Copyright 2019 - 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

Although my Strike 16 and 18 trimarans both have hard chine hulls, when I started developing a new racier small trimaran design I initially drew a round bilge main hull. However I thought it would be worth comparing it to a hard chine hull shape, as obviously the latter is much easier to build and trail.

Fortunately a computer makes it very easy to modify existing hulls. A couple of mouse clicks changed the spline tensions and converted a round bilge hull into a hard chine one with no other input from me, (although I did need to adjust the draft slightly to keep the total displacement the same)

I was very surprised by the results.

As I had kept the length and displacement the same the Slenderness Ratio stayed constant, as did the Prismatic Coefficent. These are two of the main factors affecting wave drag. The other major factor needed to reduce wave drag (and thus to increase top speed) is a high LWL/BWL ratio.

So my first surprise was that the WL beam of the hard chine hull was less than for the round bilge hull, hence the LWL/BWL ratio is higher/faster. Much more surprising was the fact that the wetted surface area was essentially identical, and WSA is of course the main factor affecting low speed drag.

So the implication is that a chined hull will have similar speed to a round bilge hull in light winds, and be faster in a blow.

As I say, that was for a small hull drawn essentially by the computer, not by me (so obviously the final design still needs my input). However logic says that these comparisons will also apply to larger boats. And it did show that a hardchine hull is better than people think, and that a double chine hull (as used on Flica, Romany etc) is better still. Certainly it has convinced me to use it for my new racing trimaran.