Riveting plywood to aluminum boat

Hi all,

First off, I know nothing but theory on how to rivet. I removed the old decking from my aluminum boat (it was 3/4" treated plywood), and it was fastened using rivets. I measured the rivets and the diameter was somewhere between 3/16" and 1/4" (my micrometer is broken).

My local hardware store sells a pop riveter for 9 bucks. It says that it will handle up to 3/16" rivets. Do you guys feel 3/16" rivet would work? It seems to be common size on-line. And I'm guessing I need a rivet that is at least the size (length) of my replacement plywood decking (3/4" plus the width of the aluminum I'm fastening too.

Anyone have comments or tell me where I can find these rivets (am I even going down the right track?)?

 

You need a rivet length that will pass through both materials and have enough room for the "mushroom" on the inside. Decrease the rivet spacing by 30% from the original on center spacing to accommodate the smaller diameter rivet. In other words if the original was riveted every 6" then you need to place a rivet every 4". If the holes are torn up a little or you feel you need additional support (not a bad idea with aluminum rivets) then use a washer on the mushroom side, which spreads out the load a good bit. Drill just big enough of a hole to pass the rivet, no bigger.

 

A couple of observations. Number one, what are you doing? Replacing the plywood? Modern plywood is pressure trreated with a new, highly corrosive chemical soup due to the environmental problems associated with the old pressure treated.
It would make sense to epoxy the ply areas to be riveted to seal the wood against the aluminum.
Then, what metal are the rivets made from? Aluminum, again, will corrode, so stainless is the only choice. Stainless rivets will fare better in pressure treated wood. Another option (one I like) is to use marine fir plywood, epoxy-seal that, and attach it with lots of stainless self-drilling/tapping sheet metal screws.

Alan

 

PAR, thanks for your tips. I went out today and purchased a riveter that will accommodate 1/4" rivets. It came with a few sample rivets, and I confirmed that they are the same size diameter as what was used in my boat. The only problem is that none of the local shops around here have long enough rivets.

Alan - Yes, I'm replacing the plywood decking. I purchased a sheet of 3/4" plywood from a local lumberyard based on the suggestion from a local boat shop. They didn't mention this problem, and I hope it's not that big of a deal. I already cut the wood and doubt I can return it. I was planning on using Aluminum rivets, as that is what the boat shop recommended. I will purchase and use the epoxy. Can you recommend a specific kind of epoxy?

Speaking of stainless - I thought I read somewhere in one of the threads here at boatdesign.net forum that stainless steel rivets can chemically react to aluminum as well. Can someone substantiate this for me?

Thanks for your help guys!

 

foxcf,

Yes stainless steel is going to celectrically react with the alloy, but they are much stronger and if you use a product like Duralac (anticorrosive compound) when it is inserted into the alloy, it will be good for many years ahead.

You have not mentioned just how big this boat is, but I am guessing it is about 20 feet (3/4"ply), so go with the 1/4" rivets, coat them with Duralac and happy days mate. Do coat the ply before hand as stated above.

 

Fox, you might locate a store that sells West Sytem epoxy. It's pretty user-friendly. There are several hardeners. Choose one for your temperature range.
A couple of coats should do.

Alan

 

Coat the plywood after you've drilled for all your holes and made all the cutouts. Coat the wood until it is uniformly shinny all over. If there are less shinny places, coat it again. This usually requires three coats as two will get 95% shinny, but will have some slightly "dry" areas.

As you're installing the rivets, dip each one in fresh, unthickened epoxy just as you insert it into the hole. It will help bond it, if it tries to "circle" out some years from now. Rivets come in standard sizes, but are most commonly available in the shorter lengths. You may have to contact "Fastenall" or maybe a mail order supply house like McFeelys to get the longer length.

The plywood you bought is likely not PT, just a good grade of exterior fur, which is just fine for your application.

Don't be tempted to cover the plywood with indoor/outdoor carpet, unless you just want to do this again in a few years. There are several options for covering the plywood. Paint is the simplest method, typically with a textured additive added to or applied over (my preferred method) the paint. There are some marine grade stick on or glue on types of covering, though I've never liked any of them much. These are basically vinyl flooring for a boat and not very good. I've seen some nice jobs done with a painted base and stripes of textured taped applied in a creative way. These tapes seem to curl at the corners with lots of use after several years. A teak or pitch pine veneer can be applied for the "yacht" look, but likely way past what you think about this little boat. Good Luck . . .

