Wood-epoxy #2 – Epoxy-Ply Lapstrake

Epoxy-ply lapstrake construction is basically the modern equivalent of clinker construction. Like clinker it produces a strong, relatively lightweight structure.

The strength of a clinker structure is that the planks lap each other and are fastened to each other along the laps, as well as being fastened to the frames – in the case of clinker nearly always steam bent timbers. So the laps essentially form longitudinal stringers. And the fact that they are all fastened together along the laps provides significant structural stength and torsional resistance. This often allows for a lighter structure than the equivalent size carvel hull skin.

Epoxy bonded ply lapstrake improves substantially on the structural strengths of traditional clinker. And additionally it possesses all the advantages of an epoxy bonded and coated monococque structure that I discussed in Why Wood-Epoxy.

Plank laps for ply-epoxy lapstrake are usually about three times the plank width. So 6mm planking will typically have 20mm laps; 9mm planking will typically have 30mm laps; and 12mm planking will have 40mm laps and so on. This gives a good bonding surface and forms an appropriate size longitudinal stringer. The inner plank at each lap is bevelled off so that the outer plank sits nicely on it.

The number of planks can be quite variable with ply-epoxy lapstrake because plywood gives a much greater possibility for plank width. So, at one extreme there are perhaps four quite wide planks, which is looking almost like multi-chine construction, except that the style of the design will actually most always be rather different. At the other extreme, there will be pretty much the same number of planks as there would be in traditional clinker. I tend towards the latter – rather a lot of planks – because I think it suits my design style and structural preferences better.

One area where ply lapstrake has a big advantage over traditional clinker is at the plank ends. At the ends, the inside faces of the planks have to fit flush down on to the faying surface of the stem – and similarly on to the transom at the aft end. For this to happen, the inner plank has to be rebated out to take the outer plank; otherwise the would be a "channel" through which water would leak. With traditional clinker this is quite difficult to achieve and it is perhaps the weakest part of the construction method (not structurally weak – just weak in the sense of being prone to error in the accomplishment of it). With epoxy resins, one can use the structural gap-filling properties of the modified resin to achieve a strong leak-proof job.

Small epoxy-ply lapstrake boats do not need much in the way of permanent transverse structures. In a dinghy for example, the thwarts and thwart knees, plus the quarter knees and the breasthook will usually give adequate support to the hull skin. It can be conveniant to build them on simple permanent frames but it is not structurally necessary to do so.

Larger boats – say over 4.50m (about 15') – will need some permanent transverse structures. These can be quite widely spaced frames and/or bulkheads. Frames can be deep simple ply frames like Design No 074, or shallower laminated frames like Design No. 119 (and many of our other designs). And we have recently been developing shallow ply sandwich frames constructed from 3 or more layers of ply – Design No. 152 is an example of this type of structure.

Personally, I like laminated frames! But there are undoubted advantages to ply sandwich frames. One big advantage is that they can be CNC cut directly from DXF files – a big saving in build time (the same is true of deep simple ply frames). And they can be directly assembled. No setting out or measuring is necessary; peg holes in the components locate them all in their correct positions relative to each other. If I were building a boat myself right now, I would almost certainly go down this route.

Once the permanent frames and bulkheads are set up on a building jig, the hull is planked, starting from the backbone and working up to the sheer. The process is quite straightforward. In our instructions we give full details of how to derive the plank shapes quite simply and accurately on the job. And increasingly, we are developing the plank shapes and, again, providing these as DXF files, from which the planks can be CNC cut, which is a considerable saving of build time.

A final aspect of epoxy-ply lapstrake (entirely lacking in traditional clinker) is the ability to fillet joint the plank lands (the corner formed by one plank to the next at each lap), internally and externally. A fillet is a cove shaped application of thickened epoxy bridging an inside corner. Filleting is usually carried out after the first epoxy coating. These epoxy fillets add considerable strength to the plank lap structure and are an important integral part of the epoxy-ply lapstrake system.

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.

© George Whisstock. This article is for information only and may not be comercially reproduced in any form or used in any way without permission.