The Latest "Skinny" on Cold-Moulding

[mcm]

Originally Posted by PAR

The original double planked hulls (Ashcroft method) used a few different things between the layers. In the modern versions of this method, glue is used.

Next the outer layer goes over top of the first, covering the seam of the inner layer.

This method eliminates much of the need for a internal frame work or structure (frames, floors, stringers, etc.) Once the glue sets, you don't need the fasteners.

Once the glue sets?? I thought you liked this process because it saves one from epoxy exposure!

Obviously then, the expansion and contraction of the outter exposed planks isn't weakening the glued lap-joint with the inner planks.

Or, are the outter planks sealed with glass and epoxy.

I guess I'm going to have to read-up on double-planking before I'll really begin to understand it.

Oyster,

Tongue n cheek huh? OK, I'll take another look.

But I still like those photos in your posts that picture the frames and the strip-planking process.

One picture worth a thousand words.

[PAR]

The Ashcroft method used solid lumber planking with tar, varnish, lead or tarred paper in between the two layers. These were hard fastened to frames. Much like the glued lap strake method, the modern double plank method uses glue, which replaces the fasteners. This glue can be epoxy, but doesn't have to be. The use of epoxy firms up the structure sufficiently enough that most of the frames can be eliminated, with proper engineering. Glue lap boats have a similar benefit, as a result of epoxied seams, firming up the structure.

Solid lumber planking may move a bit, but plywood will move much less with changes in moisture content. Most builders have found that epoxy coating is over used and rated. Thin cloth coatings, set in epoxy only offer abrasion resistance. Encapsulation is only as good as the embalming job performed during construction and the maintenance of that coating to prevent moisture ingress. Don't get me wrong, some methods rely heavily on epoxy, that's fine, but many methods can live with it or without.

In most wooden boat construction types, the planking is a consumable product, like a spark plug. It has a life span and then needs to be replaced. Cyclic loads on the fasteners and supporting frame work eventually requires refastening the planking, which is usually just a short iron shot from replacement planking operations. Last summer, I replaced 47 year old plywood planking. It wasn't bedded at the seams, just painted with red lead. I seriously don't think that epoxy would have prolonged it's life any more, given the same amount of care it received in it's life. That planking was 20 years past it's expected life span, which ain't bad for plain old fashioned, clenched lap plywood. I've seen a fair number of plywood boats of that era (late 50's early 60's) and the coated ones died quicker. I've not seen any 50 year old epoxy coated or cloth set in epoxy boats yet, but I imagine they will have a similar fate. Not near as bad as the poly coated jobs of the 60's and 70's, but likely not as long as the more traditional methods used in the 40's and 50's, regarding coatings on wood, solid or other wise.

Solid lumber planking can be glued (with or without fasteners) or bedded (with fasteners only). Plywood can be bedded (I suspect), but I've never seen it, just glued (again with or without fasteners). Scantlings will determine the amount of framing structure necessary for a particular design.