Amateur Boat Building

Do not run your hand along the edge of the plywood and other things you need to know about this stuff
by Jim Betts

"Plywood," says one boat builder, "is just a bunch of splinters glued together." Well, that is so, but it gets more complicated because plywood is a lot of information not stuck together and some degree of research is needed. Let's get a few basics out of the way and then go into a broader look:

1. No matter what it says on the sheet of plywood, anyone with a rubber stamp can label the material anything they want. Yes, there is counterfeit plywood. IABBS has been asked about "Boat Grade" or "Boat Building Grade." The American Plywood Association (APA) says there is no such thing.

2. Graded plywood varies from manufacturer to manufacturer. So AA grade (the best), may look super, or semi-super, or below-super, or just not super at all, but more like trash. There is no industry law about this, only the APA's standards, which are voluntary. Plywood is made in the U.S. and many other countries and they each have their own way of labeling any products.

3. Yes, Marine Grade and Exterior use the same waterproof glue. The main difference is that Marine has solid inner plies where Exterior can have voids. This is especially important when the plywood is bent, because the voids will give a weak area that will not bend in a uniform manner. (Quite frankly, I am tempted to sum up this story with USE MARINE GRADE! But that's too simple.) But look at the economics: If you are building a boat that will cost $5,000 all-up (materials, motor, sails, etc.) and you use 10 sheets of plywood - where 3/8 in. Exterior is about $20 a sheet and where Marine is about $40 - you have a difference of $200 for the whole boat. This is only 4% of the total cost of the boat. For $200 you will sleep better - especially if you are on the boat!

4. Plywood is sold under a number of names of the wood used; Fir, Mahogany, etc. Then there is Luan, which is about the same as Okoume (o-KU-me), both of which are really Philippine Mahogany, which is not Mahogany at all, but Cedar. (Often called cigar-box Cedar because it is used for that.) Fir is the strongest and Luan is well below that.

5. A major problem with plywood is checking. This is because it is made up of hard and soft wood. They do not hold together well and tend to separate. Fiberglass (FRP) is often used to cover the plywood in order to prevent this, or at least to hide it. 6. Some plywood boats are designed to be "encapsulated." That is to say, the plywood is coated both sides and edges with epoxy with or without the FRP. This gives a watertight coating. However, if it develops a leak, it also keeps the water in. This is where rot starts. But in general such encapsulation is in wide use and some designers specify it in their plans.

7. Then there is MDO (Medium Density Overlay) plywood. This is coated on one or both sides with a plastic sort of material that looks much like the brown paper used in grocery bags. It gives a smooth surface and is ready to paint. It costs more than Exterior, but less than Marine and certainly less than the cost of FRP treatment. The big problem here is that few suppliers carry this, so you will have to have it shipped. Harbor Sales Co. is the major source.

8. Plywood is heavy. It is 34 pounds per cubic footand Fir lumber is 32 and Cedar is 21. A 4x8 sheet of 3/8 plywood weighs 35 pounds and one-inch is 96. And a 4x8 sheet is hard to handle, especially outdoors with the wind blowing! This is one reason for plywood lapstrake designs - narrow pieces are easy to handle.

9. The number of plies (layers) is important. The 3/8 stuff you get at the local lumber yard mostly has three plies, two fairly thin outer plies and one fairly thick inner ply. This is usually soft wood. Use a pencil point or a nail to test it. Try to find five-ply in 3/8 and 1/2, and seven-ply in 5/8 and 3/4 and go for nine-ply in one-inch plywood.

10. This will outweigh points I through 9. Sad to say, the major factor is "What you can get." Or what your local lumber yard has. Sure, you can get anything from anywhere, if you pay the price. (But do go back to point 3 and note the cost as a percentage of your total cost. And consider solid lumber, which is much cheaper.) Plywood from out of town comes by truck. For the most part, they will not deliver to a residential address. So you will have to have it come to a business with a loading dock, or have it delivered to a local lumber yard that will then deliver to you. (Even amateur-built boats are expensive!)

II. Do think about other materials. Plywood was very new and exciting back in the 1930s. There are other materials and methods - cold molded veneer (make your own plywood to the shape you want see story on this in the June issue). And "real wood," which is cheaper and readily available. Boats were made of solid wood long before plywood and FRP and metal. Yes, real wood does not come in the thickness you may want, so have it cut to your specs. Local lumber yards and woodworking shops have planning mills for doing this. And fiber-glass is easier than you think. (See story on C-flex in this issue.)

Let's wrap this up before you go to sleep

Yes, you can say only so much about plywood or most anything else. It is a great material. It is used for boats, airplanes, houses, hog-feeding troughs and much more. But it is FLAT. The main trouble with plywood boats is that they look like plywood boats. No sexy classic curves, no varnished Mahogany, no subtle shapes. Designers have often bowed to the demands (read LIMITS) of plywood.

But plywood is strong, easily cut and relatively cheap. A plywood boat is better than no boat at all.

Plywood has made amateur boat building the popular hobby that it is. It is indeed "engineered wood," but other materials and methods should be looked at. And what new material is in the future? Will we be able to grow a boat the way we now grow watermelons? Hey, a 40-foot watermelon, cut in half, with a tough skin, is interesting. And think of all the eating pleasure as you hollow it out!


(Hint - Check your local yellow pages and/or call the boat manufacturers and repairers in your area and ask them where they get their plywood.)

Harbor Sales Co., 1000 Harbor Court, Sudlersville MD 21668-1818, . Free catalog and price list. All types of Marine Grade plywood, MDO and HDO. Ships worldwide.

M.L. Condon Co. Inc. 250 Fen-is Ave., White Plains NY 10603. Phone: 914-946-4111. Many types of Marine plywood and hundreds of varieties of solid wood. Free color catalog. - Sells boat plans and ships plywood to boat builders worldwide.

World Panel Products Inc., Eurolite plywood and marine plywood of many types, epoxy, teak, etc. Free catalog.

Edensaw Woods Ltd., - Phone 800-745-3336. Marine plywood and solid lumber.

Quality Woods Ltd. Box 205, Lake Hiawatha NJ 07034. Plywood and lumber.

Plywood Door Corp. Four locations (NJ, IL, TX, CA) - Marine plywood, Okume, etc.

Anchor Hardwoods Inc. Marine plywood plus other solid woods,

Boulter Plywood Corp. Marine plywood and solid lumber. 24 Broadway, Somerville MA 02145. Phone 617-666-1340.

Flounder Bay Boat Lumber. Marine plywood and solid woods, epoxy, fiberglass cloth, screws, paints etc.

L.L. Johnson Lumber Co. Marine plywood and solid lumber, - Phone 800-292-5937.

Rare Earth Hardwoods, Philippine Mahogany marine plywood, Phone 800-968-0074.

Resources International, Keruing Marine Plywood. PO Box 1306, Port Orchard WA 98366.

South Jersey Lumberman's Inc. Marine Plywood, solid lumber, oak, teak, millwork. - Mail: 6268 Holly St., Mays Landing NJ 08330. Phone 609-965-1411.

NOTE: This is a partial list. For a more complete list, go to and click on Boat Building Resources, then on Wood & Plywood, and then List of Lumber Suppliers. This site has been built up over the years and is arranged by state and city.


Most of these companies have web sites that are of general interest, though not related to boat building.

Timber Products Co.
Stimson Lumber Co. 520 SW Yamhil, Portland OR
Roseburg Forest Products
Potlatch Corp.,
Plum Creek Co.
McKenzie Forest Products,

Jim Betts