Cold Molded Planking Review

by Glen L. Witt on December 15, 2010

Many of our boat designs call for cold molded, double or multiple diagonal planking. This is a method of applying the skin or planking of the boat in multiple layers; the first layer about 45 degrees and subsequent layers approximately crossing at right angles to the previous one. Cold Molding is used on boats that have compound curves that plywood will not conform to.

Biscayne built by Bob Perkins

Bob Perkins applies first layer to Biscayne

Biscayne by John Maddox

Two layers shown on John Maddox's Biscayne

Veneers can be used however plywood, particularly when ¼” or thicker, is cheaper and easier to apply. Classic hulls, such as the mahogany runabouts, use solid wood planks run lengthwise over the laminates for a beautiful bright finish. Instead of this final wood plank veneer , the hull can be fiberglassed and painted.

Bob Perkins Biscayne

The final layer is applied to the laminates of Bob's Biscayne

Mahogany Runabout planking

Final veneers on bottom & sides of Bob's Biscayne

Neil Quade's Sea Angler

Neil Quade's Sea Angler is a 35' painted cold mold hull

A width of 6” or so is often used for the diagonal strips however, the width is not critical. In the aft section of a typical runabout, the layers can be full or partial sheets of plywood when the area is relatively flat; it all depends on the hull contour. Sheet plywood won’t bend over compound curved surfaces so the laminates are narrowed in such areas.

How wide should the laminations be? Wide enough so the edges of the strips conform to the hull surface. The strips need not be the same width. As the surface curvature increases, the width can be reduced. There is no rule that states the strips must be uniform in width; in fact a somewhat “V” shape may be desirable.

cold molded wooden boat construction

Applying "v" shaped strips for cold molding

When the edge of a laminate tends to lift from the hull surface, or along the previous plank, the strips should be narrowed or tapered in width. Changing the angle of the laminate from the “approximate 45 degrees” may be advantageous. There will be a certain angle where the laminate will adhere to the hull surface without the edges curling to any degree; that’s the angle you seek.

Use a strip of the cold molded planking stock, 2”-3” wide bent over the hull to find the best angle to apply the next diagonal laminate; there will be a “sweet spot” where the laminate will mate solidly to the hull surface with minimal force. Change the angle again on subsequent planks if necessary.

When the edges of the planking don’t mate solidly it probably means the glue will not bond to the base properly; the structural integrity of the cold molding process depends on a positive glue bond of the layers. Eliminating curled edges of the diagonal laminates will also save a lot of grinding as the surface of each layer must be fair so subsequent laminates mate firmly and provide a positive glue bond.

Biscayne built by Bob Perkins

Biscayne 22 by Bob Perkins--the results of a beautiful cold molded hull

In addition, the final surface must be smooth to receive the longitudinal planks or fiberglass covering. Spend a little more time checking and thinking and you can save a lot of work when applying cold molded planking. Come to think of it; isn’t that what boatbuilding is all about? Doing a lot of thinking before the actual doing pays off BIG!  And doesn’t that apply to most phases of boatbuilding?

  • Daniel Holmes

    This little article is full of great information. The picture are also very good.
    I’m getting ready to start the forms for my 19′ barrel back. Got about 350 board feet of strait grain douglas fir. Had a very hard time finding it up here in western New York. I’m near Lake Ontario and all the finger lakes.

    • Scott Seelye

      I am about to begin the same 19′ Barrel Back with Douglas Fir as well. We should share progress photos during the builds.

      • Gayle Brantuk

        Great Scott–we’d love to see photos of your build…

  • Mark Bronkalla

    What Glen says is exactly right. One of the harder things for me to get my mind around originally was that the planking widths and angles are all approximate. You really do “adjust to fit” and do need to avoid the temptation to force fit the planks to the curvature. If a plank does not want to lay on the curve you risk blowing it out when stapling it down (too much force needed) and additionally this tortured fit will not be helping you get a fair curve, leading to much more sanding / grinding for the next layer.
    It is truly amazing when you get the first layer on and you move from almost “imagining” the fair shape of the hull based on the frame and first see the fair curves emerge out of the planking strips. It is truly rewarding as you run your hand over the freshly sanded fair and complex curves this technique allows.

    • Gayle Brantuk

      Great feedback Mark–good to hear from you!

  • tim fweguson

    thank you love your web site

    • Gayle Brantuk

      Thanks Tim!

Previous post:

Next post: