UK-Cherub Class

Get Your Heart Racing

Fittings and Hard Points

Attaching fittings to a boat built out of light weight foam sandwich is not that difficult, however if not done properly then they will tend to move, leak and then pull out. This could take a few years or it could happen the first time you hit the water. Foam Sandwich is a great material for boats, as it produces stiff light panels, however they are not good when it comes to point loads, a small point load will tend to damage the thin skins, it can crush the delicate foam core and eventually fail. The key to attaching fittings is to spread the load out over a larger area protecting the soft core.

Screws and Bolts

Avoid using wood screws wherever possible, they rely on only a small contact area of the thread to grip, this works OK in a high density wood but in low density foam they will just pull out, particularly if the fitting is loaded in tension

Plywood core

During construction the core is locally replaced with plywood. This gives a strong area to bolt or even screw fittings to, however overtime the plywood will absorb water and rot.

High Density Foam

Core Replacement

The holes for the fitting are drilled through the top skin of the sandwich only, then using an allen key in a drill the foam is crushed around the holes, the voids are filled with filler and the fitting can be bolted in place.

Carbon/glass Backing plates

In areas where access to both sides of the structure is possible you can cut out a section of 3-4 mm thick carbon or glass plate slightly larger than the fitting to be attached, this spreads the load over a larger area.

Fronting Plates

A backing plate is made up and the fitting bolted to it, the bolts are then cut to length and the nuts bonded to the backing plate. This plate with nuts on is then bonded to the deck of the boat and glassed over the top. As the nuts are now captive the fitting can be removed and replaced as necessary. This is particularly on the false floor of a boat where you cannot get to the inside.

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Tying with Carbon

This is where you bond a fitting in place with Carbon, this is often done by wrapping Carbon uni's around the fitting and a handy tubular part of the boat, this can be good for forestay and shroud points. You could loop around the mast stump for the kicker or around the boom for the kicker. It is also possible to just use small sections of woven cloth to bond lighter loaded parts onto the hull and spars. This is particularly handy for putting small fittings onto booms and masts where you want to avoid drilling holes.

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Tying with String

Self explanatory but with the number of blocks that are designed specifically for tying in place it might be worth designing in some points that they can be tied to rather than adding weight by bolting a fitting in place and tying to that.

tech/fittings.txt · Last modified: 2013/06/25 15:55 (external edit)