On the capsizing of multihulls…

There has been scuttlebutt about the inability of multihulls (catamarans and trimarans) to self-right after capsize for many decades. There’s also sometimes a belief that it’s very easy for them to turn turtle (as capsize can be called). The general gist of one side of the fanatics is:

  • once a multihulls gets turned over, you can’t get it back whereas a monohull can (not is) be designed, built, sailed and crewed to allow (semi-)automatic re-righting. So it’s far better to be on a monohull.

The “other” side generally responds with:

  • sure, once a multihull turns over she can’t be re-righted, but she almost always floats and provides a (uncomfortable!) platform to survive in until rescue. It’s also not easy at all to capsize a multihull, and the majority you see are racers and thus not generalisable.
    Also, almost all monohulls won’t re-right either as they aren’t designed, built, sailed AND crewed to do so and when turned usually go straight to the bottom of the ocean… and even if they do re-right, they have often lost their mast and ability to provide a stable platform for their crew to survive.

Do you think Ford or Holden is best? Cats or dogs? Catholicism or Hinduism?

Decisions have usually been made and sides taken: we have decided on a catamaran and are happy. But we can all still learn, and there have been quite a few scientific studies on the ability of multihulls to not turn over. I’ve quoted and copied what I find interesting pieces from a some publications here, and I’ll add more over time if you know of others (send me an email). We are obviously interested in respect of our vessel, so there’s some comments below for her.

TLDR: multihulls won’t capsize from non-breaking waves, and can probably handle breaking waves up to their beam.

Model tests to study capsize and stability of sailing multihulls – Barry Deakin

  • This is particularly of interest as it looked at the risk of wave capsize not wind (wind capsize if much harder to measure in some ways because it depends if full sail is out, reefing, position of traveler, height of mast, fullness of sails, etc.).
    The reasons for capsize of catamarans were: wind 56%, wind and wave 16%, pitchpole 12%, wave 8%, breaking wave 4%, flooded 4%. “84% of the catamaran casualties were the result of wind induced capsize or pitchpoling“.
  • As we have a power vessel, we are particularly interesting in the non-wind capsize likelihood. We also have a very narrow beam for multihulls – 4.95m on a 15m vessel – which is unusual and would make her “extra tippy” for some pundits.
  • In the study, the #5 “narrow hull” appears to be the same as our boat (they scaled a 13.9m catamaran to 1:12 making an LOA of 1130mm; “normal’ hull separation was 367mm. Assuming a 10:1 beam to length ratio, the BWL would be 113mm so the beam would 367+113=480mm, scaled up 12x to be BOA 5.7m or 41% of LOA. The “narrow” hull had a 293mm separation which would indicate a beam of 293+113=405mm, or scaled up to 4.9m or 35% of LOA. Our boat is 14.5 LWL, 35% of which is 5.05m – very close to out 4.95m).
  • the principal conclusion of interest here is that the peak responses are significantly less than those for typical monohulls. Response to unbroken waves therefore is not likely to result in heeling to large angles.
  • The models exhibited no extreme responses, the catamarans generally contouring the waves with no evidence of a hull emerging from the water…these yachts are not vulnerable to capsize or heeling to large angles under the action of non-breaking waves alone
  • “A vessel heels in response to the wave slope ahead of the breaking crest and is then struck by the crest. The energy principally is in the broken water which travels along with the wave. There is little rotation in the wave to turn the vessel and it is pushed ahead of the crest by the broken water. Any rotation of the vessel is brought about by its resistance to movement through the water
  • The addition of the keels appeared to result in a slight increase in the vulnerability to capsize. For the narrow model it increased the capsize incidence from 14% to 60%, and for the standard model with the second VCG increase it increased from 17% to 25%

Capsize and stability of sailing multihulls Phase II (add to the first paper…)

  • Recommendation 8.4: “In severe breaking waves of height similar to or greater than the beam of the yacht…encounters on the beam or astern should be avoided
    This is interesting as it goes against the series drogue data (start here) which suggests that a drogue off the stern of a multihull is better than off the bow.
  • Conclusion 7.4: “In non-breaking waves, multihulls have lower roll responses have lower roll responses than monohulls, and no dynamic behavior was found which might affect their safety
    Yay!  Essentially the multihulls didn’t ever capsize in non-breaking waves.
  • Conclusion 7.5: “Breaking waves with a height equivalent to the beam of a catamaran…may be sufficient to cause capsize“.
    Note this is breaking waves, not just waves!

Multihulls design considerations for seaworthiness – John Shuttleworth (an important designer, but not delivered in a peer-reviewed journal/conference)

  • Tank testing has shown that capsize due to the action of unbroken waves is impossible
    which supports the first paper.
  • in most multihulls, the energy of impact is not transferred into rolling, and in fact virtually all the energy is absorbed by surfing sideways.
  • And regarding drogues or sea anchors (which are different things!)…
    Whether to deploy the sea anchor from the bow or the side of the vessel depends on the type of boat, and the conditions. Several people have written on the subject including the Cassanovas, and Dick Newick, who both have used and favour this method of controlling drift and rolling in a storm

Additional on Lock Crowther

A quote from Brian Eiland which is from Lock Crowther is:

This work [tank testing multihulls] has indicated that the well designed catamaran is remarkably safe in breaking waves up to considerable height, even when beam on, we were unable to capsize a power catamaran yacht in the largest wave which could be generated. This corresponded to a 52′ wave for a catamaran of 40′ beam. Scaling this down to a typical 24′ beam cruising cat means she should be O.K. in a 31′ breaking beam sea. An equivalent size mono-hull power boat was easily capsized by a 25′ breaking sea, and in tests with conventional yachts after the Fastnet disaster, it was found that a 40′ mono-hull yacht could be capsized in a 12′ breaking sea.

Which for our Blu Emu, would mean no concern about capsize from wave upto (and possibly beyond!) 21.5′ breaking waves.

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