It is a claim that sets my teeth to grinding.

“Heavy displacement sailing yachts are slower than light displacement vessels because heavier displacement results in greater wetted surface and frictional resistance….”  — Anonymous Internet Guru      .

 So, bear with me, while I set the record straight.

There are several reasons that — all other factors held constant — a heavy-displacement sailing yacht is, on average, slower than a light-displacement vessel. But there being a necessary connection between heavier displacement and greater frictional resistance resulting from increased wetted surface is not one them.

The fact is that wetted area (and the resulting hull friction) is not directly related to displacement (i.e., immersed volume).

Think about it for a moment. Which geometric solid has the least surface area for a given unit volume?

The answer is… a sphere.

The more the shape of a hull’s “canoe body” deviates from that of a sphere (or a semi-circle in section), the more surface area it will have for any given underwater volume (displacement).

Consider the two midship cross-sections in the following illustration. They have the same waterline beams and the same draft. But A (the heavy-displacement hull) has about 65% more immersed volume (displacement) than B.

Yet, all other factors held constant, B (the light displacement hull) has approximately 15% more wetted surface than A.

 Image 0257:

In other words, simply because A is heavier displacement does not mean that it has greater wetted surface, hence faces more frictional resistance.

The reason for this seeming anomaly is a matter of solid geometry… pure and simple. Hull B deviates further from being a sphere (or a circlular shape in cross-section) than does A.

So why then are most light displacement yachts faster than heavy displacement ones, especially in light airs?

Primarily, because their overall weight is less. And the power required to move a yacht at a given speed is directly related to its weight. Moreover, larger immersed volume requires pushing more water out of the way (wave-making resistance) which is the dominating resistance to be overcome in a sailing yacht.

But wait! Didn’t I say that light displacement yachts move better than heavy displacement yachts in light airs? And isn’t it in light airs at slow speeds when frictional resistance is most important?

Correct. Confusing, perhaps. But correct.

The fact is most light displacement yachts do have less total wetted area (and so, less frictional resistance) than heavy displacement vessels — but not as the result of having less immersed volume (displacement).

This apparent conundrum stems from the fact that, when designers and naval architects use light displacement hull forms, they also generally eliminate large portions of the underwater profile traditionally associated with heavy displacement vessels — the deeper forefoot and long straight keel running the length of the vessel and ending aft at the rudder.

The next illustration offers a clear picture of just how much underwater profile may trimmed away when going to the kind of high aspect ratio fin- or bulb-type keel associated with modern light displacement hull forms.

Image 0258:

The result is that the light displacement hulls do end up with less wetted surface but — and this is an important “but” — not because lighter displacement, in and by itself, results in less wetted surface. 

Myth busted. Case closed. — Phil Friedman

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