Crouch formula C factor # for flat bottom hulls

[Delaney]

What would be a reasonable C factor for something like a 19 or 21 foot flat bottom hull like a Carolina skiff. I was maybe guessing 200 to be conservative. It also seems like hp effects the C because of the great increase in hp to gain little speed. Does a lower hp that moves a hull 25 mph have a higher c than a 50 that moves it 34? Using engine test data and back calculating it sure seems to and makes sense since you have to triple hp to double speed. Thanks for the help.

[Yellowjacket]

Crouch's formula non-dimensionalizes power to weight ratio, and while it includes many other factors it is relatively accurate in that speed of a planning hull is a function of power squared. So changing the power doesn't change the coefficient, for a given coefficient the speed change will be equal to the square root of change in power. Only when you start to go very fast does Crouch (and Savitsky for that matter) break down, so the concept is valid. According to Crouch's formula you have to have 4x the power to double the speed, which is correct.

Since Crouch's coefficient also includes a lot of other things, such as drag of the shafting system, CG location, aerodynamic drag ect., it is dangerous to assume that you will have the same results by just estimating a coefficient. It is better used to predict a change in speed for a change in power. Now, if you have a coefficient from a very similar hull then you can get in the ball park using Crouch's formula. Better to use a Savitsky calculator and put in more information such as CG location and other stuff. That will get you closer to what you are trying to predict.

[Alik]

Coefficient in Crouch formula depends on length also... For 20' boat with normal deadrise I to take it 160...170.

[tom28571]

200 seems to be a little optimistic for a general purpose skiff. I would think that Alik's number of 160 might be ok. If your boat is perfectly set up, light weight for its use and optimally designed, you can shoot for a higher constant. No matter, the correct number will be known when you run the boat

[gonzo]

Since you are talking of a production boat, it is easy to figure out. Clear the C factor in the formula and plug in all the other data which is known.