For the OZ PD Racers we built we originally chose 4mm plywood for the hull.
people involved in the class thought we were crazy as most were using
plywood around 8mm and still having some structural problems with hull
twist under sailing loads and excessive movement of the flat hull
were quite confident that the 4mm would provide enough strength.
But we were a bit shocked when we saw the light shining through
our bargain basement plywood (pic right - the sun is behind the boat).
There was no middle veneer in some places.
The combination of the poor ply quality allied with its thinness resulted in Peter putting his foot through
the bottom of one of the boats (he doesn't have my catlike agility!). I
ended up replacing the bottoms with 6mm Gaboon Ply - not only is it
thicker but it is also made of 5 layers of timber - making it much
tougher than the normal 3 veneers of the original 4mm bottom..
All other parts of the hulls are 4mm and have stood up very well.
So we the rules for the OZ PD Racer will require 2 x sheets 4mm plywood 1 x sheet 6mm plywood.
Below is our original discussion BEFORE we ran into the problems above.
Also see the EPOXY and Boatbuilding FAQ for detailed information about the thinking and use behind the modern construction methods involved in the OZ PD Racer Regarding the
thinner ply we are using in Australia - it was partly chosen because
they were the cheapest true 8ft x 4ft sheets we could get locally on
that day. Under $20/sheet.
A lot of ply these days is
2400mm x 1200mm - which is just under the imperial sheets but
unsuitable for the PDRacer as it would add quite a bit more labour and
material wastage to get the 4ft width. - though you can use the measurement tolerances.
Using the thinner ply - Sides, bulkheads, decks and transom have turned out fine – quite stiff and strong enough.
the current longitudinal bracing of the bottom it is OK (being fitted in the pic right), but making the bottom panel
of 1/4" (6mm)would have been better - but that would have doubled the
price of one of the sheets.
fact springing the extra $20 for
the thicker bottom panel would have been a good idea - it would have
eliminated the longitudinal runners I have just finished gluing to the
bottom of the hull, for a labour saving and would only add an extra
pound and a bit (0.5kg).
(and that was all written before Peter put his foot through the hull!)