Repair & maintenance: Lark Dinghy

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Lark Dinghy - Bouancy Tank Issue

Printed From: Yachts and Yachting Online
Category: General
Forum Name: Repair & maintenance
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Printed Date: 16 Aug 25 at 5:11pm
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Topic: Lark Dinghy - Bouancy Tank Issue

Posted By: radixon
Subject: Lark Dinghy - Bouancy Tank Issue
Date Posted: 26 Mar 07 at 7:57am

Right, I have aquired a Lark Dinghy complete with all the sails, Mast fittings etc but the issue I have is the hull has two large cracks one either side down the bouancy tank, where the tank meets the hull floor. (Lark is Baker design (original)). Crack is from the Mid Ship (halfway) to the stern of the boat. Its not a wide crack, but it is long.

Q.
1) Do you think its worth repairing it?
2) What about the presure build up inside the tank when capsized once repaired, as I can only repair from the outside.
3) Do you think a repair and then bouancy bags will solve the issue.

Any help/advice would be appreciated.

Replies:

Posted By: JimC
Date Posted: 26 Mar 07 at 8:47am

Very very familiar problem in a lot of classes. You can repair it from the outside, but its not great. opu haven't got a lot of choice though. Buoyancy bags sounds like a nightare.

Where this job always goes wrong is insufficient preparation. You need to clean up every trace of paint, old repair, gunge grot and everything else at least two inches each side of the join both on floor and tank side. Sand and/or grind down so that glass is just thinking about being exposed. Go right the entire length of the tank - if it hasn't failed it will soon. Scrape out all the gunge in the cracks that are there so that there are no bits in.

Make up a mix of epoxy and filler, very strong on microfibres as thse gives flexible strength. You want to push this into the crack as much as possible to get some grip in there. Then make a fillet say about 1/4 inch radius the entire length along the join.

Now you need to reinfoce the entire joint with glass. Woven tape is good, but in many ways ordiary glass cloth cut in strips so that the fibres are diagonally across the strip is much better, but much more hassle. Don't even consider chopped strand rubbish! You probably want two layers of glass for the conventional tape, and the strip need to be at least 4 inches wide. If you are cutting your own make one strip wide than the other. Make sure that the glass is only to your prepared surface - if it starts on unprepared it will tend to peel off.

Tidy up and paint... In future be very carful about jumping on the floor next to the tanks, keep the feet a bit nearer to the middle...

Posted By: radixon
Date Posted: 26 Mar 07 at 7:55pm

Thanks Jim for this, does anyone else have experience of this problem?

Did you repair it and was it successful?

Thanks

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Posted By: olly_love
Date Posted: 26 Mar 07 at 9:19pm

i have repaired things like that on larks.

we had two options

1. rebuild the tank and support it from bothsides.( it is quite structial)

2. slap shed loads of resin and glass over it and then sand it flat. seemed to work well as it didnt break there again.

good luck with it anyways and the older larks are good to sail.

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TWO FRANK-Hunter Impala

Posted By: Rupert
Date Posted: 27 Mar 07 at 9:31am

I've done this repair on various boats too. Jim's approach will certainly be the best, but if you only put one layer of narrower glass tape on, it will still probebly last as long as many other parts of the boat. The thing to remember is that it is very difficult to make resin stick to gelcoat (the shiny outer layer) so you do have to sand that back as far as possible. I have done the job only using the epoxy fillet, but I've yet to see if it stands the test of time!

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Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686

Posted By: Paramedic
Date Posted: 29 Mar 07 at 12:09am

DO NOT USE EPOXY!!!

The boat is built out of polyester resin. This means that the best repair will be with polyester resin. The fillet option is good, but use polyester/microfibres, not epoxy.

Epoxy is harder than polyester, and what happens is that the seam will just break again at the new weakest point (Usually what you've just done!) and you'll end up chasing the repair around the boat

Note that you should only use Microfibres, glass bubbles or colloidal silica with polyester as it eats microballons because the styrene dissolves them.

This is a common problem with old larks, good luck with the fix :)

Posted By: Garry
Date Posted: 01 Apr 07 at 6:21pm

I agree with Paramedic use Polyester. This is fairly easy to repair clean out the crack, dry, wedge open and pour in your microfibres/resin mix which should be fairly stiff close the crack and allow it to go off. Then for extra strength you can lay up some glass. If you can get to the inside of the tank through a hatch then fillet that as well.

Alternatively you can grind out a shallow V and fill with chopped strand matt, grind flat then build up slightly bonding to the surrounding area to spread the load.

You need a repair that doesn't require bouyancy bags. It should be fairly easy to make it as strong as before with good preparation.

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Garry

Lark 2252, Contender 298

www.cuckoos.eclipse.co.uk

Posted By: redback
Date Posted: 01 Apr 07 at 10:30pm

Many designs fail in this area since the tanks are important to the stiffness of the hull and particularly the floor. So buoyancy bags are not a solution. I have seen many repairs that haven't worked because the gap must be clean and dry. So I echo Jim's comment about it all being in the preparation.