No butts about it ...

[brian.cunningham]

Well I see why CLC prefers scarfs to butt joints. I’m trying to align the panels for my outriggers, and I’m having a devil of a time doing it. I used fiberglass butt joints, where you butt the panels together and use layers of fiberglass tape to bound the two panels together. I wound up with a really stiff joint but that’s the problem, they’re too stiff. That’s why I couldn’t get the panels to torture correctly. Even when I reduced the beam of the bulkheads for the floats, the panels wouldn’t take the shape. The cross section would be terrific at the bulkheads. But the minute it came across a butt joint, it would flatten right out. That’s why that shear clamp scarf blew out, the butt joint is stronger than the shear scarf!

I used three layers of glass at each but joint. 2in 4in and 6in wide tape, so it’s feathered out. The end thickness is a great as the plywood.

OK give up on torturing the panels and go for a simple Vee shape. Well I’ve still got a problem. I followed standard practice and staggered the butt joints so joint they wouldn’t align on purpose

Port float
4 ft, 8 ft, 8 ft port panel
8 ft, 8 ft, 4 ft starboard panel

Starboard float
8 ft, 8 ft, 4 ft port panel
4 ft, 8 ft, 8 ft starboard panel

But the butt joints are flattening the panels out, and since the joints are staggered, the centerlines of the hulls are weaving back and forth like a snake! It’s not much 2 degrees, but over 20ft it really shows up.

I could jig the deck to force it to shape, but it will just pop back after it’s removed from the jig.

I could pull the panels apart, and stitch them together so the scarfs are aligned

Port float
4 ft, 8 ft, 8 ft port panel
4 ft, 8 ft, 8 ft starboard panel

Starboard float
8 ft, 8 ft, 4 ft port panel
8 ft, 8 ft, 4 ft starboard panel

This should result in symmetric hulls, but there’s no way they will match each other. On the other hand I’m only going to have one hull in the water a a time. But now, I have to figure out how much aligning the scarfs is going to weaken the structure, and then calculate what I need to add structurally ( read more weight ) to make up for it.

Thoughts?

[videoguy]

Brian I always use scarf joints on the dory's I build. But I rember reading in one of d paysons books he butts the two pices of ply togther and dishes out the joint useing the edge of a random orbit or a disk sander. puts on one layer of tape.
And after it sets flips it and does the same on the other side.I don't think it's nessary to use that much tape on the butts. Once you get the whole thing together you will be suprised at how strong the boat becomes.

[Mike Vogdes]

How bout grinding off say, half of the glass, do your torture, then reglass the areas.

[brian.cunningham]

Video, the ply I'm using is kinda thin. Neat Idea though.

I took another look at the joints, they could be smoother. I'll try thinner them out and see what happens.

[George Roberts]

Both scarfs and butt joints are stiffer than the virgin plywood. Both make flat spots.

There are several ways to resolve your problem. I will only provide one.

1) Use a single layer of 3 or 4oz glass on both sides of the plywood. Extend the glass as far as possible. Bend the plywood as soon as the epoxy is hard and before it is fully cured. Add more glass after the hull is the proper shape.

[ion barnes]

If you glass only one side, you will find too that the panels will bend more easily one direction than the other. I can not tell which would be better but...my guess would be with the glass on the outside of the curve. One layer only.

[Hughman]

Try this: partially set up your panels on the bulkheads, i.e. partly bent. Get a heat gun and soften the butt joint so it will take the stress as you finish the bend. I think you might have to use a support panel (to provide a fair radius) on the outside of the bend as it cools in its new shape. hope this helps.

[ 01-02-2003, 09:18 AM: Message edited by: Hughman ]

[brian.cunningham]

I tried it last night, sanded off a layer of glass. still way to stiff. :(

My outriggers are 20ft long, hence all the glass.
I still don't trust them being next to each other, so I'm keeping them staggered. But that's what's really causing the problem. I get fair curves they just don't match each other side to side!

I'll have to make up some sort of jig. I could use a strongback, but how would I pull it out after the deck is on? Perhaps a deckbeams at each buttjoint. Then secure it to a "kingplank" and pull it till it's symetric?

I'm putting hatches on it anyways, so the additional weight isn't that much.

[ 01-02-2003, 03:19 PM: Message edited by: brian.cunningham ]

[brian.cunningham]

I got this from the multihull boatbuilder mailing list, seems to make a lot of sense.


Brian,
I presume you are using what is known as a "Dynamite Payson" joint. I fiddled with these on a rowboat about two years ago. They are stiff!. And if you try to make them less stiff by using less glass they will fail (with a loud pop!) under load.

The problem seems to be not only the higher modulus of the glass as opposed to wood but the fact that the glass is applied on top of the surface. This effectively takes the compression and tension loads farther away from the neutral axis - sort of an I-beam effect.

One solution is to hollow out the joint before laminating so the glass lays closer to the neutral axis. Be careful not to grind too much of the outer ply away or the joint may then fail at the edge of the glass. It's still going to be a guessing game. I suppose fiber orientation is a factor as well.

Doug

[Ian McColgin]

Start over with scarf joints. Maybe you can save enough plywood around the butts that you're only buying one new sheet but whatever.

By the way, I never observed a hard spot from a proper 8 or 12 to 1 scarf joint well made with a thin glue line.

It's a pain and you'll have to use the wasted ply for something else - there's always something else - but you want it right. Butt joints only work where the bend is unimportant.

G'luck

[Armedmariner]

I'm building a lumber yard skiff (www.oldwharfdory.com) and it is supposed to be done with butt joints. I did it with butt joints and they opened up on me as I was winching the sides in. I then put 6" wide 24 oz biax cloth on them and that did the trick. Very stiff joints though.

From now on, I promise you, I'm scarphing my plywood. There's no reason to butt join plywood panels. I am kicking myself for not scarphing to begin with even though the plans called for butt joints. Oh well, live and learn.

Sam Devlin told me in his book to scarf and how to do it but I didn't listen. I thought I was smarter than him I guess. Dummy me.

[brian.cunningham]

I did the jig thing the other night, temporay setup, just to see if I could jack the hull back into shape.

IT WORKED!! Yipee!!

Now I have to come up with a more perment setup.

I'll have to fillet and tape the keel, and see how much it changes once the jig is removed.