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Calculated Propeller Slip for Twin Outboards
|Author||Topic: Calculated Propeller Slip for Twin Outboards|
posted 09-26-2003 06:59 PM ET (US)
Can anyone tell me what percentage of propeller slip one should expect from a Whaler rigged with twin outboards with an/or without a Whaler Drive?
posted 09-27-2003 12:19 AM ET (US)
There are so many scenarios on different typs of props, theres no way to get an honest answer, such as, if your using 4 or 5 blade props with little pitch, you will have less slippage then you would with 3 blades with lots of pitch..
What engines are you running & what kind & size of boat ?
What type & make prop are you thinking of ?
posted 09-27-2003 06:22 AM ET (US)
I sea trialed a 27 Whaler WD with twin 2002 Evinrude 225s (very smooth engines, I was impressed). The were turning old 15 x 17 stainless steel propellers. They exhibited what I thought was a great deal of cavitation (wake was extremely turbulent at all speeds). Top speed at 5500 rpm with 4 people and about 170 gallons of fuel aboard was 34 knots (39 mph). Plugging the data into the propeller calculator suggests that at WOT, there is 18 percent slip. That seems high to me and may confirm excessive cavitation.
The head mechanic at the yard says the boat is not propped correctly and recommends that the motors should be turning 14 3/4 x 19 Vipers and that the cavitation is caused by too small propellers. I wonder whether the cavition may also be caused in part by excessive toe-in or out alignment of the motors.
Plugging in performance figures into the prop calculator for all my single engine Whalers, I usually end up with calculated slip of about 6 percent. So I am wondering whether a twin engine configuration should see a different percentage of slip or expect a slip in the range of 6 to 10 percent if the engines are set up right.
posted 09-27-2003 08:49 AM ET (US)
Plugging some twin engine Yamaha and Bombardier data (wish Merc would provide some) into the propeller calculator this morning for boats other than classic Whalers suggests that the slip range is something like 4 to 8 percent. Is that a range that should be expected out of a Whaler with Whaler Drive powered by twins? Out of a Whaler 27?
posted 09-27-2003 01:10 PM ET (US)
Interesting on the cavitation bit.
My 23' Conquest cavitates coming on to plane (twin 150's with 15.5x17 3blade stainless) and have always been wondering how it would perform if I could find a different prop arrangement. Chuck Bennett told me though that Whaler tested about every prop there was and said what came on it was the best overall performance.
When I run my props and numbers on the calculator, I come up with a negative slip which I don't understand the meaning of.
posted 09-27-2003 04:34 PM ET (US)
Can't help you on the negative slip except to suggest you go back and look at the accuracy of your measured inputs. GPS speed measurement can be off due to currents and pitot tube and paddle wheel speed sensor aren't necessarily that accurate.
posted 09-27-2003 07:28 PM ET (US)
Data entered into the Propeller Calculator which produce negative values for calculated SLIP are probably in error. If you are certain of all measured values (RPM, MPH) and of the RATIO number, then most likely the rated PITCH of the propeller is understated.
The propeller pitch may actually be higher than represented by the propeller manufacturer due to progressive pitch or cupping.
The PITCH number is supposed to represent the amount of advance the propeller would make if turning in a solid, as a screw advances into a workpiece. In propellers with progressive pitch and cupped blades the rated pitch is an approximation, as the actual pitch changes over the blade surface.
posted 09-27-2003 08:43 PM ET (US)
Jim also makes a very good point here that I neglected to mention. For example, the pitch on some Powertech props is understated but they point that out to consumers in their marketing literature.
posted 09-27-2003 10:45 PM ET (US)
If the props your running are stock s/s run of the mill from OMC, your mechanic is right about them being the wrong ones.
By going to a high performance prop you should darn near eliminate any cavitation or venting.
One thing I don't understand what your tech said is, the props your running are to small ?????, but he recommends a smaller diameter & more pitch,...small means diameter which lifts the boat & pitch is thrust to push the boat.
I really don't recommend the powertech props at all.
Those engines are sweet.
It's my understanding that those engines are max at 6,000 rpms not 5,500 & by adding 2 inches more pitch [ from 17 to 19 ] your gonna loose at least 300 more rpms with 19 pitch.
posted 09-28-2003 07:04 AM ET (US)
The owner was reluctant to go above 5500. However, from the throttle position it looked like it had plenty more to go so. I currently don't know what the WOT engine speed is but I suspect it is probably 6000 or above with the ability to cavitate.
