Have you thought of changing your prop?

'Propping' White Maiden, my twin engine BWSeaCat

Changing props on White Maiden was not quite as trivial as my previous boat that had an outboard, as I had to have standard props modified. Basically the standard 12" (Diameter) x 13 3/4" (Pitch) was over pitched and trying to push too much water per revolution. I could only get the engines to a max of 4700 rpm and therefore could not reach the ideal 5500 rpm. The next standard prop at 14" (Diameter) x 11" (Pitch) prop allowed me to exceed 6000 max rpm if I kept pushing it, which is worse for the engines.

So I needed a pitch that was not pre-built and readily available at something like 12" x 12 1/4", but I could order these through BHG of Lymington. These modified props allowed me to rev to 5200 rpm and were fantastic. Fuel economy is superb on this boat.

Read on about my previous experience and the theory if still interested.

On my old Shetland 570 single engine

On my previous boat a Shetland 570 I changed the engine from a Yamaha 60HP 2 stroke (High Thrust) to a standard Yamaha 60HP 4 stroke. To get the best from your engine you have to get the boat going along at the Maximum recommended RPM’s with Wide Open Throttle (W.O.T). In my case this is now 5500 to 6000 rpm. Ideally when the boat is relatively light with a clean bum it should do 6000RPM. When I first tested Sundowner with the new engine I could only run at a max of 5500 revs W.O.T.

So following advice and having some previous experience I knew that to achieve the manufacturers recommended W.O.T. maximum RPM’s I needed to move down on prop size. I swapped the 11” x 11 5/8” aluminium prop for a 11¾” x 10” aluminium prop. What a difference! It now gets ‘up and out’ of the water very quickly and onto the plane very smoothly indeed. Previously with the slightly bigger prop it struggled just a little, just at the point, where the boat wanted to get ‘up and out’ of the water. I can appreciate this much more now as I had the same problem with my old 2 stroke engine, as that would only reach 4900 - 5000 rpm when re anti-fouled but then it would always go down to 4700 after being on the mooring for a month or two. I guess it is a bit like cycling up hill in the wrong gear.

With my new (ish) 2002 engine ‘re-propped’. I could push along at 24Knts (27.6mph) with the small prop as oppose to 22.5Knts (25.9mph) with the same load and in the same sea conditions on the old prop. It feels right and I recommend that if you are NOT pulling max revs or close to it and your engine is new or nearly new then you should re-consider your props pitch size. It is likely to make a big difference.

There must be some risk to doing this on an older engine that has never seen its full potential, as everything tends to wear in. The huge advantage to me after the change was that she did not struggle when my whole family were aboard. We ran well at 24Knts and 5700 rpm (WOT) easily over to Yarmouth on a sloppy day when the bottom was not that clean either. The next test was to beat and not succumb to the weed and barnacle growth building on the hull over the autumn and winter period. She did not struggle when towing the donut or pulling my buoyed anchor out with the engine from greater depths. I can tell you that it now feels more responsive and somehow like a more powerful engine, even though the prop is actually smaller. If you want more technical information on this then read on.

What Yamaha say about their aluminium props

Although aluminium is the lowest-cost option for most engines, Yamaha aluminium propellers provide better performance than other aluminium propellers because of their precise matching to Yamaha engines. Their lightweight and lower cost also makes them an excellent choice as a spare propeller, in case the primary propeller is accidentally damaged striking an underwater object.

NOTE: Speed ranges vary for different models because of different engine operating ranges and different gear ratios
. It is generally best to pick the lowest pitch propeller that produces engine speeds near the maxi
mum engine speed ranges shown in your Owner's Manual. The propeller speed ranges shown are only a guide. More accurate information for a particular boat may often be obtained from the boat manufacturer directly or by visiting the Yamaha web site at www.yamaha-motor.com and checking out the Performance Bulletin section. The best way to select the best propeller for a particular boat and customer is by testing various propellers on the boat.

