Hull speed: ploughing through or planing over water.

by Han
(Brittany, France)

Before I write my next article about my experience with the FD (Flying Dutchman) I would like to explain about the dynamics of hull movement through water, because that is seldom explained nowadays to young sailors; they take planing for granted.

Before people built engines in boats, the hull speed was low, now easily to be calculated by way of a simple formula. (Search for "hull speed" on Wikipedia) In reality you can see the maximum hull speed of an "old fashioned", motorized hull: accelerating from speed zero, you see a bow wave appear; then a second wave appears, moving backwards as the hull speed goes up; while the bow wave remains stationary directly under the bows, the second wave moves backwards until it is directly under the stern (the stern wave). The length of the hull seems to be moving with the wave trough in-between, and is still level.

When the propulsion is increased, the speed increases very little, but the fuel consumption goes up steeply; the bows rise, the stern goes downhill, moving ahead of the stern wave, and the hull is no longer level, it's going uphill and a lot of the propulsion goes into a verical, instead of a horizontal movement.

Observing that, the first builders of powerboats (in Italy and Switzerland) tried (and, at length, found), a hull-shape allowing the speed going up until the hull planed on top of the bow-wave.

That, for an engine-propelled hull, took a lot of power, because the propulsion is brought to bear below the waterline, while on a hull with sail-propulsion the forward-driving force is way above the waterline, so at least in theory sail-propelled hulls should be able to plane much easier.

The real sailors soon saw the effects of the new hull-designs and improved on them to suit sail-propelled hulls and presto! came up with designs allowing wind-propelled boats to plane, albeit with some difficulties. For example, they had to be urged on by a tug at the mainsail-sheet, or the crew-weight had to be moved forward.

Shortly after the second world-war new hulls were designed, until, early in the fifties, the Flying Dutchman appeared on the water.

See my next article on the FD, soon to appear on this site.