The hull form used in a particular catamaran design will have great bearing on both its performance and carry capacity as discussed in the previous tech tip. The slenderness of the hulls will greatly impact both the performance and the carry capacity, however, the sectional shape of the hull – meaning, the shape if you sliced the hull up like a loaf of bread – will impact the usable space in the hull. Cat hulls have been traditionally slender, high length to beam ratio hulls with slack bilged section shapes which minimize wetted surface. As discussed before, the high length to beam ratio affects their carrying capacity, but the low wetted surface section shape affects the usable space within the hulls for accommodations. In the past years, lead by developments in the small high performance A & C cat classes, hull section shapes have changed to more of a boxy, full shape. In these boats the shape tends to help these lightweight race boats lift up on top of the water; not quite planing but certainly lifting as opposed to digging a deeper hole in the water as their speed increases. The correct question now is, “How do these lightweight hull forms apply to your cruising cat?” The short answer is that they are not directly applicable in terms of lifting your 45,000 lb cat, but when these hull forms are used in a measured manner, they can increase both the carrying capacity and the usable space in the hulls.
The new hull form developed by Bruce Roberts incorporates this idea. The hull’s beam has been increased somewhat while increasing the fullness of the sections to allow the hull to carry more weight and to increase the usable “square footage” in the hull. The slipperiness of the hull is slightly compromised, but on the order of 1/10’s of knots in light air – a time that many cruisers will fire up their engines. Another feature has been added to the new Bruce Roberts cat hull form to help offset the slight increase in wetted surface; a slight chine in the aft half of the hull. The chine will help to separate the rushing water from the hull sides aft and reduce the wetted surface.
This new hull can be found in our brand new CAT 1300, 1500 and 1700 sailing cats. There are other manufacturers on the market that are beginning to use these ideas in their cats; check them out as well. Just as described before, the final hull that you choose needs to be tailored to the building material and the use for the boat. Lightweight options like fiberglass or carbon fiber can use a less buoyant hull shape. The same holds true for a race or semi race cat. However, a true cruising cat will require a hull of ample buoyancy to perform well.