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Old 15-10-2008, 14:08   #1
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Hydrodynamic and weak spots

I have some ideas about the answers, but it would be nice to read your opinions on these two questions also...

1. What hydrodynamic (etc) rules should a cat follow, to be as seaworthy and comfortable as possible? How does beam over all, beam of each hull, hullshape, lenght, bridge clearance etc contribute (or discontribute) to seaworthiness. As for sail area and mast height it seemes clearer (and that's not a hydrodynamic issue), but the rest...
I read somewhere that the 2:1 lenght-beam proportion would be optimal. For what, and if so, why so? Looking at modern cats, that don't seem to be the fact, they rather seemes to approach 1:1 (slightly exaggerated).

2. Where are the weak spots in a cat? Where do they break if they do it? Are modern cats built to be able stand on the ground without support?

Trying to dig deep into cat constructions,

Rolf
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Old 15-10-2008, 14:37   #2
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Modern cats don't break unless they hit something hard. Structurally I'd say they all could take to the ground fine, but if they have fixed rudders, or saildrives and no mini-keels they would need some support to protect those appendages. Some boats have rudders that can handle supporting the boat too. Boats with mini-keels can usually stand on their keels fine, although balance fore and aft can be an issue if the keels are short.

The idea behind a boat being longer than it is wide is so that in the event of being overpowered and at risk of capsize, you can turn downwind, lessening apparent windstrength. The boat being much longer than wide will have a greater fore and aft "righting moment"

There is a modern tendency to make cats wider which makes them roomy and allows them to carry bigger rigs and sail faster. It's possibly a result of the way marina berths are charged for - by length.

Most aspects of the hull design are a compromise IMHO. I like the idea of hulls having bouyant ends and little rocker (high prismatic coefficient), I believe it reduces the tendency to pitch fore and aft.
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Old 18-10-2008, 11:19   #3
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To wide...or not

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Originally Posted by 44'cruisingcat View Post
The idea behind a boat being longer than it is wide is so that in the event of being overpowered and at risk of capsize, you can turn downwind, lessening apparent windstrength. The boat being much longer than wide will have a greater fore and aft "righting moment".
Ok, I agree in that. But when will a cat become to wide, can it be to wide and how will it react in the sea? "Nervous" sailing? Less risk of being overpowered by sidewind.
If you take any cat, longer than wide of the given reasons, and then widen it more and more, to gain space inside, what will happend? Compared with a board, the wider and longer it gets, the more stable, but on the other hand you don't sail a board, at least not with the widest side ahead.
Rolf
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Old 18-10-2008, 13:38   #4
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I think there are many ideas out there on cat design - not all of them agree. Good sources of info include Charles Kanter's book, Shuttleworth's website (John Shuttleworth Yacht Designs Ltd.), and Richard Woods' site (Woods Designs Sailing Catamarans). Enjoy.
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Old 18-10-2008, 14:14   #5
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I'll add a link to a forum where this subject is discussed regularly.

Boat Design Forums
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Old 19-10-2008, 10:03   #6
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The simple truth is that there have not been enough pitchpole accidents to justify all these fears. And if you filter out the absolutely inevitable occurances, the discusion becomes totally academic. Those are the wave induced accidents where the vessel is overrun by a breaking wave higher than the boat is long. This will do the deed for anything afloat, up to and including the Edmund Fitzgerald [as one theory claims].

I greatly respect Charles Kanter's experience, but I believe that his position on 'over-square' catamarans arrises more from respect for older style cats than objective science. I believe that mast height, or more precisely the height of the CEP (center of effective pressure) in relation to a function of the Center of boyancy vs the center of horizontal resistance is the key to predicting the onset of a roll over, and that the ability to recover before that point is reached is a function of the vessel's agility. For me, ease of reefing matters far more than the beam to length ratio. I suspect that the reputation of older designs stems from the fact that the need to reef became urgently apparent on these boats before it became too late. The PDQ cats can be reefed by one person from the cockpit from head to wind to somewhere around a beam reach.
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Old 19-10-2008, 10:45   #7
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Quote:
Originally Posted by tolly View Post
Ok, I agree in that. But when will a cat become to wide, can it be to wide and how will it react in the sea? "Nervous" sailing? Less risk of being overpowered by sidewind.
If you take any cat, longer than wide of the given reasons, and then widen it more and more, to gain space inside, what will happend? Compared with a board, the wider and longer it gets, the more stable, but on the other hand you don't sail a board, at least not with the widest side ahead.
Rolf
I think we might be confusing things a bit. I am reading two different measurements for a catamaran being discussed here.

There is the length to beam ratio and then there is the length to hull width ratio.

For length to beam ratio the compromise is righting moment (good because it means more speed) versus resistance to pitchpoling. Most manufacturers are finding the optimal to be around a 2:1 ratio.

