Sheeted outside, oiled inside. Good idea?

For an epoxy strip planked hull of 37 ft, using red cedar or any other decent wood,would it be possible to finish outside by covering with a layer or two of epoxy and glass fibre, but keeping the inside surface untreated?
I like the natural look of wood, and if possible i would like to oil it and leave it at that. But i am hesitant regarding the consistancy of breathability, and resistance to rot. One side of the wood will be saturated with epoxy,and will be sort of sealed, wereas the inside will be mostly breathable.
If you think this is not a good idea, i can live with a coat or two of matt varnish for the inside, but not glossy epoxy.
Appreciate your comments.

 

Several issues. Cedar is unique, and you'll find that it has properties that allow certain things to be done that might kill other woods.
On a boat having a hull thickness of an inch, say, the glue between planks (strips) is going to be fastened with nails. The nails provide the tensile strength (in addition to some frames), so the additional layers of glass on the outside are for watertightness, abrasion, and some extra stiffness (and even more tensile strength).
You are correct, that you are allowing the hull to take up ambient moisture.
Your sealing the outside, however, allows the moisture to come and go from one side only. This is not necessarily a big problem if it is kept well sealed inside. If the transfer is gradual enough, the cedar in partcular should handle the movement.
At a certain moisture level, wood fibers can be crushed by being limited in movement. Strength is lost. While cedar soft and resiliant and therefore better at resisting this damage, most other woods suffer to some degree from confinement through moisture cycles.
I'm assuming, too, that specifications do not require skinning the inside with glass, and that other means to provide tensile strength are there in the frames and fasteners.
To seal or not to seal is a good question. Moisture trapped from a scrape outside is repaired and some moisture remains within. Or bilge water causes lower areas to remain at very high moisture levels. Not sealing the inside allows moisture to escape in addition to taking it on quickly.
Older strip builds of your size used no epoxy outside. The boats gained a lot of weight through the season, but otherwise, those hulls usually did very well (though they are difficult to repair).
My only suggestion would be the "better safe than sorry" don't epoxy the outside in the first place unless specified (for reasons having to do with structural integrity).
Then use a harder wood for the hull strips, something that can take some hard knocks, if abrasion is a concern. Maybe Spanish cedar.
PAR will no doubt weigh in on this one, since he loves questions like this, and knows a lot about the subject.

Alan

 

Yep, Alan has it about right. Much depends on the type of strip planking intended by the designer. There are several different types, ranging from traditional strip with no epoxy or sheathings at all, to cored striped, where the strips serve as a separating core between sheathings.

Personally, I'd sheath the inside of the hull with a light weight fabric (4 ounce). This will provide the pretty, natural beauty of the wood, without seeing the fabric, offer some abrasion resistance and seal up the hull completely.

Moisture on the inside of a boat is usually more damaging then on the outside. Sweet water in the bilge does more harm then the mooring she's hanging on.

Check the plans or consult with the designer about the sheathing schedule. They'll know how much the sheathing incorporates into the integrity of the hull.

I'm of the school that prefers any wood which has epoxy on it, must have epoxy on every square inch, including fasteners holes, or you're inviting moisture in, but providing little possibility for it to escape. This promotes problems in most cases, I've found.

 

Three points:
- Red cedar is very strong in fibre direction, but looses strength if it gets wet.
- Red cedar is "toxic". You should cover it for the sake of yout own health
- Red cedar is soft and can´t withstand local loads.

I personally would do the following:
1. Coat the hull from the inside with a epoxy based colour or varnish to preserve the "wood-look"
2. Coat the hull from the outside with glasfibre and epoxy.

If the hull is build out of several layers of strips (with different fibre orientation) You can omit the glasfibre for weight reasons. But be aware of contacts with hard objects. The hull will be relatively "soft".

If everything is done properly the hull can survive 25+ years easily ;-)

 

Thank you all. Most helpful.
Alan and PAR, since you did not touch upon the varnish option for the inside, may i take this as,you do not prefer or recommend varnish to epoxy?
Also, if Red Cedar is not ideal, can you recommend a suitable wood which is light in colour and has a pleasing texture to look at. I love Mahogany, but when left as it is, it darkens the interior of a boat too much. I also think it is more pleasing to have a bit of varnished mahogany trim here and there than having it all over the place. Too much of a good thing sometimes kills all the excitement. So the wood i am looking for should be there with all its understated glory, and must not be overpowering the entire interior.

 

Hi Omeron. Mahogany usually lightens over time, rather than darken. Varnish can be applied over epoxy and light cloth, and should be if it won't be painted. Epoxy degrades from UV rays, even limited rays that find their way below through ports.
White cedar and Atlantic cedar are both excellent boatbuilding woods. Phillipine mahogany (really a cedar) and Spanish cedar are also good and harder (heavier) than North American varieties.
Never compare varnish to epoxy. They are not interchangable options. Varnish passes moisture quite easily, while epoxy is amazingly water resistant.
I think it best to paint the hull's inside surfaces after epoxying (if you epoxied the outside too). It's a huge amount of work to keep the inside free of marks and imperfections as you build. You can't fair except by removal of good wood because the fairing compound is opaque.
You are right that too much wood below makes it dark. Why not paint the hull inside and concentrate your resources on some nice mahogany details here and there?

Alan

 

Yep, all epoxy coated surfaces have to be protected from UV, as Alan has mentioned and why I didn't bring it up, it was assumed on my part.

You can have the varnished look, but it will require continuous upkeep to insure the clear finish looks good.

Most people love the idea of a clear finish, then the reality of a clear finish sets in and they paint it, after a few seasons of trying to keep up with the varnish.

Again, all of this is immaterial, until you establish what the sheathing schedule is designed to do. What do the plans say about the sheathing schedule? In other words, what is the outer and inner (if required) fabric layup called for in the plans? Without this information, we're shooting in the dark (you are too).