in this category. This stainless steel propeller did not have the vent holes on the hub which are now a standard feature on the MIRAGEplus series. It is also set up for use with the Flo-Torq II hub adapter, which enables fitting it to the OMC propeller shaft. The diameter of this propeller was very large, and it was almost an interference fit into the propeller aperture on the big OMC V-6 gear case. The blade tips just cleared the cavitation plate. Actually, after some use, a wear line appeared in the trim tab aluminum insert in the cavitation plate. I assume this was caused by water pressure off of the blade tips and not from actual metal-to-metal contact, as the propeller blades showed no sign of abrasion.
The three-blade MIRAGEplus propeller gave very good performance, but its diameter was so large that is was practically an interference-fit on the lower unit. The blade tips were just a whisker from hitting the anti-ventilation plate. This older propeller has no vent holes for the Performance Vent System (PVS), now standard on this model.
Photo Credit: JWH
Despite the small pitch rating, this propeller gave a very good performance. It came within 1-MPH of being the fastest propeller, and it had excellent mid-range speed. At 3500 is has the boat going 25-26 MPH, a very comfortable cruising speed, and it showed astonishingly small SLIP numbers. My feeling is the actual pitch of the propeller must be a bit greater than the 15-inch rating. This would tend to produce the low SLIP numbers the tests showed. The performance at 3500-4000 RPM is excellent, and this is the region in which a great deal of the time underway would be spent when on plane.
The large diameter seemed to work well with the boat. The propeller maintained a good grip and did not blow-out despite the relatively high mounting of the engine.
July 23, 2004, Orchard Lake Mercury MIRAGEplus 3-blade SS 15-3/4 X 15 (No vents) P/N 48-19838 Fuel: about 1/2-tank Canvas: Flying top stowed in Up position Gear: Minimal Winds: 10-15 MPH RPM MPH SLIP % 1000 5.5 28.0 1500 7.8 31.9 2000 9.8 35.8 3000 20.5 10.5 3500 25.0 6.5 3500 25.9 3.1 4000 29.5 3.4 4000 30.0 1.8 4500 33.0 4.0 5000 36.0 5.7 5200 37.3 6.0 5500 39.4 6.2 July 18, 2004, Lake Michigan, Door County Wisconsin Mercury MIRAGEplus 3-blade SS 15-3/4 X 15 (No vents) P/N 48-19838 Fuel: 3/8-tank Canvas: Flying Top, Windshield, Curtains, UP Gear: Normal Cruising Gear Winds: 5-10 MPH RPM MPH SLIP % 1500 7.5 34.5 2000 10.0 33.1 3000 19.8 11.7 3500 24.9 4.8 4000 28.8 3.7 4500 32.5 5.4 5000 35.9 6.0 5200 37.1 6.6
In these two sets of measurements you can see the effect of increased boat weight and wind resistance from the canvas rigging. The wide-open-throttle engine speed decreased 300 RPM, the SLIP numbers are higher, and the boat speed dropped over 2-MPH. All of these effects are reasonable and anticipated.
Unfortunately in the testing I failed to make close observation of another parameter: wake making. After trying a few different propellers I began to notice that they differed not only in the speed they produced, but also in the way they carried the boat. Propellers that provided better lift tended to raise the whole boat out of the water more, and this resulted in a smaller wake. With the stern not digging a hole behind the boat, the height of the wake would decrease. This seems to naturally imply a more efficient hull with less drag, which can be seen in the reduced wake height. More of the force from the engine has gone into moving the boat forward, and less force is wasted in creating wave action in the wake. Regrettably I did not make consistent observation of this. My recollection is that this particular propeller produced a small wake and tended to carry the stern higher at hydroplane speed.
The test data from Door County was collected before the engine received a close look from Dave Zammit, the OMC Master Technician. He replaced the faulty voltage regulator, gave the engine a de-carbon treatment, and checked the throttle linkages on all carburetors. The boat might run even better with this propeller than it did during these tests. This propeller is a very strong candidate for the "ideal" propeller, except for the problem of its large diameter. The fit is so close it scares me. I plan to check the casting on the trim tab insert to see if a few more thousandths clearance might be obtained by grinding it down slightly to better fit into the recess in the lower unit casting. Another alternative may be to grind a small relief groove into the trim tab insert itself to create greater clearance from the blade tips.
