Need fastener recommendations please

New guy here. I've been a long time lurker though!

What size screws would be good for fastening 1/2" marine plywood to 3/4" pine frames? I will be using traditional bronze screws from Jamestown. I'm thinking #8 screws perhaps?

I'm building an 18ft jon boat that's 6ft wide using probably a 35 hp. outboard that will see some pretty rough service. The screws will be glassed over with 6oz. cloth.

Any information would be much appreciated! Thank You!

 

1.5" #10 would be the usual recommendation and welcome to the forum.

 

Par, you are a wood specialist, would stainless be as good as bronze, cheaper?? So long as it is 316

 

Good point Barry. Personally I'd use rolled s/t 316 A4 although 304 A2 is OK for a freshwater boat. At least they won't corrode away inside the timber like bronze does over 30 years or so...
If you ever anticipate removal, then slot heads are better than Pozi - more torque can be applied. If you can source the hybrid slot/pozi heads they are OK, but not sure I've seen them in csk yet.

Depends how deep you want to sink the screw heads as to length. 1.5" sounds a bit on the long side for a max depth of 1 1/4" thickness? 1" would be better? Make sure you 'flag' the drill too. Assuming the ply is glued and screwed No 8 or No 10 are fine IMHO, but if bronze I'd go for the No 10s.

 

If PAR doesn't mind - you can check this discussion: http://www.boatdesign.net/forums/boat-building/using-stainless-steel-screws-wooden-boats-49954.html

 

I wouldn't say that stainless wont corrode...besides building boats, I am also a beekeeper, and I use stainless screws to put boxes together. After a few years sitting in the pine wood, the screws are pretty well rotten, if not rotten completely through, the threads are pretty far gone, and the only thing holding the box together is the exterior wood glue (which is still holding strong)

Keep in mind it's not submersion in fresh or even salt water marine environment here, it's the pine wood that eats them in the case of the bee hives. I have no explanation for it, but I have seen it happen for many years.

 

It's not stated but I assumed the frames were on edge and not the flat.

 

Wood is made of cellulose, which is a polymer made of sugar molecules. In the presence of moisture these molecules decompose and release acetic acid, which is an electrolyte (though weak).
Besides that, wood also contains a range of minerals like potassium ad chlorides, which also act as electrolytes.
And there are chemical preservatives (often copper-based) which are used against fungi, and which also create electrolytically active ambient inside the moist wood.
So, a moist wood has all that it takes to corrode a metal (water, electrolytes and areas with different oxygen content), even if placed in a non-marine environment.

If you want to learn more, you might check these papers, for example:
http://www.npl.co.uk/upload/pdf/corrosion_of_metals_by_wood.pdf
http://cdn.intechopen.com/pdfs/46241.pdf
https://www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr220.pdf

Cheers

 

Yeah I knew it was still galvanic corrosion, just wasn't sure from where. Anyway, regardless of the processes involved, I just didn't want people to think that stainless is immune from corrosion. It resists it, but it still happens...even in non marine environments like my beehives.

Just FYI, not that anyone really cares, but I don't use treated wood of any kinds in my beehives.

 

That is absolutely true. Check the link in my post #5, I think you will find it interesting and will see how the problem is dealt with in the boatbuilding industry.

 

Thanks all for the meaningful discussion! My frames are 3/4 x 3" yellow pine turned on edge so 1.5" lengths are just right. I think I would feel better about using #10 screws.

Again, thanks for the advice everyone!
Bud

 

stainless is a cheaper choice, but they will eventually rust unless encapsulated. I have never seen a stainless fastener rust in an encapsulated hole, but plenty that have with no protection.

 

Paul,
Cheaper than what? and how long does that material last in comparison?

Scot

 

Stainless is cheaper than bronze. Both (if installed properly) will out live you. Most stainless screws are 304 (18-8), though 316 and 316L are available, just not as commonly as 304. The higher grades of stainless do last considerably longer, particularly in marine and alkaline environments. If the wood is epoxy encapsulated, in particular the holes epoxy bonded, stainless (304, 316, etc.) will live for decades, without noticeable degradation. I recently pulled a transom apart, that I had replaced in the early 90's. This was a time when I still left the fasteners in place (I generally don't any more) and each was in an epoxy filled hole and none showed any signs of corrosion. Conversely, I've pulled bronze fasteners that had be "de-zinced" from continuous immersion in salt and electrolysis after only a few years.

In the end, it's all about the protection and environment. Corrosion can occur on all metals, if placed in a less than desirable and/or unprotected environment.

 

How does dipping the screw in epoxy work for bonding? Do you have a recommended process for bonding holes and fasteners yourself?