A Full Displacement hull is the theoretically purest of designs meant to travel efficiently through water by gently displacing the liquid in which it floats. Importantly, the chines of true displacement hulls are rounded the entire length of the vessel. This roundness offers little resistance to forces which is critical to its efficiency. However, forces intent on inducing roll, whether under way or at anchor, can have their way. In following seas, they handle well. Krogen and Nordhavn brands are examples of this design.

 

The speed in knots of a displacement hull vessel is calculated at 1.3 * LWL½ (square root of the waterline length of the boat). Relatively little power is required to maintain a boat at this hull speed. Single engines in the 150 – 300 hp range are the norm. Twin configurations are in the 200-300 hp each range. Most 40’ – 60’ displacement trawlers have hull speeds of around 7.8 – 9.5 knots. Fuel consumption can range from 4 – 8 gph in most cases. When greater speed is required, however, the hull must change to a planing or semi-displacement hull.

In a Semi-Displacement hull, the chines harden and the bottom flattens in the first third of the boat. Large engines (typically twin 400 – 800 hp) are installed. They can drive the boat faster, even if it does not actually plane, or ride up upon the surface of the water instead of partially displacing the water. Fuel consumption commonly ranges from 12-30 gph except at wide open throttle when it can double. Turbulence at the stern as the displaced water tumbles back onto the transom creates drag. Following seas hit the square stern of the boat and sometimes make handling a bit of work. Grand Banks, Marlow, Fleming, Offshore, and Outer Reef are some examples of these hulls. They routinely cruise in the mid to high teens and often can top out over 20 knots.

We describe Howard Chen’s Selene hull design as having the best characteristics of the full displacement hulls without some of the disadvantages. The chines harden up about two thirds of the length back from the bow and the bottom flattens out. This reduces the tendency to roll in beam sea conditions under way, or in a surge when at anchor. Some models of the Selene (53, 59) can be built with the optional Cruiser Stern which provides rounded sides and aft section to disburse forces from following seas and thus have the same seakindly characteristics as the full displacement designs. It is standard on the Selene 48. It also increases LWL providing a marginal increase in hull speed.