Copyright 2017 - Woods Designs, Foss Quay, Millbrook, Torpoint, Cornwall, PL10 1EN, UK
  • home built Flica 37

  • plywood Romany 34

  • lightweight 14ft Zeta mainhull

  • Strike 15 trimaran at speed

  • 28ft Skoota in British Columbia

  • 10ft 2 sheet ply Duo dinghy

  • 24ft Strider sailing fast

  • 36ft Mirage open deck catamaran

There is always some confusion between what is meant by the "weight" of a boat and by it's "displacement". However the differences are actually quite simple.

The weight of the boat is exactly what it says, how heavy it is when lifted out of the water by a crane.

The displacement is the volume of a boat floating to a specific waterline (remember Archimedes) and so varies depending on the actual hull draft of that specific design.

A boat in fresh water will displace more than in the sea, yet it is the same weight. That is because salt water is more dense than fresh. Look at people floating in the Dead Sea, they float much higher than in a bath, yet they weigh the same.

These days I specify the displacement for the designed WL (DWL), ie the one you see on the drawings. So that displacement will be a combination of the weight of the boat plus the load it carries to make it sink down to that waterline.

In the past I would calculate the empty weight of the basic bare boat carefully, and then give a maximum loading, which might mean the boat floated deeper in the water than what is shown on the drawings.

In other words I gave my early, pre CAD designs a weight estimate and also a maximum safe loaded weight which may or may not be the one that matches the DWL. My newer CAD designs have a total weight of boat plus loading to match the DWL. But I assume that some people, including me, will overload their boats so that they sink deeper in the water.

So, as an example. The weight I give for Skoota is based on our own boat. If you look at the just launched photos you will see it is floating with the antifouling line about 120mm out of the water. We put that line at 40mm above the 2.3cu.m line (or 2300kgs for easier visulisation). So checking the lines plan I found that it related to about 1700kgs boat weight when empty. Right now we are pretty much exactly on the 2300kgs line.

The drawbacks to overloading are obviously that the boat is slower and less comfortable to sail - more bridgedeck slap and the boat ploughs into waves rather than rides over, so it's a wetter ride. But it isn't a structural problem as I always design in enough strength to take a large overloading. As a result, in 35 years I have only once had a structural beam failure, but the owner then admitted he deliberately made the beams undersize to save weight.