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Old 06-10-2017, 16:54   #106
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Re: Modern cats

How about we step back for a bit.
The "equation" is a kludge. The basic terms of weight*beam/(sail area*COE_offset) do not give you wind speed.
The value of 14 is a kludge, as is the 0.6 safety factor.
The "equation" is, however, a very useful kludge. Would a naval architect use it to design a 100 foot catamaran? Clearly no.
Its is a rule of thumb that yields the "recommended" reefing points. If the dynamic stability is 18 knots, will the boat immediately flip at 18.2 knots? No.

Weight is good for comfort but bad for everything else associated with actually sailing.
What the "equation" tells us the is that weight*beam (good for stability) is opposed by sail area*COE height (bad for stability).
Given just the terms shown, greater stability implies worse sailing performance unless you increase beam. Which of course can't be done after launch.
You can increase weight but would you accept the penalties of direct loss of speed, more frequent motoring, greater slamming etc etc?
You can also reef. As in, design sails for 70% max sustained wind speeds and then reef as needed. Reefing lowers sail area and COE, simultaneously (assuming its not an in-mast reefing main).
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Old 06-10-2017, 17:26   #107
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Re: Modern cats

Take two identical cats be it an Oram, Seawind or Condo your choice. One is at manufacturers Light ship ready to race. The other Loaded to 120% of its recommended load (that I remember is OK from a previous thread you just need to raise the water line)

Now same sail plan on each and increase the wind speed until one capsizes, which boat will it be? The heavy one or the light one.

Quote "Rubbish. Weight is not your friend, a lighter cat wont blow over easier in gusts it will accelerate."I beg to disagree with the above statement, but am willing to listen to your arguments as to why the heavy one will flip before the light one does.If that is what you believe.
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Old 06-10-2017, 17:33   #108
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Re: Modern cats

I'm pretty sure they're not calling the concept absurd, I think they're referring to the statement that an increase in beam only counts for half of the added stability vs an increase in weight. Also that added weight is good on a cat.


I have a very hard time believing that added weight has a greater effect on stability than added beam. Think of it practically from the point of rotation. A cat capsizes by rotating over one hull, which never leaves the water, so the weight of that hull is immaterial to stability. But a sail is always centered, so an increase in beam creates a larger lever that the sail has to work against, both to lift one hull and to force the other hull down, working both against the weight of one hull and the buoyancy of the other.
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Old 06-10-2017, 18:14   #109
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Re: Modern cats

Lets take another look at it
lifting torque = SA * COE
torque down = half weight * beam

so if you increase weight by 10% only 5% is available to increase torque down -- the other 5% does nothing.
if you increase beam by 10% the torque down is increased by the full 10%.

just to be clear, I agree with you.
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Old 06-10-2017, 18:22   #110
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Re: Modern cats

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Originally Posted by UpOnStands View Post
Lets take another look at it
lifting torque = SA * COE
torque down = half weight * beam

so if you increase weight by 10% only 5% is available to increase torque down -- the other 5% does nothing.
if you increase beam by 10% the torque down is increased by the full 10%.
Exactly. Polux is saying that weight counts double when it comes to stability. I think that's what CC44, Factor, StuM and others are calling rubbish.


I also wonder if the added weight has a negative effect on buoyancy in the leeward hull, making a capsize somewhat more likely? A cat will heel slightly before flying a hull, the greater the buoyancy of the leeward hull, the greater force required to heel the boat. Of course lighter weight means once the buoyancy is overcome then less effort would be required to fly the windward hull, and weight isn't directly tied to buoyancy, so I don't know how much of a factor it would be. Or if it's even something that could be accounted for.
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Old 06-10-2017, 18:48   #111
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Re: Modern cats

Quote:
Originally Posted by UpOnStands View Post
Lets take another look at it
lifting torque = SA * COE
torque down = half weight * beam

so if you increase weight by 10% only 5% is available to increase torque down -- the other 5% does nothing.
if you increase beam by 10% the torque down is increased by the full 10%.
But Polux tells us that it's "weight" * "half beam", not "half weight" * "beam" and like you, makes the false statement that the order of calculation makes a difference.

