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Bill Baillie
05-21-2003, 09:07 AM
I have a question about laying venners in a cold molded hull.
Some builders say that if you are glassing and painting the hull, then laying the last
layer of veneer fore and aft is not necessary as you cannot see them. Other builders
say that they always lay the last layer for and aft, even under glass and paint because
the shape of the veneers will "telegraph" through the epoxy/ glass and paint leaving a
hull that may not look the best.

It seems to me that if you take care to secure the edges of the veneers and make sure
that the hull is properly faired you should have little to telegraph through. However, some
of the writers (eg. Ian Nicholson) emphasize this point very strongly. On the other hand, Hub
Miller in "The Laminated Wood Boat Builder" says that the choice to lay the last layer
fore and aft is strictly an aesthetic one. Miller's boats are glassed and painted, yet he
still shows the last layer running fore and aft.

Does anyone have any direct experience of both approaches? If so, I would appreciate
your comments and opinions on the matter.

Buddy Sharpton
05-21-2003, 09:22 AM
I have experience with two cold molded hulls, one 22 foot or red cedar, and one 15 foot of mahogany, both sailboats, both diagonally molded.both four 1/8" layers, both painted not finished bright. The 22 was glassed inside and out, the 15 glassed outside, epoxy only on the inside.
Here was my experience. No matter how carefully faired and finished- one for additional use as a plug for a fiberglass mold, the other could have passed for a fiberglass boat- the perfectly smooth surface eventually becan to show the seams of the last layer. The wood inside does change in moisture content as water vapor can pass through the epoxy. The wood fibers do swell, the epoxy wood flour filler in the seams does not and thus the slight depression. No big deal but damn it , it's there and didn't use to be. m
I believe it was Paul Gartside who recommends a outside layer fore and aft( and I'd add all of the same width) so the grooves look like planks if detected.

Bill Baillie
05-22-2003, 10:55 AM
Thank you for your reply.
None of the books that I read had any explanation for the appearance of the seams on the outside of the glass with the exeption of Sam Devlin's book which dealt with it at length. In Devlin's case he spent more time discussing the telegraphing of the weave of the fibreglass to the surface when hulls are covered with dark coloured paints that absorb heat from the sun.
I guess my construction plans will provide for fore and aft planking on the last layer and I will have to calculate the amount of wood to use with this in mind. I think I'll re-read the sections in the Hub Miller and John Guzzwell books about preparing "master strakes" for planking the topsides and below the water line.

It sounds like you used cold molding of wooden strakes exclusively when you built your hulls. Have you any experience with the combined strip/ veneer process? It sounds interesting but I have yet to talk to anyone who has actually used it.

Bill

imported_Conrad
05-22-2003, 11:19 AM
I don't think it matters- if you post-cure the hull properly. The reason you see the plank seams is that epoxies don't fully cure until they have either been warmed to a critical temperature, or a great deal of time, months/years has gone by. As the cure is completed, the epoxy shrinks, as do all the paint and fairing layers you so carefully put on and sanded down. And since different materials shrink at different rates, the uneveness, including plank edges, etc. shows up.

The best builders avoid this by tarping and heating their hulls to temps in the 140 degree range at different times in the building process, and keeping them at these temps for 12-24 hours. For example, once to shrink/consolidate the wood/epoxy matrix and again just prior to the final pre-color sanding. In our auto business we follow the same procedures on our best restoration jobs, or those that required above average amounts of filler. That way the finish looks just as smooth and fair after 6 months in the sun as it does the day it leaves the shop.

While moisture moving through the epoxy may have a very small impact on the eventual appearance of plank seams, by far the bulk of the impact is from the eventual curing of the epoxy and its resultant shrinkage. I'd go with the layout pattern that makes sense, then rent a couple of drywall heaters and build a tarp tent- you'll get the best and longest lasting finish ever, even with dark, heat absorbing colors- because the post cure shrinkage has already taken place!

[ 05-22-2003, 12:24 PM: Message edited by: Conrad S. ]

Bill Baillie
05-27-2003, 11:57 AM
Conrad,
You mentioned that the full cure can take months to years at normal temperatures; can information about this be found in the manufacturer's specifications? I don't recall seeing anything on the samples of epoxy that I bought to do some trial runs. I did some tests with cedar strips to see how much filler would be needed in the epoxy between strips so that it was thin enough to penetrate and hold the strips together firmly while at the same time being thick enough so that it wouldn't run down the side of the hull as I built the boat. I made up a few test panels and had no problem with the seams showing even after the panel had cured for several weeks. Mind you, they were kept in my gargage and never exposed to direct sunlight.

Bill

imported_Conrad
05-27-2003, 02:39 PM
Bill- my understanding is that it's more a case of the pattern of use. If a car/boat stays in the garage and never warms up past a given temperature, then the ultimate extent of possible cure/shrinkage will never be reached. Additional cure and the resultant shrinkage remain a possibility until the resin has attained and held for some time the maximum required temperature, typically around 140 degrees for the epoxies we all use. You can easily see the effect by fairing a panel covered with coarse cloth or roving, then painting it black and setting it out in the sun for a day or two, preferably after a week or so of intial cure. Your once smooth surface will show the cloth pattern as well as any irregularities in the underlying substrate, say gouges in the wood or a discontinuity like a plank seam.

There are two reasons for not painting a hull black or other dark colors. The first and most often experienced is this post-cure experience, where the plank seams, cloth pattern, or other irregularities suddenly appear in the once perfect paint job. The second is that all resins have a temperature at which they begin to soften and lose strength, around 135-140 degrees for the least expensive epoxies. Manufacturers often list this upper limit. As a practical matter this really isn't a concern to boat builders since its pretty difficult to get a boat that hot unless you're spending a lot of time near the equator. And the epoxy doesn't suddenly lose all strength, it just begins to plasticise with increasing temps.

Since shrinkage is obviously a concern in the auto business, (and my business in particular that serves a high-end client), it has been well studied and written about in journals and manufacturer's "help" sheets. Boatbuilders have been a bit slower to recognize, understand, and find strategies to deal with it. Professional Boatbuilder magazine has however had a number of articles that deal with the issue.

Again, when I build a boat I simply figure out what planking pattern makes the most sense in terms of material usage, ease, and strength, then go ahead with it- and make sure to bake the thing overnight with a couple of drywall heaters. The paint jobs have always held at the level we originally decided to pursue, even with the darker traditional greens and blues. Good luck!