Multihull Capsize Prevention <split>

Thanks Autodafe, for your even handed comments, and thanks Catsketcher Phil for your comments based upon experience rather than conjecture.

Most of use are trying to be unbiased and provide information that can be helpful to others.

 

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I on the other hand am trying to Obtain information that can be helpful to others

Specifically about how the Buccaneer 24 Trimaran is claimed to have a charmed life regarding capsize and pitchpoling.

If this is really true, do any of you have an explanation?

There have been several statements by B24 builders that they are going to extend the length of the floats forward with the intention of preventing diagonal capsize.

However the risk exists that doing so will cause the boat to trip, due to the extra moment, as the extended nose of the floats digs into the water, or the back of a wave.

This happened to Logan Apperley's Kraken 40 when he suffered a diagonal capsize from a bullet gust in Manly harbour NSW.

This design conundrum need addressing.

 

Extending a trimaran float forward will help to resist a diagonal capsize, that is a matter of hydrostatics. You probably would not want to extend the bow of the float forward of the bow of the main hull because you would then be 'wasting' LOA - apart from anything else, some of the operating costs of a yacht are a function of LOA, so within a given LOA you probably want as long a main hull as possible.

One point to watch out for is that if you extend the floats of a trimaran without making other structural changes, you will increase the stresses that the structure could experience in extreme conditions. That may or may not be a problem for the particular craft you have in mind. As mentioned in another post here, I did some preliminary stress analysis on a proposed catamaran design and found that the 'close to diagonal capsize' situation produced a torque load on the hulls that resulted in rather high stresses in the vicinity of large hull hatches and windows. In that case the stresses due to bending in the cross beams were not so bad since the design allowed for deep box section cross beams rather than the ally extrusion type of cross beam.

As for bows digging into the back of a wave, when this happens there is an increase in the hydrostatic lift on the bow area, possibly together with hydrodynamic downforce as waterflow is deflected upwards by immersed deck surfaces. I think I can safely say that for any normal sailing speeds and bow shape (i.e. boat speeds, rather than aircraft speeds and reasonably high bows), the hydrostatic upwards force will exceed any hydrodynamic down force, so even though the bow may look like it is 'digging in', it is actually producing a net upward force that is resisting diagonal capsize. This situation could change once the process of capsize is well underway and the craft is at an extreme pitch and roll angle, but then it is too late anyway.

For the capsize event that you mention, perhaps the float bow would have dug in even sooner had it been shorter?

 

I guess one reason the B24 has such a good record is because they have relatively small main hulls and therefore can't be loaded down with clutter and junk - and are saved from capsize problems because they are forced into being light boats, therefore can accelerate away from hard gusts plus having buoyant floats set quite wide (for their time) to provide good stability.
We came close to burying Miranda one time during a two handed race, fast shy spinnaker reaching when accidentally the halyard was released, so the spi flew way out to leeward, still full and drawing hard, but the B24 handled the contretemps with no problems aside from burying the leeward float deep and perching us at a high dihedral until the sail was controlled. That's the closest I've been to tipping a B24 but Sam Tucker will have far hairier stories on his overpowered Green Death Machine - seen to suddenly halt amidst foam and spray, then back out to go his merry way on numerous occasions.
John Perry, here is a long float Tennant trimaran; there were a couple built here, also another Tennant was his long float Demon Tricycle - all good boats with unblemished histories.

 

Yes Gary. I can see Miranda in the background there.

The interesting thing about Tennants 'Wild Thing" was that at an overall length of 40' the main hull was only 35'. The float bows were well in front of the main hull.

But it had a long bowsprit. (Prodder).

I guess Malcolm had a reason for that, but I don't know if he ever divulged it. :?:

 

More drifting?

To me it seems that monohull is much more "fluctuating" concept compared to multihulls. Same difference comes in these conversation when we discuss multis. Differences are so little inside the multi "box" that it allways ends comparing against mono's. If we were talking about mono's it's about high vs low aspect keels, AVS, GZ curves, cockpit arrangements etc etc, but not comparing to multi's.
For me a good monohull blue water cruiser is not even close to most modern production boats, they are more something btw racer/coastal cruiser to pier queens. Anyway don't see how it helps mee to know that Beneteau flipped in the Caribean if I wan't know how to save myself in a capsized multihull in mid of Atlantic..

Just saying..

