Epoxy Sheathing

I know this subject has been touched upon but I would welcome any advice. I am responsible for the strip back to the bare wood of a 65 foot long keel former race boat, built in 1968. The cold moulded mahogany construction is in good shape. The boat is to be repainted in the same colour dark navy which is not the greatest in warm weather and a recent paint job has cracked in the direction of the plank. Once dry should we epoxy sheath her and with what grade of scrim/system? I welcome any advice/direction for research.

 

I am in London if you need to hire a marine surveyor/shipwright to inspect the boat and make a job plan.

 

Thanks for that. Trying to gather as much information as I can about the merits/disadvantages of sheathing via the net at this stage.

 

If you are only looking for stabilization of the surface and waterproofing, 10 oz boatcloth and marine epoxy should be adequate. However, don't hold me to it. I would need to see it to give a definite answer. "Marine" epoxy is simply formulated to be used under less than perfect conditions.

 

You can sheath the boat if you like, but I wouldn't recommend it before asking why. It adds a great deal of effort to the refinishing of the boat and isn't without potential problems as well. Why do you want to do this to an old war horse? I'll second giving Gonzo a call to discuss your needs.

 

Par/Gozo thanks very much. She was built in 68 in cowes. Although cold moulded she has had her beam increased by the adding of horizontal veneers. In the main the paint has cracked along these lines, due I suspect to moisture being trapped from the previous paint job and delamination of the glues behind. It has been recommended that these veneers be removed once the paint comes off and we go back to the original shape only marginally less beam. I understand that a sheath provides nothing structural to the boat. Do people sheath the boat to provide purely a better paint finish? I don't think water ingress would be a factor. If the boat is properly dry prior to painting surely the paint shouldn't crack provided we manage the direct sunlight when laid up on the pontoon. Regards and thanks

 

sheathing

Warren125,

You beat me by a day to ask the same question

I have tried all the threads and have not had much luck.
I am re-building an old tri thats ply wood. it has been sheathed in the past and has caused a lot of problems. due to water ingress through fittings and had rotted the ply behind the old sheathing (not epoxy)
I am looking for advice as to what weight of cloth to use if any.
I have read somewhere that under 450 gsm there is no real structrual benifit. I don't really want to put heavier gsm cloth on as It will subtantially increase weight. ( I estimate 70 kilos).If there is no benifit.
I am going to epoxy and tape all seams, coat the whole boat in straight epoxy and then was wondering if I put a layer of finishng cloth on just to reinforce the epoxy(30 gsm) before another 2-3 cots. I do understand that 300 gsm will give impact resistance but is it worth it at that weight.
ooohhh questions questions

Please any advice from any one would be helpfull.----of and pleae can you give the weight in gsm as I have NO IDEA what the conversion is from ounces

Kim

 

Unless the veneers are quite thick (not very likely) then the increase in beam is insignificant and not a concern.

Stripping the hull of these veneers seems a lot of work, but if the outer layers is not well stuck, it has to come off. Once off, you may find the reason they used another veneer layer. I can think of only two reasons they did this: one is they knew the boat was lightly built for her racing days and added the additional veneers to stiffen the hull, once her racing career was over. Or the hull had some other issue, maybe damage or surface defects, which was easier to just cover with a layer of veneer, rather then smooth out with putty.

I suspect the boat was built lightly and needed the extra longitudinal stiffness of the veneers to "firm 'er up". This would be typical of a racer that was expected to be successful. Other areas of the design may also be fairly lightly built too, such as the spars or framing.

To fix it, you probably need to remove the veneers, but it really should be looked at by a pro (call Gonzo) so you can have a true idea of what's going on, instead of the guess work of a pro that's 3,500 miles away. You may be able to fill the seam gaps and call it a day. It's hard to tell from here.

Light weight sheathings will impart no strength to the structure. Impact resistance can be very slightly improved, but more so if the sheathing is on the inside of the hull, not the outside. Generally, a sheathing, with regular fabrics improves abrasion resistance and waterproofing. You need to use special fabrics to improve these qualities, such as polyester cloth, non-woven directional fabrics rather then woven or the exotics like Kevlar, before you start making big gains in strength, abrasion resistance, etc.

