Using Crouch's Formula to Predict Waterlogged Hull

Optimizing the performance of Boston Whaler boats
Striper15
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Using Crouch's Formula to Predict Waterlogged Hull

Postby Striper15 » Sat Jul 29, 2017 11:32 pm

Crouch's formula is used to calculate the speed potential of a hull based on HP, weight, and a "hull constant". From my research, I believe the hull constant for Boston Whaler hulls is typically 180 to 200.

Crouch's Formula:
The Square Root of (Total Shaft Horsepower / Weight ) x Constant = Speed

I was wondering if it might be possible to predict water in a hull based on this formula and solving for weight. I realize that the hull factor is the wildcard but it would seem that if the hull constant for the specific BW models doesn't already exist that it could be estimated based on multiple user trials. Or, as Jim H. discusses in his topic above "For a typical Boston Whaler hull the hull factor of 180 can be used to get a reasonable estimate of hull speed potential".

Comments?

number9
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby number9 » Mon Jul 31, 2017 12:10 am

hull factor of 180

Good factor for me when used for a 18 Outrage. Hull factor not much of a wild card.

I was wondering if it might be possible to predict water in a hull based on this formula and solving for weight

There are too many other ways to easily/economically check weight and compare against known ideal weights. Formula good way to predict hp/speed and easier/cheaper than swapping motors. Doubt it's very accurate/useful for your questionable purpose.
......

jimh
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby jimh » Mon Jul 31, 2017 9:57 am

Crouch's speed prediction formula relates three elements:

--weight
--power
--speed

You can solve for any of the elements if the other two are known. The accuracy of the predicted value is the same in any form of the equation.

The hull factor is influential and is an estimated value so it's a fourth element in the formula.

jimh
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby jimh » Mon Jul 31, 2017 11:09 am

Determining water in the hull by weight of the hull is discussed in the FAQ. The method is not very precise, even if the hull were weighed on a very accurate scale, unless the hull weight is grotesquely higher than expected.

jimh
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby jimh » Sat Aug 19, 2017 9:54 am

Striper15 wrote:Crouch's formula is used to calculate the speed potential of a hull based on HP, weight, and a "hull constant". From my research, I believe the hull constant for Boston Whaler hulls is typically 180 to 200.


Could you describe your research into the range of hull constants for Boston Whaler boat hulls in more detail? I came to the same conclusion, and I am curious to know what method you used to arrive at the same results.

Yellowjacket
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby Yellowjacket » Sun Oct 01, 2017 2:22 pm

There are too many variables in setup that can creep into the equation. Prop pitch, trim angle, weight distribution and too many other variables would distort the effect of the overall weight. Unless you're carrying a few hundred pounds more is isn't going to show up.

If you've lost speed and know that the engine is still strong then you could compare the speed before and after and that might be valid.

But trying to compare it using a generalized coefficient is a bridge too far.

jimh
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Re: Using Crouch's Formula to Predict Waterlogged Hull

Postby jimh » Tue Oct 03, 2017 6:08 pm

The fundamental problem in the proposed method to deduce boat hull weight due to added water remains: how much should the boat weigh if there were no water absorbed? Any method of deducing presence of absorbed water that depends on measuring hull weight has this limitation: without knowing the original weight of the hull at time of delivery from the manufacturer, it will be impossible to know if the present weight represents a gain in weight due to absorbed water. This problem exists as a fundamental stumbling block, without consideration for use of the Crouch performance prediction.