The load imposed on the engine by the propeller is a function of the propeller design, including the material used, the number of blades, the diameter, and most importantly the pitch. The weight of the boat and the design of the hull are also factors in determining the load created by the propeller.
CALCULATING A TARGET BOAT SPEED
The first step in propeller selection is to estimate what the expected boat speed will be. The boat speed is a function of three elements: the power of the engine, the weight of the boat, and the design of the hull. It has been observed that a close approximation of boat speed can be determined by assessing the power(HP)-to-weight(LBS) ratio to the 0.5-exponent (or the square root), and by using a coefficient factor (C) related to the hull design. From these three inputs an estimated speed in MPH can be determined from a simple formula:
- SPEED = ( HP/LBS )^0.5 * C
The hull coefficient factor, C, is determined from observations, and for a typical monohull moderate V-hull planing boat a value of C=180 is a good choice for calculating the speed in MPH. This formula assumes the propeller will be operating efficiently. Some flatter-bottom hulls may prove to be faster, so a value of C=190 is more appropriate. For exceptionally fast boats which when on plane have only a very limited amount of their hull still in the water, values of C =200 or greater may be more appropriate. For expected boat speeds in the range of 30 to 45-MPH, however, C=180 for moderate V-hulls is a good choice, and C-190 for hulls with a flatter planing surface is an option.
This method of estimating boat speed was proposed by naval architect George Crouch. On that basis I have created a calculator that facilitates predicting boat speed (or any of the other three variables) which I call "Crouch's Calculator." I first published this calculator many years ago, and the reaction to it was generally very positive, with many people impressed with how the Crouch method could predict boat speeds that were actually obtained by many boats and engines.
To complete the first step in propeller selection, one should get an accurate figure for the total boat weight, and accurate engine power output value at full-throttle, and use either 180 or 190 as the hull coefficient (as appropriate to the hull design), and let the calculator produce a target maximum boat speed.
My implementation of the Crouch method in an on-line calculator is available at
Crouch's Calculator
https://continuouswave.com/calculators/crouchCalc.php
In addition to providing the calculator, the above resource also explains further where the method was originally published and what assumptions are made in the method.
An alternative method to estimate what the target maximum boat speed should be is to use existing reports of performance for the same boat with various amounts of engine power as a guide. In the case of a current model boats from Boston Whaler, a detailed performance report is usually provided and can be used as a guide for setting a target boat speed.
Once a target boat speed has been established, the next step is to calculate a propeller pitch that will produce that boat speed.
CALCULATING PROPELLER PITCH
In order to calculate a suitable propeller pitch to permit a target boat speed to be reached, several important inputs are needed. The engine manufacturer's suggested optimum full-throttle engine speed range at which the engine produces its peak power output is usually provided in the engine specifications. This value of engine speed is used to select the proper propeller.
All outboard engines will use a gear reduction to reduce the speed of rotation of the propeller shaft compared to the engine crankshaft. This reduction gear ratio must be known in order to estimate a propeller pitch.
A further input is a value for the characteristic called SLIP, which represents how much less than the actual speed-of-advance will be in water for compared to a screw in a solid material. For optimum-performing propellers, the value of SLIP will typically be around 10 or less.
With these four inputs, engine RPM, engine gear RATIO, SLIP = 10, and a target boat speed in MPH, a propeller pitch is easily calculated. To facilitate the calculation, I have published a Propeller Calculator for many years. See
Propeller Calculator: MPH
https://continuouswave.com/calculators/propCalc.php
The particular feature of my Propeller Calculator is that it can calculate any one of the five values, based on the other four as inputs.
To demonstrate the method I use this data as an example:
To estimate boat speed:
HP = 225
LBS = 4400
HULL FACTOR = 180
The calculated boat speed is then 40.7-MPH
To estimate propeller pitch:
RPM = 5200
RATIO= 1.845
SLIP = 10
MPH = 40.7 (the value calculated by the Crouch Method)
The calculated pitch is then 16.9-inches.
The above data is from testing with my own boat, which is using a 17-pitch propeller. As the data confirms, the outcome in actual use is a very close match to the calculated values.
Using this method a reasonably good estimate for selecting a propeller pitch can be made. This value can be used as a starting point for selecting a propeller. Actual on-water testing will then be necessary to confirm that the boat can accelerate to the target boat speed and at that point the engine speed is in the desired engine speed range specified by the engine manufacturer.
If the engine speed at full-throttle is below the manufacturer's suggested range, the propeller load must be decreased. This is usually done by reducing the propeller pitch.
Similarly if the engine speed is too high, either at the maximum allowed speed or being limited to a safe speed by a rev-limiter, the propeller load should be increased.
An old rule-of-thumb is that changing the propeller pitch by two-inches will change the engine speed by 400-RPM, which will work in either direction according to the relationship that more pitch means more load or lower engine speed and less pitch means less load or more engine speed.