Myths and Mis-information

Marine plywood is more rot resistant than regular exterior plywood. - False. The difference is primarily in the quality of the inner veneers.

Cheap plywood can be made into marine grade by coating with epoxy. - False. The difference between exterior grades and marine grade is structural. No amount of coating will make up for poor quality inner plies.

Adding flotation to a boat increases bouyancy. - False. I'm sure if people really thought about it they would know the answer to this one, but a pound of feathers is still a pound; bouyancy is decreased by one pound. Flotation only adds bouyancy when the boat is full of water.

A pontoon boat will have less draft than a flat bottom boat of the same weight and length. - False. I have included this one because I have had to explain it so often. If you compare a 4' x 10' box that weighs 100 lbs with a 4' x 10' deck with two pontoons that weighs 100 lbs; the pontoon configuration will sink lower in the water (have more draft). Bouyancy is determined by the volume of an object in the water and the weight of that object.

I can make my plywood boat stronger by adding fiberglass to the outside. - False (mostly). In a plywood boat, the strength is in the wood. Fiberglass adds a tough surface to protect the wood from abrasion and to prevent water from seeping into the boat from poor planking junctions. It also allows the builder to get a better finish on his boat. If you want to make the boat stronger, increase planking thickness. Adding enough fiberglass to make a structural difference is both expensive and adds considerable weight. In stitch-n-glue construction, fiberglass laminates take the place of a wooden member, fastening, and glue; which is structural, but fiberglass coating of the plywood only serves the purposes described above.

Much of the above is covered in other places on our web site, but the questions still come. Send us your boatbuilding True or False questions and we'll include them in the next WebLetter.

Back to WebLetter 11