Aluminum dory skiff

dangers of p.t. lumber

Hi John, thanks for the heads up about potential corrosion problems that might arise if you combine pt wood and aluminum. It got my attention so I went out and looked at a 20' jon boat that I owned for about five years and sold to a friend. The boat was new in 1979 and has been sitting on a aluminum float on trailer with 4x6 pt bunks covered in poly carpet. The bunks are screwed directly to the aluminum h-beam. I rebuilt the trailer when I got the boat and found no damage to either the trailer or boat although the axel was pretty much gone. It seems that 27 years would be enough time for the " pt to eat the aluminum like candy" but it didn't. I enjoyed the links you led me to. I think the transom damage was text book example of stray current electrolysis, that outboard is nothing more than a big generator and I'm guessing that the problem lies in the grounding to the aluminum hull. The hull corroding adjacent to the pt bunks on the other boat I can't explain, I would love to see pictures. I put together a little experiment just for the fun of it, maybe I'll learn somthing. It's a 5 gal bucket full of salt water, into the salt water I put a piece of 3/16 alum. screwed to a piece of pt plywood with ss screws, see picture. I'll pull it out once a week a take a look, let's see what happens, Regards, rusty

 

Rusty, Thanks for the informative reply! As a "store-bought" aluminum boat owner, I've not had any corrosion problems, but I keep my tin boat well rinsed off after a dunk in the salt. I had heard of people bottom painting their aluminum boats with paints that contain copper and having problems but it's all hearsay. I'm not a chemist, but I'm hoping for a long life for my aluminum fishing partner so I thought that I should pass those posts along in case they're based on fact. I look forward to reading more about your experiment, and your beautiful project. Nice Work!

John T.

 

P.T. vs Aluminum

Hey John, Thanks for the reply. I want you to know that I'm not totally convinced that Pt wood dosen't have a corrosive effect on aluminum, that's why I'm doing the experiment. My experience and intuition tell me that people are jumping to conclusions about a subject that can be pretty complicated and might not always result from the obvious. I'll let you know how the experiment goes. regards,rusty I posted the first part of this reply this morning, now it's late afternoon and I spent an hour or so doing research. I'm a contractor and this morning I got a unit of pt 2x8"s in and pulled one of the tags off the end of a board. It says on the tag not to use it in direct contact with aluminum! All the treaters say the same thing about acq treated wood, they go out of their way to say don't use it in contact with aluminum. I wondered why I'm not seeing a problem when I've used aluminum on stall doors, bar grates and other things I hang on AG structures I build. It's not the copper in the treatment that causes the problems but instead it's the alkalinity in the acq. ACQ does contain more copper than traditional cca treated wood and that might not help. Pressure treated plywood is still treated the old fashioned way--with CCA. I wondered why the plywood/aluminum experiment wasn't even showing discoloration after a 5 days in salt water, I think I have the answer but I'm going to do a parallel test using a piece of 2x8 which I know to be ACQ. In any case I'm going to lay off the ACQ. Thanks for drawing it to my attention, I've learned a lot and hope this whole thing isn't too boring. regards, rusty

 

Rusty, Please let us know what you find out. It's not boring at all.

John T.

 

Aluminum vs P.T. update

Hey everybody, It's been 3 weeks since I started the experiment to try and create corrosion between a piece of 3/16 5052 plate and a piece of 3/4 acq p.t. plywood placed in a bucket of salt seawater. The piece of 3/4 p.t. dosen't seem to have much appetite for the aluminum. If you look at the contact surface between the two you can't hardly tell they have been screwed together, only the faint outline and a very light stain that wipes off with a finger, the shine is still on the aluminum. If you close your eyes and rub the surface you can't feel any texture. What was interesting though was the condition of the stainless steel screws I used to stick the two together. The day after I started the test I noticed that the heads of the screws had turned from a typical shiney ss look to a dull grey. When I removed the screws to have a look one broke off and showed a rust stain a the point of failure. The other screw came out but the part that was in the pt plywood was dark grey almost black. The approx. 1/2 inch sticking through the back side of the p.t. was normal looking. see picture, regards rusty

 

Here's the pictures of alxpt

 

Hey Rusty,
Google "nobility of metals". There is a galvanic series of all metals (a pecking order if you will ) and it just so happens aluminum and copper are mortal enemies.Aluminum being the loser. If you want to wreck an aluminum boat , throw a handfull of pennies in the bilge.

 

A penny for your thoughts

Hi Dick, I appreciate your heads up about the tables of nobility relating to metals. I have to admit I'm pretty familiar with them. I would absolutely love to see some pictures of these two metals locked in mortal combat. There is a direct relationship between the size of the anodic metal and the more noble metal. Did you know for instance that common alloys 5052 and 6061 aluminum can and often do contain up to 10% copper, Do you have any first hand experience with a hand full of pennies destroying an aluminum boat? regards, rusty

 

Rusty, how about an update. Is the boat in the water yet? What does it weigh w/o motor. how's it handle at low speeds, high speed ?

 

Boatbuilding progress

Hey OpenBoater, thanks for the inquiry. I haven't done much to the boat since I got back from Tennessee. This is a really busy time of the year for me so I don't often get to do what I want to. The questions Relating to P.T. wood and it's effects on aluminum kind of stalled me out there for a few weeks, I wanted to find out for sure whether I wasn't doing somthing I would regret down the road. Your interest in my project and the pictures of a pretty little wood boat in the wood boat forum inspired me to go out and work on my boat for a few hours yesterday, it felt good, thanks! I spent yesterday removing the 2x2 pt cleats I had screwed to the vertical legs of the floor frames. I put a layer of plastic sill seal between the cleats and the al. angle, a little isolation just to be on the safe side. I'll screw the deck boards to these cleats. I'll lay 2.5 in. of foam sheet between the top of the longitudinals and the underside of the deck boards, this will give me 1.5 in. of space for a "bilge" to let water flow bow to stern. I'll do same to the sides. I'll post pictures as soon as I reach that point. Thanks again, rusty

 

Nice looking craft Rusty!! I'm in the first stages of building a small aluminum 'weekender' houseboat in aluminum. Very similar in size to your Dory. I have some plans from an old magazine that use wood, but I have very little knowledge of wood. After spending 10 years of my life working on aircraft, I am more than familiar with aluminum. I have a couple of questions reguarding your beautiful boat. First question is about alloys. You used some 6061, typically an 'aircraft' alloy, was availabilty your deciding factor? For my boat I was planning 50xx alloys and TIG welding for bonding. Can you tell me what holds it all together? From the pics it looks welded and I was curious about the technique.

Thanks again, can't wait to see your Dory afloat.

 

Why 6061-t6 for boat

Hey bgeddes, Thanks for the nice comments about my boat. You were right on about my reasons for selecting the more commonly available alloys for building my boat. In my part of the world the metal wholesalers don't stock and have almost no demand for the 5086 alloys. They would special order them for me but since I was going to buy as I build I didn't think that would be practical. Most of the "gurus'" on the subject say that 5052 and 6061 will work fine for an aluminum boat which I agree with. My background is in steel shipfitting and fiberglass production boatbuilding, this is the first aluminum boat for me and the smallest metal boat. This boat is welded together using a Hobart (miller) 250 amp mig welder and a spool gun. The shape was probably plywood to begin with and reworked for aluminum, I used the plans to create a set of offsets and station spacing and winged it the rest of the way. Good luck with your [project, Rusty