Another red loaner propeller from Dave at Lockeman's was tried. This time a four-blade from the Offshore series. Here is what the OMC marketing blurb says about these propellers:
When you need a prop that can hold course straight and true in all water conditions, trust the RENEGADE® OFFSHORE prop to maintain compass headings and stay hooked up in big water. Designed for heavier and twin-engine boats as well as runabouts.
This propeller gave a good account of itself, but there seemed to be a noticeable vibration in the boat at fast-idle/show-cruise speeds. Apparently some interaction between the engine, the propeller, and the boat produced a resonance which set the boat into vibration. The bow railing could be seen vibrating. This was an unfortunate artifact, as otherwise the propeller gave excellent performance.
It did not get the boat on plane as fast or as easily as the large diameter 3-blade MIRAGEplus, and at 3000-3500 it was just on the verge of being on plane.
July 25, 2004, Orchard Lake OMC OFFSHORE 4-blade SS 14-3/4 X 17 Fuel: about 3/8-tank Canvas: Flying top stowed in Up position Gear: Minimal RPM MPH SLIP % 1000 4.8 44.5 1300 6.2 44.9 1500 7.8 40.0 vibrates badly 2000 9.8 43.4 2500 11.0 50.0 3000 16.6 36.0 3500 22.9 24.4 3500 25.4 26.0 4000 28.0 19.1 4000 27.5 20.6 4500 33.2 17.3 5000 36.6 15.4 5500 40.0 16.0
In retrospect, this looks like a decent propeller for the boat, too. The problem seems to be the critical 3000-3500 speed, where we would like the boat to be on plane and doing about 25-MPH. The big drawback with this combination was the nasty vibration at 1500-RPM. Unfortunately the vibration occurs just at the speed you might spend much time operating in SLOW-NO WAKE zones. This propeller is worth a second look. I wish I had noted its wake making characteristics, too.
The Mercury ALPHA4 propeller is an aluminum 4-blade that is much less expensive than any of the others tested. We ran this propeller with a much heavier load than the other tests. One concern with aluminum propellers and higher horsepower engines is the potential for the blades to flex under heavy loads. This seemed to be happening here, as the propeller was on its way to a nice run until it was pushed above 4000 RPM. Above that the slip started to increase--never a good sign--and boat speed improvement was not great. The last 1,000 RPM only brought 5 MPH more boat speed. The numbers must be judged in consideration of the much greater weight load on the boat. There was probably at least 65-gallons more fuel aboard, which is over 400-lbs. of extra weight. In addition, all the canvas was up, adding to wind resistance. Finally, the boat was loaded with full cruising gear, bedding, clothing, etc., and this added at least another 250 pounds.
Even with this heavy load, the engine was able to spin the propeller to near the top of the rated power band. This probably means that when in light trim, this propeller would not be enough pitch to hold the engine speed down under 5750. Perhaps more pitch in this model would be appropriate.
This 4-blade propeller also seemed to induce some vibration into the boat at fast-idle/slow-cruise, but, perhaps because of the greater weight aboard at the time of the test, the effect was not as pronounced as with the other 4-blade propeller, the OMC tested above.
August 11, 2004, Midland, Ontario Mercury Alpha4 Aluminum 4-blade 15 X 16 Fuel: Full Canvas: All canvas Up Gear: Heavy Wind: 5-10 DOWNWIND UPWIND RPM MPH SLIP% MPH SLIP% 1000 5.4 33.7 5.1 37.4 1500 7.6 37.8 7.2 41.0 2000 9.0 44.8 9.1 44.1 2500 3000 17.7 27.6 18.5 24.3 (nice bounce in waves) 3500 23.0 19.3 22.2 22.1 4000 27.0 17.1 26.5 18.7 4500 30.0 18.0 30.0 18.0 5000 32.0 21.3 32.0 21.3 5500 33.8 24.5 33.3 25.6
(Update--May 2005) Propeller testing continued this spring with another Mercury MIRAGEplus, this time a 17-inch pitch. This propeller has a smaller diameter than the 15-inch pitch MIRAGEplus tried earlier, so it does not have the problem with a near-miss on hitting the anti-ventilation plate on the motor. The performance of the 15-inch propeller was excellent, but I wanted to see how the boat would run with the 17-inch because I liked the idea of its slightly smaller diameter. This MIRAGEplus also showed the excellent holding characteristics seen previously. The propeller never broke loose. There was no vibration noticed at any speed.
