Sunday, January 1, 2012


Bows, that is. At higher speeds, most of the energy driving a displacement hull is wasted on wave creation, including at the bow. To the extent wave making can be minimized, the efficiency of the hull increases. The addition of a bulbous bow creates a wave ahead of the ship's normal bow wave, and the trough of the bulbous bow's wave coincides with the crest of the ship's bow wave, tending to cancel the wave out. A bulbous bow lends about 5% to the efficiency of the hull, and reduces pitching as well.

Bulbous bows add to the wetted area of the hull. At slow speeds the drag induced by wetted area is the primary impediment to hull efficiency. For this reason bulbous bows are not (apparently) suitable for sailboats, which so often travel at much less than maximum speed.

The concept is said to have been first developed by David Taylor, head of ship design for the U.S. Navy in 1910.  He noticed that the ram commonly incorporated into the bow of large combatants in that era seemed to slow the ships much less than theory predicted. While slow to catch on, bulbous bows are now ubiquitous.

The Baidarka built by Alaska's Aleuts has the unique bow depicted here. While western academics assumed the shape had some totemic significance, perhaps meant to mimic a salmon's maw, some believe it represents a bulbous bow. The efficiency of these boats is remarkable, and it seems entirely possible that thousands of years of inspired design resulted in an advance that long anticipated David Taylor's discovery.

You may think there is a flaw in the logic here - that bulbous bows are efficient only in hulls driven at the higher realm of their potential speeds, and that kayaks are slow. Think it through. A container ship and a kayak both have displacement hulls. Simplifying somewhat, the maximum hull speed of a displacement hull is known to equal 1.34 multiplied by the square root of the waterline length, at which point there is a big bow wave, a big stern wave, and a deep trough amidships - think of a tug powering along. A kayak with a 16 foot waterline length will thus have maximum hull speed of about 5.4 knots. (If you're doing the math, a container ship with a 900 foot waterline length has a maximum hull speed of 40 knots.) A powerful man could and would drive a kayak at speeds approaching 5 knots, suggesting that the bulbous bow would be a useful part of the hull. Bear in mind also that when a hull is driven by muscle, even a small addition to efficiency is normally appreciated and sought after.

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