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  #1  
Old 06-13-2005
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Home Hull Construction

I''m curious, if one is making a small sailboat hull (20-25 ft)... I''m thinking of using 3 layers cross-overlapped 1/4 inch laminated ply (end thickness 3/4 inches) with 2 layers of cross-overlapped dynel/resin outer layer.

Am I way off base? What changes, if any, would be appropriate?
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Old 06-13-2005
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Home Hull Construction

You might want to read a book like "The Gougeon Brothers on Boat Construction" -- I expect that it would answer your questions, as they were pioneers in the development of cold-molded construction (which is essentially what you are talking about).

Regards,

Tim
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Old 06-13-2005
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Home Hull Construction

Three layers of 1/4" plywood would produce a pretty heavy hull for a 20-25 footer (and in and of itself, weight does nothing good for a boat). You probably could get by with a single layer of 1/2" plywood for the hull or even less depending on the quality and species of the plywood and the proposed use for the boat.

To give you an example of scantlings for a pretty ruggedly built 25 footer, attached is a part of the scantlings for the Thunderbird One Design class. <http://www.thunderbirdsailing.org/bbook/TOC.html>

3.2 Wood Hull and Deck

3.2.1 Planking, bulkheads and inner transom are to be 1/2" (12.7 mm.) plywood as specified in the official plans.

3.2.2 Cabin sole, cockpit floor and decks are to be 3/8" (9.53 mm.) plywood or equivalent suitable material.

3.2.3 Cabin top is to be constructed with two (2) layers of 1/4" (6.35 mm.) plywood as specified in the official plans.

3.2.4 Cabin sides and cabin face are to be not less than 3/4" (19.0 mm.) thick.

3.2.7 The chine shall be shaped from wood of the dimensions shown in the official plans and shall not be less than 3"(76.2 mm.) in width inside the planking after shaping.

3.2.8 Floor timbers are to be not less than 2" (50.8 mm.) in thickness

There have been many good boats built from plywood and perhaps even more bad ones. Designing a good plywood boat takes a lot more skill than designing a good boat in many other materials. Properly designed plywood can produce a reasonably strong boat for the weight but proper design takes a lot of care. Plywood boats can be quick to build, but not all that quick if you are going to glass it anyway. At that point there are other, nearly equal build time techniques that can produce lighter stronger boats for a similar cost.

Good luck,
Jeff
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Old 06-17-2005
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Home Hull Construction

I used exactly that schedule to repair the ruined deck of a 45 foot ketch.

Truth? It was INCREDIBLY strong, and VERY heavy. The big ketch could handle it like nothing. I''m not quite as sure about a little daysailor, but then, I once had a little 19 foot Cape Dory that was as heavy as a rock and sailed like a dream. I could easily handle weather that sent all other little daysailors scampering for shelter.

Double the lamination thickness in the belly of the hull to about 1 1/2" in the shape of a surfboard, with the buildup being inside, so you will have a super strong mounting pad for a keel, then search out the salvage yards for the cast iron keel off a Tartan 22.
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Old 06-27-2005
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Home Hull Construction

You may also want to check out Ruel Parker''s books on plywood boatbuilding, and the Woodenboat website. If you''re going to build in plywood, one thing to watch out for is voids. Much of the plywood available has voids in interior plies, and this WILL create problems if you use it; problems building AND problems later on. Paying extra for plywood without voids is worth it if you want anything more than a one-season punt.
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