"real" wood transom on ply skiff ?

[BrianY]

I'm builiding a self-designed row/sail skiff that will have ply sides and bottom. I have built a transom out of solid doug fir (3 VG 7/8" planks edge glued) mostly because that's what I had laying around, but now I'm questioning whether it's a good idea due to potential problems caused by the movement of the solid wood pulling against the non-moving plywood.

The boat will live on a trailer and will not be left in the water between uses.

My plan was to epoxy glue and screw the plywood sides and bottom to the transom. I'm wondering if I should a) use a different, more flexible adhesive like 5200, b) redo the transom in ply and forget about the issue all together or 3) just proceed as planned and stop worrying

Opinions?

[Thorne]

3.

;0 )

[Tar Devil]

3.

I second that...

[JimConlin]

To minimize the expansion of the solid transom, I'd coat it thoroughly with un-thinned epoxy.

[Rick Tyler]

What Jim said. And "3". You're not the first person to do this, and I haven't heard of any disasters. Keeping the transom at its current moisture content to the extent possible is a good idea.

I've always kind of wondered how the cross-grain connections at the stem and along the bottom of a transom work in a traditional planked boat. Do planks split or bulge out because of the cross-grain sometimes?

[Jeff Benagh]

Go with #3. I built my Shellback that way but with mahogany. Absolutely no problems 5 years later:

http://jbenagh.smugmug.com/photos/339675964_EvdAL-L.jpg

[S B]

I've always kind of wondered how the cross-grain connections at the stem and along the bottom of a transom work in a traditional planked boat. Do planks split or bulge out because of the cross-grain sometimes?[/quote]
Can you explain this a little better? I don't understand what you mean by cross grain connections.

[Domesticated_Mr. Know It All]

I would think the edge of a solid piece of Doug Fir would hold fasteners better than the edge of plywood.
Doug Fir is pretty rot resistant too.
It's cheaper than mahogany or marine plywood.
It's fairly light too.
3M-5200 is forever, so if you ever need to take it apart I would use something else.

[JimConlin]

I would think the edge of a solid piece of Doug Fir would hold fasteners better than the edge of plywood....
The end grain of the DF is another story, so you're still dependent on the adhesive, so why complicate things?

[David G]

Brian,

Were I starting from scratch, I'd pick another, more stable, wood for my transom. However... VG fir isn't bad for your original plan. You haven't said what the moisture content of the fir is, nor how wide (high) your transom will be, so it's hard to gauge how much movement will actually occur, but I'm guessing it'll be fine. Besides, if this is a self-designed boat, it's a bit of an experiments anyway, right?.

Did you consult any of the good books out there on constructing plywood/epoxy composite boats? Most especially, do you own a copy of The Gougeon Brothers On Boat Construction? It's a great resource. For instance, if you're concerned about the strength of your screw attachment - as mentioned above - they show you how to sink your fasteners into a pocket of epoxy (they call it an annulus), which will substantially reduce the chances of the fir stripping out or the screw pulling out.


"As a general rule, the most successful man in life is the man who has the best information" -- Benjamin Disraeli

[esingleman]

I believe doug fir hydrodynamically moves more than Mahognay especally plain sawn boards. Also edge screwing into plywood is fine as long as the screw is long enough and the hole is drilled tothe right size. I remember someone posting that they saw small cracking on a lapstrake boat near the transom of their solid transom boat when i asked the same question in a post one time. Not sure what the long term affect of these small cracks would be on a trailered boat. I am thinking they would dry out easily and not be a rot trap.

Option 1) do it over in ply
Option 2) let it go and either make repairs later, or maybe there are no repairs to be made.

i am pretty sure ther are plenty of solid transoms out there with dimensionally stable planking surrounding them, i just don't know if doug fir's movement makes it a problem.

[Rick Tyler]

Can you explain this a little better? I don't understand what you mean by cross grain connections.