 

OS7, I fairly certain the Wild Thing designs are 38 foot main hull length; the long floats also have very raked bows. The main hull and float waterline lengths are the same, or close to it. And remembering what he said, he wanted the overhanging bows to take up some buoyancy when over pressed, but the most important reason was to counter diagonal capsize. And I'm also fairly certain that the original plans did not have bow sprits; a later owner add on - which I'm sure he agreed with.
By the way, that tip truck moment with Miranda was before we made the foil and extended stern alterations, was a true blue B24, had a wire strop main traveller which gave us trouble in the two handed race; that was replaced with full width aloy beam traveller track along with the rest of the changes.

 

There is no such debate as multi vs mono in Naval Architecture. You objectively assess the vessel for the statement of requirements. If you want heavy weather safety as an intrinsic part of small vessel design then prompt self righting is a paramount requirement. No one can pretend otherwise. Just conduct a risk assessment for a small vessel in a storm.

AdHoc mentioned hazard and operability studies before as being part of NA..

If there is a possible chance of capsize in heavy weather and that capsize represents the total loss of the vessel and the highly probable loss of life then a risk assessment makes that an Extreme Risk.

I think there's a real attempt to downplay the risk by trying to subvert the risk assessment by making incorrect assumptions about other vessels or using or creating flawed statistics making invalid comparisons.

The Naval Architecture assessment of a small multihull craft in a storm is considered an Extreme risk. This isn’t grinding an axe or a biased viewpoint it’s quite a simple and sensible assessment.

Given an Extreme risk you should take extraordinary measures to reduce that risk. Designs should be tested for suitable survival tactics and good strong and reliable storm gear should be carried.

 

Yes, because it's a 24 foot sheltered water design with no engines tankage and miscellaneous cruising equipment the displacement is low compared with the float volumes. That's all.

 

Teddy
Yes it's often comparing the best multihull survival story with the worst design monohull disaster !

For a valid comparisson, comparing the best we can design within the two types and then comparing them.

I think multihull sailors can learn from monohull sailors storm tactics providing they make allowances for quitting earlier and slowing down as the sea state builds.
The performance monohull sailor can press on and even find safe tactics in breakneck downwind courses. But that's not a cruising multihull option which should retire much earlier in the game and face the music.

 

Referring to the B24, Mike said:-
"Yes, because it's a 24 foot sheltered water design."

Mike that is just your opinion as an MA specializing in big monohulls.

I have built, sailed, cruised and raced that design in the worst of conditions that Lake Ontario can throw up, and make no mistake about it, Lake Ontario is an inland sea without the long sea waves, but with nasty 3mtr short spaced waves and high winds, which can create havoc with a small boat.
I was prepared to take my B24 from New York to Bermuda with full confidence, which I wouldn't have done if I had thought it was "a 24 foot sheltered water design."

 

Speaking of safe sailing, the US Navy did studies on heel angle and found low angles up to about 10 degrees had a positive effect on crew efficiency while higher angles progressively diminished efficiency and contributed to fatigue. The theory was the low angles perked up alertness while the steep angles simply took extra effort......If most of your energy is spent not sliding it leaves much less for handling the boat.

Somewhere here someone must have mentioned that in beam on breaking seas a multihull should have the boards up. Many cats have side slipped impacts that have rolled monohulls....in either case good seamanship requires trying not to get in that position.

 

Q.E.D.

So the capsize prevention tip here is, don't have LAR keels on a cat if you want to be able to make full use this feature in a situation where all hell breaks loose . . . . :idea:

BTW, a centerboard mono in those conditions side slips as well with the board up . . .

Cheers,
Angel

 

Does anyone know the title of the Gavin Lesueur book about the round Australia race in 1988. From what I understand was that it was in rough conditions. I do know that Gavin was racing a 40 foot center pod cat in the race. Last year Gavin capsized eDoc ex top gun the big sister to his previous race boat. It was recovered the same day as it was in 30 feet of water with a 50 foot mast. It got me to thinking if only the windward center board was down at the time. It may have acted as a release valve.

Thanks Pat for your comments as to obtaining help we are all trying to do it in our own way.

The comments as lengthening the bows were also interesting one aspect to watch would be the length beam ratio on a trimaran as there is a trade off.

 

Gavin's book was "The Line" it's a good read and the competitors certainly had a hard time of it. Strangely enough (from f-boat's safety page) on a statistical basis the Farrier designs with smaller, low buoyancy floats have the best capsize record but are obviously lower performance.