Propeller Basics: Part 2

This article describes some of the different materials used in fabricating propellers, discusses in greater detail how propellers actually work, and provides a formula for calculating the speed potential of a boat based on weight and horsepower. (Readers may wish to refer to Part 1 of this article, as well.)



Propeller propulsion developed first on inboard powered boats, using a long shaft to connect the engine to the propeller. This meant the propeller was immersed continuously, and thus subject to continuous corrosion from both fresh or salt water. For that reason, propellers tended to be made from metals that could withstand such a corrosive environment on a long term basis, with alloys of bronze proving to be the most popular. With the development of outboard motors, the propeller could be tilted out of the water when not in use. In fact, most outboard boats are based on trailers and are not stored in the water at all. Corrosion resistance became less of a concern, and aluminum came into wide use as a material for outboard engine propellers.


Aluminum's strength and relatively light weight is nicely matched to the application as a propeller on outboards motors. Aluminum is easy to cast into the general shape and then easy to machine to the final product.

To protect against corrosion, most aluminum propellers are painted, generally black or white. If the propeller runs in sand, paint will be rapidly removed, giving the blade tips a distinct band of undercoat or raw aluminum. The tendency for cavitation to occur near the blade tips also works to remove paint from them. Sighting aluminum propellers with some (or most) of the paint missing is quite common at the boat launch ramp.

Aluminum is no match for rocks or even wood in the event of a blade strike, and aluminum propellers tend to be badly damaged when a blade hits an underwater obstruction. Fortunately, aluminum can be repaired. New material can be welded onto a blade, and the shape restored to the original contour. Repairs can be made multiple times, and the repair cost is moderate ($40-$60).

As outboard engines increased in horsepower, aluminum began to reach a limit. All the force being applied to move a boat is exerted through the blades of the propeller. If you have a 200 horsepower engine, all that power is coupled to the water to produce the thrust needed to move the boat by the action of the propeller blades. Thinking of it in those terms, you can begin to appreciate the amount of strength required in a propeller blade to stand up to the forces applied to it. Although you don't ordinarily think of cast aluminum as a flexible material, under forces like those of a propeller spinning with several hundred horsepower applied to it, aluminum does exhibit some tendency to alter its shape.

The strength of aluminum is such that in order to produce blades that can tolerate high pressures and horsepower, the thickness of the blade material must be increased. This is another limitation of aluminum as a propeller material because as a general rule the propeller designer would like the blades to be as thin as possible. Thinner blades have less drag and therefore are more efficient.

Stainless Steel

Stainless steel propellers are twice the cost (or more) compared to aluminum, and they weight at least double, as well. Yet in every respect except weight and cost, stainless steel makes a better propeller than aluminum.

To begin with, stainless steel is more corrosion resistant than aluminum, and it can survive immersion in fresh or salt water without protection from topcoats of paint or other finishes. The finish of stainless steel propellers can be buffed and polished to a high gloss. At extreme speeds there may actually be something to be gained from such a smooth surface. Even when not spinning, a gleaming, polished stainless steel propeller adds style to a boat's appearance.

Stainless steel is also much stronger than aluminum, approximately five times the strength. This greater strength of material permits the propeller designer to reduce the thickness of the blades. The extra strength also reduces damage from contact with underwater obstructions. Many times a blade strike with a small rock or log will result in no damage to a stainless steel propeller; the same encounter would have caused considerable damage to an aluminum blade and required repair or replacement. After a number of such minor collisions, there may not be much real difference in price between aluminum and stainless steel propellers.

There is one concern about the extra strength of stainless steel propellers. Some see the destruction of the blades of an aluminum propeller as an effective means of protection of the shaft and gearcase of the outboard. Better to buy a new prop than to replace the entire lower unit in this point of view. On the other hand, almost all outboard propellers are engineered with some type of break away hub design which provides a flexible coupling between the propeller shaft and the propeller. For many years this flexible coupling was accomplished with a rubber insert between the propeller hub and the actual splined drive shaft coupling. Should a blade strike occur, in theory the rubber would slip and allow the shaft to continue to turn while the propeller was stopped by the obstruction. A problem with this approach was the gradual aging of the rubber, causing it to shrink and become less flexible, eventually resulting in the shaft spinning free from the hub under normal loads. Newer hub designs incorporate a splined plastic coupling which is designed to break away when subjected to extreme loads like those produced by a blade impact.

The extra weight of a stainless steel propeller also creates more momentum when it is spinning and more inertia when it is not. Because of this, shifting in and out of gear may produce a large "thunk" from your gearbox.

