Displacement, Planing and Semi-displacement hulls

When buying your first yacht, it is important to know something of the different types of hull design and bottom profiles, and to understand what set of compromises these will offer you.

Hull designs can be broadly categorised into three different types: displacement hulls, semi-displacement hulls, and planing hulls.

The displacement hull displaces water equal to its total weight. As the displacement hull begins to move through the water, small waves appear at the bow and further aft along the hull. When speed is increased, these waves grow larger and the distance between them increases, until the boat is riding between the two waves at the bow and stern. At this point, the hull has reached its maximum hull speed. Adding additional horsepower (or sail power) after this point only makes the waves bigger, with very little or no increase in the speed of the boat.

There is a simple empirical formula to work out the hull speed of a diplacement hull in knots: 1.39 x the Square Root of the Boat's Waterline Length in feet. For example, if your waterline length is 100 ft, multiply 1.39 by the square root of 100 gives a hull speed of 13.4 knots.

Displacement hulls have several advantages, often overlooked by newcomers to the yacht market. They require relatively little horsepower to drive them through the water, so the fuel consumption of the yacht is much lower. Smaller engines cost less to buy and maintain. Because the yacht pushes aside the waves rather than riding over the top, its ride is smooth and seaworthy. Because there is plenty of hull below the waterline, the yacht tends to be stable and comfortable at anchor (where yachts spend most of their time). And finally the lower speed of the yacht means that appendices such as zero-speed stabilisers and azipods can be added to the design.

First time buyers, however, are too often seduced by the top speed of a yacht. If you really must have speed, then you need either a planing hull or a semi-displacement hull.

When a planing hull is either not moving or going very slowly it works like a displacement hull. However, by adding lots of horsepower, a planing hull will rise up over its bow-wave, causing the boat to displace much less water. As a result, there is much less wetted surface on the hull bottom, meaning less friction as well. The speed of the boat will now increase at a great rate. With this hull, the more horsepower you add, the faster the boat will go.

However, this great speed comes with some big downsides. The boat is now skimming over the waves which is great in flat water, but even in a mild chop the pounding and vibration become very severe. At best, this is just uncomfortable for crew and passengers, but worse is that it greatly adds to wear and tear on the yacht. If the yacht slows down and reverts to displacement mode, the flat bottom of a planing hull and short waterline length make for a rolling and pitching ride. The same can be said for the ride at anchor. The flat bottom of a planing hull also means that the yacht suffers greatly from windage, and so can be hard work to berth, even if fitted with bow and stern thrusters. High speed planing yachts are often combined with Arnesen surface drives for yet more speed, which makes the captain's job even harder when manouevring at low speed. And it is nigh on impossible to combine a planing hull with stabilisers because of the drag they cause (though I might write something about gyro stabilisers soon…). Planing hulls are generally used by smaller dayboats, but some fairly large open yachts (Mangusta and the likes) are designed to be fully planing hulls.

The semi-displacement hull is a design that attempts to combine some attributes of displacement hulls with some features of planing hulls. It is generally used in mid-range flybridge cruisers of 50-100 ft in length. The semi-displacement hull never completely planes, but relies on a flat forefoot and lots of horsepower to drive the yacht a little bit over its bow-wave, generating some lift. Effectively, the yacht is always driving uphill on its own bow-wave – so fuel consumption is very high. The yacht can be heavier than a full planing yacht, so it can be a little more stable in a sea or at anchor, but never as seaworthy as a full displacement yacht.

These basic types of hull can all be refined by the designer to seek the compromise that is required, for the perrformance and sea conditions that the yacht is expected to meet. Some hulls may be v-shaped forward and flat further aft, others may be v-shaped forward and round further aft and so on. There are also a few radical designs that step out of the traditional mould completely – eg check out the M-hull.

But the lesson here is before buying any yacht, think carefully about your real requirements and expectations. Try not to fall into the common mistake of an inexperienced buyer of being seduced by the yachts top-speed. All too often we have clients who have learnt the hard way, and come to us with a high-speed lightweight, high-depreciation, high-maintenance open planing yacht that they now want to trade in for a slow, comfortable, cheap to run, displacement yacht. As one wise client said to us recently, “If I want to go somewhere fast, I use my aeroplane. If I use my yacht, it is because I am on holiday and have the time".

Comments

[tony sansomgower]

Great site! I live on Christmas Island and am building a semi displacement cat for fun. Keep up the good work!

[Scott FEWELL]

IT IS A GREAT SITE... THANK YOU!
CAN YOU TELL ME IF A PLANING HULL CAN BE AS EFFECIENT AS A FULL DISPLACEMENT HULL AT THE SAME "HULL SPEED? (7-8-9 KNOTS)

[lieben]

Interessante Informationen.