Plywood hull with small patch of rot

[fishermantwo]

I have just bought a 30 year old 26 foot yacht. It has a fibreglass deck, balsa core double skin on top of a plywood chine hull. Ben Lexcen designed quarter tonner. The hull is Dynal sheaved and the ply looks like cedar, the bow and stern sections look like a clear varnish or epoxy finish on the inside. It all looks in very good condition except for one small patch up in the bow between the chines, there is a soft patch about 6 inches by 4. Its a bit strange for rot to appear there and there is no obvious reason for it to be there. Its too small to cut out and replace so I intend to kill the spores with radiator concentrate and when that dries, saturate the wood with Evedure and a patch of epoxy and cloth.
Any comments or better ideas welcomed.

 

[graham]

i am not a real expert on wooden boats but have owned and repaired a few over the years. In my experience you would be wise to cut the rot out as far back as you can ,dont let any rotten sawdust wood chips or anything remain in your hull. Then replace with new plywood before epoxy/ glass cloth .

Hopefully then the rot will not return to haunt you again in a year or two.

 

[Thistle]

[ QUOTE ]
Its a bit strange for rot to appear there and there is no obvious reason for it to be there.

[/ QUOTE ]
But the fact that the rot is there indicates that the wood has been damp for some time so there must be a hole somewhere letting water in. If you don't find and fix the hole I'd think the problem will simply recur at some time in the future. Remember that water has a nasty habit of showing up some distance from where it came in.

 

[snowleopard]

a few pointers-

the guys on the classic boat forum will be best qualified to help

most rot is caused by fresh water, not salt.

there are proprietory treatments that kill rot and impregnate the timber with resin to restore strength - investigate these before applying other chemicals which might prevent further treatment.

 

[fishermantwo]

Thanks for the replies. There is no leaks at all into the boat and the hull is bone dry. I'm sure its caused by condensation dripping down so its fresh water. The radiator coolant is to kill the spores, it just soaks in and runs into the timber, the glycol does the job.

 

[mirabriani]

Agree with Graham

Get more advice before proceeding

Regards Briani

 

[William_H]

My thoughts would be to sand/grind out the rotten wood as far as possible then treat the remaining wood. When it is all dry I would use successive layers of fibreglass cloth or woven rovings (not chopped strand mat) saturat3ed in epoxy to build up the structure to original dimensions.
This way you can sand down to the inside of the dynel if necessary and can have shamfered edges which could be awkward to fit with a ply patch. Keep it all warm for drying and hardening the resin. regards will

 

[oldharry]

[ QUOTE ]


most rot is caused by fresh water, not salt.



[/ QUOTE ]

Sorry Snowleopard, water as such does not cause rot. To deal with rot you have to understand that it is one of a range of a fungal infections of the timber. The rot spores feed on the timber destroying its structure, and therefore its strength.

There are a variety of types of fungus, but the most common are Dry Rot, and Wet Rot - this being most commonly found in boats for obvious reasons. Wet rot spores are nearly always already present in timber and only need moist conditions for it to start making a meal of your boat. Fresh water is particularly favoured by rot spores so that rainwater lying in the boat or seeping in through damaged joints are sheathing will 'activate' rot much more quickly than salt water. This is why rot often starts in the upperworks first.

Plywood is, because of its construction, particularly prone to attack, as the spores will be trapped between the laminates, which is why plywood boats need particularly careful maintenance. Sheathing with Dynel or similar if done properly goes a long way towards protecting the hull, but has the disadvantage that once water has penetrated, it is trapped creating ideal conditions for rot! It is also more difficult to repair. Sheathing with Glass fibre and Polyester resin will 'kill' a plywood hull in 2 - 3 years, as the polyester resin does not stick properly to wood, the join fails as soon as the hull starts to work, and water finds its way in within weeks. Even epoxy sheathing requires the hull to be thoroughly dried out to a maximum 15% water content in the ply - which is about the level to exepct with new ply from the store. It also means that sheathing a hull after it has been in the water, is rarely succesful.

