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Centreplate case buttress, Repairing
(this page)
This page shows one way of repairing a cracked centreplate case buttress. This cracking of the buttresses seems to be a design weakness in the early Mk 2 Luggers as it is a frequently reported problem.
This photograph shows the starboard side of the Lugger centreboard case. This is supported by three GRP buttresses; one at each end and a central one. It is the central buttress (arrowed) which exhibits stress cracking. The central buttress on the port side (not shown) shows identical cracking. The buttresses at each end of the centreplate case have a stronger construction and show no cracking.
These close-up photographs show the cracked GRP on the port (left picture) and starboard (right picture) centreplate case support buttresses. It's interesting to note that the cracking occurs in almost exactly the same place on each buttress.
This is a typical example of GRP stress cracking where the cracks develop at right-angles to the stress. The stress is caused by lateral movement of the steel centreplate against the sides of the case slot whilst sailing. This lateral movement causes the sides of the case to repeatedly flex with the forces concentrated between the rigid and flexible areas of the supporting buttresses. I have also observed crew, sitting on the windward side decks, bracing their feet against the centreplate casing when the boat is heeling when being sailed hard into the wind - which will also contribute to the flexing.
The GRP of the central buttresses is not of sufficient thickness to withstand these flexing stresses and has cracked.
The simplest method of making a repair is to cut away the cracked GRP and replace it with a thicker laminate, at the same time eliminating the tight curve of the buttress which is a weak point concentrating the stresses. The following tools & materials are required;
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Project Outline
These sketches show the stages in making the repair to the buttress. Essentially the cracked laminate is removed and replaced with a thicker, stronger laminate. The tight curve is also a weak point concentrating the flexing stresses - this is replaced by a gentler curve which spreads the stresses more evenly over the laminate. (Click on image for larger view) |
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Removing Cracked GRP with a Powerfile
A Powerfile makes quick work of cutting away the the damaged GRP. (Click on image for larger view) |
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Damaged GRP removed
The damaged GRP has been removed with the Powerfile. Note how the edges have been chamfered for maximum surface area adhesion with the new laminate. (Click on image for larger view) |
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Support for Fabrication of New Laminate
A short length of plastic drainpipe was used to make a support for making the repair to the buttress. A hole has been cut into this to enable a piece of wood to be inserted at right angles to form a support for the buttress cross-piece. The gaps between the wood and the drainpipe have been filled with plasticine to prevent leakage of resin. (Click on image for larger view) |
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Support Ready for Laminating
This picture shows the drainpipe in position temporarily held in place with two self tapper screws. The tape-covered wooden insert fits beneath the existing buttress cross-piece. Before commencing the laminating, the area beneath the repair should be covered with polythene sheeting to guard against inevitable spills. Likewise the stainless self tapper screws should be taped up to protect them from resin so that they can be removed and the drainpipe tapped free, after the laminate has cured. Note that if you are using Epoxy resin then the drainpipe should be covered with parcel tape or similar, to prevent the resin adhering to the plastic. (Click on image for larger view) |
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Fibreglass Options
The photographs show the two different fibreglass cloths to be used in the repair. The woven cloth (shown on the right), is the strongest of the two fabrics and a single layer is the first to be used for the repair. Purchase a length of about 0.5 metre in length and 10 cm width. The chopped strand mat (CSM), shown on the left, forms the bulk of the repair and about 1 metre of weight 300g/Sq Metre will be sufficient. The decision now has to be made as to which resin should be used. I opted for Polyester layup or laminating resin as it is the cheapest option and cures relatively quickly (30 - 45 minutes). However, it has to be said that Epoxy resin is better as it has superior adhesion to existing GRP and cures to a much stronger laminate. The down side is that it is more expensive, requires much longer to cure (12 hours plus), and a higher temperature. Note that if you are using Epoxy, you must ensure that the chopped strand mat is Epoxy compatible (i.e. is emulsion bound & not powder bound). Blue Gee sell CSM which is compatible with both Polyester & Epoxy resins. The woven cloth is compatible with both resins. (Click on image for larger view) |
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Laminating
Cut a single piece of woven cloth to size so it will cover the surface of the drainpipe and overlap the adjacent chamfered surfaces of the GRP. Similarly cut about 10-15 pieces of CSM to size. Ensure that all surfaces adjacent to the repair are covered with polythene sheeting to guard against inevitable spills. Next, thoroughly mix about 80ml of resin with the appropriate catalyst/hardner (If using West Epoxy this is in the proportion of 1 part hardner to 5 of resin; for polyester resin the proportion is usually in the order of 2% catalyst to resin). Mixing can be done in a plastic (polythene) or waxed cardboard drinking cup. Once thoroughly mixed, use a 1" stiff paint brush to apply a liberal coat of resin to the pipe & ajacent GRP. Lay the woven cloth in this resin and stipple down. Apply more resin until the cloth is thoroughly saturated (no more white areas), and use the brush to stipple out any air bubbles. Apply more resin on top of the cloth and then start laying the pieces of CSM. Again stipple down each piece of cloth & ensure saturation and removal of air bubbles. Gradually build up the laminate until it is of an even thickness of approx 5-6 mm. It doesn't matter if there are irregularities as these can be ground out when the laminate has cured. Mix up more resin & catalyst if required. When the laminate has cured, remove the tape covering the self tapper screws (you may need to use the Powerfile to expose this if the tape has been covered with resin), and remove them. You should then be able to tap one end of the drainpipe with a small hammer and ease it free. The photograph shows the cured laminate with the drainpipe removed. The repair is now ready for trimming & sanding. (Click on image for larger view) |
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Laminating Completed
The repair can now be trimmed and sanded smooth using a combination of a Powerfile and a sanding block with grade 80 aluminium oxide paper. (Click on image for larger view) |
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Finishing Off
Finally, the repair can be finished off by painting the sanded surface. It doesn't much matter what colour the paint is as the repair is hidden beneath the floorboards! You could use a bilge paint such as International's Danboline. As I had some suitably pigmented gelcoat left over from other repairs on the boat, I used that for painting the repair. If you do use gelcoat then several coats are required and, as it is not self levelling (remains uneven after painting), it will need sanding flat using wet and dry paper (used wet). That essentially completes the repair which took much less time to do than to describe it! (Click on image for larger view) |
I shall monitor the repairs carefully and if the cracking re-appears I will update the information on this page accordingly. If this does happen, it is not a big deal to start again and use Epoxy instead of Polyester resin for the repair.
Project started 2 July 2006, completed 7 July 2006