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Old 14-06-2008, 03:07   #76
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Originally Posted by 44'cruisingcat View Post
This is almost too stupid to be worth responding to. Do you really think there are CRUISING boats that will capsize in 13.6 knots of wind? Seriously?

MMMM, 44' cruisingcat, as you learn more about multi-hulls you will recognise that the part of the example you quote is probably not those for a heavy cruising boat. The displacement figure of 2750 kgs is probably a dead give away. James was merely using different vessel designs to demonstrate the effects these changes make on capsizing wind speed. Try reading the article on the web site shown in the post, it will help you understand.
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Old 14-06-2008, 03:28   #77
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I suggest that you read the link that Catty provided before you start calling people here and James Wharram "stupid". You clearly do not understand what is being said .
I have read the link in the past - this analysis was taken in 1990 and things have moved on a little since then. Tony Grainger (Aus multi designer) has calculated quite different outcomes for his boats for example. I know of no production multi of any size above 10 metres that could capsize in under 20 knots - which is where I am having trouble with intrgrating modern cats with James comments.

I think a give away for the age of the analysis is that James suggests CAt B with a beam of 5.3 metres is wide. Its actually norm or narrow these days.

Anyway, YOu guys who want to self right, please contiunue to buy and sail the boats you want, to those that want pure speed go right ahead with your kite boards and similar - to those that want performance more than comfort then a tri rather than a cat (and that a big generalisation I know) is for you.

I have owned them all (well I havent had a proa yet) and I have made my choice, I would really love to have people treat my choice with respect. I will certainly treat your choice with respect, and in some cases even admiration.

My money - my choice - my boat.
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Old 14-06-2008, 03:47   #78
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Reading the result without looking at all the data doesn't make sense either.
cat A has a a very narrow beam and cat b has an enormous amount of sail in relation to its wieght.
My cat is lightwieght with a largish sail area but the numbers come up at a lot more than these two examples.

Getting back on topic.
I agree there is a huge difference between a knockdown and a capsize but I get the impression people think that a mono knocked down is of little consequence. I would think that if one wasn't prepared for and aware of the impending situation having the world turn 90 degrees could be quite painfull.

Mike

Mike, the formulae require inputs in imperial dimensions. ie sq ft, lbs etc.

I plugged in the dimensions for a cosmos 11 that ie Google'd up and got a dynamic stability of a bit over 16 knots with working sails and lightship. I suspect this is the down side of sparkling performance. We would be interested to crunch your boat specs if you would like to publish them.
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Old 14-06-2008, 04:00   #79
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I have read the link in the past - this analysis was taken in 1990 and things have moved on a little since then. Tony Grainger (Aus multi designer) has calculated quite different outcomes for his boats for example. I know of no production multi of any size above 10 metres that could capsize in under 20 knots - which is where I am having trouble with intrgrating modern cats with James comments.

I think a give away for the age of the analysis is that James suggests CAt B with a beam of 5.3 metres is wide. Its actually norm or narrow these days.

Anyway, YOu guys who want to self right, please contiunue to buy and sail the boats you want, to those that want pure speed go right ahead with your kite boards and similar - to those that want performance more than comfort then a tri rather than a cat (and that a big generalisation I know) is for you.

I have owned them all (well I havent had a proa yet) and I have made my choice, I would really love to have people treat my choice with respect. I will certainly treat your choice with respect, and in some cases even admiration.

My money - my choice - my boat.

Factor, no one is telling you your choices are wrong. We are simply presenting facts that people new to catamarans can read and analyze. There is no need to defend your choices.

No, physics has not evolved that much in such a short time. The wind is still trying to capsize the vessel and the righting moment is resisting it.


If you read the article James states.......

Since 1977, this dynamic formula concept, after much initial argument, has been accepted. It has now been generally agreed amongst designers, that taking 60% (x 0.6) of the Static stability allows for a suitable safety factor. So, the Dynamic Stability (i.e. maximum mean wind speed it is safe to sail in before reducing sail) is found as follows:
Static Stability x 0.6 = Dynamic Stability.

unquote

As James states the dynamic formulae evolved after a spate of capsizes in less wind speed than the standard formulae predicted.
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Old 14-06-2008, 05:29   #80
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Originally Posted by Factor View Post
I have read the link in the past - this analysis was taken in 1990 and things have moved on a little since then. Tony Grainger (Aus multi designer) has calculated quite different outcomes for his boats for example. I know of no production multi of any size above 10 metres that could capsize in under 20 knots - which is where I am having trouble with intrgrating modern cats with James comments.
The static stability formula is when the windward hull just lifts from the water (it is obviously a generalisation and one assumes applies in flat water) - he does not say that happens for either of the examples given in under 20 knots if that is what you mean in the above quote; Cat A is 30.4 knots and Cat B is 22.5 knots. Also note, in case missed, that the beam in the formula is hull centreline to hull centreline.

