People are right to be concerned about bridgedeck slamming on catamarans. Having said that, it is usually a comfort problem rather than a structural one. Fortunately it is, or should be, a thing of the past.

Early catamarans had low freeboard, partly because of the heavy materials which were all that were available at the time, and also because low freeboard boats always look better. The latter is still true today of course, but people have now got used to seeing high boxy catamarans.

In an attempt to cushion the slamming, many builders, and Prout Catamarans in particular, developed the central nacelle. Prouts took this nacelle to it's extreme and their last designs were more like three hulled catamarans with the nacelle in the water at rest. I took this photo (below) when cruising in Venezuela. A Prout is in the foreground while a conventional high bridgedeck catamaran is in the background.

I have never believed this approach to be a good one. I always say that for offshore sailing one should be able take a conventional inflatable dinghy under the bridgedeck. I sailed the S Atlantic from Capetown to Rio in a Norseman 43. Its bridgedeck was very low (but it looked a very nice boat due to its low freeboard) and it was very uncomfortable living on board. A few years earlier I had sailed from the UK to the Canaries in another low bridgedeck catamaran (again not one of my designs). The slamming was so bad that we were unable to use the saloon table as plates would jump off as we hit every wave.

The Gemini 105 has a notoriously low bridgedeck, in part to keep freeboard hence weight hence costs down. We have been kept awake by the slapping from one that was anchored next to us, while this from one of my customers "I was aboard a 105 last week in Waderick Wells, Bahamas, and was unsure if it was polite to break conversation whilst she slammed while on a protected mooring, or pretend it wasn't happening as I guess their dealers must?"

I took the photo, below, while sailing in the Greek islands. This is a large (over 45ft) catamaran but I do not know what design. Clearly there is not enough bridgedeck clearance!! Yet if you look at the transoms they are still out of the water, so the boat cannot be considered overloaded. A boat like this should have at least 600mm clearance - implying at least 500mm (18in) more freeboard is needed to make it safe and more comfortable. This is not a boat to take to sea, no doubt it was bought because the owners liked the space below and they never took it for a test sail before buying.

Much of this slamming is self inflicted. Imagine two hulls close together pitching into a wave. The water they displace has to go somewhere, and it piles up just as the bridgedeck sails over it. Clearly a wider hull spacing will turn a narrow high peaked mountain of displaced water into a low flat molehill. A wide knuckle and flared hull will also help reduce the size of the induced wave.

The best solution is to start the bridgedeck well back and have it low only where needed. That's why most offshore catamarans have nets or trampolines forward. The boats to avoid if you plan any offshore sailing are those with bridgedecks taken right to the bows.

Furthermore, the water that goes in at the bow also has to come out at the stern, for as the bows pitch out of a wave the sterns will pitch in. So bridgedecks should also be high near the stern. Have a look at the stern of a Prout catamaran, you'll see there is very little space for the bows waves to get out. No wonder they are so noisy to sail - and the waves trying to force their way out must slow the boat down.

I do fit a nacelle on some of my designs, notably on the Gypsy and Romany. I accept the compromises as I wanted standing headroom in a small boat. But I designed these nacelles as footwells so they are as small as possible, (they are only 600mm/2ft wide). I sailed my own Gypsy and Romany thousands of miles and didn't find slamming to be a problem. Mind you, I also had a Veed bottom to the nacelle. Had it been flat then I am sure the slamming would have been noticeable.

So my newer designs (like Transit, Vardo and the powercats) don't have a flat bridgedeck bottom, it's slightly Veed

Of course the smaller the boat the bigger the problem. That's why I don't have any bridgedeck cabin designs with full headroom under 30ft. If you have 6ft headroom on  a 30ft boat then scaling it up to 40ft would give you 8ft headroom - more than enough! So a 40ft boat can easily have over 2ft bridgedeck clearance yet still look in proportion