 

I have a treated ply hull, waiting to be finished. The main hold up is "WHAT" to put on it, HOW to tape the seams and what to paint or coat the hull.
You REALLY need to spend some time reading & researching this new pressure treated crap. We can thank the soccer moms for this s**t, by the way I'll wager you (like me), will contact no one who says their product is suitable for PT limber.
Epoxy manufacturers I've called, say there is no garruntee on the epoxy adhering to the new PTs.
Not only are the chemicals HIGHLY corrosive to aluminum in particular, but they also saw fit to add paraffin (wax), to the mix. Hence the adhesion problems.
I'm 21 phone calls into this mess, IF you find something, please, contact me. At present only 2 fasteners are acceptable for PT. SS & "hot" dipped galvanized.
.
Considering the cost, time & effort of "maybe" using alum pop rivets, Use one or both of the 2 just mentioned or use a better choice of materials. Cheaper in the long run.
You don't give the age of the original deck. If it is old, it was non corrosive to your support members. If you now lay the new PT ply onto these members, it will eat them up. SO, add a rubber strip to the already convoluted assembly procedure. Not worth it I think.

 

Ted, the original poster makes no mention of PT, nor would I suspect the boat shop, where he got the plywood recommendation, would suggest this material.

There's nothing "convoluted" about coating plywood with epoxy, nor doing so after it's finial cuts (fastener holes, etc.). I also don't see any issues that may be considered "convoluted" for the seam treatment.

You are correct (my assumption about you concerns with aluminum and ACQ) in that ACQ (pressure treating chemicals) will eat aluminum pretty quickly and it's not a recommended fastener for Alkaline Copper Quaternary treated materials.

 

=========="the old decking from my aluminum boat (it was 3/4" treated plywood), and it was fastened using rivets."============

You're right, he made no direct mention of ACQ. I drew the conclusion from 3 sources. 1. "treated" usually means PT in the USA. 2. As a redo, he will probably go back with treated ply, as that was what he found as original. Perhaps unaware of the changes. 3. ACQ will be what he finds, (most likely), in his local market, in Missouri.
If not, then certainly what you suggest for standard outdoor or marine grade ply, will add to its life.
.
Convoluted, (complicated; intricately involved ), was not a criticism of your methods. However, the steps you recommend do require a fair bit of material juggling, timing, prep, & skill. Not impossible, but indeed, convoluted. Sorry, No insult intended,

 

Believe it or not, I was told to buy what you guys are referring to as PT. I went back to the boat shop, and was told by the owner that I shouldn't have been recommended PT after all. The gentleman I originally talked to seems not to know what he was talking about.

I suppose rather than risking using the PT, I will get regular plywood, and treat it with epoxy resin. I was recommended the following for the epoxy: http://jgreer.com/

I also found the extra long 1/4" rivets I needed from http://www.bylerrivet.com/
They seem to be fairly priced including the shipping costs.

Anyone need 4 sheets of 4x8 PT plywood here from Missouri?

 

One has to hunt for decent plywood. An indication of the number of voids found in the inner plies is seen along the edges so examine the edges carefully!
Yeah, you can get lucky with exterior plywood. The grade "Exterior" ought to be stamped right on the plywood. Look for fir for strength but pay attention to the flatness of the sheets. Twist is okay but not lumpiness or serpentine edges. Don't be satisfied with the first batch you look at. Shop around.
I'm usually not super concerned with the outer surface if it's hidden---- voids there are sealable.
For a utilitarian craft, marine plywood can be overkill based on balancing the components of the whole. If you save $80.00 per sheet and you use four sheets, that's a big savings on the hull you might replace for $800.00.

Alan

 

Thanks much for your helpful hints. I'll give these suggestions a try.

One last question - Alan, you mentioned above, "Look for fir for strength but pay attention to the flatness of the sheets.". I know how to look for flatness, but how do you pay attention or know its strength?

 

What I think Alan was referring is, use Douglas fur, so you'll have the strength, rather then some of the junk woods they use in lower grade plywood panels. If it's a panel rated for "ground contact" or "exterior" then it will likely be Douglas fur or western larch, both of which will be fine.

In a stack of plywood, you may (or may not) find one or two exceptional sheets that get past the inspectors. Check the edges (as Alan suggested) for overlapping veneers, holes (voids), etc. The surface of the panel should be flat, without any waves or obvious bumps, dips or buckles which indicate something isn't right inside the panel.

 

Yep, well put by PAR. Fir is very strong and reasonably rot resistant, so look for that species in an exterior-rated grade (meaning laminated with water-resistant glue), and beyond that, be discriminating---- what looks good IS probably good; fewer voids along edges indicates better inner plies, and stay away from pieces with squirrelly edges.