Wood-epoxy #3 – Cold MouldedCold Moulded can cover a variety of hull skin structural systems. The uniting feature is that they all produce a smooth round bilge hull and as such can be considered the modern successors to traditional carvel construction. In this article we will be discussing the four most common cold-moulded systems.
As with all wood-epoxy boat building systems, the hull skin should receive a minimum of three, preferably four, coats of epoxy. This adds strength to the timber itself renders the entire structure essentially waterproof and, more importantly, vapour-proof.
1. Strip Plank + Diagonal VeneersThis is perhaps the most common system for cruising yachts. It is suitable for all sizes of yacht and can easily be scaled up for very large vessels.
Generally the inner skin is made up of relatively narrow, relatively thick strip planks. Just to get a handle on this, on a 5.00m - 6.00m (17' - 20') sailboat the strip planks might be 6mm x 30mm wide. On a 6.00m - 7.50m (20' - 25') sailboat they might be 8mm x 35mm. On a 7.50m - 9.00m (25' - 30') sailboat they might be 10mm x 40mm. And on a sailboat up to 12.00m (40'), the strip planks might be 12mm x 40mm. On a sailboat up to 15m (50') we would be thinking of something like 16mm x 45mm. These sizes are not absolute in any way – just an average example.
The sub-structure for this type of construction is generally well-spaced transverses (frames, bulkheads and floors) with a laminated backbone centreline structure and a deck-edge shelf.
The strips are available commercially – or you can machine them yourself quite easily, provided you have the right machinery (circular saw and planer-thicknesser for square-edge strips; plus a spindle moulder, or powerful table-mounted router, for profile-edge strips). Three types of strip are in common use:
Square edge. These are simply rectangular strips. The cross section of the strip is generally narrower than the profile-edge strips because they are usually edge-nailed together as the strips are applied to the hull. So, mostly, square-edge strips won't be wider than about 30mm. The strips are epoxy bonded and fastened to the sub-structure, and edge-to-edge to each other. The tiny gap that will exist on the outside between the strips (because of the curvature of the hull) easily fills with thickened epoxy during the bonding process. The main reason for edge-nailing is to keep the strips running flush with each other between frames.
Concave/convex edge. These strips have one concave and on convex edge. The advantage of this profile is that generally the strips will remain flush with each other between frames, because the convex edge nestles in the concave edge of the strip beneath it. Edge fastening is not required. And edge-to-edge bonding is easier, less messy and less wasteful, because the concave edge of the strip (already bonded to the sub-structure) retains a nice bead of thickened epoxy.
Tongue and groove edge. These strips have a groove on one edge and a tongue on the other. The profile of the tongue and groove is such that the strips can rotate a litle, one on the other, so as to take up the hull curvature. And because the tongue and groove are a loose fit, there is room for the epoxy bonding. As with the concave/convex strip, no edge fastenings are required. Guidance (so that the strips remain flush with each other between frame) is excellent. And the groove retains a nice bead of epoxy, which makes the edge-to-edge bonding easier and less messy and wasteful.
There are several ways to fit the inner skin strips. We prefer to start at the deck edge and work upwards (assuming the hull is built upside-down). The internal appearance of the strips, running parallel to the sheer is, we think, preferable. And it is a very easy way to start. Because the hull girth increases from the stem to about amidships, and then decreases again towards the transom, the strips will require some degree of edge bending. As you move up the hull from the sheer, the amount of edge bend will generally increase, to a point where it may become difficult to edge bend them any more.
At this point we generally fit a "stealer" or two. These are tapered strips (generally full-width in the middle and tapering off to the ends) which will reduce the amount of edge bend required. Often a stealer is required at about the turn of the bilge. Sometimes you can get nearly to the keel without a stealer.
The inner skin provides the longitudinal strength of the hull skin and a big part of the general strength and impact resistance of the hull skin. To provide additional skin strength and bulk, and especially torsional resistance, a minimum of two diagonal veneer skins are bonded on at approximately +45º and -45º angles.
The diagonal skins usually use 3mm veneer or 3mm ply. These are sawn to about 100mm - 150mm widths. They are epoxy bonded and (generally) stapled to the inner skin. The staples can be left in the first veneer; they are generally removed from the outer veneer. To facilitate removal, the staples in the final skin are usually driven through a tape (12mm plastic parcel binding tape is ideal for this).
As with the strip planking the diagonal veneers won't lay exactly straight. So they need to be edge-bent, or edge-fitted. Edge-bending is generally not very successful for more than two or three strips, so we usually edge fit the veneers. This may sound tedious but in practice it is very quick and easy. Some builders will temporarily fit a whole batch of veneers and then epoxy bond and staple them in place; other builders fit, bond and staple each strip, one at a time. Strip by strip is probably the easiest and fastest way to go for an individual builder. Air bagging can be used to bond a whole batch of pre-fitted strips at one time, but this is generally beyond the scope of the home builder, although the technology is very simple and straightforward.
