Do I need to Fiberglass both sides of the hull?

[Solon]

I recently built a Martha's Tender rowboat, following the instructions in a series in WoodenBoat magazine. The boat is built and ready for varnish and paint. Per the instructions, the hull is planked with quarter inch marine grade fir plywood and sheathed on the outside with a layer of fiberglass but not on the inside. My plan is to paint the hull with a marine grade paint, inside and out. A friend of mine who builds and repairs cedar-strip canoes for a living said that I was asking for trouble if I only glassed one side of the hull. He has seen hulls of cedar-strip canoes buckle when the hullls sat too long between being glassed on the outside and on the inside - the uneven moisture gain and loss presumably the culprit.

Is the hull of my Martha's Tender going to be OK if it is glassed on the outside but not on the inside, but painted outside and in? It's too late to fiberglass, but I could coat the inside of the plywood with epoxy if necessary. What do you think?

[JimD]

I would do what ever you can to keep moisture off the wood on the inside. A careful and thorough coat of epoxy on the inside would be a good idea imho. Some of my (plywood) boats are glassed on both sides, some just glassed on the outside and coated on the inside.

[Bruce Hooke]

I don't think you'll run into the problem strippers run into with the wood moving and buckling. Plywood, because of the cross-grain layers, does not shrink and expand the way solid wood does.
So, it comes down to whether you want the added protection offered by an epoxy coating. If this boat is going to sit outside at a dinghy dock or something similar, where rainwater may be swilling around inside until someone comes to bail it out, then the epoxy coating may do a good bit to keep rot at bay, especially if it is likely to see hard use and somewhat lax maintenance on the paint. At the other end of the scale, if it will live in a garage or someplace similar when not actively being used then it really doesn't matter. In that sort of use the boat will never be wet long enough for rot to get a foothold.

Some argue that leaving off an epoxy coating on the inside allows water that gets past the epoxy barrier on the outside to escape. I think this is bad logic for a number of reasons:

1. The glue lines in plywood pretty effectively stop water migration from one side of the plywood to the other.

2. The real issue is water getting into the wood from the inside.

3. There is lots of evidence that boats thoroughly coated with epoxy both inside and out last very well.

And one last thought...it is a royal pain coating the inside of an already built boat with epoxy, so I would only do it if you really feel that it is important. Sanding epoxy is already a pain, and sanding around chines, frames, etc., multiplies the pain by a couple of orders of magnitude!

[Rum_Pirate]

In answer to your initial query " Do I need to Fiberglass both sides of the hull? " I think it would be worth the 'trouble and expense' in the long term.

[pipefitter]

I only glassed the interior where water might sit for a time in the bilge to keep the plywood from checking going thru dry/wet cycles and just to keep the superficial topcoats from cracking along with it.With marine grade fir,I would consider doing this if you frequently get water in the boat. Paint will last alot longer over glass,especially on fir ply.The lightest epoxy compatible cloth obtainable would suffice.1088 plywood can do with just epoxy coating but with fir,I would glass it just for the paint life factor alone. I used some "marine grade" fir on my boat and after seeing what the unglassed scraps did compared to the unglassed 1088 ply,I am glad I glassed it.You will have visible cracks in your paint in no time at all.

[Gary E]

Maybe just the Right side ,,,so like the NASCAR cars,
it will take the abrasion as you hit the walls ??

ohh..just a rowboat... a new 250 HP outboard would make that run nice...

[Todd Bradshaw]

His comparison between plywood and stripper constructions doesn't hold water....as it were. The inside glass is critical to the structure of the strip canoe. Without it, the loss of structural strength is off the charts. On a plywood hull, the glass sheathing is more of an abrasion guard. It provides almost no strength unless you plan on jumping up and down inside the boat, putting the outer glass in tension. Sheathing both sides would strengthen the hull, but most plywood boats are designed to stand alone structurally without the glass contributing. If you did need more structural strength on a ply design, using thicker ply is nearly always a more efficient way to do it from a cost, weight and labor standpoint. A good job of epoxy sealing on the inside is likely a good idea, but you shouldn't need fiberglass in there.

[Bob Cleek]

Now, don't you wish you'd built a boat out of real wood?

[Bruce Hooke]

pipefitter raised a good point. This is marine grade fir. It WILL check unless it is fiberglassed, which does then expose it to water intrusion via the checks. Even an epoxy coating alone (with fiberglass) won't solve the checking problem. Since you can't fiberglass, this is probably a moot point, but it is something to know about. An epoxy coating might at least cut down on the checking but as I said before, it will also probably be a royal pain to install if the boat is basically done at this point.

[Thorne]

One alternative to consider might be either CPES or thinned epoxy on the inside, if it would be too difficult or expensive to fully epoxy the interior.

If you use CPES, be sure to do it outside on a breezy day, wearing a good respirator with new organic filters -- the solvents in that stuff are nasty!

