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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Analysis and Comparison Cruising Multihulls 2013
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Thread: Analysis and Comparison Cruising Multihulls 2013 Reply to Thread
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Topic Review (Newest First)
02-23-2013 02:46 PM
PCP
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by smj View Post
Not many studies around on cats. I had read that one many years ago and I had read it again. In my opinion the study, that is very interesting even if not always enlighten, suffers from a flaw: They determined that in the vast majority of capsizes wind was the main factor in what regards to capsizing and then when studding rolling and capsizing trough testing they considered only waves that have a pretty low incidence in capsizing catamarans, except when associated with wind.

I confess that I don't also understand this conclusion:

"84% of the catamaran casualties were the result of wind induced capsize or pitchpoling, whereas only 47% of the trimaran casualties were directly attributable to the wind. This does not indicate that trimarans are less vulnerable to capsize by the wind, because there are twice as many trimarans as catamarans in this sample"

They are talking about percentages so if the numbers are meaningful the fact that they have more samples from one than another is not relevant in what concerns percentages.

In fact it makes sense to say that Trimarans are less prone to capsize than catamarans by the wind. A simple compassion between two typical stability curves will show that while a cat after reaching its max righting moment (between 10º and 15º of heel) will lose rapidly stability, a Trimaran has a much more wider range of stability and even after reaching max RM, the loss of stability is much more gradual.

That's why when a cruising cat lifts one hull from the water it is in a dangerous situation (a side wave can complete the loss of stability) while on a Trimaran sailing with one ama out of the water is normal and even with the central hull partially up (light on the water) the boat is still on control. this gives much more time for a sailor to react in a trimaran than in a cat. That is also why big ocean racing multihulls are today almost all trimarans.

Of course, they found out in that study (with testing) what it was obvious and I had already said here:

"Of the various catamaran configurations tested, the higher VCG (vertical center of gravity) conditions and the narrow beam configuration proved most vulnerable...
Although the narrow beam model was 20% narrower than the reference design, it is by no means unrepresentative. The length to hull separation ratio of the narrow model was 3.1 ..Previous tests with monohull models, indicated that, in general, they could be capsized by a breaking wave of a height equal to or greater than the beam of the yacht. "


and here he can see a basic difference between monohulls and multihulls, I mean on the last paragraph. Since the size of the wave, among other factors, relates with the beam of the boat in what regards the size needed to roll it, multihulls are much more resistant to be capsized by a breaking wave.

That is why when we study the factors that lead to the roll of monohulls we have by far breaking waves while on multihulls that is very rare (but not unheard) being by far the main factor, wind or wind associated with waves, a factor that has practically no relevance in rolling monohulls.

Here we can see that through these stability curves (all cats):



We can see that the values of Max GZ go from 1.8 to 3.0 (that has to do with beam). The cat they use as basic model (From where were derived the other models, with less and more beam, higher or lower CG) was a scale model of a 13.6m cat. A monohull with the same size has a Max Gz between 0.8 and 1.2. Since for the same size cats and monohulls have approximately the same weight that can gives us an idea of the different static stability and about the potential to carry sail.

Off course, the cats have to maintain a much bigger safety margin of security in what regards the amount of sail it is safe to carry. On a monohull a broach and a knock down is not a problem, on a cat it means a capsize.

Also, as was expected, they found out that Keels have a negative effect in what regards capsizing multihulls, but not with the relevance that Tropicat seem to attribute to them as a causing factor:

"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%. ...These results support the theory that it is the resistance to sideways motion that provides the couple to convert the breaking wave energy into rotation."

As we can see only on the narrow than average Cat the effect of the keels was very significant. On the standard model the increase was only 8% and in a sportive cat, that is wider than the average it is expected the increase would be even smaller. besides this is an old study from the nineties. Today almost no cats use keels, but daggerboarders that have a smaller lateral resistance so it is expected the negative effect in what regards stability to be smaller.

Regarding trimarans, not surprisingly and contrary to the wind factor, they have found that they are slightly more prone to capsize with a breaking wave, found out that smaller floaters in proportion with the main hull will make them easier to capsize by a breaking wave. Here, contrary to catamarans, the weight is a negative factor (and that's why I like trimarans).

"Of the trimaran configurations, those with the smaller floats were most vulnerable, with a 28% capsize rate for the standard displacement and a 38% capsize rate for the higher displacement....The tests confirmed the common opinion that small floats tend to become fully immersed if the yacht is struck by a breaking wave. Their high resistance to sideways motion then encourages rotation."

Regards

Paulo
02-23-2013 10:22 AM
smj Another study on capsize.
http://www.wumtia.soton.ac.uk/sites/default/files/STAB2000BD.pdf
02-15-2013 06:31 PM
PCP
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by johnnyquest37 View Post
"A good way of having an idea of an absolute static stability measure on a multihull would be to know what the intensity of wind would be necessary to capsize the cat only with the wingage over the mast and cabin."

My point is that "absolute static stability," which I take to mean "angle of vanishing stability" is calculated simply through geometry, for both monohulls and multihulls. If you want to calculate the force required to tilt a hull to the point of vanishing stability, then add displacement to the equation. Divide that force (taken from the RM curve) across the area of the mast to yield how much wind is required to flip the boat. I'm certain that no conceivable amount of wind pressure along the mast will be sufficient to flip a multihull going on bare poles, since the maximum sail area is calculated so that it is smaller than the force required to cause the boat to flip.
Windage is not only on the mast but on the cabin and on free-board.