Test conditions were with the boat in its lightest possible configuration. There was literally no gear on board, only one person (me), and very little fuel in the tank (3/8-th indicated). If there was ever going to be a moment of light loading, this was it. It would be a chance to see how far into the upper RPM range the 225-HP Evinrude could wind this big propeller
The fuel in the tank was left over from the previous fall, and also the engine was running on double oil. (This was due to the tank having been pre-mixed with fuel and oil as a safety precaution when re-fitting the oil injection tank last fall.) I don't know if this had any affect on the engine performance. The motor seemed to run fine, although it did smoke a bit more than usual.
The temperature was in the 60-degree range, winds were about 10-15 MPH, and the lake had just a very small wave pattern on it, nothing at all for a 22-foot Whaler hull to react to.
One significant difference in this test was the addition of a NAVMAN 3100 fuel flow meter with instantaneous calculation of fuel efficiency in miles-per-gallon. The fuel flow meter is linked to a WAAS-enhanced GPS which provides speed data, and it displays fuel consumption in either gallons-per-hour or miles-per-gallon. As a result, the test results also include information on fuel consumption at various engine and boat speeds. The results are quite surprising: the worst fuel economy is actually at displacement speeds!
The complex shape of the three bladed MIRAGEplus propeller is hard to capture in a photograph.
Photo Credit: JWH
May 10, 2005, Union Lake, Michigan Mercury MIRAGEplus Stainless Steel 3-blade 15-1/2 X 17 PVS VENTS: Full plugs in place--no venting P/N 48-18278A46 Fuel: 3/8 Canvas: Flying top stowed in UP position Gear: None--boat literally stripped Wind: 10-15 Crew: just one aboard RPM MPH SLIP% MPG 1000 5.9 31.8 1.7 downwind 1500 8.0 38.4 1.5 downwind 3000 23.0 11.4 2.05 upwind 3100 23.3 13.2 2.1 downwind 3500 27.3 9.9 2.1 downwind 4000 31.5 9.0 2.1 upwind 4500 35.0 10.1 2.0 downwind 5000 38.4 11.2 1.8 upwind (WOT) 5400 41.3 11.6 1.85 downwind (WOT and TRIM UP)
The results are quite good with this propeller, and actually it hit the all-time highest speed recorded with the boat. However, this speed was downwind, and with the boat at least 200-pounds lighter than it had ever been tested before. Both of these are factors in producing higher speed. The SLIP numbers are excellent, and again the MIRAGEplus seems to be optimized for best performance around a boat speed of 30-MPH. That is fine with me, as that is the speed at which I probably do most of my cruising. Unfortunately, the engine could not wind this propeller to its maximum rated speed (6000-RPM). Flying downwind with the engine trimmed as much as possible, it did hit 5400 RPM, but upwind it ran out of oomph at 5000-RPM. Considering the unrealistically light load, this was not a good sign. With the boat loaded for cruising and its fuel tank full, it would be reasonable to conclude the engine would never see 5000 RPM with this propeller. That is probably too slow, and it is a good indication the propeller is still a bit too much pitch for this boat and engine.
I also tried to make some assessment of the wake produced by this propeller. The wake seems to be about "the usual" size, or perhaps a bit smaller than I remembered. This wake making analysis a bit less scientific than the other data reported here. I will have to work on some technique for properly measuring it. One idea that has come to mind is to perhaps throw something overboard in the wake and then photograph it. This would provide a repeatable record of the wake. Unfortunately, I got that idea after making this test run.
Update--September 2007: I tried the Mercury MIRAGEplus 17-inch pitch propeller again, suspecting that perhaps it might run better due to some improvements in the engine, that is, we might be able to hit some higher speeds. The results are somewhat enigmatic, as the SLIP numbers seem much higher than the last test data. The data:
September 9, 2007, Orchard Lake, Michigan Mercury MIRAGEplus Stainless Steel 3-blade 15-1/2 X 17 PVS VENTS: Full plugs in place--no venting P/N 48-18278A46 Fuel: 1/8 Canvas: none, flying top down and stowed Gear: Light Wind: 10-MPH Water Temperature: 82°F Air Temperature: 72°F Crew: 2 RPM MPH SLIP% MPG 1000 5.2 39.9 1.75 1500 7.1 45.3 1.5 3200 17.0 38.6 1.6 3400 23.4 20.5 2.0 (on plane and slowing down) 3500 24.0 20.8 1.95 3500 23.4 22.7 2.0 3700 26.5 17.3 2.0 3800 27.0 17.9 2.1 3800 26.6 19.1 2.0 (UPWIND) 3900 28.4 15.9 2.2 4000 28.0 19.1 2.05 (UPWIND) 4000 29.5 14.8 2.0 4500 32.8 15.8 2.0 5000 37.0 14.5 1.8 5500 40.0 16.0 1.86
The higher SLIP may be due in part to the engine being trimmed higher. As compared to the prior test data, the water temperature is certainly much warmer, and this also may figure into the SLIP of the propeller. The boat was probably running with at least 200-lbs more weight than in the prior test from added crew and gear aboard. This could account for a slight decrease in top speed.
