Types of Hulls - 2 : The Displacement Hull

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In our second part of the Types of Hulls series...we talk about the Sumo Wrestler them all - the Displacement Hull !


Pushing it's weight through the water, displacment hulls are the Gentle Giants who are are often misunderstood by prospective buyers since they are not meant for SPEED - that may often tickle the senses of many.

DISPLACEMENT HULLS

Displacement hulls typically have a rounded bottom with a tear drop shape running bow to stern. Displacement hulls "displace" or move, an amount of water equal to the weight of the boat.


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.

HOW DOES IT WORK?

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.

ADVANTAGES OF DISPLACEMENT

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.

Most matured buyers of boats go in for displacement hulls.