Power Boat Hull Types    >>   SEMI DISPLACEMENT HULL

As the name suggests these hulls fit neatly in between the displacement and the planing hull types. The stern of the Semi-displacement hull is lower and designed to be always below the water. The hull can be round bilge form but is generally of the 'Hard chine' type. These hulls have less fullness than a full displacement hull. The chine line runs aft with a small curve from where it enters the water and on back to the transom. The hull sections are designed with a moderate Vee.

The semi-displacement hull will out perform the displacement 'Hull Speed' rules and will accept additional power to convert it to additional speed however there are limits to this benefit. Generally speaking for vessels with 30 ft 9.1 M to 60 ft / 18.3 M waterline length, you should only consider Semi-displacement hulls if your speed requirements do not exceed 12 to 18 knots. As you have seen with displacement hulls additional power is wasted, however with semi-displacement hulls often the extra power may be utilized to advantage. If you already have access to a certain size of engine; or you already own the engine(s), then this factor may assist you in making the decision as to which type of hull best suits your situation.

As with Displacement hulls, Semi-Displacement hulls can be driven harder, but at the expense of greater fuel consumption and again the stern will tend to dig in at higher speeds. Existing semi-displacement hulls can be made to achieve extra speed with the same horsepower by adding trim tabs or planing wedges at the stern. The trim tabs and the wedges will be fixed after trials are completed to establish the best angle. In no case should you try to improve the performance of your hull in this manner without the assistance of professional advice. If you are building a Semi-Displacement hull, you should try and keep the weight to reasonable levels. The Semi-Displacement hull is a good weight carrier but it takes additional power and fuel to get the best out of an over weight boat of this type.

This is the type the writer would personally choose when planning to undertake extended cruising, that is cruising that regularly involves cruising distances of over 100 miles from home base.