Join Date: Feb 2000
Location: Annapolis, Md
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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.