Stainless steel is more expensive than aluminum and more difficult to cast and machine. This tends to drive up the manufacturing costs of stainless steel propellers. The goal in choosing a stainless steel propeller is usually improved performance, and thus stainless steel propeller designs tend to be more refined. Such shapes may be more difficult to produce, as well. Thus both the actual costs to produce and the market premium on performance work together to increase the price of stainless steel propellers.


Propellers are also manufactured from non-metallic materials. In low horsepower applications (less than 15 HP), simple plastic molded propellers are often used. Recently a number of "composite" propellers have been marketed for use in mid-range horsepower outboards. Non-metallic propellers are not subject to corrosion from the water, but they may be affected by prolonged exposure to the ultraviolet rays of the sun. They generally cost less than equivalent aluminum propellers. In case of impact with an underwater object, the blades are likely to be heavily damaged. This is often cited as an advantage in that the composite propeller absorbs all the energy of the impact and does not transfer it to the shaft and gearcase. Some composite designs have featured individually mounted blades which can be replaced one at a time as needed. Since no repair of composite blades is possible, being able to replace just a single blade helps to keep the replacement costs lower.

The comparatively low cost of non-metallic propellers makes them attractive for use as spares. Mercury makes a line of such propellers in various sizes just for that application. Some are rather large and would not be practical for long-term use in high-horsepower applications, but they do provide a low cost alternative as an emergency spare. To distinguish them from their other products, Mercury molds them in colors of bright oranges and yellows.


Coupling power from an engine to a propulsion mechanism is an interesting application of physics. We are all familiar with the operation of automobiles, where a transmission is interposed between the engine and the drive shaft. The transmission provides three, four, five, or more gear ratios to better match the engine's torque to the load at varying speeds. There is also a clutch which can introduce some slipping to permit the load to be gradually applied to the engine. All of this is necessary because the engine's output of torque is quite low at lower engine speeds (RPM), and the engine must be operated at a range of speeds (power band) where it can produce sufficient power to turn the drive shaft and propel the vehicle. The gear ratio of the transmission is varied to create a range of vehicle speeds produced by running the engine in its power band. Thus the engine tends to run at a near constant speed, and the gearing of the transmission is varied to accomplish the desired vehicle speed.

On a boat, none of this takes place! The engine is coupled to the propeller shaft by a simple Forward-Neutral-Reverse transmission, which provides only a single gear ratio in each direction. The clutch mechanism provides no slip; it simply engages or disengages. Further, the design of many outboard engines results in their power being developed in a rather narrow range of engine speeds, and only at high engine shaft rotation. Were the drive mechanism of the boat, the propeller, to be a tightly coupled mechanism, most outboard engines could not produce enough torque at low speeds to turn the propeller shaft. Putting the engine in gear would result in the engine stalling immediately.

Fortunately, the characteristics of a propeller provide a solution to this problem. At low propeller shaft speeds, the marine propeller has a great deal of "slip". The propeller turns, but it does not develop much thrust, and therefore it does not require much torque to revolve it. The reason for this is the angle-of-attack that the blades make with the water. At rest and at low forward boat speeds, the angle of a blade's chord with respect to the direction of travel is often as high as 45-degrees or more. At this high angle of attach, the blade is "stalled" and it does not produce much lift (forward thrust). This fact works quite well for boaters because it allows the outboard engine, which is unable to produce much torque at low speeds, to be able to turn the propeller.

As the boat moves forward, water is drawn into the propeller. Although the angle of the blades remains the same, the effective angle-of-attack is reduced by the movement of the propeller through the water. Thus, as the boat picks up speed, the propeller blades reduce their angle of attack and come out of a "stalled" condition, and they begin to produce more "lift" or thrust. At some combination of fixed blade chord angle and boat forward speed, the effective angle-of-attack of the propeller blades reduces to near an optimum angle, generally about 3 to 4 degrees. At these optimum angles, there is very little slip, and the propeller is developing optimum thrust.

As the propeller shaft speed has been increasing, the torque required to turn it has been increasing, too. Fortunately, our outboard engine has been increasing its crankshaft speed and it, too, is entering its optimum power band, producing maximum torque and horsepower to match up with the increasing load from the propeller.

This theoretical stuff may be a bit confusing. Let's look at some real-world numbers. I carefully measured the crankshaft speed and boat speed of my engine-propeller combination at several different revolutions per minute. From this data I was able to derive the "slip" of the propeller. The results were exactly in accordance with the theory described above.

