Repairing rotten cored GRP

[Anthony]

Hi,

Well my rebuild of an old Datleline Bounty 17' powerboat continues, but I have discovered that the transom is (was!) a mixture of balsa and plywood cored, which has now rotted away.

Here are a couple of pics of the area in question.

I have removed some patches of the internal GRP layer, and as you can see the wood in the areas near the bottom has totally disintrgrated, whilst further up the (balsa?) wood squares seems pretty sound. The central area of the transom (that the outdrive is mounted on to, to the left in these pics) has a thick piece of flat ply sandwiched in it, which appears rotten for the bottom few inches, and pretty sound from there up.

So my question is, what is the best way to rebuild the balsa cored areas to the sides? It is curved in one direction so I was thinking of cutting say 1" wide strips of ply (so they can accomodate the curve) and glassing them in under a new layer of grp?

Do you think its worth replacing the central plywood section, from what I cen see most of it is ok, but certaily the bottom, and a little of the outsides are rotten, and it is after all what supports the outdrive and I am worried about the alignment going off under load due to flexing. It will be a pain to have to remove the outdrive mounting, but I already have the engine half out on a hoist, and suspect that its best to just do it all.

Anyone experienced in these sort of repairs out there that can offer some advice?

Many Thanks,

Anthony

 

[Anthony]

Quick update:

Just done some more digging around with sharp implements, and founf that there is no plywood left in areas on other side around outdrive mount so its going to have to come and and all be replaced :-(

Anthony

 

[boatmike]

That looks pretty bad. Often it is not worth doing half a job when it is like this. Assuming that the outer GRP skin is still OK and that the rotten bits will come away from it relatively easily, I would be inclined to remove the lot unless you are totally sure that what you leave behind is quite sound. The ply is probably in areas where it could be in compression, such as through bolted fittings and / or outboard clamp areas, everywhere else you could replace with block balsa which will take up the shape without cutting. Bed this down ideally on crestomer paste, but if you find this too expensive thicken some normal resin with filler and bed the core down on this. When totally cured glass over with alternate layers of CSM and biaxial cloth which (after grinding it back to clean) should be continued right over the transom and for at least 4 inches down the hull sides forming a new boundary angle.

 

[Poignard]

The technique for renewing a rotten transom core is described in detail in "Fibreglass Boat Repair & Maintenance" published by WEST and available from chandlers.

 

[roly_voya]

Having had the problem once would it be worth renewing with foam core rather than balsa?

 

[ThreeSummers]

How difficult would it be to take an angle grinder to said transom and cut it out? Build a new transom with some marine ply, then glue and tape it in place of the old one. It wouldn't take too long, and is probably quicker and easier than trying to 'repair' the existing transom.

How sure are you that the core of the hull bottom and sides are intact?

 

[Niander]

Dont frighten him!

 

[ThreeSummers]

[ QUOTE ]
Dont frighten him!

[/ QUOTE ]

Not trying to frighten anyone... The sides and hull should stand more chance of being solid than the transom. After all, the transom has holes drilled through it all over the place, whereas the hull shouldn't have.

Most problems seem to come from holes that are not sealed properly before something is bolted on - just using SikaFlex on, for example, an outboard mounting is not enough. Those holes need to be sealed with epoxy or some such before the SikaFlex gets squeezed in.

imo, of course.

 

[oldsaltoz]

G'day Anthony,

Looks like a rip out and start over to me.
If the outside of the transom is still looking OK you might find it less work to remove the inside only, This has the advantage of not having to re cut holes that may effect the alignment. cut the outside off if it's past it's use by date but keep in mind the repairs will cost more if the outer skin is removed.

I find the best kit to remove a transom is a plunge router, set to not cut the glass on the other side, run around the edge using the inside of the hull as a guide, this will let you remove the transom and leave a bit of spare around the outer edge that you can damage because it will be trimmed again later.

Make sure you leave at least half an inch of the original transom glass on the inside. This will help to align and hold the replacement transom.

Ply is needed to support the loadings of thrust on the transom, but it is not at all water resistant, so, must be treated before it goes back in. Epoxy coating is the best and you can improve the penetration by adding 30% by volume of Methylated spirits by volume after mining the epoxy resin and hardener; apply extra coats to exposed end grain, apply about 2 coats wet on tacky after you have applied the thinned resin, Avoid drilling any holes at this stage if possible.

I prefer to replace the whole transom with a single sheet of ply but this is not always possible, curved transoms need more work. One way to form a curved transom is to first place a flat sheet in the area the out drive passed through (or the outboard bolts on) then cut sections of play small enough to follow the curve as required. To maintain strength and integrity you should cut a 'V' on the joining edges to form a joint =<<=, this is filled with epoxy resin mixed with Micro-Fibres (NOT Micro-Balloons). also apply this mix to the skin the ply is placed against. Tip: do not over clamp epoxy joints, you must leave a layer of glue or the joint will be weak.

You can use Micro Balloons to fair the inside prior to glassing if the steps at the joints are to be smoothed.

Prior to re glassing you must lightly sand the epoxy, just enough to remove the high gloss and grind off any paint on the area just outside that being glassed. Clean the area after grinding and you are ready to start glassing.

I only epoxy resins for this type of work and so do not use any CSM (chopped strand mat) because it has large voids and wi8ll not only waste epoxy resin but be weaker and heavier than if proper epoxy resin cloth is used,
When applying epoxy and cloth remember you are looking for a one to one ration by weight of resin and cloth, so you need to roll it out with a threaded roller. Standard resin and CSM has a ration of three resin to one mat and is heavier and not as strong pound for pound.

Back to drilling holes:
Using the old holes in the remaining transom skin, drill the holes out at least 10 mm over size, then wet the inside with some epoxy resin, then using the same batch of resin add some Micro fibres (Not balloons) to the resin and fill the holes with this mix, put a bit of tape over both ends when filled and after cleaning, because micro fibres are very hard to sand when cured.

After curing for at least 24 hours re drill the holes to the required size, this will leave a 5mm layer of protection around the hole, self tapping screw holes must be predrilled to avoid cracking of the filler; large holes (more than 8mm) should be 15 or 20 mm oversize before filling, this helps to prevent crushing on the transom when bold loading is high.

I hope this helps, no doubt I missed some detail but someone will let us know in the next few posts.

Avagoodweekend......

 

[Seven Spades]

It begs the question as to whether it is worth repairing.

 

[cortez]

try this
http://transomrepair.com/zk/fullrepair.shtml

 

[ningcompoop]

"remove wood with chainsaw, carefully" /forums/images/graemlins/shocked.gif

 

[Anthony]

Hi all,

many thanks for all the tips! In answer to some of your questions, only the ransom is cored, the rest of the hull is solid, and in good condition. The outer skin of the transom is also in great condition, its only the wooden core thats rotten, and its only held in place by some CSM. The rest of the wood in the boat (floors, side panals etc) had also turned to mulch but have already removed them, and should be easy to just laminate in new ones.

The transom doesnt have a top as such, its moulded straight up into the deck (inboard not ouboard), must have been a split mould as not even a hull deck join, so cant dig out from top.

Again thanks for all the advice, will go back and have a good read thru, and get ordering some large quantaties of resin and mat...

Anthony