Here is the Theory

Propeller Selection

It is important to us, as boaters, to understand the basics of propellers. This will enable us to determine the propeller that is best suited for our application. The following is only a guide; to save time, ask your marine dealer or boat manufacturer for advice on selecting a propeller.

The propeller on your outboard motor is the means by which the horsepower, developed by the engine, is converted into thrust to propel your boat. As such, its care and selection is very important to insure continuous service and satisfactory performance.

Several propeller options are available to you, if you want to change your performance for better top end speed or to increase load carrying performance.


Composite and plastic propellers are generally used for emergency situations. Aluminium propellers, being the most common, have reasonable durability for most applications.

Stainless steel propellers are stronger than most all other propeller materials. Some advantages are longer life and high hydrodynamic efficiency derived from stiffer blades.


Diameter is simply the width of the circle described by the rotating blades.


Pitch is the angle of the blades expressed in inches of theoretical travel in one revolution of the propeller. This propeller has a pitch that will theoretically result in 24in. of forward travel in one revolution. In use, the propeller experiences slippage so that its actual travel per revolution is less than the stated pitch.


Rake is the measurement of the angle of the tilt of the blade's tip toward or away from the gear case. The angle is measured on a line extending from the centre of the hub through the centre of one blade.

Propeller Cup

Cup is the added curved lip on the trailing edge or blade tip. This added curvature will increase pitch when added to the trailing edge and increase rake when added to the tip. Cupping a propeller will cause a decrease in RPM. The actual amount of RPM decrease is dependent on where, how much, and the quality of the cupping. Cupping also tends to decrease ventilation and allows higher trim angles and transom settings.

Propeller Hubs

Most propellers have a splined bushing in the hub that attaches the propeller to the propeller shaft. The bushing is mounted to the propeller with flexible rubber. This rubber acts like a shock absorber. If the propeller strikes something hard, the rubber flexes and helps protect the gear-case components from damage.


There are several specifications that are used to define a finished  propeller. Besides the part number, a propeller is normally identified by two numbers, such as 13 x 9, followed by a material identification, aluminium or stainless steel. In the number sequence 13 x 9, the first number is the diameter, and the second number designates the propeller pitch.


Steering can be a real chore with a dual engine power setup. When the props turn in the same direction, the boat tends to list and steer off course. Keeping an even keel and true course requires constant attention, especially in choppy water and high winds. Much of that problem can be solved by having two props turn in opposite directions. In other words, counter-rotation. The major advantage of counter-rotation is its ability to enhance performance by reducing steering effort in all RPM ranges.

Most single engine boat setups normally operate in forward motion using clockwise rotation of the engine and gear-case. Although counter-clockwise rotation setups have been used since the creation of the outboard engine, the use of counter- rotation has become more prevalent in the last decade. This increase is largely due to the manufacturing increase of larger twin engine recreational boats and performance boats.

To better understand how counter-rotation enhances performance, you must first understand what happens under normal conditions using clockwise rotation propellers. Clockwise rotation propellers, when turned in the same direction, will tend to list or walk to the right side of the direction they are moving forward in.

The illustration below shows two effects of clockwise (Right Hand) propeller rotation. The listing of the propeller to the right, pulling the gear-case in the same direction, and the effect of propeller torque, causing the boat to roll over to the port side.

The illustration below shows how clockwise (Right Hand) propeller torque forces a boat into a right hand turn.

Left-handed or counter-clockwise rotation is usually accomplished by using a special gear-case configuration that rotates opposite of engine rotation under normal operation. Counter-rotation is mechanically achieved by driving the gear clutched to the prop-shaft in counter-clockwise rotation when shifted into forward gear. A counter-rotation gear-case is completely different than a standard gear-case, as it is comprised of special parts that provide strength and durability to accomplish this reversed rotation.

To select the optimum propeller for the use of your boat and motor combination, keep the following guidelines in mind:

1.     Have a selection of propellers to test with.

2.     Use an accurate tachometer to measure RPM.

3.     Use an accurate speedometer or some means to measure boat speed.

4.     Safely test each setup at W.O.T.

5.     Make sure every test is with identical settings. (i.e.: Boat load, trim angle, engine height, water conditions, etc.)

