Slamming

Dear David

You have a talent for asking questions that require a long and complex answer. I hope you are sitting comfortably.

Slamming happens only in a short uncomfortable chop and not in large ocean swells. Also only going to windward. It is to do with the hull design and clearance from the water. Usually the bigger the boat, the less the slamming. There is little you can do to change that, except in the way you sail. In essence most catamaran owners simply accept a certain amount of slamming, and minimise it by taking a different course to the waves.

My understanding is that catamaran design has evolved dramatically over the years. The early ones in the 60s and 70s were very narrow and the bridgedeck clearance from the water was not very high. The narrowness led to instability and boats had a tendency to capsize. Water rushing throught the gap between the hulls when the boat was moving tended to be forced upwards (due to the confined space) and would slap on the accommodation part of the boat. In small boats the accommodation tended to be fairly far forward, to maximise the space available, so it accentuated this.

The next generation of catamarans (HT included) were built wider to solve the instability, but still had low bridgdeck clearance and a tendency to slam.

The newer and usually bigger boats including the French designs have a much higher bridgedeck clearance. They can afford to do it on bigger boats as they can manage to make the whole thing much taller, without the boat looking out of proportion. Also they can have sufficient accommodation further aft in the boat, which leaves the water more space before it hits the accommodation block. The down side to the boats having higher bridgedeck clearance (and usually with standing headroom in the accommodation) is that the boat has substantial bulk which it presents to the wind. Running downwind it is its own sail, before you add any canvas. Upwind and at anchor it has a lot of bulk to fight against the wind.

Pats early designs (prior to the OT38 and T36) are somewhere in the middle ground. Small boats, maximising the accommodation, wide enough to be stable but not tall enough to totally prevent slamming. The boats are not helped by the tendency to nod in an awkward sea.

The design of the bow and stern are of similar size and profile to give them excellent sea keeping in big seas, but they can see saw in just the wrong conditions. Broader sterns would help this, but you can help simply by adding just a little more weight aft and keeping the bows light. Don't overdo the aft heavy though, or you are into a whole new scenario. The hulls on the older cats and Pat's in particular have bows that sheer, leaving the waterline length much less than the length overall. HT Grey Dove has altered the bow shape (and so have several of the Ocean Winds). This would help the nodding effect, by making the hulls less symetrical. The point of the bows, and the extra waterline length helps cut through the water, improving performance.

You asked about the slats. The original design had a wooden slatted foredeck in order to keep it light. Later models changed to a solid fibreglass foredeck with no noticeable increase to slamming. The solid foredeck is a great comfort to work on in seriously bad weather, but it is heavier. Trampolines would be light, but I know which I would want to work on if the going got tough. It depends what type of sailing you are going to do. Round the cans or round the world….

Hope that helps…

Chrissie