(Update--May 2005) The tenth propeller in this series of tests was a somewhat conventional OMC/BRP three-blade from their SST series, again in a 17-inch pitch. (One important point to make: this propeller was a used propeller. Most all of the other propellers tested were either new out of the box or in like-new condition.) At first glance, this propeller looks similar to the MIRAGEplus. The SST is a half an inch smaller in diameter. The blade rake angle is about the same. The blades of the SST seem to have trailing edge cup, like the MIRAGEplus blades, but the blade shape of the SST seems a bit flatter. The MIRAGEplus blades definitely have larger area, and particularly so at the outer portions of the blade. On the SST propeller the blade shape begins to taper as the blade grows outward from the hub, but on the MIRAGEplus the blade area stays quite large in this region. These slight differences in shape turn up in the results.
The SST propeller put a slight vibration on the boat at low speeds. This may be due to the fact that the propeller was not a new propeller. Its precise history is not known, but perhaps it had been slightly dinged or otherwise unbalanced at some point. The vibration was slight, but noticeable, whereas with the MIRAGEplus I do not recall detecting any vibration at all at low speeds.
This propeller is an older model, and I do not believe it is still available. The exhaust hub has a distinct ring, a so-called diffuser ring, at the exit. The newer style SST propellers have an exhaust hub which flares slightly at the exit, more like that seen on the MIRAGEplus.
Test conditions were with the boat in its lightest possible configuration. There was literally no gear on board, however this time we were back to our standard two person crew. Fuel was about identical to the test earlier in the week, 3/8-th of a tank indicated. The fuel in the tank was again mainly left over from the previous fall, but this time we added six gallons of fresh premium fuel. This also slightly reduced the pre-mixed oil/fuel ratio.
One minor change in the boat set up for this test: a short section of fuel line was replaced. I thought the old primer bulb did not have the recommended 3/8-th-inch fittings and line, and replaced it with a new bulb and new 3/8-th-inch fuel line. I do not think the change had any effect on engine performance, although the new primer bulb works better because I changed the orientation to vertical. (See A Primer on Primers for details.)
The temperature was in the 55-degree range, winds were about 10-15 MPH, and the lake had just a very small wave pattern on it, nothing at all for a 22-foot Whaler hull to react to.
The SST propeller did not have the holding power of the MIRAGEplus. We had to run the engine at lower trim, particularly while getting up on plane, in order to prevent the propeller from loosing grip. Once on plane we could trim up, however the trim did not seem to be as effective. (This may have been influenced by the greater crew weight aboard for this test.)
The SST series propellers are stainless steel with a black teflon coating. On this used propeller the teflon coat has been removed. Note the larger difuser ring at the exhaust on the hub. This is typical of the older style SST propeller. The newer models had a difference exhaust design.