The current propellers look like old SSTs and from what I've been told they came off the original Johnson 250s (V8s).
The Evinrude 225s have a very narrow WOT range between 5500 and 6000 rpm so I think getting the props right is critical and it may be better to have a shorter pitch to make sure that the narrow range can be met under all conditions. One problem in propping this boat is that it can hold 300 gallons of fuel and so there could be a significant difference in WOT performance between full tanks and half tanks or less.
posted 09-28-2003 08:29 AM ET (US)
Sal--I was intrigued by your mention that diameter affects lift. Which way is the trend? Does larger diameter imply more lift?
Peter--All of the data that I have used with the Propeller Calculator has resulted in values of calculated SLIP at wide-open-throttle in the 5-percent range, just as you have found.
With twin engine installations, it seems like the propellers may be somewhat more efficient. I don't have a good scientific basis for this, but it is just my feeling. For example, consider the case of twin 115-HP engines versus a single 225-HP engine. The 115-HP lower units are smaller (less turbulent) and the propellers may be running in cleaner water (because they are offset from the the keel centerline). Also, the propellers are smaller diameter and somewhat higher pitch than would be on a single, so this may affect the diameter/pitch ratio.
I do recall that really high-speed propellers tend toward smaller diameters and higher pitch, so this may indicate some underlying "science" that would suggest an advantage to the twin engine propeller set up versus a larger diameter propeller of the same pitch on a higher horsepower engine.
posted 09-28-2003 09:54 AM ET (US)
My unscientific observation has consisted of comparing single engine rigs to twin engine rigs where both rigs have the same size engines and the hull weight of the twin engine rig is about twice that of the single engine rig such that the horsepower to weight ratio is about the same.
My observation has been that the twin engines will usually be fit with propellers that are one pitch size larger than the single engine. For example, my single 225 hp on the 22 Revenge is turning a propeller having a 17 inch pitch. A friend has two of the same motors pushing a 27' Grady White. His motors are turning 19 inch pitch propellers of the same brand.
One hypothesis for greater "efficiency" would be that there is more propeller surface area relative to the hull's total wetted surface area so the props get a little more traction (better bite) from the combined effort than a single propeller gets from the solo effort. Another hypothesis is that the twin motors are mounted higher relative to the keel (the "feet" are not as deep relative to the keel) and so there is less drag and therefore reduced slip. I like the second hypothesis better.
posted 09-28-2003 02:29 PM ET (US)
Peter, when testing a single engine application to twin engine applications on the same hull, it's automatic to add 4 inches of pitch to the twins over what is on the single engine.
Jim, i'm refering to larger diameter on larger hulls, smaller diameter for smaller hulls.
The larger the hull, the more need for more diameter.
The dia is the power to lift the boat in order for the pitch to grab & push the boat to speed.
If you have a 12 x 17 prop on a 25' boat, no matter how many rpms you twist, it wont get on plane.
posted 09-29-2003 11:43 AM ET (US)
On a 23 Conquest with a Hard top and a pair of 135 Optimaxs (2:1 gear) running Mirage Plus 17" props I get:
.2% at 5600 rpm and 45mph WOT with a light load.
4.6% at 5600 rpm and 43 mph WOT with my normal load.
The boat will plane on a single engine giving me 26-28 mph and 4500 rpm. This is with full fuel , 3 guys, and 200lbs of ice on board. Takes a bit of time to climb on top though.
posted 09-29-2003 12:09 PM ET (US)
Strange that your twisting those props at the same rpms with a normal load comparded to a light load at WOT.
I would think with a light load your pushing 5,800 rpms, compared to 5,600 with a normal [ heavier ] load.
posted 09-30-2003 03:22 PM ET (US)
Props should be purchased by pitch and design, and never diameter. The prop manufacturer works out the diameter problems, and you get what you get. Props should also be purchased by stated use of the manufacturer, and sometimes the diameter and number of blades varies, for a given pitch, based on the intended performance design, whether it be Ski, Bass, Offshore, Go-Fast, etc. Number of blades also influences diameter.
