Displacement or plane hull? - Page 3 - SailNet Community
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post #21 of 35 Old 01-26-2009
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Yeh....I guess that’s right....I forget the big difference between traditional and modern designs when it comes to overhangs.

The difference between my LOA and my LWL is around 9ft on a 48ft LOA boat.

BTW my L/D is 295 ....W/O beer!

James S
S/V Arctic Lady
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post #22 of 35 Old 01-26-2009 Thread Starter
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As I understand it, the Elements book is based on designs rules from a 100 years ago, so I thought it be worthwhile to check on them. Supposedly, the book has been updated over the years. But I guess my suspicions were correct that its rules are a bit out of date. It's still an interesting read.

I did a quick survey of marina depths at various spots using google. An 8' draft seems acceptable in most places, but there looks to be quite a few marinas that are only 6'.

The sketches I made for a 38ft has very little overhang. I'm trying to maximize the LWL and discourage the bow from riding up on incoming waves. It also made for a bit more room up front.

Just out of curiosity, I'm wondering if anyone has some opinions on the SWATH-style catamarans. The torpedo hulls look like an interesting way to punch thru rough water and gain some speed as well.
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post #23 of 35 Old 01-26-2009
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A comfortable “sea kindly" boat is important to me.....AND SO IS DRY.

A white knuckle wave piercing ride would be fun for about oh say…a couple minutes…. but then I want to drink my coffee and have a sandwich.

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S/V Arctic Lady
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post #24 of 35 Old 01-26-2009
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INetRoadKill, I guess I just have to ask, what the heck are you doing?

On one hand you are mentioning exotic multihull hull forms that are best suited as power driven commercial craft and in the same post you mention what I assume is "Skene's Elements of Yacht Design"? These are such far extremes of thinking that I am perplexed when you say "sketches I made for a 38ft has very little overhang." In other words, are you attempting to design a boat? And more to the point, are you attempting to design a boat that you plan to build?

And if you are designing a boat, are you designing a multihull, or a planing boat, or a semi-displacement monohull?

I only ask because the weights and drafts that you are throwing around while mildly heavy for a monohull are dangerously heavy and deep fopr a multihull and are way too heavy to every get up on a plane under sail.

And there there is Skene's. Skene's was published in a number of editions over close to a 100 year period and frankly was outdated decades ago. Its not that the most recent versions aren't useful as reference tool, but so much has been learned about motion comfort and seakeeping that it is not all that useful as a design textbook.

And that gets me to my point of asking if you plan to build what you are designing. Traditional hull forms and rigs were pretty easy to design. Its not that bad designs weren't done, but frankly it was easier to get it right if you spent some time looking at what had been done before, did your calculations rigorously and paid attention to your drafting. Modern designs, whether they are multihulls or monohulls require a lot more skill to design. They are just plain less forgiving and so if you really plan to design a boat that you plan to actually build, I suggest that you look into WESTLAWN Instutute of Marine Technology, Westlawn Institute of Marine Technology because whatever one of their courses cost it is way cheaper than building a not all that well designed 40 plus footer.

Respectfully,
Jeff
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post #25 of 35 Old 01-26-2009 Thread Starter
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I'm just exploring options right now. Nothing is in stone. Right now, any idea is up for consideration -- within reason. Granted, going too far off the beaten path is probably not a good idea. I guess it's confusing trying to figure out what I'm up to at this point based on the questions I've posed. But it's mostly just brainstorming.

The design I'm toying with now is a rather conventional monohull. But I've always had a soft spot for catamarans which is why I was asking about them. I know the swath-style hull on a catamaran probably isn't going to tolerate the heeling forces from the sails very well. I was curious if it was a good open water performer in terms of speed and smoothness.
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post #26 of 35 Old 01-26-2009
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Originally Posted by InetRoadkill View Post
I'm just exploring options right now. ....
... I've always had a soft spot for catamarans which is why I was asking ...the swath-style hull on a catamaran
Can't believe I'm posting a pic of a powerboat, but is this what you mean by "swath" design? This boat is a passenger ferry between St Maarten and Saba.


Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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post #27 of 35 Old 01-26-2009 Thread Starter
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Can't really tell from that picture, but it might be.

A swath hull is roughly a pontoon boat in which the pontoons are submerged and torpedo shaped. I believe the idea is that they don't make bow waves so they are not limited by hull speed and since they are underwater, they don't care about waves. They are probably too unstable for use in a sailboat application since there is little change in buoyancy with depth.
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post #28 of 35 Old 01-27-2009
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Are you actualy thinking about putting a mast on a sub....?
Should be a smooth ride

James S
S/V Arctic Lady
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post #29 of 35 Old 01-27-2009 Thread Starter
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I bet it would be smooth. But I'm not going on deck to hoist those sails.

Actually, I'm just trying to get familiar with different hull shapes and their advantages and disadvantages.
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post #30 of 35 Old 02-01-2009
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build baby build.
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