Fortunately Plywood is also the easiest boat buildong material to repair, and anyone with a modicum of carpentering skills, coupled with modern glues and sealants can make a sound repair. The Eventide Owners Group boasts that the beauty of their plywood boats is that they are 'infinitely repairable' for the backyard boat restorer.

Dealing with Fishermantwo's particular problem is very straightforward unless it is in an area where there is a hard curve in the hull. Cut out the damaged area, plus at least 6 inches all round. This is the only way of ensuring you have removed all the contaminated timber. If dealing with Dry Rot you need to go much further - up to 18 or some say even 24 inches. Treat everything with a good proprietary antifungal timber treatment. Radiator fluid does work provided it is Glycol based, but not as well as proprietary fungicides like Cuprinol which soaks in better.

You then have the choice of doing a 'simple' repair - fashioning a piece which exactly fits the hole, putting a backing pad on the inside, then gluing and screwing everything up. Several hours hard work filling and rubbing down will produce a good final finish.

Otherwise if your carpentry is up to it do an 'invisible' repair: This basically involves fashioning a scarph joint at least twice the depth of the timber all round the hole, and gluing the repair patch into it. This is a lot more time consuming, but done properly, it is virtually impossible to see the repair once painted up. The wider the scarph, the stronger the repair will be.

If the repair has to be done art a point where there is a hard curve in the ply, it is often better just to start again with a new sheet of ply.

Finally as has been noted elsewhere in these forae, obtaining decent quality ply is by no means easy. Modern 'Marine Ply' to BS1088 is rubbish as far as boatbuilding is concerned. You need to obtain ply that is constructed of laminates of equal thickness (not a thin outer veneer with a thick 'coreboard' often of inferior timber), and is the same quality of hardwood throughout all the veneers. Sources are few and far between, but if you are near Portsmouth, Kings Boatyard in Emsworth supply some of the best ply I have seen.

Good luck! /forums/images/graemlins/smile.gif

PS just re-reading your post, the most likely cause of the damage is penetration or failure of the sheathing allowing water ingress at that point.

 

[snowleopard]

[ QUOTE ]
Sorry Snowleopard, water as such does not cause rot.

[/ QUOTE ]

it's like saying that oxygen causes rust , not salt water: true, but a bit pedantic. wet rot spores flourish in fresh water but not salt. rot will set in rapidly if rainwater is allowed to collect. conversely, old wooden boats would be sunk in salt water to preserve them if they were not to be used for a long time.

 

[oldharry]

OK so I am being pedantic - SWMBO would agree! /forums/images/graemlins/grin.gif

But understanding the cause goes a long way towards an effective cure. But I agree - in a plywood boat fresh water has much the same effect as battery acid as it triggers the rot. /forums/images/graemlins/frown.gif

 

[snowleopard]

further to my previous point, "Gitrot" is still available which kills rot spores and consolidates the soft wood - i first came across it in 1972 but have had grp boats since then.

 

[fishermantwo]

Thanks again for some excellent replies. OldHarry thanks for that detailed reply. I will kill the spores first then fill the damaged area temporally and worry about cutting a patch out later when on the slips next. The bit that I'm concerned with is a flat piece in the bow, below the water line right in the middle between the chine. I run a 60 year old timber fishing vessel, with all its original planking. The cabin is plywood sheaved in glass and I am forever having trouble with rot there. This yacht has cedar ply which must have been expensive at the time.

 

[oldharry]

It would be easy and worthwhile to put a 'doubler' over the whole affected area inside to carry any stress loading back to sound timber. Clean off any paint or varnish, and apply plenty of fungicide before your start.

Plywood hulls often have much thinner hulls than planked boats because they depend on the curvature and shapes to provide the necessary strength, a bit like modern cars. Any localised weakness due to rot needs to be compensated for.