A measure of your ease of motion through the water … how low can you go?

We struggle to find ways to reduce our footprint as we travel. Striving to save the planet is hard work when you're designing, building and voyaging in modern high performance yachts. But there are steps we can take in the right direction without building our boats out of biodegradable materials or figuring out how to turn our old boats into compost for orange orchards.


For a start we need to stop thinking about our boats simply in terms of length and how much stuff we can carry in that length. We need to be mindful of how much air and water we are  pushing around as we travel. Whatever it is we need to bring along; sleeping cabins, bathrooms, sun decks and water toys; the answer is to use efficient hull forms and spread the superstructure lengthwise rather than stacking it wedding cake style. Thoughtful design is part of the solution.


There's a pretty basic formula that measures this ease of motion through the water. It's the displacement to length ratio (DLR). It compares hull length against the vessel's weight and it's a reliable measure of how much resistance we are pushing against in the water as we travel.  It can be a reasonable indication of air resistance as well because if you're heavy and you're not that long it's a fair indication that you have a lot of superstructure above the waterline.  More weight drives the number up. Longer hulls bring it down. The lower the number the better.  


A low DLR comes with a host of benefits. Lower fuel costs, extended range from a given fuel capacity, less sail area to drive the boat if it's a sailing boat. Longer hulls provide a more comfortable ride. And if your profile above the water is modest you have a lower centre of gravity which means less roll, less pitch and less energy needed for forward motion.


The displacement to length ratio is a little but more complex than just the length or the weight 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 any particular 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.


This online calculator makes it easy: