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02-03-2008, 09:51
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#1
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Registered User
Join Date: Jan 2008
Location: Denmark
Boat: FP Tobago 35
Posts: 721
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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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02-03-2008, 21:14
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#2
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Registered User
Join Date: Nov 2007
Location: Brisbane
Boat: Lightwave 45, sold nov 2020.Previous self built Roberts 36
Posts: 217
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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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02-03-2008, 22:14
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#3
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Registered User
Join Date: Dec 2007
Posts: 340
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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.
__________________
cat skin hat
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02-03-2008, 23:08
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#4
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Registered User
Join Date: Sep 2006
Posts: 9,398
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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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03-03-2008, 00:56
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#5
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Registered User
Join Date: Jan 2008
Location: Denmark
Boat: FP Tobago 35
Posts: 721
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Quote:
Originally Posted by Glenn C
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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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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03-03-2008, 01:00
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#6
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Registered User
Join Date: Jan 2008
Location: Denmark
Boat: FP Tobago 35
Posts: 721
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Quote:
Originally Posted by 44'cruisingcat
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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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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03-03-2008, 01:11
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#7
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Registered User
Join Date: Dec 2007
Posts: 340
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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.
__________________
cat skin hat
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03-03-2008, 01:45
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#8
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Registered User
Join Date: Nov 2005
Location: At the intersection of here & there
Boat: 47' Olympic Adventure
Posts: 4,851
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Quote:
Originally Posted by catty
, 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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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 ‘build’ 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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03-03-2008, 12:29
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#9
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Registered User
Join Date: Jan 2008
Location: Finland
Boat: Catharsis 27 under construction
Posts: 33
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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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03-03-2008, 12:51
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#10
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Registered User
Join Date: Jan 2008
Location: Denmark
Boat: FP Tobago 35
Posts: 721
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Quote:
Originally Posted by terhohalme
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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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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03-03-2008, 14:44
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#11
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Registered User
Join Date: Sep 2006
Posts: 9,398
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Quote:
Originally Posted by Nordic cat
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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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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03-03-2008, 14:51
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#12
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Registered User
Join Date: Sep 2006
Posts: 9,398
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Quote:
Originally Posted by catty
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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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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04-03-2008, 03:17
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#13
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...
Join Date: Feb 2007
Posts: 483
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Quote:
Originally Posted by Nordic cat
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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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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04-03-2008, 06:05
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#14
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Registered User
Join Date: Jan 2008
Location: Finland
Boat: Catharsis 27 under construction
Posts: 33
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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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14-03-2008, 07:49
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#15
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Registered User
Join Date: Jan 2008
Location: Denmark
Boat: FP Tobago 35
Posts: 721
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Quote:
Originally Posted by ireaney
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Many thanks Ian,
I have ordered both books, they should be arriving next week.
Alan
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