From Soundings Magazine
A semiplaning hull such as this is supported by both buoyancy and dynamic lift.
That brings us to this month’s topic, although the preamble was necessary to fully appreciate what the semiplaning hull can do. Any design, whether displacement or planing or in between, has capabilities and limitations. What’s helpful is to understand what they are so you can make an informed choice about which combination of compromises produces the boat that best meets your needs.
A semiplaning hull is simply one that operates in the gray region between displacement and full planing, a speed range at which the hull is supported partially by buoyancy and partially by dynamic lift. It is further made gray by the observation that a 40-foot hull at 20 knots is running at an S/L of more than 3, so it is, in fact, fully planing. But 20 knots is close to the top of its speed range, not the bottom, and that makes a very big difference in the boat’s performance, as we will see.
In the simplest terms, the semiplaning hull bears little resemblance to the displacement hull, but quite a lot of resemblance to the planing hull. As such, let’s compare the semiplaning hull with its full-planing counterpart.
Bilges and chines
For a semiplaning boat that routinely cruises between 12 and 16 knots, either a round bilge or hard chine hull will do the job. At 16 knots and higher, however, the hard chine is the way to go. That’s because hard chines start producing significant flow separation — deflecting water flow and spray away from the hull surface — at these higher speeds.