Thursday, 12 July 2012

Improving the front beam

Here's the front beam of the cat:

There's a bridle from the middle of the beam to close to the water line on each hull. I don't like this set up.

The angle of the bridle seems too small - the forestay is under great tension when the boat is sailing to windward, and the tension in the bridle must be greater still given such a small angle. I suspect the hull fittings must be under great load, and though the moisture meter doesn't suggest any water leaking around these fittings, it just seems a matter of time. The builder of this boat wasn't too clever about keeping water out of other places! Like where the beam passes through the hull for instance:

I've scraped away some gel coat at the top of the join. There's a fillet of resin and glass around the beam, but the resin is polyester, not epoxy, and there are places where the fillet is pure polyester resin with no glass or filler in in - it is just brittle resin, with very little strength.

Water has entered the join on each side. The beam passes right through the hull and is bonded to thick plywood above and below the beam. This is a very strong arrangement - or it was, until water softened the plywood. It had even begun to rot in the starboard hull.

I climbed in the little compartment in the forepeak (really small, as the bottom half is a watertight compartment filled with foam) and loosened the bridle attachment fittings. Both fittings were quite bent:

Those really aren't the best fittings for the job anyway. They aren't designed to withstand such forces from an oblique direction.

Time for a total redesign. Googling Richard Woods and Sagitta, I noticed that on the first of these boats that he built, he had a seagull striker rather than a bridle. This is an arrangement I much prefer - no through hull fittings near the water line, no wires below to cause trouble with mooring buoys, anchor ropes, people in dinghys, or my head, when I am walking around under the boat and forget the wires are there.

Anyway, here's my plan for retro-fitting a seagull striker:

I've drawn up plans to hand over to an aluminium fabricator. All the tangs and fittings for rollers are welded to two-part sleeves that can be bolted over the beam. I have anchor rollers on each side as well as the central one - you often need more than one roller, but positioning them here allows me to also use them for a sea-anchor bridle. The sea-anchor lines can be easily led back to the sheet winches.

I'm replacing the plywood around the beams, but will be using epoxy. The plywood will get a couple of coats of epoxy before I glass them in, to ensure they don't become wet.

I'll make a small epoxy/glass strand and filler fillet round the beams where they enter the hull - for strength rather than waterproofing. Over the join, I'll make a thick fillet of Sikaflex to seal it.

That should fix it!

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