How Do You Scarf 12mm Ply Bulkheads?

Thanks, guys. I'll try that.

I guess I'm just worried that my half bulkheads won't line up perfectly when the two hulls come together. More needless worrying?

Probably, huh?

 

Do you have to glass them in completely or can you just tab the outboard edges. That would give you a bit of slack.

 

Hoyt: There can't be "pre-scarfed" bulkheads. Each hull is made by making a half a hull like you see in the picture in the first post. Each of these half hulls like in that picture will have half bulkheads installed in them while still in the mold. Then, two half hulls are joined together at the same time the bulkheads are joined together, all at once. A half hull with half bulkheads will be sitting in the mold and I'll lower another half hull, with its own half bulkheads down onto the one in the mold, joining the whole thing at once.

Unless I'm misunderstanding what you mean by "pre-scarfed" I don't think you are taking the method of having half bulkheads installed in half hulls before joining them into account in this post. Or... maybe I read the post wrong.

 

Good question.

You're supposed to glass and cove them in, but tabbing might work.

The main point of having them there before those halves come together is to support what is essentially, just a bunch of Corecell with glass only on the inside. That's all it is when it comes out of the mold. It's a limp noodle.

The bulkheads are put in place early to keep the shape of the half hull when you are moving it around in and out of the mold. I plan to also run a longitudinal beam from bow to stern, attached to each bulkhead, to keep the bulkheads in position so the hull doesn't fold up in the middle.

 

Can you put the whole bulkhead in one half and then lay that over the half hull in the mold. You could then tab it all together. It seems like the designer is not too clear on the method of construction.

 

The designer has very few instructions on how to build the boat. This is a big annoyance to me as an inexperienced builder. There is a set of plans and about 5 pages of how to set up your mold and put in strip planking or vertical foam. Then... he just trails off and there is little instruction.

Your idea of doing one half with full bulkheads would work...

The order of manufacturing is:

Port hull, inboard half - could add full bulkheads
Starboard hull, outboard half - could add full bulkheads
Port hull, outboard half - drop port hull inboard half with bulkheads onto it
Starboard hull, inboard half - drop starboard hull outboard half with bulkheads onto it

Wow! Problem solved, I think. Argggg... except I don't have the lift height on my indoor cranes to lift a half hull with full bulkheads up and over the mold with an existing half hull in it.

I'll have to think that one through a bit, but maybe there is some way...

 

Got it, I think:

How about just butt joining and taping the bulkheads that are only in one hull? As in, I have a half bulkhead on each half hull, but instead of scarfing them, I do the butt joint and taping. This make sense because these in-hull bulkheads are under compression and tension loads, not sheer loads.

Then, when it comes to the main strength bulkheads that run from one hull to the other hull, I can build those outside the boat and use scarf joints. I can then lift them up with my shop crane and into place when the two already-joined hulls are in place, but before I put the deckhouse on.

That'll work!

 

Use the Payson butt joint or the modified Payson butt joint. It's as strong as a scarf and doesn't require careful machining. On 1/2" plywood you'd use 3 layers of 12 ounce biax (45/45) on each side. 6" wide tape will be more then enough, stagger the over laps and you're good to go.

If you use the modified version (shallow depressions for the 'glass) then you can make perfect seamless joints.

 

Wow, that's a pretty good butt joint, Par. Thanks. I could definitely use that on the bulkheads.

Can I ask a theoretical design question about the Payson vs a scarf?

The scarf joins each and every ply in a sheet of plywood to its respective member on the sheet your are joining it to. I think that's the idea of it. Each ply lines up and bonds with its counterpart on the other side of the joint.

According to the picture of the Payson Butt Joint I found searching that you put up, wouldn't the Payson be bonding all but one or two plies in the very core of the sheet? Those one or two plies would still be un-joined and just "along for the ride" in the joint, right?


Link to Picture of Payson Butt Joint


You don't really have to answer if you don't want. I'm just curious why my designer is a scarf joint fanatic.

It seems the Payson Butt joint is the closest thing to a scarf that I've ever seen though.

 

Concur. Given the method described, butting the vertical joint and glassing is incredibly easy. Your designer may have something else in mind but maybe he expects you to scarf two halves together in situ, which is absurd. While it could be done, the job would be extremely time consuming and as PAR says, a glassed butt joint is as strong as the scarf---- and besides, a bulkhead wouldn't be stressed like an outer hull panel. Any force big enough to compromise a taped joint on a bulkhead would already have pretty much destroyed the hull anyway. Especially if there is no opening in the bulkhead.
Regarding lining the two sides up and the risk of misalignment, I can promise it will happen if you have already attached both halves of the bulkhead, so I'd wait to tab in the second side. Once the two hull halves are together, fit the 2nd half bulkhead fore and aft to line up perfectly and clamp a straight edge across top and bottom, using temporary screws. Then spot glue both bulkhead center joint and bulkhead to hull sides and when solid, cove and tab the works.

 

If you read the text along with that image post you'll find how it's made. Since that post I've modified it further by back cutting the two butting pieces, slightly. This lets epoxy down into the center of the joint and it's also how I do fillets. It serves to seal up the end grain and tie all the veneers together in one, homogeneous epoxy/fabric bond.

Why your designer is married to scarfs, is likely why I was for many years (read decades), I wasn't well versed in the Payson butt joint yet. I've tested this joint (as have many others) and it's bullet proof if you use enough fabric and over a large enough area. If you attempt to break it, the plywood fail, not the joint. Biax is preferred, as it more closely matches the elongation properties of epoxy, so both hold on right up until failure.

Send me an email and I send you one of the instructional sheets I supply with all my plans, which covers fillets and Payson joints.

 

That looks really solid!

I guess something beats a rabbett, after all(but a rabbett is still great in other less demanding applications ).

 

Learn something new every day. Cat , cyanide gas sounds bad, will I have the same problem with divineycell if I have to heat it.

 

Or if you cut or machine sand it.

 

You need room with forced air ventilator.