Some Real-World Data

Before I present the results, here is some information about how the data was measured and how other data was deduced from those measurements:

The PROP PITCH was that advertised by the manufacturer for the propeller in use.

The ENGINE SPEED was observed on a reasonably accurate tachometer.

The SPEED OF ADVANCE was based on the technique described in Part One of this article.

The OBSERVED SPEED was measured using a GPS navigation instrument. Water conditions were a slight chop and some other boat wakes with a breeze. The results are a rough average to two runs in opposite directions. Conditions were not ideal, but the data is believed to be fairly accurate.

The SLIP was calculated by subtracting the OBSERVED SPEED from the SPEED OF ADVANCE, dividing the result by the SPEED OF ADVANCE, and expressing it as a percentage. This is the propeller designer's definition of slip.

Here are the results. Notice how the slip decreases dramatically with forward boat speed. There is also a peak in the slip at around 2000 RPM.

Propeller Performance BLACK MAX Aluminum 3-blade
Propeller Test 5/2001
Boat: BW-15 Sport 1976
Engine: Mercury 500 50-HP
Prop: 3-blade BLACK MAX 10.125 Dia. Cupped Blades
1000 6.6 3.7 44.2 Displacement
2000 13.3 6.4 51.7 Plowing
3000 19.9 12.5 37.1 Almost On Plane
3600 23.9 19.3 19.1 On Plane
4000 26.5 23.0 13.3 On Plane
5000 33.1 30.4 8.3 On Plane
5400 35.8 33.2 7.3 WOT


Interpretation of Results

The propeller tests show very good agreement with the theory of how marine propellers operate, and well they should! One nice thing about the Laws of Physics: they seldom mutate. If measurements are carefully made, they tend to show behavior predicted by nature's laws.

As for the peak in the slip at 2000 RPM, this is due to an increase in hull drag caused by wave making. The boat is being driven in displacement mode above its natural "hull speed" and the result is a very large stern quarter wave. This wave making action dramatically increases drag on the hull, and thus induces additional slip in the prop over that seen at other hull speeds where the hull drag is lower.

The results also show that the engine crankshaft speed was able to advance to 5,400 RPM. This is very close to the maximum recommended speed of 5,500 RPM, and, therefore, the propeller has been correctly sized to match the horsepower and the load.

Using Published Results

I was enjoying the JUNE 2001 issue of SPORTFISHING BOATS magazine, reading their test results for a new Boston Whaler CONQUEST 26, when I realized that they had provided me with most of the information I needed to analyze that boat's propeller performance. Getting the gear ratio for the big OptiMax engines from the 2001 Mercury catalogue, I was able to compute the prop slip for the twin engine Whaler with Mercury MIRAGE stainless steel 3-blade propellers.

Propeller Performance MIRAGE SS 3-blade
Propeller Test BW-26 Conquest 6/2001 SPORTFISHING BOATS Magazine
PITCH=17 MIRAGE 3-blade & GEAR REDUCTION 1.75:1 Twin 200 HP Optimax
1000 9.2 5.4 41.3 Displacement
2000 18.4 7.9 57.1 Displacement
3000 27.6 9.7 64.9 (Wake must be huge!)
3500 32.2 27.3 15.2 On Plane
4000 36.8 32.2 12.2 On Plane
5000 46.0 40.8 11.3 On Plane
5500 50.6.8 45.8 9.5 WOT

Talk about having to get over the hump to get on plane, the propeller slip at 3000 RPM is a whopping 65%. That boat must be creating a rather big wake just prior to jumping on plane.

Now that we know how fast the boat actually goes and how much the propeller is slipping, can we determine if our boat-motor-propeller performance is appropriate for the hull and horsepower being used? Some form of speed prediction algorithm is needed.


Boat speed is ultimately a function of weight and horsepower. Over many years of observation, some general approximations of speed potential versus weight and horsepower have been developed. Here is one such relationship, initially suggested to me by Clark Roberts, a long-time Whaler owner and performance enthusiast, based on the well-known Crouch's Speed Predicting Formula

Crouch's Speed Predicting Formula

A great deal of original work with propeller design and study was done by George Crouch. Crouch developed a speed predicting formula for planing hull boats. This formula assumes that the boat has been equipped with a proper propeller so that the overall efficiency of the propeller is in the 50- to 60-percent region. The essence of the relationship is that speed is proportional to the ratio of horsepower to weight by an exponential factor of 0.5 (i.e., the square root).

