Increase efficiency by reducing hull resistance

21 January 2015
A reverse bow lengthens a boat where it matters most: at the waterline. Water will climb this hull so forward sections of the deck need to be covered, as in motor yacht A.

There are several types of resistance (or drag) when a yacht moves through waves. The total resistance is made up of four components that vary according to the speed of the vessel.

At lower speeds, frictional resistance comprises more than 75 per cent of the total picture, with wave making and eddy resistance comprising most of the remainder. Wave making is the curl size and characteristics of the water as it leaves the hull, while eddies are low pressure swirls or vortices appearing behind protrusions in the hull below the waterline. Air resistance makes up the last part of the picture, but in most slow-speed cases it rarely rises above three per cent.

At higher speeds the picture changes. At a displacement hull's maximum speed, wave making is the largest component of the hull drag and may be up to 80 per cent of the total. Frictional drag has decreased, but eddy making and air resistance are higher.

When a boat is on plane, the hull is supported by dynamic lift. In this mode the struts, shafts, and propellers may comprise up to 70 per cent of the total resistance, with the hull and air resistance making up the remainder.

Because wave making drag is reduced for long thin structures, hulls that have longer waterlines as a percentage of their overall length (such as axe or reverse bowed vessels) will go faster with less power than will shorter, beamier hulls with flared bows.

Plus, of course, the longer a hull is, the greater its top speed in the displacement mode according to Froude's law which states that displacement speed is directly proportional to the square root of the waterline length, or (1.34 x vLWL). Because of overhangs, this number if often expanded to 1.5 x vLWL. The only time a displacement yacht will exceed this figure is when it is powering down the back of a wave and gravity might temporarily accelerate it.

When a vessel's bow drives into a head sea it can do two things: go through the wave or over it. Flared bows are designed to go over the wave, but in order to do so, they need additional engine power to first drive the yacht up the face of the wave and then over it. Wave piercing bows tend to go through the wave and usually have the main deck located high enough that the crest of the wave will not climb upwards onto the deck.

Originally published: MegaYachts volume 14: Concept Design Construction (2013)

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