Multihull Capsize Prevention <split>

Thanks for that corley, Now I just have to try to get a copy again.

In terms of the Ian Farrier boat is it also that the boats could be more talkative due to the reduced beam and float volume (you can see whats happening). It tells you over a wider period, where as with a wide beam high float volume you get a narrower range of communication (It will fly the main hull with slight float submerging).

It will be interesting to see how the Ian Farrier race only boats go, both in performance and safety with the higher performance levels

 

Size is the single most important factor in multihull inversion prevention. A 24 foot light displacement craft is quite vulnerable offshore. You’ll find yourself in a survival situation much earlier in bad weather.

Sure great feats have been achieved by people in small boats but they don’t set the standard, rather they defy the odds.

Pitchpole over one hull (or diagonal capsize) is the risk with speed downwind sufficient to bury the leeward float. The most important factor is the immersed float volume relative to total displacement. But also it depends on speed and what sort of 'diving plane' effect is achieved.

Also consider that small boats can have their sheets thrown off easily and people can shift their weight to good effect, this is significant when two men could represent 20% of the vessels own displacment .

 

There seams to be a small flaw in mikes theory by the name of Samnz. If you were to follow the theory he should have pitch poled may times over. or is as mike will say it is because he can release the sheets. Which then blows the dragonfly capsize out of the water with a contradiction.

Could part of the trimaran issue be float length combined with a square length beam ratio, It interesting to not that they are lengthening the bows on the main hull now.

 

Here some info, links to the books are at the bottom. Amazon says too have all three of them.

For the thread this might be a good one . . .

Gavin LeSueur - Multihull Seamanship - An A-Z of Skills for: Catamarans and Trimarans - Cruising and Racing

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Good luck !
Angel

 

That's not very logical. It's much easier and safer to quickly release the sheets on a small boat. Whether you choose to do so or not is another issue entirely.

 

Mike, I have a lot of respect for your opinions and experience but I feel that you are over simplifying.

A typical risk assessment is a very crude tool. They are good in that they get people thinking about the job and what could go wrong, but at their heart it boils down to an arse-covering exercise.

Each hazard is considered by risk and severity, with 4 or 5 "levels" for each. An acceptable outcome is not that the sum total hazard is minimised, but that all the little boxes are a "low" hazard after controls.

An event with a 0.001% probability and one with 0.00001% probability both have a "low" probability and are treated the same. They shouldn't be.

An event where the consequences had a 5% chance of death and one with a 95% chance of death would both have "extreme" severity, and would be treated the same. They shouldn't be.

As a sailor I want a boat that minimises my chance of injury or death, not one that allows a NA to document that "all risks were identified and all reasonable effort made to control each risk". The concepts are related, but different.

George

 

Thanks Angelique I already have the Multihull seamanship book by Gavin LeSeuer.
I think it would be fair to put Gavin in the experienced category, having raced in the eighties and cruised with his family until recently racing top gun a 50 foot Crowther pod catamaran.

It would be interesting here from any one with experience on the Malcom tannant boats wild thing or demon trycile as they have floats extended past the main hull.

I understand there may be a photo of the wild thing in Australia pitchpoling on the float how ever like a lot of multhulls it just pop back out.

 

Thanks George for your comment, I think part of it is we are getting tired of all these negative comments, being put forward, when everyone else is trying to be constructive.

 

Something I don't remember seeing yet on this thread is much discussion of wing deck windage as a factor.

A number of capsize accounts mention wind under the bridgedeck and trampoline appearing to contribute to inversion.
After cyclones a few cats anchored in sheltered waters (that is, no large waves) have been found inverted, still at anchor, with wind under the wing deck the only likely cause.

So for capsize prevention:
1. use an open, free flowing mesh for trampolines; and
2. for cats, minimise bridgedeck area.

The wing deck of some boats is larger than the double-reefed sail area, so the effect can be significant.

Some mention is made here in NEMA minutes:
http://www.nemasail.org/pdf/Winter2005.pdf

 

Here some pics of Gavin's adventures . . . .

Cheers,
Angel

 

Thanks angel for posting the link.

