Fixing a leaky plywood boat

[Zen Zero]

I have an EM850 from the Italian boatyard Sibma (http://www.sibma.it/occasioni.htm - you can see photos of a new one here!).

The boat was built from marine ply about 20 years ago and is bearing up quite well, but ... when it rains, the water comes in! And it is a royal pain in the posterior avoiding the drips while holed up during a Mediterranean thunderstorm.

I think water has found its way under the coamings and other deck furniture, down through the nail holes that fasten the deck ply to the hull ply and thence into the cabin. In another area water comes in where the cockpit floor meets the vertical boards that rise to meet the deck.

My plan:

Deck leaks:
remove coamings and all deck furniture
remove rotten plywood (there isn't much of this in the small area I've so far checked)
paint exposed plywood with epoxy resin (I use C-Systems 10 10 CFS).
fill dimples with epoxy paste (10-10 thickened with "filler powder").
replace coamings over a generous layer of epoxy filler paste (Question 1 - is it better to use flexible goo like sikaflex?) holding coamings in place with weights until epoxy sets thus eliminating screws (Question 2 - do I need the screws, they seem to be just a good way of letting in water?).
Paint with epoxy compatible paint.

Cockpit floor:
rip up cosmetic teak planking (done) clean up for reuse
clean, scrape, curse away glue used by previous owner to fasten teak planking
remove cockpit drain fittings *
strip off all paint
paint with epoxy resin
Question 3 do I apply glass cloth here? If so how much (just tape the joints, or make a glassfibre basin out of the whole cockpit)?
Replace teak planking on bed of thickened epoxy resin.
Fit new cockpit drains.

* my prime suspect for how the structure got compromised in the first place is that the drain fittings were left flush with the original floor, the previous owner who fitted the teak planking drilled a couple of big holes in the corresponding planks and bunged up the gaps with black goo, the water got underneath and couldn't get out again.

My main worry with using glass cloth is creating an impermeable membrane under which the water will eventually find its way (because it always does) and do damage.

Many thanks for your advice.

Zen

 

[Zen Zero]

No takers?

 

[Searush]

Give it time, few people on here will have expertise in plywood repairs. You could try the Classic forum.

My only comment is that fresh water (ie rain) is the big killer for plywood, especially when it soaks into end grains - where the joints are! I did some repairs on a small dinghy & ended up having to replace 2 entire panels.

Also, grp sheathing was hailed as a wonderful cure all 50 years ago, but it turned out to agravate rot in many situations. You will have to ensure any end grain is sealed & that no water can get under any sheathing.

Best of luck with what you are planning.

 

[oldharry]

As Searush says, thiswould be best posted on the Classic Boat forum where there are a number of experts on plywood boats.

Anyway: Cutting back rot in ply you need to go at least 500cms back from any damaged area because rain water penetrates the end grains of the interior laminates carrying rot inside the board and reducing the strength substantially of a sheet that looks otherwise sound.

The big problem you are going to have is to get the ply back to a 'dry' condition. For overcoating with epoxy the moisture content of the timber should be no higher than 15% - roughly the same as a new sheet of ply direct from the ssupplier. Higher moisture content will get trapped in and can either induce rot or actually cause the epoxy joint to fail. However maybe your fabled MED sunshine CAN acheive that level of drying out! Certianly very difficult to do it in UK without extra heat.

No DONT use epoxy for bedding things. Wood expands and shrinks as it gets wet. Epoxy filler doesnt, so it will break away and cause worse problems. Use a polyurethane type 'goo' which will move and stretch with the timber. To a lesser degree this is also true of glued seams. Most of us prefer to use a good poyurethane glue such as Balcotan which allows the timber to expand and contract. Cheaper too!

Using screws? Definitely yes unless you can be absolutely sure that the glue seam is in full contact all the way. Particularly important if you use poyurethane glue as it is not gap filling. The screw heads should be set slightly below the surface of the timber, then filled and sealed. It also provides a blet and braces fastening should (when) the seam fails again.

And yes you are absolutely right. Dont sheath anything: Two reasons. One already mentioned: the sheathing is rigid, the timber moves as it becomes damp. The joint fails, and 2 moisture gets underneath, is trapped and destroys the timber.

Epoxy is not the wonder 'cure-all' timber or ply, sadly. Applied to a newly built boat that has never seen the water, it works extremely well, and there are various epoxy/wood sheathing systems that have proved 100% effective on new builds. As a repair medium the problem is getting it in to EVERY end grain and gap. One single gap causes no end of problems! It IS possible to use it, and there are people on this board who will tell you they have with considerable success, but mostly on small dinghies or trailable boats that do not spend their entire life afloat. It is also expensive. Much more so than other materials which are half the price and just as effective! On a boat this size, nearing the end of its life (ply boats of this era have a working life of 25 - 30 years) the cost effectiveness pof repairs needs to be thought about.

Now I will worry you a little bit more. The boat is 20 years old. What glues were used in its construction? Glue technology was not so well understiood in the 1980s, and after 20 years there could be further problems with glue seams failing over the next five to ten years in both the deack and the hull. If good quality glues were used, then you should be OK. This is why there are so few plywood boats left from the boom of the 60s/70s, while the GRP versions are still going strong.

Whichever, Good luck. Nice looking boats from the pic you showed.

 

[Pete54]

The best of luck, the previous replies are pretty much on track. Whilst there may not be much rot evident I would suspect it would be much better to remove all of the plywood that has been affected - right back to the first joint. One of the posters above has it - water travels along the end grain and there is nothing more depressing than having to re-address an area you thought you had repaired.

I had an old Debutante and the only 'solution' ended up being stripping the majority of the deck to finally get to sound timber.

With the new timber yes epoxy coat it to seal it, but sikaflex is the correct option for bedding everything. You can scarf joint new plywood into an existing structure - but it does require real accuracy and actually it is much quicker and easier to replace larger areas. The only drawback is that it is likely you will find the gunwales may have a lot of damp in them - and that raises the ante quite a lot.

Plywood is a great material but once it deteriorates repairs are problematic. If it is only 20 years old then you should be ok in terms of the glue. Before the 70s there was a lot of glue being used which absorbs moisture and fail, casein type glues are notorious for this. The aerolite and similar glues should be alright.

 

[SvenH]

When preparing to remount deck hardware, try to raise all holes so that expected water flow does not have access to the holes. Also make sure that water gets no opportunity too pool anywhere.

It's the steady flow or pooling water that causes most problems.

 

[Zen Zero]

Thanks very much for all this useful advice.

The job is started. The cockpit sole is almost done, I just have to fit new drain fittings and then glue down the teak planking. The coamings are proving a lot more difficult though, i've broken 3 screwdrivers and buggered-up the heads of a similar number of screws trying. I'm currently hunting down my old impact driver (from 1970s motorbike days) to see if that will shift them.

Under the nonslip paint on the deck, I'm finding small patches of soggy plywood (despite 3 weeks of dry sunny weather) my plan is to let them dry out then impregnate with thinned epoxy resin and then fill with epoxy filler. I thinking of a way to raise the coamings by 2mm or so on a bed of epoxy filler so that water does not flow underneath them again.

Can't wait to get back in the water!

 

[oldsaltoz]

Any screws or through bolts in ply should be drilled out oversize by 20mm or more, then completely filled with a mix of epoxy resin and micro-fibres, after curing drill the required size hole, this method ensures no water can contaminate the ply.