Page 28: of Maritime Reporter Magazine (September 2002)

Naval Architecture • Marine Engineering only 13.3 ft. (4.4 m) versus 29 ft. (8.8 m) and the depth of hull would be only 6.1 ft. (1.8 m) versus 13.1 ft. (3.9 m). Clear- ly one shape does not fit all vessels.

Since vessel displacement is roughly proportional to the length cubed, we can group vessels by a so-called displace- ment-length ratio (DLR), which is defined as:

DLR = Displacement (long tons) (waterline length (ft)/100)3

By this measure, the tug has a dis- placement-length ratio of 588 vs. 216 for the tanker. In other words, the tug is quite heavy for its length. Typical of vessels with high displacement-length ratios (over a value of 400) are tugs, fishing vessels, and some yachts. Most slow passenger vessels have moderate displacement-length ratios (values between 200 and 300) while fast vessels generally have low displacement-length

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For speed-length ratios from zero to approximately 1.3, the weight of a ves- sel, at speed, is supported by buoyancy forces. Operating at a speed above 1.0 times the square root of the waterline length, greater speed typically means a long, slender hull. This is shown in the transatlantic liners of the 1930's such as

Normandie, which has an operating speed of 29.5 knots - provided by a hull that was approximately 962 ft. (293.2 m) on the waterline; giving her a speed- length ratio of 0.95. Its ratio of length to beam was 8.7:1 and its displacement- length ratio was 76.7.

Where the weight of the hull is sup- ported by both buoyancy and hydrody- namic forces, as in a planing hull, the hull shape is markedly different. The width of the hull relative to the length increases in order to provide more lift- ing area to get the hull out of the water.

Typical of monohull planing boats are length to beam ratios of between 5:1 and 4:1. The hulls have significant deadrise in the bow to prevent pounding in waves. The bottom lines aft of amid- ships tend to be of a relatively constant shape, providing good water flow. The stern is a transom shape to provide a clean separation of the water flow.

Again, the hull shape changes as the operation changes.

Figure 1 on page 27 provides some comparison between various hull shapes, sorted by displacement-length ratio.

What About Multi-Hulls?

Shortly after our coastal dweller mod- ified his basic log shape, his neighbor probably lashed other timbers onto a log and pronounced his invention of a newer, better hull shape: the outrigger canoe. Since that time there have been a large number of vessels built with two or more hulls. Are two hulls better than one? Let's see what our key numbers tell us.

For low speeds, the resistance of a hull

Circle 341 on Reader Service Card or visit www.maritimereporterinfo.com 28 Maritime Reporter & Engineering News