Photo Credit: JWH
May 15, 2005, Union Lake, Michigan Evinrude SST Stainless Steel 3-blade 15 X 17 P/N 391290 Fuel: 3/8 Canvas: Flying top stowed in UP position Gear: None--boat literally stripped Wind: 10 RPM MPH SLIP% MPG 1000 5.0 42.2 1.7 vibration 1500 7.3 43.8 1.6 vibration 2500 13.0 39.9 1.5 coming off plane 3000 19.0 26.8 1.8 3000 19.5 24.9 1.9 3500 25.0 17.5 2.1 4000 29.0 16.2 2.3 best MPG! 4500 32.8 15.8 2.2 5000 36.0 16.8 2.1 5500 40.0 16.0 1.9 upwind (WOT) 5500 41.0 13.9 1.9 downwind (WOT) June 4, 2005, Lake St. Clair, Michigan Evinrude SST Stainless Steel 3-blade 15 X 17 (Second propeller--"The River Propeller") P/N: xxxxx Fuel 5/8 Canvas: Flying Top Rigged Gear: lighy Wind: S 10-15 This test is in open water, and in wind, waves, and current RPM MPH SLIP% MPG 1500 7.8 31.9 1.6 upwind 3000 19.0 26.8 1.8 upwind 3500 24.5 19.1 1.8 upwind 4000 29.0 16.2 2.1 upwind 4500 32.0 17.8 2.0 upwind 5000 36.0 16.8 2.0 upwind 5500 38.6 n.a. against current 5500 40.1 15.8 n.a. with 1.5 MPH allowance for current
The results were good with this propeller, and it came close to the new high speed mark (41.0 vs. 41.3 MPH) for the boat. The SLIP numbers are revealing. This propeller does not seem to be tweaked for best results until boat speed exceeds 40-MPH. With this boat and motor, we barely get into that speed range, and as a result the SLIP numbers are generally higher than optimum over most of the cruising range. The engine was able to wind this propeller up to 5500 RPM, both upwind and downwind. This is just at the range of wide-open-throttle engine speed we'd like to see, particularly when the boat is so light. Again this points in the direction of trying a propeller with slightly less pitch.
Curiously, this propeller turned in the best fuel efficiency: 2.3 MPG when running at 4000-RPM and making 29.0 MPH. This shows that running an engine with the right propeller can boost fuel economy. Running with too much propeller pitch can hurt fuel economy and also lug the engine down, causing other problems like excessive exhaust soot and carbon build up in the cylinders.
The SST propeller has a rubber lined hub, and it had very smooth and quiet shifting in and out of gear, much smoother than with the plastic hubs on the Mercury FLO-TORQ series hubs used with the MIRAGEplus. Another noticeable difference in the SST propeller is the flare to the exhaust hub. The SST has a larger diameter "diffuser" shape molded into the exhaust hub, while the MIRAGEplus has a less dramatic flare into a slightly larger diameter at the exhaust exit. This has been changed on the newer SST propellers, and the exhaust does not have the distinct diffuser ring shape.
The wake assessment by eyeball showed this propeller had a very small wake at high speeds, definitely smaller than the MIRAGEplus wake. From this I infer that there is some stern lift in this propeller. The boat trim also seemed to reflect this. The boat ran at a very flat angle when on plane. (That would be another good datum to record in these tests; perhaps next time out we can rig an inclinometer on the boat.)
Blade Area Comparison: SST vs MIRAGEplus
Using some kraft paper I traced and cut out the blade shape of the SST propeller. Then I positioned this shape on the blade of the MIRAGEplus for comparison. The larger blade area of the MIRAGEplus is easily seen. Most of the difference is in the trailing edge of the blade where the MIRAGEplus has additional area and a sharply cupped edge. This probably accounts for the better "bite" the propeller provides in aerated water.
Photo Credit: JWH
We were able to try a third SST three-blade, this time a 15-inch pitch. (We have now tested 19-, 17-, and 15-inch pitch propellers in this series.) This propeller was also a used propeller; I think it has been pushing boats around for over 20-years! It was in decent condition, although there was some minor repair seen on one of the blades. The test environment was not as controlled as in most previous tests. We were on the extreme southern end of Lake Huron just offshore from Sarnia, Ontario, where the lake funnels into the St. Clair River. There is some current in these waters, and it appears to have had a slight effect on the measurements. We took upwind and downwind readings and averaged them.
The propeller allowed the engine to wind to near red-line maximum, topping out at 5,900-RPM. That is quite fast for a big 3-liter V-6, but within specifications for the engine, whose maximum rated speed is 6,000-RPM. I would not want to run it at that speed for very long. It seemed to be really wound up and humming along!
The boat was not quite as light as in tests No. 9 or 10. We had more fuel aboard, and slightly more gear (as well as two people). The canvas flying top was up, and this adds some air resistance, however it is probably about as much as when the canvas is stowed in the up position. The propeller shows good efficiency as indicated by the SLIP numbers. They get down into the single-digit range. That is always nice to see with a single engine set-up. It is quite common to get single-digit SLIP numbers on dual engine propeller set-ups, but with single engines we have seen plenty of propellers whose SLIP stays in double-digits throughout the test range. We did not notice any tendency for the propeller to blow out and lose its grip, although we did not have any rough seas to test in.
