Twister Class Association: News Archive

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Shroud plate fastenings

This query arrived via email during the summer from Ian Cox (who has since sold Suju). What follows is an amalgam of different solutions and opinions.

It has been suggested that GRP Twisters shroud plate fastenings require beefing up to be absolutely secure.

Apart from the usual arrangement of shrouds being attached to the deck by bolts through to substantial backing plates underneath there is a thought that fibreglass "knees" should be installed internally at the six shroud anchor points which effectively give additional bonding of the deck to the hull. The backing plates are then placed on the horizontal (-ish) part of the knee under the deck.

Has any member of the association any view on this or experience of having to put "knees" in and if so what exactly did they do?

I think the perceived problem arises from modern boat construction where the shrouds are tied into a frame sitting on top of the keel/fin/or whatever. Twister shrouds are just fastened through the deck and stress could cause deck fracture cracks or, at worst, a lifting of the deck from the hull. However in talking to other Twister owners the problem has not really arisen.

A note from Sandy Thomson, Skyros:

A less than friendly surveyor suggested that the existing chain plate arrangement on Skyros was more than unsatisfactory. As in all Twisters, probably, there are ‘D’ bolts through the deck with a thin stainless plate acting as a washer on the underside. My solution to this is shown in the photo. A hardwood pad was back drilled, so to speak, and two SS studs screwed through. Brackets from 40mm x 60mm strap manufactured (3 pairs) to take the ‘D’ bolts at the top and those from the pad at the bottom. The pad was then epoxied to the hull with filler.

This is a copy of the note in the May 2002 magazine from John MacMullen:

Correspondence from several members suggests this is an area of concern so here is a brief description of what we did to reinforce our cap shroud chain plates:

Críonna had always shown slight signs of stress cracking on the deck moulding around the port cap shroud chain plate but we kept a careful eye on it and I put the damage down to the years she was an RNSA boat. Nevertheless we decided to beef up the below deck reinforcement which only consisted of a pretty mingy piece of stainless strip under the nuts.

We got the plates made up by a local fabricator from 5mm stainless plate. On our boat some of the fastenings for both the centre mooring cleat and one of the stanchion bases interfered so we ended up having to drill a number of holes to suit. We did this by taking out the offending fastenings and then offering the reinforcing plate up to the chain plate. Once it was in position we ran a drill down the other holes to mark the positions and then took it out again to drill on the bench. (making the holes a little oversize really helps!!).

You will probably find, like we did, that the underside of the deck in the area you are interested in is anything but true and flat. Our solution was to mix up a quantity of the proprietary polyester woodfiller we use for work and generously ‘butter’ the top of the plate. Then offer it into position and pull it up to achieve satisfactory alignment under the deck by using the chain plate nuts. You have to work fast as the stuff sets quickly. Make sure you use enough so you get 100% fill. The excess squashes out around the edges and is easy to trim off neatly before it cures fully. This way you get 100% contact area with no effort!. Finally drill down through the cured filler for the other fastening holes, re-bed the deck fittings and fasten them.

A reply from Don Henley, Jacobi:

My boat Jacobi was fitted out originally with fibreglass knees and vertical stainless steel chain plates. The installation was quite substantial - the chain plates are about 200mm x 40mm x 6mm bolted through at least 12mm of fibreglass with hardwood pads on the opposite side to the chain plates. The plates protrude through the deck and the sealant has not given any problems since I have owned the boat (1995).