View Full Version : Cold molding

11-03-2003, 09:39 PM
The plans call for 900 sq ft of 1/8" mahogany. Veeners cost way too much to buy outright. And it seems like alot of saw dust (waste of mahogany) to cut my own.

Perhaps this is WB blasphemy- but can I substitute a good quality plywood (like Sapele) for the veeners?

Tom Wilkinson
11-03-2003, 09:52 PM
Where are you located and what kind of price are you looking for. I have a source for some 1/8 mahog at a decent price but I would have to buy quite a bit. I have considered buying the lot and reselling what I don't need.

11-03-2003, 09:54 PM
A suggestion: We built our cold-molded boat using plywood scraps purchased from Chesapeake Light Craft in Maryland. They make kayak kits and have lefover scrap that you can buy for cents on the dollar. Good Luck, Mitchell in CT

11-03-2003, 10:13 PM
Stupid question--could 1/8" doorskins be used for cold-molding?

solent sailor
11-03-2003, 10:48 PM
Not a stupid question at all. We have used mahogany doorskins for all sorts of boats. Purists will say that you can only use "marine quality" plywood, but in reality, wood is wood, and if its epoxy saturated, which 1/8 will be, itdon't matter none.

As long as you get the joints pretty fair, whatever you use is immaterial as its going to end up as a composite anyway. If you were using 1/4 it might be more of a moot point as the saturation would probably be short of the full depth of the material. However, this stuff is going to be so full of epoxy goo that water ain't getting in anyway, especially if its sealed both outside and in. The doorskin simply becomes the "mat" as it were.

11-03-2003, 11:38 PM
but in reality, wood is wood, and if its epoxy saturated, which 1/8 will be, itdon't matter none.
No, this is not the case. There is a vast difference between a piece of mahogany and a piece of cedar. Wood is not a homogeneous material, (unlike steel or aluminum) strength and stiffness vary with grain orientation.

The designer of your boat is relying on however many layers of 1/8" Mahogany running in the specified direction. By substituting plywood you have (perhaps) half the grain running in the wrong direction and cut up into short sections with no continuity.

You can build a boat out of "mat", but you will need a fairly thick lay-up to gain any structural stiffness (very heavy). This is why the boat is built of wood, it's light, very stiff and quite strong in certain directions, it will also flex and springback if required. The epoxy is to keep the wood dry.

So the answer is no, do not substitute plywood for solid stock.

All the best, Tad

Bob Perkins
11-03-2003, 11:47 PM

What type of boat are you building?

Check with the designer, but 3mm ply of good quality (BS1088 Okume for instance) is very strong, and laminated it will be as strong as Mahogany I'm sure.

Also, Boulter plywood (Somerville MA) sells this stuff - I ended up getting mine there (25 sheets). Ask for the banged up stuff, they will make you a deal on that over the *good* stuff. You are going to cut it up anyway and hide it inside layers. I saved good $$ over full price.

I just started my cold molding myself and I'm using the exact same technique.

[ 11-03-2003, 11:57 PM: Message edited by: Bob Perkins ]

11-04-2003, 02:08 AM
I think that the diminishing availability of 1/8" veneers has driven the trend toward strip composite construction, where there's a single layer of strips running longitudinally and glass in the other directions. Seems that the use of non-longitudinal veneers has come to be limited to larger boats where the layers get thick enough to be practical to re-saw. See the recent WB article about the boats of the Brooklin Boat Yard.
To me, plywood strips make little sense.

11-04-2003, 10:20 AM
As it happens, my 15' Marsh Cat was made from 900 square feet of 1/8" mahogany venner laid in four diagonal alternating layers with epoxy. Richard Vogel has a Marsh Cat also, built of four diagonal applied layers of 1/8 marine mahogany plywood and epoxy. In both boats, one half of the individual mahogany fibers are running at right angles to the other half of the fibers. True, the plywood strips have two interply bonds of waterproof glue factory applied under heat and pressure, not epoxy slathered and stapled down, but that appears to have made no difference.Both methods resulting in a strong, durable hull with the outer hull sheathed in light fiberglass, primed with epoxy, and given an LPU paint finish. Both boats used 1/2" marine mahogany ply for decks by the way.
Hope this helps.

11-04-2003, 12:03 PM
It's the Marsh Cat I'm hoping to build. I've been doing some internet research and found many cold moulders use plywood veeners- but these are generally for larger boats. Does Richard Vogel visit WB?

On Vacation
11-04-2003, 01:38 PM
solent sailor
Member # 7509

posted 11-03-2003 10:48 PM
Not a stupid question at all. We have used mahogany doorskins for all sorts of boats. Purists will say that you can only use "marine quality" plywood, but in reality, wood is wood, and if its epoxy saturated, which 1/8 will be, it don't matter none."

Bad information :eek:
Purest hell. Wood is wood, some better than others. Some glue, in plywood, is better than others. Make damn sure the glue, in the laminate of the plywood, is not valued less than the other glues for the faces. This will bring down the value to your boat, and your life, given stress conditions to it.

"Penny wise, and pound foolish"

Edited to add some of the thinner cheaper doorskin types luan or like types of oriental panels, are filled with powdered pulp cores.

[ 11-04-2003, 02:33 PM: Message edited by: Oyster ]

11-04-2003, 05:26 PM
Richard Vogel does occasionally visit the WoodenBoat site. That's how he found me to ask a few questions. He's probably in the member profile list. The email is richard vogel@tufts.edu. He bought his boat from a man who built it in Pennsylvania. I spoke with him when he listed it in the Cat Boat Associations web site for sail dept. I was checking on it for a friend who liked my MarshCat enough to think about acquiring something similar. Sounded to me like he got more enjoyment out of building boats and was selling his after a couple of years ownership for an incredibly low $3900 to buy materials for his next boat project. My friend decided she didn't really want to undertake boat ownership and passed on it. Richard found the listing, queried here about it's construction from 1/8" plywood, and decided to buy. He's had here this season and outside of running into a centerboard swelling/ trunk clearance nuisance, he reports he's had a lot of fun with here this first season. Doug Gray also visits here and on the Catboat site, and the Cat Boat Association site as well. He did his MarshCat with a slightly taller mast and 189 sq ft of sail area. Do the search thingy for MarshCat and you'll find lots of useful conversation about this great little giant killer of a daysailor/camp cruiser. I absolutely love mine.