The propeller appears to be about 2-MPH lower than the 17-inch pitch of similar size and design. It gets to almost the same final top speed because it lets the engine wind 400-RPM faster.
One odd thing I noticed with this propeller is the engine did not like to run between about 1500 and 3000 RPM. This was a band of engine speed where the engine seemed very unhappy. It felt almost like the spark plugs were fouling. (The spark plugs may have been fouling. I had been running the engine on double oil for several hours previously. Also, the water temperature was very cool, about 46°. This could have led to the engine running colder than normal, and this may have aggrevated plug fouling.) The engine ran roughly at this throttle setting, and would smooth out once the throttle was advanced to 3,000 RPM or more. I wonder if the lighter load from the low-pitched propeller was contributing to this. It almost felt like the engine wanted to be under more strain to run properly in this region. Because it was under a light load it seemed to be missing a cylinder or two. Perhaps it has to do with the exhaust pressure or crankcase vacuum. I would be interested to hear comments about this. In general I think that 1800-2500 RPM is a range of speeds that most two-stroke outboard motors want to avoid. These are speeds where most boats are in transition from displacement mode to hydroplane. The heavy load on the engine during this transition can cause it to be lugged down. That is a situation where lean combustion in a cylinder can lead to overheating and a blown engine in a hurry.
One other difference in this test: the fuel itself. We added about 30-gallons of fresh gasoline to the tank. This effectively diluted the pre-mixed fuel we had been running on during tests No. 9 and 10 back to full-strength gasoline with just a small amount of pre-mixed oil in it. (In case you've lost track, the fuel in the tank was pre-mixed with oil when we did some work on the engine's VRO oil system. Using pre-mixed fuel was a safety precaution in case there was any problem with the VRO.) By this change we'd expect to have less chance of spark plug fouling. On the other hand, the plugs do have quite a bit of running time one them (probably over 100 hours) and they may be due for a change. And they've been running on a rich oil:fuel mixture for some time, so perhaps they are getting a bit oily. I did not have a chance to check them. I guess the acid test will be how the engine runs next time out (with a different propeller). If it still has the hesitation, that probably points to the plugs as the problem.
The fuel economy numbers are within five-percent of the best readings seen so far. We don't have enough experience with the fuel flow transducer to know if it is accurate to less than five-percent. This could account for some of variance seen. The speed at wide-open-throttle (WOT) is over 40-MPH, which is about as fast as any propeller has given us.
I include pictures of the boat's wake and the relationship of the engine's lower unit anti-ventilation plate to the water. The wake seems fairly moderate with this propeller. I think the propeller has some stern lift which helps to reduce the wake height. The AV plate is definitely running above the water, where it should be when on hydroplane.
This is the boat wake at approximately 28-MPH on hydroplane. The wake is moderate with this propeller and speed. What bothers me is the amount of foamy water and propeller thrust breaking through to the surface. I think this is an artifact of the high mounting height created by the Whaler Drive with single engine and 25-inch shaft. That's the Ontario shoreline on the right and Stag Island on the left.
Photo Credit: JWH
The anti-ventilation plate is above water on the trailing edge. This is with normal trim, that is, not trying to trim out excessively to raise the bow. The trailing edge of the AV plate has a pointed shape. The boat is on plane at about 28-MPH at this point.
Photo Credit: JWH
With this rather high mounting of the engine, there is not much spray coming off the lower unit. The turbulence on the port side near the lower unit is from a SONAR transducer. A new location is planned for the transducer. Speed here is in probably about 22-MPH.
Photo Credit: Steve Farnsworth
May 21, 2005, Lake Huron, Michigan Evinrude SST Stainless Steel 3-blade 15 X 15 P/N 174926 Fuel: 5/8 Canvas: Flying top Gear: Light Wind: 5 Air-temp: 70°F Water Temp: 48°F UPWIND DOWNWIND AVERAGE RPM MPH MPG MPH MPG MPH SLIP% 1000 4.0 1.2 5.8 1.7 4.9 35.8 3200 18.7 1.8 3500 21.5 1.9 23.0 1.9 22.3 16.6 4000 26.5 2.1 28.0 2.1 27.3 10.6 4500 30.0 2.1 31.5 2.1 30.8 10.4 5000 33.3 2.0 34.5 2.0 33.9 11.2 5500 37.0 1.8 38.0 2.0 37.5 10.7 5750 39.1 1.7 40.1 8.7 5900 41.1 1.8 8.8
It may be interesting to look at the test results as plot of the boat speed as a function of the engine speed. The results are organized by propeller type. First, the Evinrude SST series propellers, then the Mercury MIRAGEPlus propellers, and finally the four-blade propellers.
