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Old 02-03-2008, 10:51   #1
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Underwater lines for a performance cruiser

I have been reading up on some of the literature on hull designs, but there is not much out there regarding cats. So any pointers and feedback will be much appreciated.

By the way this is a bi-rig design without foresails, so I need to ensure easy tacking - so any input on this would be great.

My design has about 52 cms/21" draught without boards. LOA 49'/1490cm

LWL = 47'/1430cm. Transoms are 8"/20cm free of the water at rest. Hull beam is 110 cm/ 3'7". Fineness ratio is 1:13
Prismatic coefficient = 0.575.

Any comments on these figures?

How far back should the max wlbeam be? Also is there an advantage to having the max draught at this position?

Looking forward to feedback from the great knowledge base here.

Regards

Alan
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Old 02-03-2008, 22:14   #2
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Hi Alan,

have you checked out the multihulldynamics.com website ? There is a wealth of comparative performance data, (along with the formulaes used so you can plug in your own numbers to see where the boat fits on the trend line graphs) and I have found the guys really responsive to specific questions,

Glenn
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Old 02-03-2008, 23:14   #3
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Underwater lines of performance cruiser

Whilst we are on this subject , does anyone know what the advantages of a completely flat bottom, carried all the way forward to the bow are? As we see on some of bob orams designs. Whatever they are, I suspect you have got to have your fillings glued in well to appreciate them, especially after a long windward passage.
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Old 03-03-2008, 00:08   #4
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Which of Bob's designs would that be? I haven't seen any that are like that, but possibly I don't know his designs as well as you appear to. (Although I have looked at the 38'er and the 62'er and I am building the 44'er - all of these are quite round for about the first 1-1.5 metres aft of the bow.)

I have sailed to windward in one of his boats, in pretty crappy conditions, 25-30 kts, wind against 1 - 2 kts tide in shallow water, and the boat performed very well for the conditions - better than an other wise fairly similar round bilged design I have sailed on, but admittedly a little more noisy. Bob's boats tend to have a fairly deep forefoot, which possibly helps by keeping the bow in the water more of the time.

Alan, regarding your figures, you have the transoms 200mm clear of the waterline (presumably DWL at max displacement?) - I was wondering why?
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Old 03-03-2008, 01:56   #5
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Hi Alan,

have you checked out the multihulldynamics.com website ? There is a wealth of comparative performance data, (along with the formulaes used so you can plug in your own numbers to see where the boat fits on the trend line graphs) and I have found the guys really responsive to specific questions,

Glenn

Hi Glenn,

Yes I have, I have plotted it in against the Shuttleworths, Kelsalls, Crowthers and a few others.

The work they do is great, but they do not really know things like prismatic coeff., hull beam etc. so it can only be used as a guideline. As their information is primarily from sales material, it is probaly a bit flawed, as most designers, and builders especially don't give out this information easily.

One has to be a bit careful in these comparisons as they do not take different rig types into consideration when calculating stability index etc. so some of the figures come out a bit wrong.

I would like to post those files here showing the comparison between a number of boats, but am not allowed to due to their copyright. Maybe I will drop them a mail and ask, as that would also show what they offer.


Thanks for the suggestion.

Regards

Alan
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Old 03-03-2008, 02:00   #6
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Alan, regarding your figures, you have the transoms 200mm clear of the waterline (presumably DWL at max displacement?) - I was wondering why?

I have been lead to believe that this will give the least drag and turbulence. I think it was in an article by Morelli and Melvin some years ago..

When sailing at speed I expect that there no longer will be clearance, but we have not yet done any simulations/calculations.

Anyone have a good explanantion of why or why not?

Regards

Alan
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Old 03-03-2008, 02:11   #7
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Correction

I stand corrected, So what are the advantages of a totaly flat bottom to within 3 feet of the bow. Gennuinely interested, . I suspect the flat forward sections will stop any freefall abruptly. Was this the intention? I notice many open water boats avoid flat fwd sections. Is it just a compromise for ease of amateur construction.
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Old 03-03-2008, 02:45   #8
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, So what are the advantages of a totaly flat bottom to within 3 feet of the bow. Gennuinely interested, . I suspect the flat forward sections will stop any freefall abruptly. Was this the intention? I notice many open water boats avoid flat fwd sections. Is it just a compromise for ease of amateur construction.
Oram designs aim for simplicity and cost savings - the flat bottom is also the cabin sole (straight from his site: My boats tend to be cheaper because we simply Ďbuildí less boat, eg. we have no floors in the hulls (you walk on the heavily reinforced and flat keel panel) - that saves about $1000 in materials and days and days in labour.)
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Old 03-03-2008, 13:29   #9
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The best catamaran hull analyse I have found is by Alexander Simonis in:

Claughton, Wellicome & Shenoi: Sailing Yacht Design: Practice, ISBN 0-582-36857-X
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Old 03-03-2008, 13:51   #10
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The best catamaran hull analyse I have found is by Alexander Simonis in:

Claughton, Wellicome & Shenoi: Sailing Yacht Design: Practice, ISBN 0-582-36857-X

Many thanks Terhohalme.

I just looked it up, 187£, pretty steep price! How long is this article by Simonis?


I remember and article by Lavranos i think, anybody have that available?

Regards

Alan
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Old 03-03-2008, 15:44   #11
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Originally Posted by Nordic cat View Post
I have been lead to believe that this will give the least drag and turbulence. I think it was in an article by Morelli and Melvin some years ago..

When sailing at speed I expect that there no longer will be clearance, but we have not yet done any simulations/calculations.

Anyone have a good explanantion of why or why not?

Regards

Alan
You are giving up a considerable amount of waterline length, especially if the hull rises gently to the transom, which is desirable. I don't really know what advantage there would be.
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Old 03-03-2008, 15:51   #12
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I stand corrected, So what are the advantages of a totaly flat bottom to within 3 feet of the bow. Gennuinely interested, . I suspect the flat forward sections will stop any freefall abruptly. Was this the intention? I notice many open water boats avoid flat fwd sections. Is it just a compromise for ease of amateur construction.
By having flat bilge panels and no floors, you save the weight and cost of the floors, and also weight, cost and windage by having a lower sheerline for the same amount of headroom.

As I said previously the boats are still comfortable enough to windward in quite unpleasant wind/tide conditions.
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Old 04-03-2008, 04:17   #13
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Many thanks Terhohalme.

I just looked it up, 187£, pretty steep price! How long is this article by Simonis?


I remember and article by Lavranos i think, anybody have that available?

Regards

Alan
Alan, You can get the book for almost half the cost on Abe Books, look at this link, hope it helps.
AbeBooks: Search Results - Claughton and Sailing Yacht Design

Ian
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Old 04-03-2008, 07:05   #14
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The article is 30 pages case study for a modern charter catamaran. There is lot of other good stuff also in the book. 187£ though... uuh...
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Old 14-03-2008, 08:49   #15
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Alan, You can get the book for almost half the cost on Abe Books, look at this link, hope it helps.
AbeBooks: Search Results - Claughton and Sailing Yacht Design

Ian

Many thanks Ian,

I have ordered both books, they should be arriving next week.

Alan
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