mast section aluminum paint anodize weld

Discussion in 'Metal Boat Building' started by Charly, Feb 3, 2013.

  1. Charly
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    Charly Senior Member

    Please excuse the thread title, but it sums up my ignorance on the topic

    I will soon be tuning my attention towards the building of a bow tube for my 36 beach cat. The plans call for an oval shaped mast section, about 10x5 inches, lying on the flat, with a back cut angle on each end, and an aluminum chainplate welded into a vertical groove that is cut on center, also at each end.

    The chain plate function will be to accept a pin that goes through a pair of angle brackets mounted on the deck on each side, and on the inboard end of the chainplate, to accept a turnbuckle for a wire that runs at an angle to the gull strikers, which are welded to the middle of this mast section. It sounds complicated to write it all down, but most of yall have probably seen this a million times already.

    OK my first concern is strength and quality of the mast section, and how to judge it. With all the salvage opportunities, I figure I should be able to find what I need ~ a twenty ft section, somewhere nearby.

    Questions: does the mast section have to be without any previous drill hoes in it? There will be a doubler welded for a couple of feet across the very middle, at the base of the gull strikers, but thats all.

    How much weakening if it has previously drilled holes for , say a steaming light, etc.? Is there a general rule? Can such holes be patched?

    Is there anything else I should look out for?

    Can pitted places from previous corrosion be patched?

    The designer calls for all pieces to be anodized after all drilling and cutting etc, has be done. this is a big piece. I imagine it would be expensive to get it to someplace and have it done? or not?

    Paint. If it is painted, does it still have to be anodized? can a backyard doofus like me clean, etch, prime and paint the thing?

    What else? should I fill it with foam or something to keep water out?

    Thanks for any help!
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Charley,
    Answers as best I can make them.
    First, the few number of holes, the better. A few small fastening holes may not be a problem. The more holes and the bigger they are, the less desirable the part is. The forward cross beam, I presume, has the headstay mounted to it at the center, so it will see a considerable load. This should cause you to look for the best quality spar that you can find.

    You do not mention if the designer has also specified wall thickness for his 10" x 5" section. Wall thickness is also important. His specifications should also include the Moments of Inertia of the section, and these are usually published along with section shapes so that you can more easily and reliably make a match. That is, you want to match all his specs in size and wall thickness. If in doubt, go slightly bigger and thicker. You can go slightly smaller and thicker if the moments of inertia are the same or greater.

    Holes are difficult to repair one by one. If there are alot grouped together, or if they are big, they should be covered by a welded or fastened doubler plate. However, be very careful about welding, and consult with the designer about what he intended for strength. Once you weld aluminum you reduce its strength considerably. If the designer intended unwelded properties and then you go weld something to it, you are greatly reducing the spar's strength below the intended strength. You can get that strength back by heat treating the part--putting it in a large oven at about 400°F for a few hours (consult with an aluminum fabricator) and so you would need to take it to a facility that can do that.

    The same is true with corrosion. If there are pits or large corroded areas, you can't repair them easily. Best to steer away from any sections you find that are badly corroded. Chances are that if you see corrosion on the outside, there may very likely be hidden corrosion on the inside. Get as pristine a section as you can find.

    As for anodizing, it is probably cheaper than you might realize, but make some calls around your area where there might be aluminum anodizing available. The spar will be completely chemically cleaned before it gets anodized. These are all done in large chemical baths.

    If you don't want to anodize, or if it is too expensive, then yes, you can paint with proper etching, priming and painting. It does the same thing, prevents the spar from corroding. Polyurethane paints are the ones of choice for aluminum spars.

    I hope that helps. Good luck.

    Eric
     
  3. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    Paint the mast with a aluminum MCU (moisture cared urethane) such as aluthane (tm)

    google aluthane - it is a product well loved by its customers.
     
  4. jonr
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    jonr Senior Member

    IMO, small holes are easy to fix by welding. But do heat treat the entire part.
     
  5. Charly
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    Charly Senior Member

    Thanks Eric. There is no wall thickness specified, and the 10 x 5 dimension is just a scaled approximate. The designer calls for Bow Tube moments 25 X 11, and compression tube moments 4 x 6. But if I understand you correctly, these numbers are all I really need? As long as I know the manufacturer of the mast section, and find the corresponding moments of inertia tables for that particular model and size, as long as the two numbers are greater than 25 x 11, then, I'm in business? I was just looking at Kenyon's page, and their 8 X 5.2 section has moments listed at 31.7 and 15.10. So this one would be adequate although is is actually a smaller tube?
     
  6. Charly
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    Charly Senior Member

    Thanks guys.

    Can the heat treatment go AFTER the paint, or is it too hot?

    If a piece is already anodized, or partially anodized, will the normal acid etch prep it OK for paint?
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Correct, the moments of inertia are the controlling factor, so as long as the section meets or exceeds those, it is acceptable.

    As for your paint question, heat treatment must go before the paint. Consult with the paint supplier regarding the proper prep procedure for painting over anodizing.

    Eric
     
  8. dinoa
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    dinoa Senior Member

    Maybe sleeving would be preferable to welding. External or internal stock aluminum tube of corrosion resistant alloy, slit sleeve if exact dimension cannot be found or machined sleeve.



    Dino
     
  9. Charly
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    Charly Senior Member

    Well, I have yet to touch a piece of aluminum. I am still fiber glasssing and fairing the rest of the boat! Almost done.

    I want to go ahead and order some stock to use for the awning, as working conditions are getting pretty hot here, so it will be first.

