Crossbeam Design / Loft / Cut

[brian.cunningham]

Well I finally finished my design for SWIFTWOOD's crossbeams (akas). Yippee! After doing some CAD work on them, I went back to making models, as this is a better way of seeing how the ply will curve.

I was going to do simple tapered hollow crossbeams, with mounts that rise up to meet them, but after running the numbers, it was lighter to have the beams curve down to meet the floats (amas). Looks like the ancients had it right.

The final design is a tapered gullwing, 18ft wide, 3ft tall, with 28in of wave clearance. They're 16in tall at the kayak and taper down to 8in at the floats (amas), the depth is constant at 1ft. The front beams are triangular in cross section. This gives the front face an angle to hit the waves with. The aft crossbeams are located all the way at the transoms, so I gave them a rectangular cross section for strength. The front beams are a lot trickier to design, as some of the surfaces are conic sections.

Unfortunately the digital camera I have can't really take a good picture of the models, but I'll have pictures of the real thing soon, so no matter.

I lofted the side pieces out tonight on the plywood, and after cutting them out the port and starboard sides matched.

[brian.cunningham]

Update,
I'm getting a lot of checking while cutting out some of the pieces. The curves pieces are hardwood plywood, but the straight pieces are exterior grade plywood, those are the ones that are checking.

I left an 1/8in when I cut them out, I can plan then down when I go with the grain, but the minute I go accross the grade it checks.

So the gameplan is to go ahead and give them a coat of epoxy. I wanted to wait so I didn't use as much epoxy. Of course, since I'm staining the exterior grade plywood to match the hardwood plywood, I have to stain it first.

I've did the ones I've already cut out last night.

[JimD]

I know its against the law, but tried a belt sander? Its my most used tool, great for fairing, especially plywood.

[brian.cunningham]

I thought about sanding it down.

The wood is so soft, I guess it wouldn't eat up too many sanding pads.

[brian.cunningham]

Well, one thing's for sure, epoxy seals the wood.

I stained the wood outside, and waited for it to dry before taking it in. It was dry, but the stain still stunk to high heaven. Once the epoxy was on it though, all that smell went away.

http://www.imagestation.com/picture/sraid107/p55422ec3f75e058ed76607eaedf26daf/f958d932.jpg

[brian.cunningham]

Ah, the fun of working in limited space.

Nothing like coating 6 sheets of plywood with epoxy one side at a time to build character smile.gif

[brian.cunningham]

I hate head colds. :(

My head has been so stuffed up I haven't been able to work on the boat at all.

[Ken Hutchins]

Brian, head cold, or is it a reaction to the epoxy?

[ 04-14-2004, 08:52 AM: Message edited by: Ken Hutchins ]

[brian.cunningham]

I'm one of those guys who practically use a space suit when I work, and seeing I haven't use epoxy in a while, head cold.

It's starting to clear up.

[brian.cunningham]

Cold clear up enough to get back to work on the boat.

OMG big difference between cutting plywood with the saber saw that's been coated with epoxy, and plywood that's not been coated, hardly any tearout.

It's a little harder on the blade, you need to reduce speed and go slower, but it's worth it.

[brian.cunningham]

Worked outside yesterday.

Nice having all that room, and being out on clean fresh air, instead of down in the basement.

Got all the side pieces cut.

Here's 2 of the gullwing sections, 19ft tip to tip.
http://www.imagestation.com/picture/sraid113/p1f65349846b453fe0e4b9640d27d313d/f8eb5931.jpg

Each of these were cut from a single 4x8 piece of plywood.

These two will be joined by a foot wide strip of ply to form the aft crossbeam.