Types of Hulls - 3 : Semi-Displacement Hulls

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In our last and final post of the Types of Hulls series - we talk about the combination of the Planing and Displacement Hull - aptly known as Semi-Displacement Hull.


The semi-displacement hull is a design that attempts to combine some attributes of displacement hulls with some features of planing hulls.

Menorquin Yachts popularly use these sort of hulls and the M/Y Galactica is the first super-yacht to officially use this design.

SEMI-DISPLACEMENT HULLS

Also known as Semi-Planing Hulls, 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.


The typical semi-displacement hull is a development of the pure displacement hull - with sharp and deep forward sections fairing into full rounded aft sections. The major difference between displacement and semi-displacement hulls is in the buttocks.


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.

A semi-displacement hull is essentially a displacement hull with straighter buttocks ending at a round-bottom.

HOW DOES IT WORK?

Generally, displacement speeds are considered those below a speed/length ratio of 1.5. The semi-displacement region extends from 1.5 to about 2.5. Above this is the pure planing region for normally proportioned monohulls.



By definitions, the speed of a semi-displacement hull is limited unless huge amounts of horsepower are applied, simply because you are always pushing the boat through the water. The deep-V holds world records for speed and endurance in rough water.

Semi-Displacement Hulls are used for good sea-worthy, bigger boat designs at lesser running costs.