Regardless of which factor is "halved", both of you are incorrect.

See the numbers in post #102.

It doesn't matter whether it is displacement or beam. A 10% increase in either will give a 10% increase in "torque down".


Edit: You really should have included some sort of "sarc" smilie there. I thought you were serious.
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Old 06-10-2017, 18:50   #112
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Re: Modern cats

Quote:
Originally Posted by UpOnStands View Post
Lets take another look at it
lifting torque = SA * COE
torque down = half weight * beam

so if you increase weight by 10% only 5% is available to increase torque down -- the other 5% does nothing.
if you increase beam by 10% the torque down is increased by the full 10%.
Not sure where you got these formula. They are at odds with James Wharram's, which give equal value for weight (mass) and beam (leverage).

Please don't go off at tangents such as speed, wave action, winch size, etc.

Wharram's calculation of static stability is "converted" to dynamic stability by a factor of 0.6.
"UpOnStands" calls this a kludge. I don't know the meaning of this term, but assume it refers to an estimate of the many different forces that act together which may reduce theoretical static stability in a real sailing environment. I wouldn't contradict Wharram's estimate. Others on this forum will no doubt.
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Old 06-10-2017, 18:59   #113
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Re: Modern cats

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No, not absurd. In line with Archimedes' lever principle, where beam is the lever and the boat is the mass.

Using James Wharram's formula: in catamarans with identical beam and sail area, adding 10% weight will increase static stability and hence dynamic stability by 10%.

More weight equals more stability. Which is Polux's point.
What we're disagreeing with is Pollux claiming that weight has double the effect increasing beam has. How you can even compare totally different properties measured in different units is baffling for a start. Does one kilogram of extra weight increase stability more than one metre of increased beam?
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Old 06-10-2017, 19:22   #114
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Re: Modern cats

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Originally Posted by 44'cruisingcat View Post
What we're disagreeing with is Pollux claiming that weight has double the effect increasing beam has. How you can even compare totally different properties measured in different units is baffling for a start. Does one kilogram of extra weight increase stability more than one metre of increased beam?
That's what percentages are for
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Old 06-10-2017, 20:02   #115
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Re: Modern cats

Quote 44C "What we're disagreeing with is Pollux claiming that weight has double the effect increasing beam has."

That may be your take, but Factor's claim that "a lighter cat wont blow over easier in gusts" seems to be denying that adding weight to a cat will increase its resistance to blowing over, his words.
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Old 06-10-2017, 20:50   #116
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Re: Modern cats

How about this.
As a designer you find you have 100 kgs of surplus buoyancy.
You can widen the bridgedeck/beam by 10% or keep the buoyancy and tell the buyer that he can load an additional 100 kgs of weight.
In terms of stability, which would be more effective? BTW. The original beam is 5 m and displacement is 4000 kgs.
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Old 06-10-2017, 21:09   #117
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Re: Modern cats

Can a heavy cat even flip when only wind is at stake? I thought most cruising cat would rip a sail or loose the pole if badly appointed for the conditions...
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Old 06-10-2017, 21:36   #118
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Re: Modern cats

Has anyone heard of a Wharram flipping? genuine question.
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Old 06-10-2017, 21:55   #119
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Re: Modern cats

Quote:
Originally Posted by daletournier View Post
Has anyone heard of a Wharram flipping? genuine question.
Yes, there have been a few.
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Old 07-10-2017, 02:40   #120
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Re: Modern cats

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Has anyone heard of a Wharram flipping? genuine question.
Interesting question.

For many years the word was that no Wharram had ever been capsized through
wind or wave action.

The most capsized Wharram is probably the Tiki 21. Also one of the smallest. At least 3 have capsized that I know of.
This design " Cooking Fat" has completed a circumnavigation.

Another larger wharram capsized a while ago in Windy Wellington NZ.

As we all know, many Wharrams are not built to plan, so It would be interesting to know how many of these "Wharrams" that capsized were actually Wharrams.

For all the capsizes, Wharram catamarans are the safest with the lowest capsize ratio by a country mile .
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