6.     Power trim equipped engines should be tested at their optimum trim angle. Optimum trim angle is the highest trim position that can be run without excessive venting (slippage) either in a straight line or in turns.

Check your Operator's Manual and note the RPM of the engine at its rated horsepower and the recommended operating range. Your objective is to check to see that the propeller allows the engine to run near the rated RPM, but under no circumstances outside the recommended operating range.

Make several runs in opposite directions, and determine the maximum RPM and speed. If RPM is lower than recommended, select a lower pitch propeller and make several runs recording RPM and speed.

Testing Guidelines After testing is complete, if results show:

1.     RPM is too low at W.O.T.

o        Reduce pitch and retest

2.     RPM is too high at W.O.T.

o        Increase pitch and retest

3.     RPM is within recommended range

o        Verify RPM

The correct size propeller will provide the best overall fuel economy and performance.

Things to Remember

1.     Engine RPM at W.O.T. must be within the recommended operating range with the rated horsepower RPM as the target number (see specification in Operator's Manual).

2.     Increasing or decreasing propeller pitch directly affects engine load throughout its RPM range.

o        A high load (high pitch and/or diameter) on an engine will result in lower engine RPM throughout its entire RPM range for a given throttle setting.

o        A light load (low pitch and/or diameter) on the engine will result in higher engine RPM throughout its entire RPM range for a given throttle setting.

3.     A boat's hull design may limit its overall performance; even a highly efficient handmade propeller will not overcompensate for an inefficient hull design.

4.     Transom height, angle, and engine trim angle all affect propeller performance.

5.     Atmospheric pressure, temperature, and humidity all affect engine performance, which directly affects propeller performance.

6.     Salt water is more buoyant than fresh water; this may cause some hulls to run faster than identical freshwater setups.

7.     Water conditions can play a big part in boat performance and propeller efficiency.

8.     The boat's load and position of the load can significantly affect performance.

Ventilation and Cavitation

Occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate
Large plate cast into gear case housing directly above propeller. Helps reduce surface air from being pulled into blades.

The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in non resistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapour pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:

1.     Ventilation can lead to excessive slippage.

2.     Cavitation can lead to ventilation and/or slipping.

Operating Range

The following chart illustrates a typical outboard motor horsepower curve. The curve represents data that is determined by running an engine on a dynamometer through the RPM range up to W.O.T. (Wide Open Throttle). This chart indicates a peak of 50 HP at 5000 RPM. 50 HP would be the engine's overall power rating.

The recommended W.O.T. operating range is determined to permit operating latitudes without sacrificing engine durability or performance.

The recommended W.O.T. operating range for this motor would probably be 4500-5500 RPM.

The operating range is stated on the specification page of the Operator's Manual and is used when selecting a propeller.


Check your propeller often for nicks, rolled tips, or bent blades. Any distortion from normal will cause a loss in performance and can create vibrations harmful to the engine. A propeller with worn blades will allow the engine to accelerate beyond the recommended operating range which can result in damage to the engine.

Keep the bottom of the boat clean. Tests prove that cleanliness plays an important part in boat performance.

A fouled bottom, an accumulation of marine growth, moss and barnacles in sea water, and the accumulation of dirt, slime, lime, and other matter in fresh water, is the major cause of poor boat performance. Cleaning the boat's bottom frequently during your boating season will greatly improve boat performance.

To show the dramatic effect cleanliness plays, a boat was left anchored in salt water for 40 days. The running tests were made with a 35 HP engine which was removed from the boat after each test. A three-man load was used.


·         At beginning of test . . . 25.0 mph (40 km/h)

·         After 10 days in water. . 21.5 mph (35 km/h)

·         After 20 days in water. . 18.5 mph (30 km/h)

·         After 40 days in water. . 13.0 mph (21 km/h)

 Back to Small Boats Index