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KKat · 05-11-2011, 01:45

I have just completed helping a friend construct a plywood catamaran. I am now about to start on building my own. Two questions.(1)On his boat all the ply was glued to stringers and bulkheads using drywall screws every 75mm to pull the joint tight. Once the glue was dry these screws were removed. If stainless screws are used in strategic areas and well countersunk would it not be a good idea to leave these screws in and cover with the epoxy finish? (2) What exactly are epoxy fillets, how wide should they be, and can a filler such as sawdust be used to bulk up the epoxy for the fillets?

Blue Stocking · 05-11-2011, 01:53

The answers to these very basic boat building questions will be contained on your boat drawings, and will be specific to its design.
Good luck

**skipmac** · 05-11-2011, 04:39

Stainless wood screws are not good to use in many areas of boat construction, especially areas where they will be in a damp environment, like screwing ply to the frames in areas below the waterline. You should understand that stainless steel is not impervious to corrosion just resistant to corrosion. Stainless needs a constant supply of oxygen from fresh air or water flow around the screw (or nuts, bolts, fittings, anything made of stainless steel) to stay stainless.

In a damp plank it will not get this and suffer from crevice corrosion, basically rust away over time.

Fine sawdust could be an acceptable thickener for epoxy fillets. A fillet in general refers to filling inside the corner angle where two boards or steel or other kinds of material come together. So instead of a sharp corner inside the hull where the plywood is attached to the frames you would fill that corner to form a fillet, sort of like caulking around the side of a bath tub. The purpose is to add strength to the joint or, in the case of your plywood hull, to also seal the joint between the frame and the plywood to prevent moisture from collecting in there and rotting the wood.

As Blue Stocking suggests, this should be spelled out in detail in the building plans for the boat. Unless you are a naval architect or hire one to make modifications I highly suggest you follow the plans exactly. Some home builders want to improve the boat by making some parts bigger or stronger. Don't. This can actually cause weak spots and give you the opposite effect.

Good luck with your project.

gravosa · 05-11-2011, 04:50

Quote:

Originally Posted by kwhitehead

I have just completed helping a friend construct a plywood catamaran. I am now about to start on building my own. Two questions.(1)On his boat all the ply was glued to stringers and bulkheads using drywall screws every 75mm to pull the joint tight. Once the glue was dry these screws were removed. If stainless screws are used in strategic areas and well countersunk would it not be a good idea to leave these screws in and cover with the epoxy finish? (2) What exactly are epoxy fillets, how wide should they be, and can a filler such as sawdust be used to bulk up the epoxy for the fillets?

no disrespect to this forum and members, you should try here to get your answers...

Boat Design Forums

sdowney717 · 05-11-2011, 05:09

try wooden boat forum
The WoodenBoat Forum

**boatman61** · 05-11-2011, 05:09

I would not stress to much about Epoxy Fillets... as long as you follow the prep work well they should be stronger than spaced screws that may normally do the job...
The bond is all the way reduces stress points drasticly...
Wharram Cat's are built using the stitch and glue method (Copper wire threaded through holes to hold panels in place) and they turn out just fine... even the H beams...

cburger · 05-11-2011, 05:22

Srews and epoxy will be stronger than just epoxy alone. If you can afford the extra cost of the screws then no problem countersinking, plugging and leaving them in. Use a high quality fastener like silicon bronze, for the marine environment.

David_Old_Jersey · 05-11-2011, 06:06

Quote:

Originally Posted by kwhitehead

I have just completed helping a friend construct a plywood catamaran. I am now about to start on building my own. Two questions.(1)On his boat all the ply was glued to stringers and bulkheads using drywall screws every 75mm to pull the joint tight. Once the glue was dry these screws were removed. If stainless screws are used in strategic areas and well countersunk would it not be a good idea to leave these screws in and cover with the epoxy finish? (2) What exactly are epoxy fillets, how wide should they be, and can a filler such as sawdust be used to bulk up the epoxy for the fillets?

Are you (and freind?) building in Malaysia? Something I have been (half) thinking about........a Wharram Tiki (21).

On the Epoxy / screw thing I would stick to the plans. I would also suggest you doing some test pieces with Epoxy (filets and no filet), with screws and with both.....and then see how hard they are to seperate (start by hand, and then work up

). You will discover that with epoxy the weakest part is the wood.

A filet is basically a run of epoxy along the corner of the L shape (both sides). How deep likely in the plans, but probably only talking a cm or so.

A far better description (than mine!) here:-

Fillets

Likely a forum specific to your boat around somewhere on the internet - if it is a Wharram then there certainly is, and if not you could always use their forum for general construction thoughts / ideas.

Anyway, what are you building?

**GordMay** · 05-11-2011, 07:39

See West System’s Epoxy Use Guides
➥ WEST SYSTEM | Use Guides

Including “Bonding (Gluing) & Clamping”
➥ WEST SYSTEM | Use Guides - Bonding (Gluing) & Clamping

And “Bonding fasteners and hardware”
➥ WEST SYSTEM | Use Guides - Bonding Fasteners & Hardware

capt.bobfm · 05-11-2011, 07:46

Don't depend soley on epoxy for structural support in a wooden structure. Even if the wood is fully saturated with epoxy, there will be some movement between the sheathing and the stringers (although very minimal). Ten years or so down the road, things could start to come apart and if you use some mechanical fasteners now, you'll be much happier later when you're in 20 foot chop and the boat is racking in every way possible.

Blue Stocking · 05-11-2011, 08:09

The care in c/sinking of screws is critical so as to not initiate veneer delamination on plywood by punching thru surface later.
Most fillets only adher to surface layer.
Some deck screws, Splitstop IIRC, have rounded edges to prevent breakthrough.

oldjags · 05-11-2011, 08:35

Quote:

Originally Posted by Blue Stocking

The care in c/sinking of screws is critical so as to not initiate veneer delamination on plywood by punching thru surface later.
Most fillets only adher to surface layer.
Some deck screws, Splitstop IIRC, have rounded edges to prevent breakthrough.

I'm not sure how you countersink without going through the surface layer, but I understand what you're saying. It's easy if you have the friction clutch too tight on your electric drill to drive the screw all the way through the plywood. Oops!

If you're completely encapsulating the plywwod and framing with epoxy, I wouldn't worry about just leaving the drywall screws in place and epoxy over them. If you're still worried, use corrosion-resistant outdoor decking screws instead of drywall screws.

If water does eventually get in there, the wood will rot away long before the screw corrodes. If you use a clear finish on the wood instead of painting it, you'll be able to see if water is penetrating and address the issue before rot sets in.

Blue Stocking · 05-11-2011, 08:48

Splitstop have a head shape which when tightened carefully, crimp the surface veneer instead of cutting thru it.
They are used nowadays with synthetic deck lumber to minimise sharp edges underfoot.