Page 29: of Maritime Reporter Magazine (September 2002)

is primarily due to the frictional resis- tance of the water flowing past the hull.

This frictional resistance is proportional to the surface area of the hull. With two hulls, a catamaran has more surface area than a monohull of the same displace- ment. Therefore, at low speeds, a cata- maran is less efficient than a monohull.

It does have greater stability however due to the spacing of the hulls. A designer may use a catamaran hull for a slow speed vessel where there is a need for greater stability or where there is a need for a large deck area.

At high speeds the catamaran design divides the weight of the vessel and wave making resistance between two demi-hulls, both of which are relatively narrow for their length. Let's take the example of a 135-ft. (41-m) catamaran with a waterline length of 115 ft. (35.2- m), a speed of 36 knots, and a full load displacement of 170 metric tons (167.3 long tons). Each demi-hull has a speed- length ratio of 3.35 and a displacement- length ratio of 54.4. The beam of the demi-hull is 9 ft. (3 m) giving a length to beam ratio of 11.7:1. Note that the hulls are long and narrow and that the weight is relatively low compared to the size of the vessel. They are also spaced far enough apart that the wave train from one demi-hull does not affect the other demi-hull. The shape of the main deck with a beam of 34 ft. (10.5-m) also gives a good layout for passenger spaces.

Like any other hull type, there are dis- advantages to the catamaran configura- tion. The narrow shape of the demi- hulls makes for restricted access to the propulsion machinery.

The hull will be subject to slamming when the height of the seas exceeds the air gap clearance between the cross structure and the design waterline. The fine shape of the bows can cause the bow to plunge into waves rather than rise over them and the stresses induced by two demi-hulls working in a seaway can lead to structural cracking.

Lastly, the performance of the vessel is very sensitive to weight so careful mon-

September, 2002

Naval Architecture • Marine Engineering itoring of weight growth is essential.

So, What's the Perfect Hull Shape?

The answer will be different for differ- ent operations. If you are thinking of a new vessel design or about modifying an existing one, I suggest talking to a vari- ety of designers. Each one has their preferences and their areas of expertise.

If someone is trying to convince you that there is a break-through design that is better than anything out there, be cau- tious. As we have tried to show above, there is no one hull shape or type that will meet all needs.

Designers have lots of good tools to analyze vessel performance in order to optimize hull shape for seakeeping and resistance. Talk to them and get edu- cated about the options for hull shape and their costs. Size up those logs and pick the one that's best for you.

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