Talk:Displacement–length ratio

The Displacement to Length ratio presented on this page comes from US Sailing (or similar national governing body) and is a reasonable method for comparing boats. However, look at Wikipedia’s Boating Encyclopedia and you see that Displacement can be different depending on which entity is using the word displacement. The actual literal definition of displacement is the actual weight at any given time divided by 64 (in salt water) since that will be the VOLUME of water that is displaced.

The intention of D/L is to be a unitless number, which is why the LWL (Length of the waterline) is cubed. So a “more true” definition of D/L would be

Weight in Pounds divided by 64 (in salt water) which would then be in cubic feet.

Divide that number by LWL cubed which would also be in cubic feet.

The reason that the National Governing Bodies typically add a factor is because the resulting calculation doing it the "literal" method will result in a very small fraction (typically less than .01). Folks just aren't used to dealing with fractions or decimals. Since the unitless number is just for comparison, it makes sense to "boost" the value into the hundreds.


 * I get a factor of 28566.36600991 to convert the ratio based strictly on LWL, displacement, and seawater density to the value produced by the boosted formula.  That seems weirdly arbitrary.  Was it based on getting boats to average around 100, or something like that?--Tedd (talk) 06:50, 17 January 2013 (UTC)

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