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Old 26-12-2014, 18:41   #151
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Re: Capsized Atlantic 57 Salvaged

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If a trimaran has low buoyancy amas wouldn't they tend to bury and lead to pitchpoling? I think it's a boat to boat comparison not tri to cat comparison.


Yes off course, a well designed trimaran is assumed at that means amas with a correct buoyancy. I did not wanted to go very technical about it, since I believe it is boring to many, but here it is the legend of the above picture:

"Fig 2. shows the curve of righting moment versus angle of heel for a typical modern 35ft catamaran and trimaran racer cruiser to my design. The trimaran (Type 3 high buoyancy amas) has an overall beam of 32 ft. and the cat ( type 6 ) beam = 23ft. The trimaran has less accommodation, and is lighter than the cat, but because of the wider beam it has greater maximum stability. It is important to note that the max stability of the tri occurs at around 20 degrees angle of heel, while the cat has a max at about 6 degrees. If the buoyancy of the ama is reduced below 100 % of the weight of the boat (as in type 2 above), the maximum stability will be reduced not only in proportion to the reduction in buoyancy in the ama, but also by the effect of added apparent displacement from the downward pressure from the sails at high angles of heel. At 20 degrees this would cause a loss of righting moment in the order of 20%. If the ama buoyancy was only 80% in the first place, the total righting moment would be only 60% of an equivalent trimaran of type 3".

The author, John Shuttleworth defines a modern well designed trimaran as:

"Light weight (due to the use of composite materials). Large Sail areas. Wide beam (L/B < 1.5 to as low as 1.0 in smaller boats). High buoyancy amas (up to 200% of displacement). Pitching very well controlled by use of different hull shapes on main hull and ama. Sailing attitude well controlled on all points of sail. Low windage. Dramatic improvement in structures due to use of Computer aided design, and better understanding of composite materials."
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Old 26-12-2014, 18:50   #152
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Re: Capsized Atlantic 57 Salvaged

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Originally Posted by Polux View Post


Yes off course, a well designed trimaran is assumed at that means amas with a correct buoyancy. I did not wanted to go very technical about it, since I believe it is boring to many, but here it is the legend of the above picture:

"Fig 2. shows the curve of righting moment versus angle of heel for a typical modern 35ft catamaran and trimaran racer cruiser to my design. The trimaran (Type 3 high buoyancy amas) has an overall beam of 32 ft. and the cat ( type 6 ) beam = 23ft. The trimaran has less accommodation, and is lighter than the cat, but because of the wider beam it has greater maximum stability. It is important to note that the max stability of the tri occurs at around 20 degrees angle of heel, while the cat has a max at about 6 degrees. If the buoyancy of the ama is reduced below 100 % of the weight of the boat (as in type 2 above), the maximum stability will be reduced not only in proportion to the reduction in buoyancy in the ama, but also by the effect of added apparent displacement from the downward pressure from the sails at high angles of heel. At 20 degrees this would cause a loss of righting moment in the order of 20%. If the ama buoyancy was only 80% in the first place, the total righting moment would be only 60% of an equivalent trimaran of type 3".

The author, John Shuttleworth defines a modern well designed trimaran as:

"Light weight (due to the use of composite materials). Large Sail areas. Wide beam (L/B < 1.5 to as low as 1.0 in smaller boats). High buoyancy amas (up to 200% of displacement). Pitching very well controlled by use of different hull shapes on main hull and ama. Sailing attitude well controlled on all points of sail. Low windage. Dramatic improvement in structures due to use of Computer aided design, and better understanding of composite materials."

I guess that means boat to boat
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Old 26-12-2014, 18:52   #153
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Re: Capsized Atlantic 57 Salvaged

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Originally Posted by Polux View Post
Regarding this :



You said:



It seem to me that the logic conclusion was that you do not release the mainsheet on critical situations since you were saying previously that "If you'd only sailed a monohull you'd be unlikely to know how much difference the long traveller on a cat makes to this scenario. On a mono it's quite frightening, and potentially dangerous"

If you did not understood what I was saying and if by the fact that a cat has a bigger traveler than a mono you see that as a potential dangerous factor, it seems to me that it can only happen if when in trouble you release not the mainsheet but the traveler.

Well, given that after all you release the mainsheet and not the travel I cannot see how you see a smaller traveler as a potential dangerous shortcut, anyway, if I do not understood I am sorry but it seemed all very confusing.
If you could be bothered to read the post which included the part you have highlighted in bold text, you'd see the 'scenario" I was talking about was accidental gybes, ie the two words IMMEDIATELY PRIOR to the part you highlighted.


I wasn't talking about overpowered situations.
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Old 26-12-2014, 18:54   #154
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Re: Capsized Atlantic 57 Salvaged

Polux I still would like to know what you have. I like all boats, mono ,multi and even stinkboats and your looks really cool.

Way back someone mentioned the uncomfortable seasickness with a lightweight performance cat. Most cats are not in this category with the obvious exception of this A57 and a few others. Heavier cats don't surge as much and I'm sure the C471 is like this even while offering better than average performance.

Everything is a compromise.
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Old 26-12-2014, 18:58   #155
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Re: Capsized Atlantic 57 Salvaged

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Yes, the traveler.