For a Whaler like Peter is considering, a prop like a Mercury Mirage Plus, or Bombardier equivalent if available, sounds appropriate for big V-6 twins. They are made for this kind of installation, with lots of blade area to carry the load with the necessary lift. The OMC Viper, previously mentioned, doesn't sound right, as it is the same as a Mercury Laser II, which would not be correct for twin 225's on a 27 WD. They are a lightweight hull speed prop. Recently I encountered a fellow with a pair of 250 Yamahas on a 28' Grady Sailfish, a big heavy boat. He was running Mercury Mirage Props, which he said GREATLY improved the performance over the standard Yamaha SS props that came with the rig.
I urge readers to check out propeller design descriptions and applications at mercurymarine.com. There is much to be learned there.
I think Peter is misusing the word "cavitation", when he really means "ventilation", which is air being drawn into the prop blades, usually from too high of an engine installation for the prop's design.
posted 09-30-2003 06:19 PM ET (US)
Thanks for catching that Larry. I meant ventilation. I'm going for another sea trial with the Vipers and I'll report what I find. Compared to the old propellers that were on there, the Vipers look much more substantial. The mechanic sounded like he knew what he was talking about so we'll see.
posted 09-30-2003 07:52 PM ET (US)
I must agree with IHG as the Viper props are "not" on my very favorite list.
I feel they are far over rated.
Bombardier OMC dosen't & didn't make any props that compared to Mercs, Merc props are by far superior.
Omc prop blades are thick & clumsy, looks like they belong on an electric fan in the bedroom.
Merc props are also better then Stiletto props but Stiletto props are pretty close to mercs design & cost less then 1/2 the cost, thats why I usually say they are the best bang for the buck.
posted 09-30-2003 08:45 PM ET (US)
With the Opti 135s, I run up against the rev limiter at 5600rpms. To get 45 mph, I really have to trim the boat out and need nice calm water wit a bit of wind ripple. The boat will still pull 5600 rpm with the heavier load, but the top speed is down and you never get the "aired out" feeling. With a max load of 7 people and full fuel, I can only turn 5400 rpm.
posted 09-30-2003 09:35 PM ET (US)
Bill, sounds like your proped perfect.
I think you mean the engine runs up to it's maximum rated rpms [ 5,600 rpms ] not it's rev limiter because the rev limiter will shut the engines off if you hit that rpm which I think is about 6,200 rpms.
Thats a heavy deep "V" hull & those 135s are working pretty hard to twist the pitch your now twisting.
The only way to get more speed is bigger engines to twist more pitch.
Really Bill, it sounds like your dialed in real close now for those engines.
posted 10-01-2003 01:14 PM ET (US)
Peter - I may have given you wrong information on the OMC props, about which I know very little. But I believe the OMC "Raker", not the "Viper", is the equivalent to the "Merc Laser II". I just remembered that Kingfish is running one of these on his 225 Evinrude, with excellent results. To me, it seems like an excellent prop for light, high powered Whalers. It could be that the Viper, assuming it's 3 bladed, is similar to the Merc "Mirage Plus". Don't know that prop, or it's blade configuration, at all.
Whether Merc performance props are better than OMC's performance props, I have no idea or experience to compare. Sal has much more experience than I in that comparison.
I do know that a boating companion just replaced a conventional Yamaha SS prop on his 150 Yamaha, 19' Outrage II, with a Merc 4 bladed "Offshore", and the performance improvement is amazing.
posted 10-01-2003 02:46 PM ET (US)
Larry, thanks for the continuing advice.
Years ago the 150 Johnson on my 18 Outrage was turning a 21 inch Raker (the one with the exhaust ports). The engine was mounted two holes up and that boat moved very well to say the least. I understand that the Raker is their "bass boat" prop and is advertised as "a great prop in surfacing conditions, especially when the boat is running over 50 m.p.h. Perfect for bass boats and high performance boats." Clearly, the Whaler 27 is not the right application for the Raker.
The Viper, on the other hand, is advertised as "excellent on any recreational boat, from runabouts and cruisers to deck boats." Strikes me as a cruising propeller which is exactly what is needed for the Whaler 27. It appears more specialized than the current offering of the SST which is what is on the motors now. I think the SST's are just a cut above aluminum propellers.