He suggests using "Dynamic Stability" (being 0.6xStatic Stability) so there is reserve stability with the full sail plan up for the expectation of gusts - that is where the wind "less than 20 knots" you mention comes in. Using the method one would maintain full sail plan on Cat A in average wind 18 knots (and would be safe for up to 30.4 knot gusts unreefed) and for Cat B full sail plan up to 13.6 knots (and would be safe for up to 22.5 knot gusts unreefed).

Now on the face of it those figures (without considering the boats themselves at all) seem reasonable. Taking a mono there are many which one would reef around 18 knots if there was likelihood of 30 knot gusts, and lighter large sail area (non cruising) ones where one would maybe want to be reefed much earlier than 23 knots if there was an expectation of such gusts if there was no one on the sheets and the autopilot was steering - now that is in boats which have no fear of capsize at all but would be sailed that way purely for the sake of good management.

Maybe it could be argued that his factor of 0.6 in getting to the "Dynamic Stability is too low - but I think good sense means one would adjust according to the circumstances. If the cat (according to the formula) just lifts the windward hull at 30 knots when carrying full sail plan then if the average wind is 18 knots and no possibility of anything like 30 knot gusts (this would be a very common circumstance in tropic and sub tropic waters and maybe also settled temperate oceanic) then one could manage the sail plan less conservatively. But if there is possibility of the gusts hitting 30 knots then one would manage the sail plan conservatively and that would be so in many temperate regions (in my home waters gusts hitting 40+ knots in 15 knot average winds are absolutely commonplace and we have experienced gusts of over 60 knots from around 15 - 20 knot average which are a problem to manage even in a mono that has no possibility at all of capsizing in them).

I think the difficulty is perhaps some are seeing what Wharram has said as being strict design criteria which if not followed the boat will capsize. Whereas what he is talking about are guidelines for the conservative safe management of the boat.

It would be interesting to see what cat owners come up with for the "Static Stability" for their own boats (that being when the windward hull just lifts clear), modern and not so modern, cruisy or racy, using the generalised formula in the link to Wharram's article that Catty gave.

Maybe some would give it a go? Maybe some could comment as to how accurately it predicts when their own windward hull just lifts? I for one would have a genuine interest in the results.
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Old 14-06-2008, 06:32   #81
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Mike, the formulae require inputs in imperial dimensions. ie sq ft, lbs etc.

I plugged in the dimensions for a cosmos 11 that ie Google'd up and got a dynamic stability of a bit over 16 knots with working sails and lightship. I suspect this is the down side of sparkling performance. We would be interested to crunch your boat specs if you would like to publish them.
Could we crunch Your boats figures (this is not a wind up not having a go etc - catty has said in a previous post that he has a multi, so I figure we can start there) - and cant start with my boat cause its a tri, and the wharram formula certainly has no capability to examine various tris.

Oh - and the other thing the wharram formula takes no account of is underwater shape, James boats are prewdominantly v hull, whereas a round hull with mini keel will operate diffenerently and a round hull with boards etc will operate differently again. The latter will side slip more easily than the former two.
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Old 14-06-2008, 06:48   #82
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Factor, Quite a pointless question. That is like saying, how many people have died in car accidents, between cars that are white and cars that have purple pokadots on them. There maybe a larger number of multi guy's on this multiforum, but it does not mirror the number of monos vs multis. Multis are still very much a minority out there and especially if you go back 18yrs.
Cant agree - its nothing like black and white cars at all. The reality is that I know of no deaths connected to sailing a multihull in Australia in that time, I know of many connected to monos, - now that doesnt prove to anyone including me that monos are dangerous but it certainly gives rise to some considerations as to what amounts to safety aboard. If the 18 years is an issue - fine pick any time period you think better represents the idea.

The issues of reduction in likelihood of going overboard, the reduction in tiredness due to level sailing leading to clearer heads for decision making can all be linked to type of boat.

And yes there are more monos that multis for sure, but up our coast there are probably as many of each actually cruising as distinct from floating around the bay. Indeed at places like lizard island, on any given day the multis can outnumber the monos.

Again I'll say it, I respect every persons decision to sail what they like, I can categorically prove that Monos sink. Doesnt mean they are an invalid choice any more than proving that cats and tris capsize makes them an invalid choice.
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Old 14-06-2008, 06:57   #83
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Again I'll say it, I respect every persons decision to sail what they like, I can categorically prove that Monos sink. Doesnt mean they are an invalid choice any more than proving that cats and tris capsize makes them an invalid choice.
Well said.

Time to go sailing, fair winds all.

Joli
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Old 14-06-2008, 07:03   #84
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Could we crunch Your boats figures (this is not a wind up not having a go etc - catty has said in a previous post that he has a multi, so I figure we can start there)

Factor, when you preface statements with " this is not a wind up" it makes me wonder. So to save you the time struggling with the formula I'll do that part for you.

