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Cold Molded Construction

Cold molded hulls have been built since the technique was used for World War II planes—often using casein or urea-based glues. Today, one would use epoxy almost exclusively, benefiting from its superior bonding, gap-filling, strength and water resistance.

The method requires a fairly substantial framework with fairly close-spaced longitudinal stringers typically being laid up over a substantial number of transverse frames. Although there are a lot of wood parts involved, there is generally not too much fairing required IF the transverse frames are accurately cut to the right outline to allow for the stringer thickness.

The temporary framework (or jig) will commonly allow for a permanent gunwale to be fitted as well as a keelson, as the diagonal veneers will be permanently attached to these.

Wood for the veneer strips is very critical. It must be cut flat with even thickness, be well seasoned and resistant to cracking. Western red cedar and mahogany are two good choices. Some builders even use thin, high quality plywood.

This photo shows a support frame with the very minimum of stringers. Only the round sectional shape made that possible and typically they would have about ½ the spacing shown.

The pre-cut veneers (2–6 mm thick depending on boat size, bilge radius and stringer spacing) will typically be laid up about 35-45 degrees from the vertical and stapled to the stringers where they cross and also near both edges.

Veneer width will depend on the curvature of the hull, with more curvature requiring narrower strips. Knowing what radius various woods can bend will help decide the strip size and thickness for a particular shape.

This table is from a fine book entitled 'The Gougeon Brothers on Boat Construction' - with their kind permission.

Veneers Thickness
(in inches)
Radius
(in inches)
Dark red meranti
Douglas fir
Sitka spruce
Red cedar
1/8
1/8
1/12
1/8
8
12
11
10
Okoume plywood
thickness (in inches)
Grain
axis
 
1/4
1/4
3/16
5/32
5/32
5-ply Parallel
5-ply Perpendicular
3-ply Parallel
3-ply Parallel
3-ply Perpendicular
24
16
16
8
6

For the first layer, the main work is to fit one edge of each new strip, to closely match the shape of the preceeding one that is already stapled in place. They are edge butted together with a little epoxy and must lie naturally together, as any forcing will cause the veneer to 'bubble up' and cause un-fairness. The angle of the first strip is important as it sets the direction for the rest. Any squeezed-out epoxy must be scraped off before curing and then the 2nd layer of veneer laid over in the same fashion, but from 70 to 100 degrees to the first one, though typically at 90 degrees. Just prior to laying the 2nd layer, the staples for the first one are removed—though some builders leave them in place if they are bronze or stainless. The second layer is stapled into the first one after first pre-fitting each matching edge. Although strips can be laid one by one in epoxy, it's generally considered more time-effective to dry-fit the 2nd layer strips using a few temporary staples and then marking each strip (going fore and aft from the starting one at midships) on removal, so that they can all be re-installed quickly with one spread of epoxy.

If the staples are arranged in a row and a strip of tough strapping material laid under them, they will be much easier to remove without damaging the wood.

(A wood lever can be made (somewhat like a large claw hammer) and if also wearing some leather gloves, this task becomes a lot easier.) At each step, it's important to keep curing epoxy from preventing the veneers from lying flat.

(This sketch also comes from the excellent book 'The Gougeon Brothers on Boat Construction' and shown here with their kind permission)

After the 2nd layer, modern builders are now often enclosing the whole surface with plastic and vacuum-bagging the bond to improve pressure between the surfaces while curing. But this requires that the working surface of the mold be made air-tight and that is impossible with an open frame+stringer arrangement. One interesting method (see URL below for Nexus Marine) is to seal the first layer airtight and then vacuum bag to that. Either way, if it can be done, vacuum-bagging is surprisingly effective and not that difficult either as long as certain precautions are taken (see separate article on this).

Some larger designs call for a 3rd veneer layer—perhaps even laid up fore and aft for appearance. In such a case, this could well be the hardest layer to fit, especially if the hull has a sharp radius in its section. Narrower strips will help in this case but that means more joints to fit. (I'd personally suggest to avoid a lengthwise veneer and plan on painting the exterior.)
Once cured, the exterior is typically sanded and at least partly faired before removal. Cold-molding is good when you have a capable partner to work with as you can each work on strips, starting amidships.

Click here for clear photos of the process:
www.sixrivermarine.com/Cold%20molding%20QA.htm
www.nexusmarine.com/odysy_const.html (with vacuum bagging)
www.bruceroberts.com.au/gallery/index.htm (see Roberts 24 and Roberts 25 wood)

Advantages are: If well executed, this system produces a very tough, long-lasting hull that is stiff and resistant to damage and rot. It requires only the minimum of internal stiffening. Still a useful option where labor costs are low or just not counted.

Disadvantages: The system is pretty labor intensive and solid veneers of moisture-resistant wood are also becoming expensive and difficult to obtain in some parts of the world. The system also requires a good, solid mould prior to construction, which has no further use after the hull is lifted off. The surface, both inside and outside, also requires a considerable amount of sanding and the outside will need additional fairing to get a professional finish. As this is more work than other competing systems, this system is now used less and less, except where labor costs are low or discounted. The Constant Camber system was first developed to try and overcome this high labor issue—see separate article.

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