[Tylerdurden]

I saw an 24' runabout I built abut 7 years ago the other day. The owner has beat it hard with little maintenance. It is ply with glassed outside and inside thinned epoxy and paint (what little is left)
It sits outside all summer and looked pretty good considering the abuse. Only checking is where he clamped the outboard like Conan the Barbarian. Told me he wants another, I told him I was too busy.
I built a few on spec. and I can tell you it must kill the real boat builders when they see their work go to hell.

[pipefitter]

I saw an 24' runabout I built abut 7 years ago the other day. The owner has beat it hard with little maintenance. It is ply with glassed outside and inside thinned epoxy and paint (what little is left)
It sits outside all summer and looked pretty good considering the abuse. Only checking is where he clamped the outboard like Conan the Barbarian. Told me he wants another, I told him I was too busy.
I built a few on spec. and I can tell you it must kill the real boat builders when they see their work go to hell.


What kind of plywood? I used only epoxy above the waterline inside and out on BS1088 meranti and it is holding up well.The BS1088 will micro check in places but it is barely visible but the fir I have wouldn't have faired so well. It came from the yard with checks already in it.Fir ply checks as you are watching it. Even if epoxy coated. The scraps of fir left outside looks like sun baked mud whereas the meranti scraps still look great albeit discolored.

[donald branscom]

If you fibergass it inside and outside how will any moisture get out?

I would let it breathe. Just paint it inside or put a couple layers of sealer on it.

[Ian McColgin]

For moisture uptake in the plywood, the glass is irrelevant but epoxy is vapor permiable - moisture can pass in and out but the in-passing is very modest and not a problem.

CPES with paint over really does the trick for inside a plywood boat.

Glassing inside can create more problems than it solves as you're glassing odd surfaces around frames and such. It's all too easy to create voids where water can accumulate.

In my experience, CPES with paint over will prevent checking of plywood.

[pipefitter]

Possibly,Ian. There is many threads on the checking subject in the search. I had remembered reading about folks trying to subdue checking on cabin tops with just epoxy,warmed 5200,gummy paint etc. It was seemingly commonly agreed that the best way to subdue it was glassing it. DF ply checks which is why they use MDO for exterior roadway signs. I am sure the DOT could afford the very best paints and sealers if that was an option. I guess the original starter of the thread was more concerned of a structural issue and I suppose just sealing/painting it would suffice in his situation.

I just remember when I bought the small amount of fir for my own boat and noticing it was checked to start with,I did not want to chase any checking around or have different sections of my boat weathering differently. I merely glassed it for a more homogenized,predictable paint surface. I merely added the "O" to my fir ply.

[Solon]

Thanks to everyone for your advice. I think I'll try to get one or two good coats of epoxy on the inside, paint, and then take care storing the boat. Again, thanks for your input.

Solon

[Bruce Hooke]

In my experience, CPES with paint over will prevent checking of plywood.

If that is the case, and I trust Ian, then that is what I would do!

As Ian noted earlier in his post, trying to apply epoxy inside a finished boat is a problem-prone activity.

[Bruce Hooke]

If you fibergass it inside and outside how will any moisture get out?

How would moisture get in?

If it does get in it would get out the same way it got in.

[Cuyahoga Chuck]

The best approach is to epoxy coat the hull inside and out. If done properly very little moisture /microbes get in. To survive the wood eating microbes need water, wood and air. Cut off any one and they die.
Applications of glass have more to do with resistance to abrasion or the need to strengthen the hull to allow it to function at high loads and /or high speeds.
Exterior glass must have "fill coats" (usually three) so that the weave is buried. Epoxy is heavier than water so extensive use of it can make the weight go up quite a bit. Whether or not the encreased weight is worthwhile is up to you.
Stich and Glue boats have interiors that are fairly easy to glass. Boats built with a lot of interior structure are almost impossible to glass satisfactorily.
If the boat is human powered or uses only a small outrboard motor there isn't much need for glass, except on the bottom for protection against abrasion.

[pipefitter]

The best approach is to epoxy coat the hull inside and out. If done properly very little moisture /microbes get in. To survive the wood eating microbes need water, wood and air. Cut off any one and they die.
Applications of glass have more to do with resistance to abrasion or the need to strengthen the hull to allow it to function at high loads and /or high speeds.
Exterior glass must have "fill coats" (usually three) so that the weave is buried. Epoxy is heavier than water so extensive use of it can make the weight go up quite a bit. Whether or not the encreased weight is worthwhile is up to you.
Stich and Glue boats have interiors that are fairly easy to glass. Boats built with a lot of interior structure are almost impossible to glass satisfactorily.
If the boat is human powered or uses only a small outrboard motor there isn't much need for glass, except on the bottom for protection against abrasion.

I agree as well that glassing the interior of a boat would be a pain and add unnecessary weight. But it does stop plywood from checking pretty much entirely. I also agree that it would have it's place moreso on a stitch and glue boat,where the material thickness is considered with the added weight and thickness of the fibreglass.

[JimD]

I glassed the inside of a framed plywood boat once. Its not that much of a chore. You just have to cut squares of cloth to fit between the frames. The small pieces of cloth are easy to manipulate.