Static stability : 29.3 knots
dynamic stability :17.6 knots

Reef at 18 if on deck and sailing to windward,15 if below decks or at night, reef early by dropping the main if running, and tow warps by 25 if deep reaching/ running on auto . Asymmetric of 3 x working sail area back in the locker by 12 to 15. Primary sheet winches are not self tailing and use 1 piece clam cleats. Note ; the wind speeds refer to max gusts being experienced at the time.

Have flown a hull a couple of times under controlled conditions (ie on auto pilot with sheets in hand) and it just so happens it occurs around 25 knots true or 30 apparent in flat water. Pretty much what James predicts. Even after 70,000 odd miles, gusty weather gives me ulcers unless we are well reefed down.

Factor if you don't like wharrams hull shape go to kelsals web site cause he uses the same formula.
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Old 14-06-2008, 07:10   #85
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Catty - could you share the actual numbers and the boat design so as I can test my theory regarding hull shape amongst other things?
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Old 14-06-2008, 07:18   #86
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Factor, I do not wish to identify myself, through my boat details, to you, after reading the threatening vitriol you regularly private post me. I simply do not feel safe.
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Old 14-06-2008, 07:27   #87
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Mate - all I asked for was the design. As for the private vitriol, I will publicly request the mods post any pms betweeen us. Would you agree to that - I think I should be entitled to defend my self against your claim. Your imputation is rejected, your assertion that I have posted any threatening PMS to you in any way is untrue.

And I must say - 70,000 miles, I dont have that on the multi log so I do value your opinion, I have been multihulling for a while now, but havent done that sort of distance, so I am certain that you have some observations that I should listen to.

All I asked for was a boat design FFS.

Back on topic - the issue of hull shape is critical. Eg i could have a set of numbers that give rise to a wharram figure of e.g. 15 knots, but if - to use an extreme I had reserve bounancy above the waterline that was extreme, eg a flare that was 10 metres wide, then she aint gunna capsize.

Now I know this is extreme - but that is just to demonstrate the concept. Hull shape, as modern designers from Antrim, to white, to grainger to jutson, to lidgard to hughes to kerr etc have identified , is critical to managing the overall issue.
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Old 14-06-2008, 11:11   #88
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Something that I had picked up on before (in discussions on CF.com) and is being touched on here................is that their is a difference in design and thinking between Wharram Cats and "Modern" Cats. (apart from cosmetic looks, plywood construction and often no deckhouse (?).

By "Modern" I am thinking more of those built within the last 10 years - given the large gap in design years.

The impression I get (and please do correct me - on any or all!) is that the Wharram hulls were narrower and more V shaped, the cross beams were designed to flex and the Cat Beam was also narrower overall in comparison to the "Modern" cats (possibly some generalisations going on here )......was it a case that designers decided that (apart from to carry extra weight / a deckhouse / have more hull space) their was part of the Wharrams design that they felt needed to be improved on? Especially when it comes to thinking about stability?

Not seeking for a spot of Wharram bashing here! - but the question comes from having read a few threads, plus I am kinda interested in maybe (next year......or the year after? ) getting one of the smaller Wharrams just for (coastal) fun - mainly cos' they seem cheap and simple enuf (and I saw a real cool Youtube clip of a 21 footer!). Of course not expecting to round Cape Horn in a 21 or 26 footer (Multi or Mono!).....so not fundamently bothered that they might not be best suited for a F12, but it made me curious as to how Cat design in general has moved on over 40 odd years and why.


And of course not getting rid of the "Wayluya" Seadog!
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Old 14-06-2008, 14:30   #89
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Factor, when you preface statements with " this is not a wind up" it makes me wonder. So to save you the time struggling with the formula I'll do that part for you.

Static stability : 29.3 knots
dynamic stability :17.6 knots

Reef at 18 if on deck and sailing to windward,15 if below decks or at night, reef early by dropping the main if running, and tow warps by 25 if deep reaching/ running on auto . Asymmetric of 3 x working sail area back in the locker by 12 to 15. Primary sheet winches are not self tailing and use 1 piece clam cleats. Note ; the wind speeds refer to max gusts being experienced at the time.

Have flown a hull a couple of times under controlled conditions (ie on auto pilot with sheets in hand) and it just so happens it occurs around 25 knots true or 30 apparent in flat water. Pretty much what James predicts. Even after 70,000 odd miles, gusty weather gives me ulcers unless we are well reefed down.
With all that you have to say about multihulls, that they are ugly, they are slow, they are uncomfortable, they are dangerous, that sailing yours gives you "ulcers", I have to ask - why do you still own one? And why ON EARTH have you persisted with it for 70,000 miles????

I have a good solid steel mono I'm trying to sell - I'd be more than happy to consider a trade.
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Old 14-06-2008, 14:50   #90
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The Wharram formula gives a theoretical number. For one thing it assumes absolutely flat sails, held directly down the centreline of the boat, so that the entire force of the wind on the sails is directed into lifting a hull, and none of it into driving the boat forward. Real world numbers are obviously much higher.
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