Capsizing A Multihull

Posted on Friday 7 March 2008

On a sailing forum, the question of capsize or pitchpoling a multihull came up recently. That’s a good question, how common is capsizing a cruising multihull? The poster was talking about two trimarans that had pitchpoled. I’d like to look at that now.

The risk of capsize on a cruising multihull is miniscule, provided the boat is sailed reasonably. Almost anyone can capsize a mulithull. Just because a multihull can do 18 knots doesn’t mean that it should be doing 18 knots in a given set of conditions. Most cars are probably capable of going 100 mph, but I don’t think that you’d be wise to do that on a icy winding country road at night in a snowstorm.

When a cruising multihull capsizes or pitchpoles it is usually due to human error—usually having way too much canvas up for the given conditions—or going to fast for the conditions. If a multihull is over-canvassed, there is a chance of the wind pushing the boat over. However, pure wind induced capsizes are pretty rare though, mainly due to the very high initial stability of most modern cruising multihulls.

More commonly what happens is the wind and waves combine to capsize a multihull. Unlike a monohull, the multihull sits on the water relatively level to the surface. With larger waves, as you might find when the wind has been blowing strongly for some time, a multihull will be angled as the wave passes underneath it. If the boat is already tipped at 15˚ angle because of waves on the beam, and the boat is over-canvassed, capsize becomes a much greater possibility.

Another danger is pitchpoling. This generally happens when a boat is moving too fast and the bow buries itself coming down a wave face. This can happen to any boat, whether it is a multihull or monohull. However, monohulls will often broach before getting to the point where they pitchpole. Multihulls, being much more resistant to broaching due to the long skinny hulls, and being capable of generally higher boat speeds, have to control their speed to avoid pitchpoling.

In the case of the two trimarans that pitchpoled, I believe both were racing at the time. Racing is the most common cause of multihull capsize, and a big part of the reason capsize is thought to be so common. Racing multihulls are generally over-canvassed with taller masts and larger sails than their cruising counterparts. Racing multihulls are often designed with smaller amas or narrower hulls, relative to the boat size, than cruising multihulls. Racing multihulls are generally sailed with a lot of sail up for the given conditions and sailed fast. All of these contribute to racing multihulls being more vulnerable to capsizing or pitchpoling.

The monohull sailor’s view on it is that multihulls have a position of ultimate stability—upside down with the rig in the water… well, monohulls also have a position of ultimate stability… sitting upright on the bottom of the ocean. I know which I’d rather be on.

There are a few monohulls which have positive buoyancy, like the Etaps, but they tend to be fairly cramped inside, due to the space taken up by the foam used to make them buoyant. They’re often fairly slow boats as well, especially compared to a trimaran, due to the weight of their construction.

One other thing to consider—most boats that go out to sea are not self-righting. The monohull sailors almost irrational fear of capsize is probably due to the fact that monohulls, when they capsize, run a serious risk of downflooding and sinking.  News stories often report about multihull sailors who survive a capsize, but often nothing is reported about a monohull, sink it will often sink with no survivors.

One story a few years ago was of the crew of a racing trimaran which had capsized, where the crew was originally thought to have been pulling a hoax because they were in such good shape—they couldn’t have possibly been on adrift for almost six months. The remains of their boat off the beach proved otherwise.

I’ve also seen monohulls get nearly knocked down at anchor because there were some swells coming in…they just started rolling, and kept rolling more and more and more… they finally left the anchorage, probably to find someplace without the swells that were causing them problems. My boat, my friend’s trimaran and another catamaran didn’t have any problems at all. Some monohulls didn’t have a problem with the swells. It was fun to watch the guys on the monohull scurry around on a rocking boat, like ants in a kicked anthill though.I’ve sailed on a lot of monohulls…and there are some I really like… but I like multihulls better.

To give you an idea, here are some numbers, comparing a Telstar 28 to a more racing oriented Farrier-designed Corsair 28.

Table comparing the Telstar 28 to a Farrier-designed Corsair 28

As you can see, the Corsair has a significantly more sail area, about 10%, for a much lighter boat. The center of effort is also a bit higher, since more of the sail area on the Corsair is higher up, due to the higher aspect ration and large roach on the mainsail. One reason the Corsair can have such a large roach on the mainsail is that the Telstar has a backstay and the Corsair does not. Also, the Corsair has a rotating mast, the Telstar does not. The jib size on the Telstar is an estimate based on the I & J measurements. I have the 150% genny shown in the drawing, and it is 274 sq. ft.

The weight of the Telstar is estimated, based on discussions with several owners, the factory estimate of 3000 lbs. is very optimistic, and the 3400 lbs. I’ve estimated is probably a bit low, especially for my boat, which is outfitted for cruising and has a bridgedeck, anchor windlass, bow roller, refrigerator and some other cruising type gear.

Below is an image of the two boats side-by-side. I’ve tried to get the two drawings to scale, but it may not be 100% accurate. However, the drawings don’t really tell the difference between the two boats. The Telstar has 6′ standing headroom throughout most of the cabin, the Corsair 28 has less than 5′ 2″. The cabin is also significantly larger in many ways, since the Telstar has a much smaller cockpit than the Corsair does.

Image comparing profiles and sailplans of the Telstar 28 and the Corsair 28

If you look closely at the image of the two trimarans, you’ll notice that the amas on the Corsair are designed to barely touch the water when the boat is at rest, where significantly more of the ama on the Telstar will be wet. The Corsair will generally have the windward ama in the air, the Telstar will not.

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