Proper screw size?

What size & length screw should I use to attach the 1/4" plywood sides and 3/8" plywood bottom of my skiff to the 3/4" wide Doug Fir frames ? The screws will be there to hold everything in place while the epoxy glue dries, so they won't be the only thing keeping the boat together. I plan to use bronze screws so that I can leave them in place set just below the surface an covered with epoxy filler.

Is there a rule of thumb for determining the proper screw size and length?

 

Well, if you don't want penetration of the tip into the boat, you are looking at 7/8" maximum, with 3/4" being the more prudent length. I would go with no larger than #10 and predrill the hole but don't counter sink the head, let the plywood crush down to fit (protecting the edge grain and keeping the ouside layer involved). If you use 7/8" you can buzz the tip off a bit to reduce the length (predrilled holes only) while getting more full width bite.

Steve

 

An Option to omit fastenings

The other alternative is to omit the fastenings and save on the budget.

Where there is adequate bonding area at the joins between ply panels through filleting or gluing to chinelogs, centreline structure deck clamp carlins etc there is no real necessity for fastenings at all.

About the only place I use them is where there is a timber member attached to the hull that is only bonded on one face and so could split away from the structure (gunwale or bottom skid) so some fastenings are a useful split preventative. But where two faces of the member are glued to plywood there is almost no chance of the member splitting.

Glassing will also prevent such members from splitting from the hull - so you can take your choice of fastening or glassing. Generally underwater members I would glass and visible ones that have an aesthetic contribution and will be left clear finished (gunwales) - I would screw or bolt.

Multis up to 55ft, Powerboats up to 50ft - dozens of smaller boats. Very few fastenings at all. Some of the boats are now 25 years old and no problems from the approach.

But apart from that there is no necessity for fastenings at all - the glued joint structure will be stronger than the adjoining plywood.

Some glass tape on the outside of the joint or glassing the bottom of the boat increases the strength of a bonded joint even further.

An important exception is with timber that does not glue well - like teak or Australian White Beech - if they are being used for structural members they should be fastened with one of the standard fastening schedules as per the above post. These oily timbers can be bonded well enough for a deck overlay with a few precautions.

See http://members.ozemail.com.au/~storerm/Faq/faqindex.html

Generally we use drywall (gyprock) screws to hold things together while the glue sets up. For structures made of ply thicker than 6mm we move up to hex headed chipboard screws.

Remove the temporary fasteners (which can be re-used) within a couple of days so the glue doesn't bond them in place as well. It is no problem if a fastening is accidently glued in - heat it with a soldering iron for about 30 seconds and it will screw out OK - it just takes some extra time - so best to pull them out when the 'pox is hard. This is usually overnight - in cold conditions sometimes a couple of days. The epoxy should feel hard and not yield when you attempt to indent it with your thumbnail.

Best Regards
Boatmik