Merrell Watercraft

Private Postings

Micro Page

Bill Sampson's
Book Reviews

Chebacco News

boatdesign.com's
Nautical and Marine
Resource List

Craig O'Donnell's
Cheap Pages

Tomboy

Slogging to Windward

AN MDO/HDO PLYWOOD UPDATE

Here we are, halfway through the year of two double aught one and well into the summer TV rerun schedule.  Before getting into the heat of the meat of June’s column let me take a minute to digress into an announcement or two:  

Our own Ed Sasser, columnist extraordinary continues to work on one of my designs, namely a Peanut 30’ houseboat.  If you want to take a peek at his project, click here to go to a little page I’ve set up to monitor his progress:

https://www.fortunecity.com/marina/pier/525/edsasser/Peanut/Page.html 

I expect that he’s much further along the road than these pictures show, but getting photos from busy Ed is like pulling teeth.  There’s also a link on this page to another that reproduces Ed’s recent article in Boatbuilder Magazine recounting his adventure in building (by his own admission) a slightly flawed Bolger Diablo.

Another unexpected announcement came from Bill Samson the world’s number one Chebacco advocate that is; he’s decided to sell his Chebacco “Sylvester”.  The main reason he wants to find a good home for his boat is because he wants to build more boats and can’t justify keeping everything.  So, contact Bill if you’re interested in acquiring a well built and scrupulously maintained Chebacco.  mailto:bill.samson@tesco.net.   Never one to be idle though, Bill has recently built a baidarka and a folding kayak.  At the moment he’s working on a boat that I drew for him.  It’s a stretched redesign (for stitch and glue) of William Atkins’ Finkeldink, which I’ve called Peach Pie.  If you want to see Bill in his element slaving away on Peachy, click on this link:

https://www.fortunecity.com/marina/pier/525/peachpie/Page.html

Bill’s Chebacco News Link:

https://members.nbci.com/billsamson/

Also, as some of you know, Jeff Gilbert has been very ill for the past few months, but I just got email from him, and he seems to be back on his feet and thinking about boats and boating again.  Now that Jeff is out of the hospital, he’s chartered a nice powerboat (1951 36 ft. Halverson shown below) and is taking his parents on a cruise.  So, all the best to ya, Jeff—continue getting better and have a nice boat ride!

If you’re a member of one or several of the Yahoo mailing lists, or cruise the Usenet in the boating venue’s you probably have noticed that certain questions and subjects come up over and over from new readers and members.  One of those that often rear its head is a question about MDO and HDO covered plywood, and it’s use as a boatbuilding material.   

The reason why I decided to address this subject is 1.) There is some new information coming from the American Plywood Association, 2.) Simpson Timber is offering a new product, and 3.) Since these columns are probably going to be archived on Duckworks Web for awhile it should act as reference and starting point for when those questions do appear on the lists.  4.) In the past I’ve spent many hours as a Consulting Industrial Engineer in the Glue Room of various plywood mills making manufacturing studies so have a good feel for the product. 

MDO and HDO are special use panels that were created by the plywood manufacturers to do some specific jobs, but have also found acceptance and interest among boatbuilders and in other crafts.  The initials MDO and HDO refer to either Medium Density Overlay or High Density Overlay.  The overlay they’re talking about comes in the form of a phenolic (heat activated) resin impregnated paper that is laminated to the faces of a plywood panel during the manufacturing process.  Medium and High simply refer to the weight and thickness of the paper panel face, but there are notable differences between these two as well.  

Originally MDO was created to satisfy the requirements of long lasting Interstate Highway signs.  It’s usually produced to rigid specifications from very high grade materials and is supposed to last almost indefinitely in extreme weather conditions with no checking and with only a coating of paint (usually two-part epoxy) for protection.  The panel is composed of fir cores and centers with the overlay over well-sanded knot-free faces.  Nowadays, the faces are only fir, but in the past thin veneers of luan were used.  The reason for the luan faces was to create an even smoother surface, which prevents the grain of the fir from telegraphing its pattern through to the surface.  The same process is used for laminating fine-expensive wood veneer faces like teak over fir plywood, but of course without the phenolic overlay.

The surfaces of the MDO are waterproof, but stippled to readily accept and retain paint.  The overlay is impossible to separate from the plywood, and if the edges are properly sealed the panel will last practically forever.

MDO is a good product to use for small boat hulls because if it’s used properly, the edges sealed and seams taped, sheathing large areas of the exterior may not be needed.

HDO on the other hand up until recently, wasn’t a good material to use for boatbuilding.  While HDO is even more indestructible than MDO, paint won’t stick to it and not much else will either.  The panel construction process is the same, the paper is a heavy phenolic-coated paper but the product is generally used to build concrete forms and molds.  The surface is made slick (I believe with a castor-oil based mold release) so that the forms and panels can be used over and over again.  

Now, however, Simpson Timber has come up with an HDO product using a heavier overlay, but designed to accept paint and reflective tape and is to be used in similar applications as MDO.  It’s called “Highway HDO” and in addition, its heavier overlay contains a greater percentage of resin for increased moisture resistance.

Whether MDO or the new HDO is for your small boat project or not is something that I can’t know or recommend.  It’s something you’ll have to decide, but it’s good to know that products like this exist for those times you may have an application where MDO or the new HDO offering would be just right. 

If you work and build with plywood, product knowledge is not only important, but also comforting to have, and for this reason I can’t recommend the following websites highly enough.  There are great product lists, specifications and product informational files that can be downloaded in both word.doc and acrobat.pdf formats.  They’re great resources and should answer any questions you may have.

https://www.simpson-plywood.com/