The other factor is hull width to length ratio. I say "hull width" so as to distinguish this from a catamarans maximum beam. The compromise is speed (good) versus the ability to carry more weight without unduely sinking the hulls further in the water (bad) and increased hobbyhorsing, also bad. A fine hull is considered one with a 12:1 ratio.
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Old 20-10-2008, 22:04   #8
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Quote:
Originally Posted by tolly View Post
Ok, I agree in that. But when will a cat become to wide, can it be to wide and how will it react in the sea? "Nervous" sailing? Less risk of being overpowered by sidewind.
If you take any cat, longer than wide of the given reasons, and then widen it more and more, to gain space inside, what will happend? Compared with a board, the wider and longer it gets, the more stable, but on the other hand you don't sail a board, at least not with the widest side ahead.
Rolf
If the only thing you changed was the overall beam, nothing much would happen. But there is a tendency to use the increased righting moment to carry a bigger rig and sailplan. Even if that didn't change, you still might be inclined to leave reefing that much later, which could make pitchpoling just a little bit more of a possibility.
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Old 22-10-2008, 03:58   #9
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Breaking wave

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Originally Posted by sandy daugherty View Post
The simple truth is that there have not been enough pitchpole accidents to justify all these fears. And if you filter out the absolutely inevitable occurances, the discusion becomes totally academic. Those are the wave induced accidents where the vessel is overrun by a breaking wave higher than the boat is long. This will do the deed for anything afloat, up to and including the Edmund Fitzgerald [as one theory claims].
As far as I know the risk of a breaking wave turning the vessel upsidedown is in almost every case extremely small, because...
A wave is not to be seen as a vertical wall, rather an increasing slope, in the end inverting as it breaks. Therefor a wave needs to bee breaking at approximately 3 times the height of the boats width, to make the boat turn over. As for a 7m wide cat it means waves that are 21m high and breaking. Hardly common in most cruising sailing areas as far as I know.
If you follow or meet the waves, the risk should be ever smaller, as the cat is longer than wide and cuts the waves differently ahead then riding sideways.
In none of these cases I consider the influence of the rig and sail. That is more a matter of good or bad judgement as a skipper.

Rolf
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Old 23-10-2008, 06:31   #10
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Charles Kanter may be 'conservative', but when it comes to stability in survival conditions that may be no bad thing. Are the risks for pitchpoling overstated? I suspect not, as even monohulls (with much more fore/aft stability) can and do pitchpole - for example, the Smeeton's experience described so vividly in 'Twice is enough'.

The preferred tactic for a cat in survival conditions is to avoid taking the seas beam on. If you are running with a storm (either with or without a series or other drogue), or set to a sea anchor off the bow, the forces will largely be fore and aft, rather than side to side. As such, in those circumstances resistance to pitchpoling is more important than resistance to capsize. This favours a lower length to beam overall ratio. Full stop.

Dealing with the issue of strength, the wider the cat the greater the tendancy towards racking and, while the boat should be engineered to cope with this, it is nevertheless disconcerting. For example, there is a PDQ Capella 36 on the hard beside my Solaris Sunstream 40 - the PDQ requires 6 well-adjusted jackstands under the bridgedeck or the interior doors will not open/close properly. My boat can simply be plucked down on some 2x6's.

Finally, it should be remembered that cats with narrower beam are also (all else being equal) easier to tack - and easier for finding docking and haul-out facilities.

All boats are compromises. The cat with the narrower beam overall will have to keep its CE and CG lower than the wider cat in order to maintain appropriate resistance to capsize. This can be achieved by utilitzing a 'Prout' style cutter rig and by keeping the heaviest part of the accomodation low (in the hulls) without a huge sacrifice to light air performance. Of course, loss of bridgedeck beam will also cost some interior/cockpit space.

Literally legions of early Prouts have circumnavigated safely despite extremely narrow beam - and as I recall, there are no reported cases of one having capsized or pitchpoled. The same cannot be said of cats with much greater beam, but also higher CE's and CG's - eg., the Lagoon that capsized in the hands of a professional delivery skipper in the Atlantic, east of Bermuda a couple of years ago.

It is often said that all boats are compromises - and nowhere is this more true than in the length/beam overall ratio.

Brad
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Old 23-10-2008, 08:43   #11
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There are lots of books, and people that give advice on this subject. Lots of it is good info.
I think a good way to judge the way different cats feel on the ocean is to sail on as many as possible. There is less theory involved in this.
You should sail on everything from a lagoon 440 to a reynolds 33. It will give a good appreciation for what hull shape, beam, weight, and rig size do to change the way a cat sails and rides on the waves.
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Old 23-10-2008, 09:40   #12
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Right you are

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There are lots of books, and people that give advice on this subject. Lots of it is good info.
I think a good way to judge the way different cats feel on the ocean is to sail on as many as possible. There is less theory involved in this.
You should sail on everything from a lagoon 440 to a reynolds 33. It will give a good appreciation for what hull shape, beam, weight, and rig size do to change the way a cat sails and rides on the waves.
I can't but agree in that. As I read in an article about how common cats are in different places: The further north you come in Europe, the less cats, till you reach Sweden were there's hardly any to find.
All to true...through the years I've sailed in swedish waters, I can remember having seen perhaps 5 cats!!! My experience is that swedes are were conservative in how a boat should look. The 52" Colin Archer I have now is in this respect (and other) a good boat.
Perhaps now you understand my problem, when I try to find a suitable cat. I will naturally try to sail as many as I can find. When I find them.
Till then I collect experience from others.

Rolf
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Old 23-10-2008, 11:47   #13
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Rolf, you are taking a sound approach to the issue. Of course, one is unlikely (and really doesn't want) to test a boat in survival conditions - so a certain amount of theory (and even some anecdotal experience) is still a necessary part of the equation.

Brad
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