A measure of your ease of motion thru the water


The quest for a low displacement to length ratio.

We generally refer to boats by their overall length. I do that myself. But if we need to find one single metric to describe the idea of sensori, then that metric is the displacement to length ratio (DLR). 

When asked what kind of boat we own, whether power or sail; the first descriptive we usually reach for is either the brand or the length, and if we put the brand (or the designer) first, then the length is likely to come second. Very rarely do we hear the displacement to length ratio put forward as a primary  factor to describe the way we travel.


OK, so the displacement to length ratio is a little but more complex than just the length measurement but if we have that figure handy and we're talking to someone who understands its relevance; it tells us more about the performance characteristics of the design that any other number.


Consider the vessel as a given mass, that is; the total weight of the boat and everything it carries. The longer the hulls for a given mass (displacement); the lower the displacement to length ratio, the more easily driven the hulls are and the more sea kindly the motion of the vessel in a seaway.



Determining the Displacement to Length (D/L) Ratio:

You don't often see  the D/L ratio published in a brochure or even in the designer's specifications, but it's not hard to figure out if know the sailing weight of the boat with reasonable accuracy.

1. calculate its displacement in long tons. One long ton equals 2,240 pounds or 1018kg.

2. Multiply the length of waterline in feet  (LWL) by 0.01 

[To convert metres to feet multiply by 3.2808] 

3. Cube the result.

4. Divide the result of 1 by the result of  3.


The formula can be written like this: D/L = DLT (disp. long tons) ÷ (0.01 x LWL)³.

Even easier; you can do a search online and find a calculator that will do the sums for you, and some of them will convert your metric measurements in the process.