UK-Cherub Class

Get Your Heart Racing

Core Repair

Whilst failures of the actual foam sandwich structure aren't common, they do happen. Here is how to recognise and fix these.


The most basic would be delamination, where the skin has seperated from the core and just needs to be reattached. This can be a bit tricky to locate one of the best ways is to take a 50P peice and use it to tap the skin. By listening to the sound as you move it around it is possable to map the damaged area. If you are sure that the damage is confined to one skin then the repair can be done without too much disturbance.

With the area of damage mapped out it is possable to fill the void with a lightweight filler without cutting too much skin away. Carefully drill two holes one at each end of the damage and fill a syringe with the light weight mix of filler, then use it to squeeze the filler into one of the holes till it comes out the other indicating that the void is full. You may need to use some tape to hold the filler in there while you wait for it to cure then all that is required is to sand the exess flat, paint and job done.

Core Failure

Slightly more complex would be a core failure where the core itself has broken up, this could be caused by impact damage from a colision or long term damage such as the point where the boat rests on a trailer The outside skin may not be damage and from the outside you may not see anything very obvious. The symptom will be simply that an area of the boat is soft, and can be pushed up and down. The area can be maped again by taping and marking with a indelable marker.

Start by cutting out the outer skin above the soft area along the failure point. You can easily identify the area where the core has broken up and isn't attached to the outer skin - its often damp and soggy. You need to take out the core for all this area. Take it back to the inner skin. If you have a router (carpentry that is, not networking!) this is great for the job, otherwise you'll need to use a sharp chisel and take great care.

You also need to see what's going on with the inner skin. This may be cracked, or it may also have delaminated. If its delaminated or cracked beyond the area of core you've already removed then you've got to cut back more. You need to and up with the core securely stuck to skin on both sides all round the repair area. Don't worry within reason) if this is an irregular shape, we'll come to that in a minute. Make sure there#s at least a couple of centimetres of good skin inside the cut away foam.

With all the affected inner skin exposed take a good look at it. Its probably damaged or distorted. If this is the case then cut it back to a couple of centimeters away from the edge of the foam (see sketch). The rest will no longer be doing anything useful, so remove it.2641-200305xxa.jpg

Sand away the outer skin for a reasonable distance around your cut out area. This must be a high load area (otherwise it wouldn't have failed!), so a couple of centimetres of exposed foam plus a five cm of taper on the actual outer laminate (as opposed to paint) will do. Try not to sand back any more foam once the skin is off it (not very easy!). You do need to cut the laminate back for a feather join both for strength and so that you can sand back the repair so there are no bumps on the skin when you finish. This will be more work and take longer than you think!

With the inner skin exposed you can see what condition its in. Its quite likely that this will have split. You need to cut back the foam to expose all of the crack plus a suitable layer all round the crack, in order to reinforce it with new layup. If the foam is no longer stuck to the skin you need to cut back until you get back to solid construction. If the crack goes up to the chine you will need to cut away enough topside foam to get a good strong repair.

You won't need a huge amount of layup on the inner skin to repair the crack, depending on how damaged the glass is of course. Use similar weight and strength layup to the original skin, but do't go overboard. Cut the glass/carbon out ready, but don't glue it on until you're ready for the foam.

You want to get a piece of foam the same thickness and density as the original. If the area you're working on is quite flat you can just cut out a single piece. To get the shape - if its irregular - put a piece of paper over the hole and trace round the edges with a soft pencil like a brass rubbing! If you're working on a very curved area of skin though, it may be difficult to get the foam to sit flush with the inner skin (unless you have vacuum bagging capability). If so then divide it into a few pieces. When its all shaped to perfection and fits snugly with the minimum of gaps then it time to get the epoxy out. 2641-200305xxb.jpg Glue on the inner skin reinforcement onto the foam as normal with some filler to fill the bubbles in the foam. Add some strong filler where it will touch the old inner skin, and while its all wet put it in the gap. The core needs to go in with the lightest possible filler mix round the edges, because this will give hard spots. Vacuum bag it down if you can, if not lots of tape, weight it down, or do what you can. Its essential that the new inner skin bonds well with the old inner skin or you will get problems and the job to do again.

When this has cured you'll probably find that the foam is slightly proud of the foam around it. Very proud if you sanded the old foam back too much. Bring it all flush, but be very careful of taking too much off. You are aiming to end up with a foam surface that's exactly where it was before the original outer skin went on. Now you're ready for the new outer skin. You want to use a smilar layup as for the orginal skin. Err on the strong side, but don't go way over the top though or you may get a “hard spot” which will crack at the edges. If you use more than one layer of cloth do successive patches in different sizes to spread the load and cover the area where you sanded back the outer skin. Consolidate it as well as you can. 2641-200305xxc.jpg Finally fair and fill and then paint. If you've got everything right you won't be able to tell where it happened and all will be as good as new.

If you're lucky this will have happened in an area where you can get to the inside skin. If you're *very* lucky to be precise in these days of false floors. If so the same applies as above, except that you work from the inside and are much less worried about fairing off the surface!

Taken from an article originaly written by Jim Champ in 2003.

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