Boatbuilding plywood

[rdbct]

Some thoughts on boatbuilding plywood selection: New arrivals to boatbuilding often are constrained by budget issues. Therefore, it is natural to think that cheaper materials would be a wise choice. MDF is not suitable for wooden boat hull construction. MDF is Medium Density Fiberboard. It has a particleboard heritage. When it gets wet it will not return to its original shape, as will plywood. MDO is a different story. MDO is Medium Density Overlay. It usually has a fir core and can have a synthetic coating on one or both faces. Sign makers like this material because it is void free, or supposed to be void free. Now a trip to the hardwood store in the west will set you back $76.50 for 3/4" MDO. This is about .050 thicker than 18mm metric plywood. I think that 3/4" MDO has about 5-7 plys. Another choice in this size is 18 mm Okume BS1088 plywood. Okume is an African Mahogany. The 18mm usually has 13 plys. The BS1088 stands for British Standard 1088. Voids are not allowed in this material. I have some small pieces of this material in my shop that I acquired from a boatbuilder in 1963. I can not tell the difference between this sample and new plywood from the hardwood supplier of today. The quality of todays BS1088 appears the same as 42 years ago. There was a time 50 years ago when I built my first boat that I thought this was expensive material. Over the years I have changed my mind. A 4' x 8' sheet of 18mm Okume BS1088 will set you back $114 in the Pacific Northwest. That is 35% more expensive than MDO. If you were going to sell your boat, or buy one, what would be your reaction when the seller said, "My boat is made with MDF / MDO / Okume?" As a PS: I have some fir plywood that is about 15 years old. It has received the same weathering as the Okume. You do not need very good eyesight to tell the difference.

[WoodButcher]

The problem with MDO is that it is not a single grade:
there is really "good" MDO and really "bad" MDO.

I have some MDO, overlaid on one side only, that
looks like absolute crap; the 'bare" side is rows of
knots (just like fir exterior) and the wood is stamped
"BC" although I do not know what kind of wood it
is. It is 3/8" and weighs about 33/34 lbs per sheet.

Just because some MDO is overlaid on both sides is
no indication that the plywood itself is any better.

I have seen people make claims that 1960's Chris-Craft
Cavaliers and lapstrake Lymans where made with "MDO,"
but I don't beleive that to be strictly true. Back in those
days, one could get "mahogeny" plywood that was overlain
with the same basic "paper and phenolic resin" coating as
MDO, but I believe that the wood itself was of much higher
quality.

http://www.europa.com/~garry/bttrnvr.htm




As far as I can tell, of the 4 (at least) grades of MDO
that Olympic Panel Products (succesor to Simpson) currently
makes, the Signal line is the highest quality. Unfortunetly, the
yards around here will only carry the cheapest of grades,
and usually only in 3/4" which is the thickness that signs
are usually made of. With some looking you can find 1/2"
but even that is too heavy for the size of boats that I
build. There are other manufacturers of MDO but their
product lines are similar.


http://www.olypanel.com/sign_making/index.php

[PAR]

MDO certainly has dropped in quality in the last 10 years, but it still can be had in good stuff. It's available in 3/8", 1/2", 3/4" and maybe thicker. It's ply count has gone down, but it carries the APA 1-95 grade, meaning it's 100% Douglas fur or western larch through the panel. It has a minimum void count and limits to the repairs on the outer faces. BS 1088 is a much high grade of sheet and understandably will look much better.

Chris Craft and Lyman's were not built with MDO, though Owens had topside planks of MDO. Chris Crafts had custom plywood made to their specs of Douglas fir. It was good stuff using good construction as I have a 45 year old 27' Chris with her original planks still in fine shape. She's seen some repairs and new garboards, but that would be quite normal. I just replanked a 1965 25' Lyman and it was all high quality Doug. fur no resin faces.

It requires much more effort and money to get good stuff currently, but it can be done.

MARINE AA FIR 4 X 10-- NO scarfs is avail. from Harbor Supply in Baltimore, MD.