CONTINUOUSWAVE

Crouch's method came to my attention in a book, PROPELLER HANDBOOK, written by naval architect Dave Geer. The original speed predictions and hull constants were for boat speeds in nautical-miles-per-hour, but I preferred to use statute miles per hour, and emended the formulas to give the predictions in that speed unit. I created an on-line interactive calculator that was quite unusual and was the first known implementation of Crouch's method in an on-line calculator.

Although another on-line calculator attributes this analysis to Mercury Marine, the method is identical to Crouch's analysis. The basis for Crouch's Calculator was the observation that boat speed is proportional to the power-to-weight ratio to the 0.5 exponent times a constant that was scaled for the type of moderate v-hull planing boat being analyzed. For a typical Boston Whaler hull the hull factor of 180 can be used to get a reasonable estimate of hull speed potential in MPH.

As mentioned in the text that explains hows the Crouch method is based, the total weight of the boat is the weight and all its contents that must be used in performing the calculation of speed.

This calculator is based on a formula developed by Naval Architect George Crouch who showed that a useful estimate of the speed of a moderate planing hull could be derived from the boat's weight, horsepower, and a coefficient factor that would scale for the hull type, propeller efficiency, and speed units used.

I am again very pleased to announce that another boat performance calculator, the Crouch's Calculator, has now been restored to operation. The new version is a complete re-write of the old calculator with a new code base. The new Crouch's Calculator can be found at

http://continuouswave.com/calculators/crouchCalc.php

Like my other calculator, the Propeller Calculator, I explain the underlying basis for the calculations and the relationship of all the parameters to each other. Unlike some other on-line calculators that perform the same calculations, I give full credit to naval architect George Crouch for the method employed and to Dave Gerr for publishing the method in his excellent book, "Propeller Handbook."

Regarding the usefulness or accuracy of the Crouch Speed Prediction Method as embodied in my Crouch's Calculator, I can offer the following demonstration.

From data for my own boat, I have established values for horsepower, weight, and speed, as follows:
HP = 225
LBS = 4300
MPH =39 to 40

Using those data, the calculator returns a hull factor coefficient of 170 to 175.

Using a median value of hull factor as 172.5, I could predict the boat speed with 200-HP: 37.2-MPH
Using a median value of hull factor as 172.5, I could predict the boat speed with 250-HP: 41.6-MPH
Using a median value of hull factor as 172.5, I could predict the boat speed with 300-HP: 45.6-MPH

Interesting, I no longer have any of my Whalers but I did try the calculator with my World Cat. Using the recommended coefficient of 200 (Long and narrower hulls with with flatter running surfaces may have a higher hull factor coefficient, perhaps 200 or higher) I came up with a theoretical speed of 40.9.

The calculated speed is what I actually get providing the estimate is in MPH. My WOT speed by GPS is 35 knots.

The term called the Hull Factor Coefficient really converts ALL the units involved. The formula works only for power in HP, weight in LBS, but can work for speed in either statute-miles-per-hour (MPH) or nautical-miles-per-hour (NMPH or KNOT) by just adjusting the coefficient.

In the example you cited, a coefficient of 200, the prediction would be intended for finding MPH. If you got 40.9 that means 40.9-MPH. Since your measured value was in NMPH at 35-NMPH, the equivalent speed is then 40.3-MPH.

For your hull, if you want to calculate the speed directly in knots you could use a coefficient of 200/1.1515 or 174.

I have never tried data from a catamaran hull before, but it appears to work well.

Of course, I assume you had good figures for HP and LBS.

jimh wrote:I assume you had good figures for HP and LBS.

I used educated guessing and advertised numbers. The boat is equipped with twin Yamaha F115. The advertised dry weight is 4,900-lbs. I estimated with 40-gallons fuel, other ancillary gear, to get a total of 5,500-lbs.