Lap joint vs butt joints for laminated stringers?

[PiratePast40]

Doing a complete rebuild on an 18' I/O runabout and wondering about acceptable methods of building laminated stringers. Will be using 3/4" marine ply and laminating with mat between the layers. Since the stringers will be about 14' long and the plywood sheets are 8' long, will need to make some joints. Will stagger the joints but wondering if I need to make lap joints or are butt joints acceptable in this situation?

 

[92excel]

Re: Lap joint vs butt joints for laminated stringers?

probably fine either way since you are staggering and doubling...

but why not spend the extra 10 minutes and just do a lap joint for that extra peice of mind... just my opinion...

 

[Utahboatnut]

Re: Lap joint vs butt joints for laminated stringers?

I would for sure go the lap joint much stronger and less chance of a kink where a butt joint would be. Plus you can use the cutoffs for other stringers and the joints will be staggerd and will be able to make better use of material.

 

[matthewp]

Re: Lap joint vs butt joints for laminated stringers?

A recent woodworking magazine (fine woodworking, I believe) rated various woodworking joints on their breaking strength. The half lap was rated as the strongest. Surprised me, I thought mortise and tenon would have been tops. I think it has to do with the amount glue surface. I'd go in for the half-lap. You're only doing it once, so....

 

[PiratePast40]

Re: Lap joint vs butt joints for laminated stringers?

Well then ... half lap joint it is - I'm convinced. Thanks to all of you.

 

[erikgreen]

Re: Lap joint vs butt joints for laminated stringers?

Most woodworking magazines don't deal with composite construction using large amounts of epoxy, FYI..

If you have to create a joint in a stringer, as long as there's some material overlapping the joint and you use epoxy with thickener so it doesn't run out of the joint, it'll be stronger than the rest of the stringer.

A lap joint doesn't make sense in plywood stringers because you're losing part of the thickness of the wood to get more area to join, and the main function of stringers is to be stiff. Stiffness is a function of thickness.

Lap joints are ok in dimensional lumber because they look neat, but they're weaker than butt blocks or laminated splices with fiberglass... two types of joint I bet you won't find in most woodworking magazines

If you have two pieces of laminated plywood to join end to end, use a butt block. If you have four pieces that make two sides of a stringer, cut them so you have two butt joints at offset locations and laminate the halves together. It's like having one big piece of engineered lumber in stringer shape.

Erik

 

[jonesg]

Re: Lap joint vs butt joints for laminated stringers?

A scarf joint is another way .
The longer the scarf the better.

 

[sschefer]

Re: Lap joint vs butt joints for laminated stringers?

Scarf it.. 30 deg is optimal. There's nothing to be gained with lap joint using plywood. When you laminate it you complete the joint with a sister, (the piece you are laminating to), same effect as a lap joint but stronger.

 

[J. Mark]

Re: Lap joint vs butt joints for laminated stringers?

Keep in mind that the wood in stringers, while providing some strength, is really a form for the fiberglass. That's why hollow stringers and foam filled stringers are used. Make a secure joint that is well glued and don't worry about whether a lap joint, half lap joint or butt block is best. Do a good job wrapping the stringers and you could get away with using balsa end glued with elmers.

Don't sweat it, just space the joints so there is no overlap and use a good glue and proper gluing technique and it will be just fine

 

[AZSenza]

Re: Lap joint vs butt joints for laminated stringers?

Keep in mind that the wood in stringers, while providing some strength, is really a form for the fiberglass. That's why hollow stringers and foam filled stringers are used. Make a secure joint that is well glued and don't worry about whether a lap joint, half lap joint or butt block is best. Do a good job wrapping the stringers and you could get away with using balsa end glued with elmers.

Don't sweat it, just space the joints so there is no overlap and use a good glue and proper gluing technique and it will be just fine

What ever you do I'm SURE it will be stronger than original.

 

[PiratePast40]

Re: Lap joint vs butt joints for laminated stringers?

Wow - lots of opinions and techniques. I really was a little concerned about doing a lap joint on plywood - essentially delaminating, and then sistering up (laminating) again. I've got it in my mind the idea that when the boat is accelerating, the motor is lifting up the transom and the transom is lifting up the stringers (and the rest of the hull) like a ladder. If I look at it that way, then the 2' joint (8' piece plus another 6' piece to make the 14' stringer and flip them over the other way on the sistered side) would be a stronger joint than a 5-6" halflap joint.

IMO - a dumb question on my part turned out to be a pretty good discussion. Or maybe it doesn't matter that much either way. Just want to "git er' done" and go fishin'!