Michael Storer Boat Design


Using Light Ply Construction


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For the OZ PD Racers we built we originally chose 4mm plywood for the hull.

Most 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 panels.

We 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 turn
ed out fine – quite stiff and strong enough.

With 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.

In 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!)

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