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mdi 02-15-2013 04:24 AM

Analysis and Comparison Cruising Multihulls 2013
 
Calvin H Markwood's 5th Edition Analysis and Comparison of Cruising Multihulls 2013 is available as a free read at this link

Analysis and Comparison of Cruising Multihulls 2013

PCP 02-15-2013 07:07 AM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by mdi (Post 990429)
Calvin H Markwood's 5th Edition Analysis and Comparison of Cruising Multihulls 2013 is available as a free read at this link

Analysis and Comparison of Cruising Multihulls 2013


Thanks, interesting stuff.

But in what regards stability the ratios they use are a bit limited. Both take into account the sail area but in extreme situations one does sail in bare poles or with very little sail area so in fact those ratios don't tell about the absolute stability of a boat but about the stability for the max sail area the cat can carry (you can always reef;)).

Multihull Dynamics, Inc. - News Article

Looking at the formulas they attribute the same importance to three factors: Length, beam and weight. That seems a bit arbitrary to me.

I remember some years ago of some huge racing multihulls (60ft) that were capsized on a huge storm, carrying no sail, just for the windage on the mast. If I remember rightly 70 to 80K winds.

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. I don't understand why that is not used. It is not difficult to computer predict that and surely If I sailed a cat I would know more about that than about a relative stability regarding Stability/sail area. I can always reef or go to bare poles.

Regards

Paulo

chucklesR 02-15-2013 08:28 AM

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.

smj 02-15-2013 09:03 AM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by chucklesR (Post 990462)
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.

chucklesR 02-15-2013 09:11 AM

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.

PCP 02-15-2013 02:45 PM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by chucklesR (Post 990462)
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 90/100 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

johnnyquest37 02-15-2013 03:09 PM

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.

PCP 02-15-2013 03:14 PM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by johnnyquest37 (Post 990642)
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

johnnyquest37 02-15-2013 03:29 PM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by PCP (Post 990645)
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.

PCP 02-15-2013 05:31 PM

Re: Analysis and Comparison Cruising Multihulls 2013
 
Quote:

Originally Posted by johnnyquest37 (Post 990651)
"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


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