May 6, 2010

Going slow in the Gulf

NOW THAT PEOPLE in Louisiana and Mississippi can’t go sailing because of the oil spill in the Gulf, they’ve got plenty of time to consider how fast their boats would go if they could sail.

The basic rule for displacement sailboats is that their maximum speed in knots, known as hull speed, is the square root of the waterline length in feet multiplied by 1.34. If you are of the metric persuasion, hull speed in knots equals 2.43 times the square root of the waterline length in meters.

Now this isn’t strictly true all the time. Non-planing boats can in fact go faster than their hull speeds for short times, such as when they’re surfing down the face of a big swell. But for most practical purposes, this rule holds good.

Planing powerboat hulls, ones that can surmount their own bow waves and skim over the surface of the water are not bound by that rule. The speed of a planing hull is determined mainly by the power available.

Your average planing hull can do 25 knots if there is about 40 pounds of weight (boat, crew, fuel, stores — everything) for every horsepower delivered to the propeller.

That speed can double to 50 knots (presuming the hull is strong enough) if the weight per horsepower drops to about 10 pounds. Pretty difficult, but plenty of boats manage it.

To a certain extent, the maximum speed of a planing hull is self-limiting because the more powerful the engine, the more it weighs and the more fuel it needs. The hull also has to be beefed up to take the strain of pounding at high speed.

However, for many sailors in the Gulf, all this is moot right now. I can only hope things change for the better soon.

Today’s Thought
Nothing is more vulgar than haste.
— Emerson, Conduct of Life: Behavior

Boaters’ Rules of Thumb, #48
“The original rule of thumb was probably the principle adopted by shipmasters that they would never approach a danger nearer than the distance that corresponded to a thumb’s-width on the chart in current use. Thus, they could navigate closer to dangers with a large-scale chart with plenty of detail than would be prudent on a small-scale chart with less.”
— Geoff Lewis, The Small-Boat Skipper’s Handbook

The body of a woman murdered 600 years ago has been discovered in an Irish peat bog.
Donegal police are now looking for a 643-year-old man who may be able to help them with their enquiries.

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