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Old 15-02-2010, 16:19   #1
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Using '2/3rds' of a Design?

Boat for sale down the deep south

"2/3rds of a of a William Atkin 'Thistle' design "

'Oslo' - 21' 6" double ended gaff cutter for sale - TradeMe.co.nz - New Zealand

What do we think of the notion of "shrinking" a designers' work.

Price seems tasty, yet I'm old enough to be deeply suspicious of my "buy_on_price_alone" motive; 'specially for deep water.

my thinking is that '2/3rds' of some design is really a complety different design, (with perhaps some similarities).

Waddya yawl reckon?

(still unable to figure out how to post a pic, or else I would'a)
*help appreciated here*
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Old 16-02-2010, 14:05   #2
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I sail a custom design which as built deviated from the plans. Everything turn out alright, but the changes were nothing on the order of a 2/3 shrinkage.

Fin keel instead of full keel. Aft head instead of forward head. Bow lengthened and short bowsprit added. Boom and mast extended. Mast moved forward. Rudder moved forward to accomodate propane tanks. Tankage added. Etc.

2/3 shrinkage is too much deviation for my taste, but that's just me. I would demand extensive test sails in a variety of weather and sea conditions. That's really what counts, IMO.
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Old 16-02-2010, 14:27   #3
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From what I know of it, as long as all the dimensions were scaled down the same way, you end up with the same boat only smaller. I've seen it done a few times over the years, and generally it works well. I've also seen them stretch a boat, but usually people only go for 10%. So if your stations are on 24 inch centers, they may shift them out to 26.4 but no more. This way you end up with a longer hull and can still use the existing scantlings. Go much much past 10% and you need to start considering a size increase to handle the increased stresses.

Anyone want to back stop me on this?

Sabre
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Old 16-02-2010, 14:42   #4
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The ratios change as a boat gets smaller. I'd be suspicious.
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Old 16-02-2010, 15:00   #5
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2/3rds scale?? - Oh well ...it sold in about 20 mins!

It be gone now
Some-on snapped it up
Good luck to them
Steve
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Old 16-02-2010, 15:04   #6
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The ratios change as a boat gets smaller. I'd be suspicious.
For some reason I'm under the impression that stability becomes more important the smaller the boat gets. That's why small sailboats tend to be fatter than long sailboats.

Stability is a function of a cubed power because the boat is three dimensional, whereas sail power is a function of a squared area. So they don't change in parallel fashion as boat size is increased or decreased.

Or something like that. Would be wonderful if Bob Perry would jump in here.

So I tend to agree with you.

OTOH, changing the size of the rig after the fact to fit the hull is not mission impossible. But if boat stability is not there, that's a much bigger problem.
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Old 16-02-2010, 16:22   #7
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Atkins, actually father and son designed a lot of boats. This may have been a smaller design that was similar in appearance, but not a true scaled down design, to the Eric/Thistle, only 10' or 1/3rd shorter. The Atkins Ingrid design is an Eric/Thistle only 1/3rd larger displacement. Other than they are both double enders, these two designs probably don't have a single line in common, however.
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Old 16-02-2010, 16:39   #8
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In general, it is very difficult to reduce a design by so large an amount and still have a seaworthy boat.

I concur with Hiracer: if all proportions are conserved, when the length is multiplied by 2/3, the surfaces are multiplied by 4/9 (2/3 squared), the volumes by 8/27 (2/3 cubed) and the stability by 16/81 (2/3 at the 4th power). Then, I'm not sure that the reduced design retains sufficient stability and sail carrying power.

In this particular case, three tons is very heavy for 21'6" length. On the pictures, the freeboard seems a bit low.

Alain
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Old 17-02-2010, 05:46   #9
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If you knew 2/3 of your telephone number; would you expect to be able to call home?
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