If you have been following the dramatic developments of the 34th Americas Cup, with 72ft hydro-foiling catamarans being tested, you may have witnessed video, (see below if you haven’t seen it), of the incredible capsize of defending Oracle Team USA 17 which pitch-polled in spectacular fashion, leaving the Aussie skipper Jimmy Spithill and his team to pick up the pieces and start again.
Though this is incredible viewing and will make for a very nervous and intense racing series next September, I can already hear the traditionalist monohull sailors preaching again from their barstool, “I don’t sail catamarans, they capsize”. This type of broad sweeping statement is of course as much ignorance as it is nonsensical. For example, in the 400+ modern catamarans that Seawind have launched in the past 20 years, they have never had a boat capsize and now days you hear of more monohulls that loose their keels and capsize than catamarans. But rather than provide counter arguments about monos vs cats, it’s probably better to understand what causes a catamaran to capsize and why not all cats capsize.
The vast majority of catamarans that capsize, usually do so by pitch-polling head over heels forward, i.e. by burying a bow and “tripping” over it, and the vast majority of these are racing catamarans. Most racing catamarans are built with very slender hulls to reduce the wetted surface area, which reduces drag which in turn increases speed. They also have large and highly efficient rigs, use daggerboards and often with a high aspect mainsail (squaretop sails – or in the case of the AC72s a hard wing sails), rotating rigs and run spinnakers or screechers. With the combination of a powerful and efficient rig with low resistance on the water from the hulls, the boat is able to accelerate very quickly to high speeds and create its own ‘apparent wind’. The ability to increase apparent wind is theoretically limitless providing the boat can keep up, and often this is where the trouble begins.
If the rig is powered up beyond a speed that the hulls are able to maintain, then the hulls can ‘trip’ the boat, usually by dipping the leeward hull which is under the greatest pressure from the rig, and then the rig continues to sail over the top and levers the boat over into the water. To prevent this, it’s up to the crew of racing catamarans to either depower the boat or rig to ensure it doesn’t get that point, or for the skipper to sail deeper to reduce the apparent wind – but when racing, things are pushed to the absolute limit.
The AC72 that pitch-polled did so after coming off the foils, at a boat speed of over 30 knots whilst in 25 knots of wind. The AC72 then started bearing away. The “bear-away” (turning away from working into the breeze, to a reach) on racing cats is the highest point of risk as the rig generates enormous apparent wind during the turn and the hulls are accelerating and catching up. Clearly the crew of Oracle didn’t execute this efficiently and went over.
The reason we very rarely see production cruising catamarans capsize is that they are designed for comfortable, safe sailing – enjoying the benefits of level sailing with large living areas. And though they are built with performance attributes, unless they have very slender hulls, daggerboards, rotating rigs with high aspects sails, it’s unlikely that they will ever generate enough apparent wind to achieve the speeds or power where pitch-polling is likely. Production cats are most at risk coming down very large waves and surfing at high speeds (over 20 knots) with too much sail up, risking having the bows dig into the bottom of the next wave. However cruising cats are also designed with enormous reserve buoyancy in the bows to counter this risk. Some cats do this through high freeboards, some with fat bows, and some with flared hulls – all achieve a similar result, but with varying levels windage and general performance. The desired result is bows that stay above the water and don’t nose dive, as there is simply too much buoyancy to allow this to happen.
Reefing also prevents capsize, by ensuring there is not too much power in the rig in large following seas. Many production cats have single line reefing systems that are easy to operate from the safety of the cockpit. Drogues are also useful in these conditions as they can slow the boat down to more manageable sub 15 knot speeds.
The other less likely scenario is the boat going over on its side due to too much wind (or too much sail area), accompanied by large waves from side on. Again, reefing minimizes this risk by reducing sail area and depowering the sails, as does sailing into or away from waves, rather than having them directly on the side. Production cruising catamarans have way too much displacement to make capsize a real likelihood, unless you are doing everything wrong – all sails up in cyclonic conditions with massive side-on waves. Techniques such as hoving-to, or deploying a sea anchor off the bow in cyclonic conditions allow cats to comfortably ride out the storm. In reality, it’s more likely that the sails will blow out or the rig will collapse (something that also hasn’t happened on any Seawind) before a production cat capsizes.
At the end of the day, high performance catamarans are designed for racing and being pushed to the limits, in the same way racing cars are, so it’s no surprise to see the occasional bit of drama. However, cruising catamarans are designed for an entirely different purpose and that is, exploring the world’s oceans and islands, entertaining and living onboard in comfort, and doing so at a reasonable pace.
Extrapolating the AC72s experience to a cruising cat is like suggesting that an F1 accident means all road cars are at fault, it just makes no sense.
Fantastic Image thanks to Guilain Grenier