The Yamaha SS propellers are nothing special. Basically, its just a cut above aluminum like the SST. I would expect that any somewhat "specialized" prop of any brand would perform better for the intended application.
posted 10-03-2003 07:50 PM ET (US)
Well I ran the Whaler 27 WD Cuddy with 19 inch Vipers and it definitely ran faster and the wake looked better. However, when I calculate slip, its still very high. Now I'm wondering whether engine synchronization or lack of synchronization is a factor and suspect that it is.
Fundamentally, I know that it is desirable to have the engines in-sync. Without a sophisticated electronic synchronizer, how does one know when the engines are really in sync? The OEM tachometers are probably not accurate enough so sound and speed measurement seems to be the only mechanism. Any advice regarding synchronization from those with twin screw Whalers would be greatly appreciated.
posted 10-03-2003 08:34 PM ET (US)
Peter, regarding twin engine synchronization, my 200HP C.R. Mercs, in 1989, came with Mercury's dual engine synchonizer gauge, which can be wired to point to either the fast or slow engine. The gauge is extremely sensitive, 25 RPM or less, and takes it the reading off the engine tach sender wires, not the tach gauge readings. It will actually show tach calibration errors! Of all the gauges on my boat, this is ABSOLUTLEY THE MOST USED gauge in the boat, and I have installed it right next to the throttles (Cetacea Page 2), for easy visibility. I liked it so well, that I added it to my 18 Outrage also. This gauge, complete with a separate sending unit, costs about $180., and should work with any brand engine. Highly recommended if you buy the boat. I would not be without mine. This gauge may be manufactured for them by Teleflex. The separate sender box, with 6 wires, mounts concealed under the dash
As for prop slippage, I have been wondering if the WD unit is giving off a lot of turbulence, causing the prop slip, since a Viper style prop should bite pretty well. But it can't bite if the water is too badly aerated. I have absolutely no experience with the 27. Just (and only) a hunch, since on the 20-25 WD boats, there is some evidence of this in the lowered top end. I recently cruised my 25 Outrage with an identical 25 Revenge WD, even with same 200HP Merc engines (except for year), same gear ratio and same pitch Laser II props, and at any given boat speed, his RPM were about 400-500RPM higher than mine, indicating more prop slip. Perhaps BW could give you some information as to whether this is the case. I do know that with my Armstrong bracket, the engines can be mounted higher and run in "cleaner" water, giving less slip and more bite.
Perhaps a removal of the WD and conversion to an Armstrong would improve things? Is the hull bottom OK, or are there any senders down there causing turbulence? Engine mounting height OK?
Full transom 27, huh? Sounds pretty nice! When I first saw your 22 up for sale, I thought we might be losing you. Now you're even going to become a convert to twin engine power!
posted 10-03-2003 09:14 PM ET (US)
Well I am now the owner of a 27 WD Cuddy. What a solid boat and great ride. The generous cockpit reminds me of a Bertram 28.
I think the performance reduction in the Whaler Drive equipped boats is because the WD acts more like a hull extension rather than as a bracket. On the 27 (and I assume this is the case with other WD Whalers), the vee of the hull is carried right on to the WD's transom. The only difference between this setup and a continuous hull is that there are a couple of "steps" at the point where the WD and the real transom meet.
Thus, the 27 WD and a 27 equipped with an Armstrong or other brand bracket are really different boats and should act differently in my estimation. The difference is that a 27 hull with WD is really a 29 ft boat verus a 26 ft, 7 inch hull with a 30 inch offset bracket. In the case of a bracket, I would assume that the motors can be mounted higher relative to the real transom than in the case of the WD. That will have a significant impact on the peformance. In summary, I think that the peformance of a 27 WD hull should be compared to 29 foot boats rather than a 26 or 27 with a bracket, like a 26 Regulator, for example.
I will be looking into whether the engine height is optimized. Currently, the motors appear to be mounted through the second hole (middle hole, or "one hole up".
Thanks for the info on the synchronizer. I'm going to look into getting something like that. Seems like a critical instrument for twins engines. I knew I was getting close to having the engines in-sync today as they started to sing a tune. That "in-sync", at speed, sound is intoxicating.
posted 10-06-2003 04:54 PM ET (US)
Sounds real nice Peter. Keep us updated on your experiences with the new boat and engines.
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