What prop should I use?
As a general comment to start off with, propellers is sort of black magic and trial and error is a big part of learning, for this reason many of the statements below are just that, general but there to give a direction to where to move as a next step.
First, determine how the boat will be used or what the normal load will be. If this boat normally operates with one specific passenger load, propeller size is relatively easy.
If it has multiple uses ranging from light to heavy loads, the selection of one or two propellers may be necessary.
The best propeller size for the boat and engine combination is based on the recommended operating range at wide open throttle(w.o.t.) for the engine, which can be found in user manual and technical documentation. This will be expressed in terms of a certain horsepower at a certain RPM
The goal in prop selection is to determine what propeller style and size will maximize performance for the boat, while allowing the engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving yet allow it to reach the RPM where maximum horsepower is produced.
Run the boat/motor at w.o.t. under normal operating load to determine maximum RPM that its able to obtain. A reading of the RPM and engine load % is necessary for this test. Adjust the motor trim angle for optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM isnot exceeded. If the test results is being able to over-rev the engine, you need to increase the pitch of the propeller.
Increasing the pitch increment by 1" will resulting approximately 200 RPM drop. If testing shows only able to obtain a RPM some what lower than the maximum rating given, the need is to decrease pitch. Decreasing pitch would increase your RPM.
Example:
• Operating Range = 4100-4300 RPM
• Target = 4200 RPM
• Tachometer Reading = 3800 RPM (- Difference = 400 RPM)
Action: decrease the pitch 2”, example SOLAS 16X193800rpm, change to SOLAS 16x17 to obtain 4200rpm
Most modern propellers are cupped for high performance. Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.
Once wide-open throttle RPM falls within the recommended range, you have a propeller that is suited correctly for the boat with respect to RPM. If the use of the boat is for multiple purposes such as fishing, cruising and crew transfer, one prop probably won't do all three things equally well., if the goal is to have the very best performance for multiple activities and the user is willing to change a prop from time to time and have two propellers then this is the most suitable solution.
It is imperative that the wide-open throttle RPM fall within the range specified by OXE as the engine manufacturer.
Why dialing in the correct propeller
Often the stock outboard props with which most out boards are equipped is a standard prop that will work for many different applications but may not work well in a specific use since they go on all different kinds of boats.
A standard prop that may or may not match the way the boat load or the usage of the boat. Since the prop has fixed diameter and pitch, it is limited in its use and may not provide satisfactory performance for every combination load that will be encountered.
One important fact to note is that the propeller moves the boat through the water at a specific engine RPM, and horsepower (HP) is directly related to that RPM. But in most instances the full benefit of the possible HP is not realized as the rpm range is not achieved. Along with the HP rating ex 300HP, equal emphasis should be placed on the RPM at which the related HP is developed, 4200RPM. This is where the propeller comes into the picture.
Outboard engines are designed to run at peak RPM for full efficiency. Excessive RPM with its increased friction and wear is harmful. It is equally harmful to run the engine overloaded to the point that it cannot achieve its rated RPM.
This results in excessive carbon build up with subsequent problems of poor fuel economy, premature wear as well as scoring of the cylinder walls and even damaged pistons.