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Construction Questions

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Q Can I use local timbers in place of the timbers specified in the design?
Q Can I use 3mm plywood in place of laminate veneers?
Q Is it possible to substitute ply "sandwich" frames (like Design 152) for conventional laminated frames?
Q I've noticed that ply lapstrake is often thinner than a cold-moulded hull skin. Why is that?
A Skin thickness is controlled in part by the panel area of the skin. Within reason, the smaller the panel area, the thinner the skin can be. Obviously we can't take this to ridiculous extremes or else we would have insufficient impact resistance and general strength.

The laps of a lapstrake skin can be considered as longitudinal stringers. So on a 9mm ply lapstrake skin with 11 planks and 30mm laps, we essentially have 10 longitudinal stringers 30mm x 18mm, incorporated into the skin structure. And the panel areas of the skin are much smaller – on average, 1/11th of that of a cold-moulded skin. So the skin can afford to be thinner than cold-moulded.

This is the principal reason why lapstrake (and its traditional predecessor, clinker) produces a strong, lightweight hull.

There is also a practical aspect to the difference in smaller boats. The easiest type of cold-moulded skin is a strip-planked inner skin, followed by two opposed diagonal veneer skins. The thinnest strip planking that one can use easily is 6mm. And the thinnest veneers that one can use easily are 2mm. So the thinnest cold-moulded skin is going to be 10mm. And often the thinnest veneers that can be obtained easily are 2.8mm or 3mm, which gives a minimum skin thickness of 11.6mm - 12mm.

Fortunately, timber is a lightweight material in comparison with glass fibre or metals, so the small extra thickness is not of concern in terms of weight for boats bigger than, say, 4.5m (15'). And because the external volume of the hull is going to increase a little, the volume of water displaced will increase a little and the floating hull will be able to support the extra weight and float on much the same waterline.

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