I had already read the article by Gavin, but unsure how to post it. The more use full part is that Gavin passed on the lessons he learned from it. It was in the Jan/ Feb Austalian multihull world magazine. How ever good photos if you pick the right depth of water.

The photo I saw of wild thing pitch poling was at about 20 to 30 degrees length wise.

 

Sorry George, but I have to disagree with you totally.

It has nothing to do with an arse covering exercise. That is only peddled by those that do not want their “system/product/design” whatever it may be, from being examined for weaknesses. It is simply rationalising events that occur, examining them to see whether such an event poses a risk to personnel and if so, how much of a risk.

I have done this many times, and done so just recently on a new product we have designed for the use of crew/personnel.

One also obtains input from “subject matter” experts in the systems, as they can see things you can’t. Such as, if there was some hydraulic piping involved, you would seek the opinion of hydraulic engineers who only do hydraulic system design. This in no way covers your arse, it enhances it, to ensure you have not forgotten anything or become too complacent thinking “its my design” I know what I’m doing!

Says who?

Once a hazard has been identified as say 0.001%, it is assumed this is because of an exhaustive review and compared to statistical analysis of similar systems to arrive at the percentage value, for that outcome.

If you do “something” 100 times in 1 year, then the 0.001% probability shall occur, simplistically, once in 10 years. This does not mean it is ignored. The very fact it has been identified as being “low” makes you aware of such an occurrence in the first place. The absence of such a study would more often than not make one ignorant or complacent of such an event.

Thus knowing it can occur, albeit it a very probability, do you choose to ignore it, or pay attention to it and what drives it...despite its low level of occurrence?

That is simple.

Going to sea has an XX% probability of death occurring. Not going to sea has a 0% probability when compared to going to sea.

Thus if you want to minimise your chances of injury or death, no matter how low or high the probability is, you pay attention to it, respect it and understand how to mitigate it should it occur. A HAZOP study of YOUR boat will do this for you…it provides YOU the information you require.

 

Thanks George

Risk assesments can be very useful tool, I think in a lot of marine design it is actually very useful and has saved a lot of life.

Declaring Risk makes people aware, no matter how much they might dislike their prejudices being challenged. A really good example is the risk of downflooding in tall ships. The risk is Extreme, and if masters actually carried that fact around in their heads and had it pasted in their stability books in bold print on page one a lot of tall ships would have been operated more sensibly. But they appear so safe, so seaworthy and so stable.....


Once a risk is identified the emphasis is on mitigating the risk rather than pretending it's not an issue.
But I do think there is a very real phenomena amongst leisure multihull enthusiast where the level of risk is subverted by invalid observations.
I could post reams of them from every multihull chat site, many magazines and marketing firms. They are very misleading and adopted as fact by many people. And people can get very upset when someone objectively disagrees. " You are being so negative "

Risk management we have covered: In heavy weather, slowing down, deploying drogues, taking boards even rudders up. reducing windage as much as possible, getting as much as possible stowed as low as possible. I'd add regular radio schedules which will ensure a more prompt response from rescue organisations too if a rescue beacon is triggered.

Then there's inverted safety, which as I have said I don't think gets anywhere near enough attention. A safe place in the upturned hull you can get into without diving, bright colors to attract rescuers, hand held VHF flares and EPIRB easily accessible.

 

Yes good point, I've been wondering how to sensibly quantify windage on not just the bridge deck but in a pitchpole and also a broaching type capsize the exposed windward float/hull as well. Really we need to accept the wave induced effects will be exacerbated considerably by the wind.

Some larger cats have developed enough lift to be blown over at anchor in hurricane force winds from ahead. I wonder if the streamlined airfoil like sections of some of the bridge deck accommodation actually works detrimentally.

 

Ad hoc, I think you missed my point totally.

I'm not saying risk assessment is bad, or pointless.

I'm saying that to meaningfully compare the overall "seaworthiness" of different boats the average risk assessment is insufficiently detailed.
A useful risk assessment in this case needs to be exhaustive, and it needs to be numerical - probabilities are not "low", "medium" or "high" they are x.x%/100,000nm.

In fact we simply don't know what the exact probabilities of many sailing hazards are, so we can't do an exhaustive risk assessment.

We still do the risk assessment, but we can't necessarily claim that boat A is safer than boat B based on the results.