Bulkhead Joinery

[Peter]

In gutting my Albin Ballad, I discovered the starboard portion of the main bulkhead is too wide to exit from any of the openings in the boat. So I'm faced with a choice of either making a larger opening, or splitting the bulkead in two ... easy choice!



When it's time to replace the bulkhead I'll need to make a joint in it to get it through the companionway. Gut feeling says make the joint a vertical one, as close to the centerline of the boat as possible. This bulkhead, and the one ahead of it in the head, support the cap shroud and foward lowers chainplates. It's through-bolted to a heavy flange glassed around the inside of the hull, and on either side of the cabin with aluminum brackets. Port and starboard bulheads are joined at the top with a bolted wooden beam. (As the Ballad has a keel stepped mast, this beam is not as heavy as a deck-stepped masted boat would be.)



My question is: What kind of joint do I make in 3/4" plywood that'll look good and be strong? I want to avoid an ugly overlapping gusset type joint ... maybe a 12:1 scarf would be best.
Any and all thoughts on this minor dilema are very welcome :-)

[ILikeRust]

I would try to find a way to hide the joint with some kind of trim element. Make it look like frame-and-panel joinery or something.

I think it would be difficult to hide an edge joint, particularly if it's going to be finished bright, but depending on what kind of plywood you'll use (e.g., mahogany veneer plywood, with its grain, might actually help hide a joint line), if you're very careful about jointing the edge very square and straight, you might be able to do a standard edge joint, as you would in making a table-top, and have it virtually disappear. If you want, you could reinforce it with dowels or biscuits.

What complicates getting a good hidden joint is the fact that you'll have to assemble it inside the boat. That makes it extra tricky. It often is challenging enough getting good, tight, invisible joinery when working in a comfortable workshop at a well-lit workbench. I dunno about crawling around inside the belly of a stripped-out sailboat!

Here's a thought - to facilitate the assembly of the joint inside the boat, maybe make a tongue and groove joint. Just make the tongue a hair shallower than the groove, so that the edges of the joint bottom out on each other, leaving as little a joint line as possible. If done right, you could make the joint disappear.

Either that, or as I said at first, find a way to cover it with a trim element - a thin strip of solid wood.

[Hirilondë]

Scarf joints are for materials that will be loaded by bending or other such forces that will try to break the joint. This is not the case with a bulkhead. I don't see any need for a scarf

What complicates getting a good hidden joint is the fact that you'll have to assemble it inside the boat. That makes it extra tricky. It often is challenging enough getting good, tight, invisible joinery when working in a comfortable workshop at a well-lit workbench. I dunno about crawling around inside the belly of a stripped-out sailboat!

This is an understatement. Ain't boats fun? ;>)

Here's a thought - to facilitate the assembly of the joint inside the boat, maybe make a tongue and groove joint. Just make the tongue a hair shallower than the groove, so that the edges of the joint bottom out on each other, leaving as little a joint line as possible. If done right, you could make the joint disappear. .

I think I would go this route. Make the joint and fit it outside the boat. Test fit the whole thing in the boat. Practice the whole install/join process without epoxy. Then when you are happy with the fit and comfortable with the process complete the glue up.

[Triton106]

Peter,

Looks like you are moving up in size. I have a crazy out of box kind of thought - it looks like you completely gutted the interior. What is stopping you from decoupling the hull and deck? It will not only enable you to get the bulkhead in in one piece but will let you rebuild and strengthen the hull deck joint. Like I said, just a crazy thought.

Best of luck,

Ray

[Peter]

Looking at Bruce Bingham's "Boat Joinery" he suggests making a bulhead from two layers of material and staggering the joint at least 4 inches, This sounds interesting until one looks at the problems of clamping two large flat surfaces together, and all the epoxy involved.

Ray: As far as separating the hull/deck joint ... I don't really want to go there, although I admit, it did cross my mind!
It's a pretty solid joint and the sealer that was squeezed out on the inside of the boat is still nice and pliable. Also, there is no evidence of leakage anywhere.

Yes, I'm leaning toward a toungue and groove joint, or rather a matching 1" deep groove in each panel and a 1 7/8" hardwood piece to act as a toungue. And a practice run on the old bulkhead might help!

[Hirilondë]

Yes, I'm leaning toward a toungue and groove joint, or rather a matching 1" deep groove in each panel and a 1 7/8" hardwood piece to act as a toungue. And a practice run on the old bulkhead might help!