SPEED = C X (HP/LBS)^0.5


SPEED = boat speed
C     = a constant which depends on hull type
HP    = shaft horsepower at the propeller
LBS   = total displacement (weight) of the boat

The value of the Hull Factor Constant depends on the type of hull. Crouch suggested these values for calculation of SPEED in knots (nautical miles per hour):


C      Hull Type

150    Average runabout, cruiser, passenger vessel
190    High-speed runabout, very light high-speed cruiser
210    Race boat types
220    Three-point hydroplanes, stepped hydroplanes
230    Racing power catamarans and sea sleds

after Crouch
as presented in PROPELLER HANDBOOK,
by Dave Gerr, International Marine, pg. 16

Because boat speeds for smaller recreational boats are often given in miles per hour (MPH), the value of C must be increased by a factor of 1:1.15 to predict speed in MPH, thus:


C      Hull Type

172    Average runabout, cruise, passenger vessel
218    High-speed runabout, very light high-speed cruiser
240    Race boat types
253    Three-point hydroplanes, stepped hydroplanes
265    Racing power catamarans and sea sleds

(the above are simply the values from Crouch multiplied by 1.15)

Based on a number of measurements and reports from fellow Boston Whaler owners, I believe the following values for C are useful in predicting the speed of Boston Whaler boat hulls


C      Hull Type

180    Constant deadrise V-bottom hull like classic OUTRAGE
200    Tri-hull older Boston Whaler hulls like early 13- and 16-foot

If a hull has a Whaler Drive, the value for C will definitely be close to 180. For notched transom boats, particularly ones with set back brackets and engines mounted six to ten inches aft, a value of C=190 will be useful. Other factors will affect the performance, including the condition of the hull bottom (i.e., has bottom paint), and the addition of drag inducing appendages like radar arches, flying tops or other canvas, and other aerodynamic drags.

In all of these cases, the weight must be the total displacement of the boat including fuel and passengers, and the horsepower must be the true horsepower at the propeller shaft. The propeller must be an efficient design that effectively converts the available horsepower into thrust, and the hull must be clean and free of drag inducing anomalies.


I have some accurate data for my boat regarding speed and weight, so let's test Crouch's speed predicting formula. First, since we can't hang the whole boat on a scale, the boat weight must be calculated:


   Hull          550 lbs  (advertised bare hull weight)
   Interior       50      (seats, console, hatch)
   Gear           50      (anchor, line, electronics, bimini)
   Engine        225      (estimate for 1976 Mercury 500)
   Battery        50      (estimate, single 24G)
   Fuel           50      (7 lbs/gal, tank, hose)
   Total Boat    975
   Crew          225
   GROSS BOAT WT 1200 lbs

Next, the horsepower must be established. Although my engine is rated as a "50-HP" motor, I am going to de-rate it about ten percent because it is from an era of crankshaft rated-horsepower, and the formula really wants prop-shaft rated horsepower. Also, the engine is 25 years old and may not be as strong as it was when new. Instead of 50-HP, I'll use 40-HP for the engine. The most important choice is the hull factor, C. Here I will use a value of 190 because the hull is not the a hybrid between the original Whaler sea-sled-style hull and the later moderate v-bottom with constant deadrise.

 190 (40/1200)^0.5 = 34.7 MPH


In a series of recent speed tests, the boat topped out at 33 MPH. This speed was obtained with a 14-inch pitch prop which is a little bit smaller than the optimum speed propeller, say a 15-inch or 16-inch. Still, the observed speed is fairly close to the predicted speed, less than five percent deviation. I have some older data that shows the boat ran about 35 MPH with the 15-inch propeller, which is very close to the predicted speed. Thus, the formula in this case seems to be quite good as a predictor of speed potential. I am sure with the right propeller, engine height, and some other tweaking there could be another mile-per-hour of speed to be had from the boat and engine.


The predicted speeds of Boston Whaler boats based on this formula and their published weights have been pre-computed and can be found in a separate article. In addition, a great deal of testing has been done on my own boat with a number of propellers (11) and based on actual weighing of the boat. Those results show good agreement with Crouch's formula when a hull factor of C=180 is used for the constant-deadrise V-hull of a classic Whaler OUTRAGE style hull with Whaler Drive.


There is probably no other element of a boat that can make as much difference in the performance as the propeller. By understanding more about how a propeller is made, how it works, and what speed potential exists for you boat, you will be better able to evaluate your current propeller and to choose new sizes and styles to improve your boat's performance.


There are still many variables and elements of propeller design and selection to be considered. These will be discussed in future articles in this series.


If you have a comment or question about this article, please post it in the Whaler Forum.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 2001 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared March 29, 2001.