Much could be said for a sheathing in Dynel or Xynole. They will offer substantial improvement in abrasion resistance, but zero increase in strength. Dynel costs about 20% more per yard then regular 'glass, but Xynole is considerably more, but it's abilities far exceeds that of 'glass.

450GSM will offer only a slight improvement in strength, but not much. You need to get into the 650 to 700 GSM range (in multiple layers) before you really start getting strength gains. This is a fair bit of weight to add to a hull.

Kim, your 300 GSM will not offer much at all in impact resistance if on the outside of the hull and a conventional weave. For your plywood boat, the taped seams with a finish cloth over sounds fine. This will not add much weight, will make the hull waterproof and offer some abrasion resistance. 10 ounces per sq. yard is approximately 280 grams per square meter.

 

sheathing

Thank you PAR,

I have understood more about it in your posting in the 2 mins it took to read than hours spent hunting .
thank you again

Warren 125,

where is the boat as it sounds an interesting project.?

I hope you get your question answerd as clearly as I have
sorry for hitching a lift on your thread.

Kim

 

Par and Gonzo many thanks for imparting your knowledge. The boat raced with the big guys in the late 60's. There are 5 or 6 layers of mahogany over oak frames, cold molded. She is a big heavy old girl, about 30 tons in fact drawing nearly 3m of draught. The veneers were added to improve handicap, part of the beam/sail area ratio that could benefit the boat under IOC regulation. They have no structural part to play and as the boat has been laid port side to the sun and 'baked' for two years. The paint has only split in this veneered area and horizontally with the veneer. Some small cracks have appeared in the direction of the plant at 45degrees around the toe rail in part we think to water ingress due to poor sealing. The starboard side is in the shade, OK in the main. The skin temp on the navy blue paint must reach 60deg C on the port side daily during the summer. Maybe the glue has delaminated along the veneers. Some moisture may be present as the paint job 4 years ago wasn't done well or climatically controlled. Traditional builders suggest on stripping of the paint that the veneers be removed totally. They were not originally there nor in the mind of the designer. A uniform cold molded surface surely is better for painting. As for the sheath, the costs are high and the 'west system' has been recommended, as of yet I do not know what this is. There will be no structural benefit, only abrasive protection. Whether the epoxy will help stop the paint cracking I do not know. I hope that in the interests of reducing cost that a properly dried boat painted in a controlled environment will be sufficient. Folk are very keen to wrap her in epoxy. Post painting a degree of topside management will be required and a way of keeping the heat down on the surface. I guess a sheath will provide a better surface for painting. Thanks guys the investigation continues, gonzo how can I get in touch for advice.
Regards to all.

 

I would resist the urge to sheath her and repair the old lass in a conventional manor.

 

Same for me. Boats with heavy lumps of wood, that are not built in epoxy, will always have some play, making the sheath of epoxy crack locally, causing water ingres. The bad thing is that this water has almost no way of getting out.

IF they really feel the urge to do an epoxy job on her, please have them do a DECENT epoxy job. Which means:

-removing all paint, the veneer, the deck, and the keel.
-have her dry out really well
-route out all seams in the hull
-detach as much wooden parts, structural members, false keel, etc as possible. Clean them, epoxy them, and glue them back in place.
-fill all routed out seams with strips of wood (mahogany) and epoxy.
-shave away the excess strips
-do a good sanding outside and inside (inside could be sand blasted)
-apply glass on the outside
-apply several epoxy coatings on the inside. Make sure to coat EVERYTHING. (also in non-accessable areas)
-reinstall (a) deck, interior, etc.
-reinstall keel

This in very short. The tricky part is the removal of the keel and the removal of several structural members. Basicly you are building a new boat. With that difference that building a new one is easier...

I would probably just do some cosmetic work. Face it, these kind of boats will never have a 100% perfect surface quality. They never had that in the past either. You cannot ask from her to age, and become structurally more sound, so skin defects will not show up.