Most of us who build boats at home do not have the facilities needed to handle 4′ x 8′ sheets of marine plywood with ease. I’ve always found it challenging to put a sheet of ¾″ or even ½″ marine plywood up onto the tablesaw to cut it to size. Thinner plywood may weigh less, but it doesn’t have the stiffness to lie flat on the tablesaw top. Roller stands, while useful in supporting the plywood, do not make it much easier to avoid binding the saw and the inevitable kickback. Besides, many of the cuts I want to make are curves—not an option with a tablesaw.
Building any boat that involves sheets of plywood is made much easier when using a technique taught to me by boatbuilding instructor Pat Mahon, now the executive director of the Great Lakes Boatbuilding School in Cedarville, Michigan.
Pat’s technique is simplicity itself: Use a 4′ x 8′ sheet of rigid foam, the type used to insulate buildings under construction, to support the plywood and then cut through the plywood and into the foam. The sheet I have in my shop right now is Corning Foamular 250, a 2″-thick sheet of extruded polystyrene (XPS) insulation (1½” XPS is shown in the photos) that weighs under 8 lbs and costs about $31. The XPS panels are stiffer and more resistant to compression than the other common insulation panels—expanded polystyrene (EPS) and foil-faced Polyisocyanurate (Polyiso)—so it won’t get badly crushed if you put your weight on it, especially if you have the plywood on top of it.
I’ve occasionally used sawhorses to elevate the foam sheet and the plywood, but it’s easier to flop the foam and plywood down on the floor than to get out the sawhorses and set everything up only to take it down 10 or 15 minutes later. Set the foam on the floor and put your plywood, best-side down, on top of the foam. If you don’t have room in the shop to do the cutting, you can use the foam sheet outside in the driveway. It will protect the plywood from grit and pebbles that would otherwise damage the plywood.
Set your circular saw to the thickness of the plywood and add an extra 1/8″ to ¼″ to ensure you cut all the way through it without causing excessive damage to the foam. The foam offers little resistance to the saw blade and helps reduce tear-out on the plywood face that rests against it. Make the cuts you need. That’s really about it.
A circular saw set to cut just barely through the plywood will cut a gentle curve without binding. By using a sharp thin-kerf carbide-tipped plywood blade (I use Diablo blades, and I’m sure there are other equally good manufacturers), you can cut extremely close to the line and minimize the amount of time you will need to clean up the curve with a block plane and sandpaper.
A small cordless circular saw with a 5 ½″ or a 6 ½″ blade can cut an even tighter curve than a standard saw with a 7 ¼″ blade. In one of Pat’s classes we could get 10 or 15 minutes of operation from the small circular saw we used, more than enough to cut the curves needed to shape each plank for the Caledonia yawl we were building.
I store my foam sheet above the overhead retractable door of my garage/boatshop. The foam sheet is easy to lift up there, and it is out of the way until it’s needed. If you don’t have space for the full sheet, cut it in half or even in quarters.
An added advantage to using a sheet of foam for a cutting surface is that you can put the foam to use in other ways after you’ve finished using it. You can cut it up with a craft knife or circular saw and use it as flotation in the boat you’ve built or glue pieces together to make a highly efficient cooler.
Pete Leenhouts, a retired naval officer, lives on the Olympic Peninsula where he enjoys building, restoring, and using his boats. His review of a clam skiff also appears in this issue.
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