It is common to incorporate a lightweight woven glass cloth (about 200-250 g/m2 – say about 6 oz/yd2) in the second epoxy coat. Note that this has little or no structural significance; woven cloths are not ideal for structural purposes because the weft-warp weave produces weakness at each crossing (just like tying a knot in a rope weakens it). The purpose of the glass cloth is twofold: it provides improved abrasion resistance; and it wicks over all the little staple holes, which otherwise can tend to produce fish-eyes in the epoxy coating around each hole
2. Spaced stringers + Diagonal VeneersThis system uses longitudinal stringers over a sub-structure similar to strip planking + diagonal veneers. The stringers are often square section and are spaced amidships at something like 150mm centres. By the nature of the shape of a boat, the stringers will get closer forward and aft, especially forward, which is structurally desirable because the hull is flatter forward and this is also likely to be the area of most impact.
Usually a minimum of three 3mm diagonal veneers are laid over the stringer system. The inner veneer needs particularly careful fitting, especially if vacuum bagging is to be employed on subsequent skins. The inner skin is bonded and stapled to the stringers. Subsequent skins are bonded to the previous skin and, once there is sufficient thickness, stapled to the previous veneers as well as the stringers. Each veneer skin is laid at approximately 45º, but in opposite directions (top forward, say, and then top aft).
Larger boats will use more veneers, and possibly thicker veneers (5mm veneers are often available as well as 3mm). The advantage of 3mm material is that the epoxy will almost completely penetrate through the veneer. So we get a true epoxy/wood-fibre structure. Thicker materials will not be so completely penetrated.
The hull skin is finished off in a similar way to a strip planking + diagonal veneers hull, with a light woven cloth incorporated in the second epoxy coating.
The advantage that this system has over strip planking + diagonal veneers is that it is lighter in weight for pretty much the same structural strength. The disadvantage, especially for a cruising boat, is that the interior is very difficult to keep clean and free of trapped water laying on the stringers.
3. All Diagonal VeneersThis produces probably the lightest hull for equivalent strength and the resulting structure is pretty much a true epoxy/wood-fibre monococque.
The hull skin is entirely made up of diagonal veneers. Usually a male mould is required, made up of temporary formers with spaced stringers fastened over them. With care in the construction of the mould a laminated backbone and a deck-edge shelf can be incorporated before the hull is skinned. And sometimes the principal frames are also incorporated into the mould. This type of hull skin structure tends in any case to need fewer transverses than the previous two methods.
The method of laying and laminating the veneers is similar to that described previously. Because there are a lot of veneers to be laid, vacuum bagging can be used with real advantage.
The hull skin is finished off in a similar way to Methods 1 and 2, with a light woven cloth incorporated in the second epoxy coating.
The interior appearance of this type of hull skin is very pleasing and easy to keep clean. But it tends to be an expensive method, both in terms of material and labour costs. For this reason it tends to be used for race boats (nowadays, mostly overtaken by exotic fibres) and small boats. Cruising boats are seldom built this way.
4. Strip Planking + Bi-Axial GlassThis method uses conventional strip planking but replaces the diagonal veneers with bi-axial glass cloth for the torsional resistance element of the hull skin.
Because glass is much heavier than wood strength for strength, the inner strip planking is generally thicker than for strip planking Method 1. On a 6.00m - 7.50m (20' - 25') sailboat we might use 16mm thick strip planking followed by one layer of 600g/m2 (18 oz/yd2) bi-axial glass cloth. On a 7.50m - 9.00m (25' - 30') sailboat we might use 16mm thick strip planking followd by two layers of 450g/m2 (14 oz/yd2) bi-axial glass cloth. And on a sailboat up to 12.00m (40') we might use 16mm thick strip planking followed by two layers of 600g/m2 (18 oz/yd2) bi-axial glass cloth.
Bi-axial cloth has two layers of non-woven rovings stitched together at 90º to each other. It usually comes in either 0º/90º (one horizontal layer and one vertical layer), or +45º/-45º format. The 0º/90º cloth would be laid diagonally – remember it is diagonal fibre direction we are after. The +45º/-45º would be laid horizontally or vertically to achieve the same effect.
Woven rovings are not suitable for this purpose because, as mentioned earlier the criss-crossing of the warp and weft weakens the fibres.
It is important to use peel ply over the final layer of glass so that a reasonably good finish is obtained. It is also important that there is a good thickness of epoxy coatings over the glass, so that subsequent sanding and filling doesn't expose the glass itself. Exposed glass will wick and tend to take up water. Unlike the non-structural glass cloth that we use on the previous methods, this glass is seriously structural and needs to be treated as such.
Though popular and perceived to be easier and faster than strip planking + diagonal veneers, when one looks at the overall completion time for the boat this is not necessarily the case. Certainly the hull skin gets built faster, but the finishing time is longer and more difficult. A hull built under Method 1 produces a very fair, clean hull, which only requires quite light sanding of the timber to get a good finish. The lightweight woven cloth incorporated in the second epoxy coating does not disturb this fairness in any way. Once the hull is covered with a heavy glass cloth and epoxy resin, it is less fair, requires more filling, and is also much harder (and less pleasant) to sand smooth to a good finish. So, in the old saying, it is probably in reality "half-a-dozen of one and six of the other"!
© George Whisstock. This article is for information only and may not be commercially reproduced in any form or used in any way without permission.