And regarding this "I'm certain that no conceivable amount of wind pressure along the mast will be sufficient to flip a multihull going on bare poles" I have said already that some years ago some 60ft multihull racers with huge beam and stability were capsized while sailing with bare pools with 70/80K winds. Latter the designers confirmed that a bit more than 70K winds were enough to capsize those boats without any help from waves.

A small cruising cat will probably not be able to sustain that kind of winds without capsizing even with bare poles.

And I am really confused with this statement "since the maximum sail area is calculated so that it is smaller than the force required to cause the boat to flip". This means that you believe that cats have no need to reef or even to sail with bare poles? Do you believe they can stand any wind with all sails up?

Regards

Paulo
02-15-2013 04:29 PM
johnnyquest37
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by PCP View Post
The same way on a monohull and a cat and your point is?

Regards

Paulo
"A good way of having an idea of an absolute static stability measure on a multihull would be to know what the intensity of wind would be necessary to capsize the cat only with the wingage over the mast and cabin."

My point is that "absolute static stability," which I take to mean "angle of vanishing stability" is calculated simply through geometry, for both monohulls and multihulls. If you want to calculate the force required to tilt a hull to the point of vanishing stability, then add displacement to the equation. Divide that force (taken from the RM curve) across the area of the mast to yield how much wind is required to flip the boat. I'm certain that no conceivable amount of wind pressure along the mast will be sufficient to flip a multihull going on bare poles, since the maximum sail area is calculated so that it is smaller than the force required to cause the boat to flip.
02-15-2013 04:14 PM
PCP
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by johnnyquest37 View Post
Stability curves are created without consideration to the force in the sails. They are produced by measuring the righting arm for each degree of heel.
The same way on a monohull and a cat and your point is?

Regards

Paulo
02-15-2013 04:09 PM
johnnyquest37
Re: Analysis and Comparison Cruising Multihulls 2013

Stability curves are created without consideration to the force in the sails. They are produced by measuring the righting arm for each degree of heel.
02-15-2013 03:45 PM
PCP
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by chucklesR View Post
Paulo,

Do mono hulls show an absolute static stability measure to show what wind speed would be necessary to capsize it with only windage from mast and cabin?
um, No.
And please, don't tell me it doesn't happen. I watch boats in the mooring field in front of my house heel over 30/40 degrees in gusts of only 40 kts. Add some wave action (say from your 80kt example above) and anyone can capsize.
No, on a class A boat no, not with wind alone. The boat goes to 90º/100º stay there till the blow passes and then comes over again.

Actually it happened to me with wind high over the charts. Some freak med phenomena. It was night I only see it coming on the radar. I don't know if it was a micro burst or a tornado. I let go the main on the 3th reef and even so the boat remained flat on the water for several minutes till the high winds passed away.

Whit a cat the same happens I mean capsize with the wind, the difference is that he will not come up again.

I am not saying that a Cat will go flat on the water with less wind than a monohull (it will depend on the cat and on the monohull). I am just saying that it will happen with a given wind intensity and that it would be pretty easy to determine that. It seems an important safety factor for a sailor to know that regarding his cat the same way that to know the boat's AVS is important to a monohull sailor.

To know what is the wind needed to capsize a cat without sails seems to me an important measure of the boat stability and one more important than a relative stability regarding stability/sail area. One can always reef or take the sails away.

I don't want with this to say that Cats are unsafe, just different.

There are however small light cats that will not have the stability (regarding the amount of wind they can take with bare poles before capsizing) to sail safely offshore. I am just saying that this would be the best way to access a Cat stability not that Cats bigger and heavier than a certain limit will not be safe offshore boats, quite the contrary.

The reserve stability on a monohull is accessed in a different way, trough a stability curve. Cats don't have reserve stability so its safety in what regards stability should be measured another way. I just pointed out the one that seemed better to me.

Maybe you have another suggestion?

Regards

Paulo
02-15-2013 10:11 AM
chucklesR
Re: Analysis and Comparison Cruising Multihulls 2013

I got hit with gusts from Hurricane Emily in the BVI's, we (on a Voyage 44) were on a ball riding flat and calm inside playing Uno.
The mono's around us all went mast down and stayed down for the 30 minute period until the wind band passed through.

It's all apples and oranges, but a lot of folks just have to argue it out.
02-15-2013 10:03 AM
smj
Re: Analysis and Comparison Cruising Multihulls 2013

Quote:
Originally Posted by chucklesR View Post
Paulo,

Do mono hulls show an absolute static stability measure to show what wind speed would be necessary to capsize it with only windage from mast and cabin?
um, No.
And please, don't tell me it doesn't happen. I watch boats in the mooring field in front of my house heel over 30/40 degrees in gusts of only 40 kts. Add some wave action (say from your 80kt example above) and anyone can capsize.
Good point. We went through a storm twenty years ago while at anchor. The initial winds from the storm were 80-90 kts and hit on the beam. The monohulls were knocked down mast in the water. They did recover quickly. On our cat we just spun around heading into the wind. This was our first cat and not what I would consider the most stable.
02-15-2013 09:28 AM
chucklesR
Re: Analysis and Comparison Cruising Multihulls 2013

Paulo,

Do mono hulls show an absolute static stability measure to show what wind speed would be necessary to capsize it with only windage from mast and cabin?
um, No.
And please, don't tell me it doesn't happen. I watch boats in the mooring field in front of my house heel over 30/40 degrees in gusts of only 40 kts. Add some wave action (say from your 80kt example above) and anyone can capsize.
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