I was thinking about using a spline instead just after I hit the submit button yesterday. Plywood does not make a very good tongue. The basic rule for splines is that it should be double the thickness of the material in width or a little more. Assuming your bulkhead is 3/4" plywood your specs above are good. It should be about 1/3 of the thickness of the material in thickness, so 1/4" for 3/4" plywood. Not only is having the depth of the 2 grooves (slots) a little extra a good idea, but having the fit of the spline in the groove a little loose. Not sloppy loose, but just a little room to assure you don't force all of the epoxy out from the sides of the spline during assembly. As with all gluing operations it is best to wet out not only the grooves with epoxy, but the spline as well before assembly. You don't need overly thick applications because of this. Even though you would be using enough epoxy just wetting out the plywood, wetting out all the surfaces assures a better penetration into the wood than having any piece/mated-surface dry during assembly.

Wear old clothes, gloves, hat and anything else you don't mind sacrificing. Even a neatness freak won't escape from this project unscathed.

[Quetzalsailor]

I'll cast my vote with the prefitted spline guys.

It occurs to me that you could cobble together a clamping jig/table such that you could assemble and glue the components in the boat. The halves don't have to be in place in the boat to be glued. They'd be hard to hold, hard to glue, hard to clamp when in place. You'd want to think this through so that you'd know that the assembled bulkhead pieces could be moved into place.

[galleywench]

Never done this on a bulkhead, but what about using biscuits to join the bulkhead. I don't think bulkheads are subject to shear or bending stress that would pull a biscuit joint apart and they are super easy to do even with a cheap $50 joiner from Home Depot or Lowes. I would think that given the poor work conditions found in the belly of a sailboat, this would be even easier to do in place than setting and epoxying a spline. The spline method would also work well, but there is a bit more 'machining' to do on the tablesaw and making sure your spline is the right size to fit the joint.

[ILikeRust]

I think either biscuits or a spline would be fine. I don't see the spline as being any more difficult or complicated than the biscuits, though. In either case, you're going to do the fine-tuning and trial fitting of the joint in the workshop, at the bench.

My own preference would be for a full-length spline rather than biscuits, but that's just me. I can think of at least two different approaches for cutting the grooves on the plywood edge: (1) tall fence on the table saw - a little more awkward, because you have to hold the panels vertically as you feed them over the blade, and keep them perpendicular to the table, but it's doable; or (2) slot-cutting bit in a router or router table. Method two would be easier to get more consistent results, because you lay the panel flat and either slide the router along the edge or slide the piece over the router table. The only thing there is that you're more limited as to the depth of the groove than you are with the table saw, but you don't really need much. I would think a 1/2" or 5/8" deep groove on either side should be fine.

I'm assuming you're using 3/4" plywood for the bulkhead panels. I would cut a 1/4" groove and use 1/4" plywood as the spline. 1/4" plywood spline will be much stronger than a 1/4" solid wood spline and far less prone to movement with humidity changes.

[Hirilondë]

I think either biscuits or a spline would be fine.

I disagree. I don't think biscuits have any use on boats. There are a couple issues at least with them. First of all they are designed to be used with water based glues as the water in the glue swells the biscuit to go from loose to a tight fit. This is how biscuits align the pieces being joined. Using water based glue and epoxy in the same joint will likely cause bubbling and over heating of the epoxy in the vicinity of the biscuit. Biscuits themselves are far less strong than wood and are used in intervals instead of the entire length of the joint. Using epoxy for the biscuit pretty much defeats the whole point of biscuits. I have been tempted to use biscuits on boats before, but I just can't think of a single application for them.

[galleywench]

I disagree. I don't think biscuits have any use on boats. There are a couple issues at least with them. First of all they are designed to be used with water based glues as the water in the glue swells the biscuit to go from loose to a tight fit. This is how biscuits align the pieces being joined. Using water based glue and epoxy in the same joint will likely cause bubbling and over heating of the epoxy in the vicinity of the biscuit. Biscuits themselves are far less strong than wood and are used in intervals instead of the entire length of the joint. Using epoxy for the biscuit pretty much defeats the whole point of biscuits. I have been tempted to use biscuits on boats before, but I just can't think of a single application for them.

Your right, I stand corrected. Strike suggestion.

[mitiempo]

I'd route a 1/4" groove in each piece and use 1/4" ply, cut so the outside faces are perpendicular to the join and use epoxy. The trickier part is making the join invisible.

[Peter]

OK, the "Splines" have it. Yes, a good piece of 1/4" ply would be fine. Maybe sand the spline a bit to make it a loose fit in the groove, use lots of thickened epoxy, Saran wrap and masking tape on both sides of the joint, and clamp it flat between two strips of plywood .... and hope the faces are flush.
I can do the gluing down below on a couple of saw horses, with a plywood sheet as a table top. There's lots of room down there right now. Next job: find some nice Sapele or ribbon grain mahogany ply.