Screw and Glue questions

I am about to commence building a custom glass over ply experimental multihull. I'm planning to use solid timber keel and stems (something light and strong - probably spruce but am open to suggestions). Then 'persuade' the ply in both planes/axes.

I'll screw and glue (1/2") ply sides to 'keel'. I'm thinking of using #6 ss screws (3/4 or 1"), like one every 6".

Some stitch'n'glue proponents who use screws for glue clamping remove the screws - why?

I am wondering about SS alloys for the screws. 18-8 is standard. 316 is 'marine', what is 410? In any case, I was planning to encase screws and timber in epoxy.

Predrilling is a pain, I found some nice self drilling screws :
/Sheet_metal_screws_Phillips_flat_head_self_drilling_Stainless_steel_410_6.aspx - self drilling.

I've also found a source for 316 SS ring nails.

Any comments on any of this?
thx.

 

If there is enough water intrusion under the laminate to cause a problem with the stainless screws or nails, crevice corrosion will be the least of your problems. Well painted steel lasts for decades in ship construction, so don't worry too much. 18-8 is 302 or 304 stainless. 316 is more corrosion resistant and harder; particularly the passivated type. 410 is harder and usually used for cutlery.

 

Gonzo, thanks.
so are you saying don't bother with stainless? Regular steel or ceramic coated decking screws would be fine if sealed?

 

There is some moisture in the wood that will eventually rust regular steel screws. Stainless, even 18-8 will provide adequate corrosion resistance under epoxy/fiberglass. The cost difference is very small.

 

thanks! good succinct info!

 

You're welcome. Post photos

 

Your description of torturing the plywood works, but is quite limited. Your screw sizes and attachment approach suggest no experience with this technique, as even a little torque on a 6" screw from "screaming" 1/2" plywood will easily pull the heads off, break the screw or have it rip right through the plywood. Temporary fasteners, typically over pads will do a much better job.

Yeah, pilot and clearance holes are a pain, but if you don't, you'll quickly realize why you need to, in the first place.

S.S. screws bonded (technical term) in timber or plywood will not rust, assuming the bonding remains in tact. Bonding is not simply coating the screw with some goo, driving them home and hoping for the best. You're best advised to download the free user's guides from westsystem.com and the epoxy book from systemthree.com. Read them over good, as they'll offer the basic techniques for taped seam construction and working with goo's.

 

Hi Par
points well taken. You're right, though I've got 40 years of construction diverse materials, precision engineering and boat experience, I have less experience with persuading ply. I avoid the term torture as I do not intend to put extreme stress on the ply - enough to gain the strength of bends in both axes, but not enough to risk the kinds of failures you describe. My instinct to use low size screws with big heads addresses this and hopefully mitigates failure along screw lines.

 

Small diameter screws with button, pan or truss heads, will just force the failure in the shank instead of the head. A #6 is a small screw for 1/2" plywood

 

410 stainless is high strength so I'd imagined I could reduce diameter one size and maintain strength.

 

Lots of people have tried to "torture" plywood using various means of force and broken their ply.

Listen to PAR and get the Gougeon book. They have successfully built fairly large multihulls in this manner, and there is a whole chapter on how to do it.
No where have I seen a solid wood keel with this method. But I haven't seen as much as PAR has.

http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf Chapter 25
FREE after I paid for 2 revisions!

 

Thanks! I _am_ listening. And I have the Gougeon book. And most of the others: Bolger, etc.
"No where have I seen a solid wood keel with this method."
I know, odd isn't it? Seriously though, I'm not thumbing my nose at tradition or experts, just having my own design thoughts. Its not really a 'keel', its an oblique rabbetted timber running about half the hull length midships along the keel line whose main purpose is to hold the side panels at a set angle (about 120 deg) prior to bending (but will provide soem longitudinal rigidity. I'm going for something between a semicircle and a deep V.

 

Are you going to do a scale model to avoid potentially breaking the plywood?
I'd like to see the model or any plans you have.

I guess you could equate the wood keel to a fillet and glass as used on a Tornado.
What about the bow and stern? What shape?

1/2 " ply must mean a large boat??

 

"Are you going to do a scale model to avoid potentially breaking the plywood?"

making a 1/4 scale model in coming days/weeks. Will post pics.

"What about the bow and stern? What shape?
1/2 " ply must mean a large boat?? "

full disclosure - 30' proa vaka. So double-ender. The general plan is to join side panels (30'x4' scarfed sheets) along the keel line midships to the 'keel piece' (about 10' long), setting the midships keel angle.
Then bend sides vertical at gunwales midships.
Then bend sides almost vertical at both ends and fasten to solid timber prow pieces. Prows/stems will be slightly flared to provide more reserve bouyancy. Then fillet the 'keel line' between keel piece and prows.

Goal is an easy-build, light and strong fast cruiser with some interior space.

 

I just don't understand why you're trying to reinvent the wheel, particularly with a much heavier than necessary approach. Given what you're building, light and stiff is what you want, so toss the centerline keel and simply tape the centerline seam, back it up with a fillet and maybe an internal batten if necessary. Use wire ties, copper wire or other "ties" if you must, though I don't very often any more, then "unfold" the bottom panels and/or side panels. Prop open and continue the filleting process. the net result is lighter and waterproof.