    My plan is to use Aluminum tubes for the four legs, which will support a wooden frame for the canvas top to stretch over. I figure I can attach the longitudinal part of the wooden frame to the upright aluminum tubes by casting up some studs, which will be formed with glass and epoxy/structuralbog. They will slip into the tops of the tubes, and be secured with aluminum pins, the bottom simply slips into a cast "boot", and secured with pins. No welding or bending.

    OK, so my question- what size tubing? I have a salt shaker here on my desk that is about 1-1/2 inch OD. That seems about right. The awning will be 8x8 ft. I can get it in 1/8" wall thickness (cheaper/lighter) or 1/4inch wall thickness (heavier more expensive)

    I know this is off the wall. Is the whole idea nuts? Any comments appreciated. It will be designed to de-mount, but I don't want the thing to fold up on me in the first thunderstorm.
     
  10. FishStretcher
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    FishStretcher Junior Member

    Be careful. The as welded strength of the alloys you will usually come across is quite low. 6061, 6063, 5052, for example. The bicycle industry knows all about this. They re solution heat treat (heat and quench back to T0 temper), then straighten, then artificially age (usually back to T6) 6061 alloy. That's a big oven, a big quench tank and a lot of straightening.

    Two armor plate/ marine alloys are harder to come by, but have MUCH better as-welded strength, and good corrosion resistance. You can tell when you machine it. Look into 5083 and 5086 alloys. And the correct filler wire. Start with filler wire 5183, but check first. The lack of heat treat with these alloys should make up for the higher price with respect to alloy 6061.
     
  11. Charly
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    Charly Senior Member

    Thanks Fishstrecher,
    A quick google turned up a source for 5083 and 5086-
    http://aluminum.adimetal.com/viewit...te-5086-h116-or-5083-h116-or-dual-?&forward=1

    As for the bowtube fabrication, the vertical end plate pieces, that bolt to the hulls, will have to be welded to the aluminum mast/tube section. For the centerpiece, I hope to make up all in one piece and either bolt or weld into place, I think will be a sleeve, with the gull striker (an inverted vee made of al tube) welded to it, and maybe a roller setup for the anchor also. I also have to attach a couple of padeyes to accept the pins for the compression tubes (fore and aft aluminum tubes). Maybe I can make it all up in 5083 or 86. Then all I have to worry about is where anything is welded to the mast/tubes. Are mast sections generally 6061? How do you tell?

    I've given up on aluminum for the awning legs. I am much more comfortable with wood and epoxy.:D
     
  12. jonr
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    jonr Senior Member

    I agree with above, it may be cost effective to make fittings out of 5083 to connect larger pieces of lower cost, heat treatable alloy.
     
  13. messabout
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    messabout Senior Member

    Charly; Have you considered getting a new aluminum tube for this application? You can get tubing in many different sizes and wall thicknesses. The I (moment) values are clearly established. The current going price for a tube of that sort, in your area, is about $2.50 per pound more or less. Standard lengths are 12, 20, and 24 feet. No holes, no anodize, no sweat. You can get the tubes in 6061 T6 alloy. You could also consider square tubing which will be easier to attach to other structures.

    As for the mast, you can remove the anodized surface with common household lye (potassium hydroxide). Mix a strong soution with lye and water. Apply it liberally with a a paint brush. In 10 to 20 minutes the anodize will be gone. You have to watch this operation carefully as the lye will attack the base aluminum if you over do it. At the first sign of the surface turning black, simply hose the soulution off with water. (This is the drill for reanodizing)

    Anodized surfaces are dielectric. That is to say, they will not conduct electricity. Thats why an old part must be stripped before it is re anodized. The welder will probably want the anodized surface removed at the weld sites. Keep that dielectric quality in mind if there are to be any lights or other electric stuff attached. Only the thin anodize film is a dielectric it does not affect the parent metal. That it is a dielectric, is why your anodized aluminum mast will resist galvanic corrossion. .....except in places where the thin coating has been gouged away. The electrolytic coating is only a few ten thousadths of of an inch thick.

    If you paint a raw aluminum part it is helpful to pre etch it a little bit. This is commonly done with a phosphoric acid solution which is not as aggressive as Lye solution.. You can get it ready to use from automotive paint suppliers. One of the common brand names is Ospho. The stuff is also useful for removing rust from ferrous metals. Just follow the directions on the bottle. (dentists sometimes use phos acid to etch a tooth in preparation for an alloy filling.... it is also a biggie in the fertilizer industry. :D)

    Incidentally, paint sticks to anodized surfaces very well. Unless it is a fairly new anodize job that has been colored. Once the anodize is deposited on the parent metal , the part is immersed in a colored dye for a few minutes. After that it is immersed in a sealer solution that insulates the dye from the atmosphere and particularly from UV radiation which would fade the dye. If the sealer is new, it does not paint as well. Epoxy paints stick to it anyway, despite the newer sealer film. If no colored dye has been applied then it was probably not sealed.

    The nearest facility that I can think of, that has anodizing vats big enough for a mast, is in Gainesville Georgia. A very substantial aluminum extrusion factory is there. Their name is Indolex if I remember correctly. Yes that's a long way from Saint Simons but they do have tanks big enough and most anodizers do not have such large capacity. Since they are a signifigant source for extrusions, you might get your tubing from them.

    There that is more than you wanted to know about anodizing. I hope that the information is useful.
     
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  14. Charly
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    Charly Senior Member

    Thanks messabout, Great information. I will check up on Indolex. I assume they are the supplier for US spars in Gainsville?
     

  15. Charly
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    Charly Senior Member

    I googled it. it was called "Indalex", which is now part of Sapa group. They have a huge website, but don't seem to sell to individuals.

    US spars in Gainesville does though.
     
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