Sure. If you make a table top, for example, with a breadboard end, you end up with a situation where the endgrain of the boards that make up the top are butted up against the sidegrain of the boards that make up the ends. In this cross-grain situation, the planks in the top will tend to split as they shrink and swell with changes in humidity if they are glued in place. Breadboard ends on a well-made table will actually float, so they don't split. I've seen tables in winter (dry) in Massachusetts where the breadboard ends stick out a half-inch farther then the edges of the table. This is because (as you already know) wood swells and shrinks a LOT more across the grain than along it.

So, in a planked caravel hull when the planks are fastened along the bottom of the transom and into the stem you have a classic cross-grain situation -- the planking lands across the grain of the transom or stem. Do planks on boats ever have trouble with splitting because of this as they dry and get re-soaked? I would guess this would be a lot more common for boats on the dry than in the water, if it happens at all. I'm not talking about planks pulling away from each other, but splitting along the plank. My guess is that it doesn't happen much because the planks are fairly narrow, the fasteners move slightly, the glue (if any) is not rigid like epoxy but flexible like beading compound, and most caravel planked boats live in the water, not on land. If splitting is a problem, it would tend to happen more with wider planks.

[S B]

Sure. If you make a table top, for example, with a breadboard end, you end up with a situation where the endgrain of the boards that make up the top are butted up against the sidegrain of the boards that make up the ends. In this cross-grain situation, the planks in the top will tend to split as they shrink and swell with changes in humidity if they are glued in place. Breadboard ends on a well-made table will actually float, so they don't split. I've seen tables in winter (dry) in Massachusetts where the breadboard ends stick out a half-inch farther then the edges of the table. This is because (as you already know) wood swells and shrinks a LOT more across the grain than along it.

So, in a planked caravel hull when the planks are fastened along the bottom of the transom and into the stem you have a classic cross-grain situation -- the planking lands across the grain of the transom or stem. Do planks on boats ever have trouble with splitting because of this as they dry and get re-soaked? I would guess this would be a lot more common for boats on the dry than in the water, if it happens at all. I'm not talking about planks pulling away from each other, but splitting along the plank. My guess is that it doesn't happen much because the planks are fairly narrow, the fasteners move slightly, the glue (if any) is not rigid like epoxy but flexible like beading compound, and most caravel planked boats live in the water, not on land. If splitting is a problem, it would tend to happen more with wider planks.
I'm sure that there is some, but how much plank spliting could be attributed to this,is impossible to determine. Traditional planked boats are at their greatest peril to cracking within the first few weeks of leaving the shed. They are usually babied through this period. Once hardened in ,cracks allong the stem or stern would be rare. Now the inside of the boat is another story, with the frame and the plank wet,the sun could split the ceiling to pieces in short order, if allowed to dry. That is why most small open caravel planked boats have water in them, there not leaky, it was thrown in to equalize the moisture content in the wood.

[pipefitter]

You should be fine. For one, it's trailered and another is, how much of the stern really gets that wet on a small sail/row skiff? I thought it ideal to not drag the stern with rowing and sailing boats? My boat has solid mahogany floors in it. By what I am hearing here, they should have exploded the seam at the chine and I am positive the bilge of my boat sees much more water than your stern will. During one of the busy hurricane seasons during the build, it had gotten about 5" of water that had leaked past the poly tarp and it was in there for a nearly a week before it occurred to me to raise the hatches and have a look. There wasn't even a water or wet stain on the plywood that was only sealed with 2 coats of epoxy.
Erster, who posts on here, has some method of using both ply and hardwood veneer and changes it up somehow at waterline. Perhaps you could put the plywood in flush with the ends of the frames and let your hardwood veneer be allowed to float past the plank ends on the outside if indeed you think they will grow that much.
There's people who build boats that float out of crappy luan plywood and they are holding together with a little common sense when according to the books, it should self destruct in humidity alone. I think most of these ideas are subject to worse case scenarios that aren't really prevalent in modern times with all of the considerations of how we store and maintain our boats. Very few are ridden hard and put away wet. You would have to be out in it for days at a time, in the rain for any above waterline parts to experience any such extremes of moisture absorption. With that said, most trailered boats will have better care and see lesser extremes than the outside of your home will.