The next two plots compare propellers of the same rated pitch. The results tend to show that the MIRAGEplus propellers seem to be the equivalent of an SST propeller with a pitch that is 2-inches greater.
If you have a comment or question about this article, please post it in the message thread reserved for this purpose in the Whaler Forum.
I conducted more propeller testing in the summer of 2006. A test series was run in which the data collected was fuel economy (MPG) as a function of boat speed (MPH). This information is presented in a separate article.
Seasonal Data 2005 ----------------- Gallons used = 508.7 gallons Total log = 941.3 miles Total hours = 67.3 hours Average fuel economy = 1.85 MPG Average speed = 14.0 MPH Average fuel flow = 7.55 GPH Seasonal Data 2006 ----------------- Gallons used = 521.3 gallons Total log = 908.7 miles Total hours = 60.5 hours Average fue economy = 1.74 MPG Average speed = 15.0 MPH Average fuel flow = 8.61 GPH Seasonal Data 2005 and 2006 ----------------- Gallons used = 1030.0 gallons Total log = 1850 miles Total hours = 127.8 hours Average fuel economy = 1.80 MPG Average speed = 14.5 MPH Average fuel flow = 8.06 GPH Seasonal Data 2007 ----------------- Gallons used = 359 gallons Total log = 609.9 miles Total hours = 60.9 hours Average fuel economy = 1.70 MPG Average speed = 10.0 MPH Average fuel flow = 5.9 GPH Seasonal Data 2005 thru 2007 ----------------- Gallons used = 1389 gallons Total log = 2459.9 miles Total hours = 188.7 hours Average fuel economy = 1.77 MPG Average speed = 13.0 MPH Average fuel flow = 7.36 GPH Seasonal Data 2008 ------------------ Gallons used = 427 gallons Total log = 843.4 miles Total hours = 68.8 hours Average fuel economy = 1.98-MPG Average speed = 12.25 MPH Average fuel flow = 6.2 GPH Seasonal Data 2005 thru 2008 ------------------ Gallons used = 1816 gallons Total log = 3303.3 miles Total hours = 257.5 Average fuel economy = 1.82 MPG (E-TEC influence!) Average speed = 12.8 MPH Average fuel flow = 7.05 GPH
Estimated Weights ----------------- (All figures are in pounds and are estimates or from published weight.) BOAT Hull: 2600 1990 Catalogue Engine: 455 Service Manual Specification Propeller: 10 1 SS and 1 Alum. Battery: 100 2 Group 24 Wet cell Fuel: 140 20 gallons @ 7 lbs./gal. Anchor: 25 16 lb. anchor, chain, rode Rigging: 50 Remote controls, gauges, wiring, etc. Canvas: 50 Bimini top, curtains, windshield, covers, etc. Gear: 300 Rear Seat, Cushions, Clothing, Tools, Cooler, etc. ___________________ TOTAL 3730 TRAILER Trailer 1200 Manufacturer MCO Accessory 50 Spare Tire, jack stand, tie downs. ____________________ TOTAL 1250 Measured Weights ----------------- Vehicle + Boat + Trailer = 10980 Vehicle = 5600 ________________________________ Boat + Trailer = 5380 Trailer = 1250 (from Estimated Weight Above) Deduced Boat = 4130 Estimated Boat = 3730 VARIANCE = 400 heavy Possible causes: --catalogue boat weight too light (very possible as these were not actual weights) --under estimate of other weights (possible) --hull contains entrapped water (unlikely given condition) --scale or measurement error (possible) --inconsistent truck weight due to passenger exit (very possible) Cruising Weight Estimate Deduced Boat = 4130 Crew = 360 More fuel = 200 More gear = 110 _____________________ Cruising Wt. = 4800
DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!
The page has been accessed times.
Copyright © 2002 by James W. Hebert. Unauthorized reproduction prohibited!
This is a verified HTML 4.0 document served to you from continuousWave
Author: James W. Hebert
This article first appeared November 21, 2004.