Screws For Aluminum Floor

[Pstoney]

Hi,
I am replacing my pontoons wooden floor with 1/8" aluminum. I got #10 x 1 aluminum screws, but cant find a bit the right size to predrill the hole. The recommended 9/64" bit, just shears the threads off, so does the next size up and the next size, the screw falls right thru the hole. Any suggestions as to what I need and where I can get them, or how I can use the ones I have? Thanks

[Lister]

At BoltDepot.com you will find everything. You can even buy one screw!
Stainless steel should be OK with aluminum.
Lister

[PAR]

1/8" aluminum doesn't have the stiffness of plywood, so what are you using as support under the sheet aluminum? Second, use stainless screws, it's the only way to get the shear strength necessary without huge diameters, self drilling will solve a lot of problems too.

[Pstoney]

I am screwing it directly to the aluminum frame itself. Wont stainless steel rust? The boat will be in saltwater too. Thanks.

[PAR]

Can you post a photo of the frame work? As that designed to support plywood, it's likely insufficient in supporting sheet aluminum. You'll probably have to increase the stringer count to compensate for the lose of plywood panel stiffness, if using 11 gauge aluminum sheet.

Stainless, as it's name suggests doesn't like to rust, though lower grades of stainless do if around salt water, so use a quality fastener of 316 or 316L stainless. Aluminum fasteners have very poor comparative shear strength.

[Pstoney]

Only from underneath aluminum already on frame. The rails that the aluminum is laying on measures 1 1/2" wide and they are 15 to 17 inches apart. Thanks for all of your help.

[Dean Smith]

a Better way use a 3/16 drill and 1/4 unc -- tap in a reversing drill, use a socket head drive, in the screw head PUT em in with lanacote and they will draw out easily . In 25 years of building alumin boats we never used self tap Screw the thread in one hit dont stop and start the drill and lube the tap

[slow fred]

The plywood gave a lot stiffness to the pontoon boat that the 1/8" aluminum won't. I have an aluminum floor on my 34' pontoon houseboat. It is extruded aluminum panels with a 1" rib every 4". Each panel is 12" wide bolted together for rigidity. When you hit a wave you don't want one pontoon going up and the other going down.

[Pstoney]

Is your aluminum in panels or is it one sheet? The aluminum that we put down is one sheet. I am hoping that will make a difference.

[PAR]

The physical properties and attributes of aluminum will not change, just because you are using a single sheet.

[Frosty]

Why dont you put a self tapper in first to make a thread then screw on your alluminium screws.

[Pstoney]

Went with stainless steel carriage bolts instead!!!! How about if I put 1/4 inch plywood underneath aluminum, would that help stablize?

[PAR]

Yes, 1/4" plywood is a good idea, but 1/2" is better, unless you use a good quality 5 ply, 1/4" plywood and most importantly glue the two together. You could use just the through fasteners, but glue would stiffen it up considerably. Screws wouldn't have sufficient "meat" to hold. Carriage bolts have a nasty tendency of "rounding out" when you get a bit of torque on them, especially in aluminum, then there's the square peg in a round hole thing.

Pstoney, I know you were looking to eliminate the plywood from the deck of your boat. No rotting, etc., but there's a reason it's so commonly used. Pound for pound, plywood is stronger then steel! It's stiff, flexible, but not ductile like metals, dimensionally stable, low cost, easy to work with, etc., etc., etc. The list is long to plywood's benefit. Yes, it can have rot issues, especially if lesser grades are used and/or it's not applied well. Then there's the maintenance issue too. Most all pontoon boats use crappy plywood, then they don't protect it as well as they could and to top it off, they cover it with a material that will trap moisture against it (like carpet), which is about the worst thing you can do to any wooden product.

I once did a pontoon deck, with the owner insistence it have no plywood. I ultimately used a laminate of 16 gauge aluminum sheet, epoxied to each side of 1.5" foam. The foam core panels were very light and strong, but it worked out to be about 4 times the cost of a 3/4" plywood re-decking.

In your case, you could probably use some extrusions under the aluminum sheet to stiffen the deck locally between the existing framing. A "C" or "U" channel riveted under or maybe a box or rectangular tubing, possably screwed under the aluminum sheet. If it was me, I'd make some bridging, between the existing framing with more of the same. Maybe just some angle stock, which of course all would be screwed to the deck sheet. Then you'd have no plywood, but you would have a fair bit more framing, which is a good thing.

[Pstoney]

Does anyone know where I can buy the U channels?? Thank you all for your responses and wisdom!

[PAR]

Try the local welding shop. Ask for T-6 aluminum 6061 and accept nothing less.