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Build a better boat from the free plans:

Can I use other materials than what is listed on the free plans?

Yes. See the “materials” help files. We recommend: Use epoxy glue instead of weaker wood glues. Replace hardwood framing with plywood and epoxy-fiberglass, see details in that section. Use thinner plywood covered inside and outside with one or several layers of fiberglass. The final panel will be stronger than plain plywood, it will be easier to bend and cut, the boat will be all fiberglass outside and the total cost will be the same than thick plywood covered with one layer of fiberglass outside.

Plywood thickness rules of thumb:

The substitution rules below are very approximate, use them as a starting point. If the plans specify 1/4” plywood, use 1/4” epoxy coated on all faces. For 3/8” ply, use 1/4” with either one layer of glass outside or one layer outside and one inside. For 1/2” ply, use 1/4” with one layer biaxial glass 12 oz. each side. For 3/4”, substitute 3/8 with 2 layers of biaxial glass one each side. All fiberglass in marine epoxy resin.

Plywood Composite Testing

Plywood framing:

The pictures below show a typical frame and a stringer.

Framing rules of thumb:

In all cases, frames spaced at least one every 36” and stringers every 24” for a panel span 24 by 36”. For small boats up to 16', displacement type, use 3/8” plywood frames on epoxy putty fillets. Planing hulls up to 20' : 3/8 with one layer glass tape each side. 20 to 25': 1/2” with 2 layers biaxial each side. Double the plywood and glass for the stringers.

This is just a rule of thumb. Boat type and panel sizes may require different scantlings. You should either calculate the framing your self or consult a designer.

Fiberglass your hull.

Protect your hull and extend it's life. If you followed our advice to use thinner plywood coated with fiberglass in and out, this is already done but if you build from the thick plywood specified on the free plans, you should at least add one layer of fiberglass in epoxy resin.

Tutorial on Wetting Out Fiberglass Cloth

Make changes to a design.

If you use the plywood framing method, you will be free to modify the superstructure at will. Other wise, base your changes on the framing shown on the free plans. Keep in mind that your boat must float in it's lines. Do not move it's center of gravity by adding to or removing the superstructure. If you add a weight forward, move another weight backwards to compensate. Free basic yacht design software will help you define the center of gravity. LINK.

Enlarge or reduce boat size.

You can resize a boat in two different ways. You can either multiply all hull dimensions by the same factor or change the spacing of the frames. In each case, try to limit your changes to + or - 10% . Some say that 20% is fine but keep in mind that if you increase all dimensions by 20%, your volume (boat displacement) will increase at the power of 3 while the surface increases only at the power of 2 and fixed weights do not change. This means about 70% greater volume for a simple 20% increase! That boat will float too high and the structure maybe weak. Stay within 10% if you scale all dimensions. You can go up to 20% by changing the frames spacing.

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