Something monohull sailors (and some cat sailors) don't realize is that on catamaran-rigged boats (3 stays, no backstay) most of the headstay tension comes from the mainsail.
Really? We quite often sail under just the headsail, even to windward. Forestay tension is just fine, having the main up doesn't make a noticeable difference. (To the forestay tension)
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Old 26-12-2014, 19:06   #156
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Re: Capsized Atlantic 57 Salvaged

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I guess that means boat to boat
If he uses stability curves they cannot come from thin air, they have to come from a particular boat but he refers those two 35ft trimaran and catamaran as typical boats:

"typical modern 35ft catamaran and trimaran racer cruiser"
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Old 26-12-2014, 19:08   #157
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Re: Capsized Atlantic 57 Salvaged

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Really? We quite often sail under just the headsail, even to windward. Forestay tension is just fine, having the main up doesn't make a noticeable difference. (To the forestay tension)
There are always exceptions.

However, this is a tip I read 30 years ago (it must have been true for the author) in a race tuning guide, re-read in the manual to my last boat, and it is true on my PDQ. While I can maintain headstay tension with the main off, and the lee shroud never goes slack, I can see a slight difference in sag with the mainsheet hard on and slacked. this slight difference moves the center of draft back several feet and makes the sail measureably fuller. You might be surprised if you look closely. It is not a huge change, but it can be material.

Perhaps this is only a racer thing. But go look. Strong wind, big main tension.
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Old 26-12-2014, 19:13   #158
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Re: Capsized Atlantic 57 Salvaged

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but you clearly didn't read my whole post

If you could be bothered to read the post which included the part you have highlighted in bold text, you'd see the 'scenario" I was talking about was accidental gybes, ie the two words IMMEDIATELY PRIOR to the part you highlighted.
I wasn't talking about overpowered situations.
Yes, I have red your post. Can you explain to me where a situation that does not result in overpower can be "quite frightening, and potentially dangerous"???? your words, not mine.
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Old 26-12-2014, 19:19   #159
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Re: Capsized Atlantic 57 Salvaged

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There are always exceptions.

However, this is a tip I read 30 years ago (it must have been true for the author) in a race tuning guide, re-read in the manual to my last boat, and it is true on my PDQ. While I can maintain headstay tension with the main off, and the lee shroud never goes slack, I can see a slight difference in sag with the mainsheet hard on and slacked. this slight difference moves the center of draft back several feet and makes the sail measureably fuller. You might be surprised if you look closely. It is not a huge change, but it can be material.

Perhaps this is only a racer thing. But go look. Strong wind, big main tension.
Certainly in what regards to cats and rigs you know more than me but it seem that it is just going downwind, where the tension is smaller that the lack of backstay would be more noticed.

Anyway some very powerful and fast monohull sailing boats (some Pogos) don't have a back stay too and they don't have problems passing without one, so I guess the problem of the tension on the mast in what regards safety without a back stay can be solved.
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Old 26-12-2014, 19:26   #160
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Re: Capsized Atlantic 57 Salvaged

A Pogo without backstay????
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Old 26-12-2014, 19:31   #161
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Re: Capsized Atlantic 57 Salvaged

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Polux I still would like to know what you have. I like all boats, mono ,multi and even stinkboats and your looks really cool.

Way back someone mentioned the uncomfortable seasickness with a lightweight performance cat. Most cats are not in this category with the obvious exception of this A57 and a few others. Heavier cats don't surge as much and I'm sure the C471 is like this even while offering better than average performance.

Everything is a compromise.
Yes I agree, everything is a compromise and one, if lucky can choose one that fits its sailing and living style. The main reason that I have learned something about boat design was to be able to choose one that fits me. I own and sail one of these, practically solo, since my wife really does not like to sail, she is a cruiser, not a sailor

Yachts and Yachting Magazine – Expert Sailing Techniques for Dinghies, Keelboats and Cruiser Racers, Bob Fisher's America's Cup Blog - Comet 41s Review

Comet Sport 41 S - Comar Yachts
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Old 26-12-2014, 19:34   #162
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Re: Capsized Atlantic 57 Salvaged

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A Pogo without backstay????
How dificult would be for you to go to the Pogo site and have a look?
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Old 26-12-2014, 19:38   #163
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Re: Capsized Atlantic 57 Salvaged

Thanks Polux. Really cool boat.
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Old 26-12-2014, 19:45   #164
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Re: Capsized Atlantic 57 Salvaged

Thats quite interesting, Carbon mast with long swept spreaders, i suspect more than 30 degrees, not really good for seting the main full DWD, but good if you are sailing upwind with those new top square mainsails, thats the only way to get rid of a backstay or the lame B&R rig, they kill 2 birds in one shoot, mast pumping and the choice to fly a roachy square head mainsail. good for pogo...
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Old 26-12-2014, 20:34   #165
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Re: Capsized Atlantic 57 Salvaged

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Certainly in what regards to cats and rigs you know more than me but it seem that it is just going downwind, where the tension is smaller that the lack of backstay would be more noticed.

Anyway some very powerful and fast monohull sailing boats (some Pogos) don't have a back stay too and they don't have problems passing without one, so I guess the problem of the tension on the mast in what regards safety without a back stay can be solved.
While 44 understood what I said, you have missed the point entirely.; It was about fine tuning, not structure.

For example, down wind is the when this does NOT matter. The jib becomes more full, which is good.
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