Stainless Steel versus Aluminium

Discussion in 'Boat Design' started by Pompey, Oct 8, 2008.

  1. Pompey
    Joined: Oct 2008
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    Pompey Junior Member

    Hi
    Rather than repair the top section of an aluminium mast, it seems little difference in price to replace the complete mast, as it is a plain tube with blocks fitted at the top. The existing tube, which is the fixed lower mast of a gunter rig (only about 10ft), is anodized aluminium, which seems difficult to get hold of in the UK. Looking at plain aluminium as an alternative, I also came across stainless steel tube, which seems just under twice the price of aluminium and seems to be about 3 times the weight.
    Can anyone advise me on the choice - is it critical to use anodized aluminium or would plain aluminium tube suffice? Also, would there be advantages or drawbacks in changing to stainless steel for the mast?
    Thanks
     
  2. eponodyne
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    eponodyne Senior Member

    Advantage--price. Strength. Weldability.

    Disadvantage--Dissimilar metals setting up galvanic corrosion. Weight (although this is to some degree offset by SS's superior strength, thinner section can be used). Different look, feel. May interfere with nav/radio/compass, depending on what you're currently using.


    You could also buy the plain aluminum tube and have it powdercoated or anodized yourself.
     
  3. Pompey
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    Pompey Junior Member

    Thanks, eponodyne, some good points - all things considered it looks like anodized aluminium will be the best in this case. I will try to find a supplier of anodized tube, because it seems having plain aluminium tube anodized costs more than the tube in the first place.
    Would you happen to know the best method of securing the stay eyelets? Would pop-rivets be strong enough?
    Thanks
     
  4. grob
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    grob www.windknife.com

    Do you know the diameter of the tube and the wall thickness, there are many suppliers of aluminium tube in the UK and you are bound to be able to find someone who could help. Try:-

    www.spaaluminium.com
    www.aalco.co.uk
    www.metalfast.co.uk

    The big part of the cost can be delivery, so pick someone local or who delivers in your area. You are unlikely to get stock aluminium over 5m in length.

    As for anodising, I would always anodise for commercial stuff but in reality if you want to save money you could probably get away without it. Anodising will slow corrosion and make the mast look alot nicer. That said plain aluminium tube doesn't corrode that quickly anyway. It sounds like you are trying to save some money so I would say anodising is a personal choice not an absaloute requirement.

    Gareth
     
  5. Crag Cay
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    Crag Cay Senior Member

    More important, if you're off to the sea, is that the alu tube is 'marine grade'.

    Then a coat of polish, lacquer or even paint (with a swill of waxyol on the inside) will see you right.
     
  6. grob
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    grob www.windknife.com

    True, Go for 6082-T6 if you can get it, and try and isolate from any disssimilar materials, especialy carbon fibre if you have any of that!

    Gareth
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Assuming you can get 6082 T4/6 then no need to anodise. If another 'marine grade' then same applies, anodising not necessary. Aluminium has it own very good protective oxide layer.

    However, what fixings do you intend to attach and how? Need to be careful of fatigue, espeically welded versus unwelded ally.

    Then it is just a matter of checking the connections for dissimilar metals, for bimetallic corrosion.
     
  8. Pompey
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    Pompey Junior Member

    Hi - some helpful information about materials and much appreciated. It sounds like aluminium tube is the best bet. Regarding the fittings, at the very top of the mast I need to fit two blocks, one for the jib and one for the gunter spar (peak). I also need to fit a block a bit lower to hoist the gunter jaw, as the peak is on a slider (for reefing). In addition I need to fit three stay eylets, so it's quite busy at the top. The original design seems to have fitted all of these blocks and stays into a short wooden section, fitted into the aluminium tube - this wooden bit is the missing bit that triggered the need for mast replacement. I would appreciate some advice on the best way to fit stay eyelets into aluminium tube - are pop-rivets strong enough? Also, I was thinking of running the halyards down the inside of the mast - any comments. Thanks all of you, your experience is a real help.
     
  9. eponodyne
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    eponodyne Senior Member

    I'm not a sailor so I can't speak to what you need to do here; but I have absolutely no faith in pop-rivets. I trust bolts; good thick bolts with a lock-washer and lock-nuts and then the bolt drilled for a cotter pin. That's what I put my faith in.
     
  10. erik818
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    erik818 Senior Member

    Sometimes dissimilar metals cannot be avoided. I learnt the easy way (asking) that you can handle the situation by galvanically insulating the two metals.

    Erik
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    I wouldn't really recommend pop-rivets, but i do know these are used and often with a reasonably long life. Probably owing to little usage! But it does also depend upon the thickness of the tube and rivet and the material properties of the rivet.

    If you can, get a qualified welder and someone you understands welding aluminium.

    Also, whatever kind of cutting you are doing, make sure that you don't scratch the surface unnecessarily leaving score marks as well as poorly cut joints. You don't want to gap fill welding.
     

  12. PAR
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    PAR Yacht Designer/Builder

    Rivets are a perfectly acceptable fastener, regardless of comments seen here, so far. They're used by just about every fabricator and spar shop around. The rivet itself can have an isolator (which very often isn't employed), usually a nylon washer, placed between the two rivet heads and the joining material, so dissimilar materials don't touch, eliminating any issues. The trick with rivets is to scale them to exceed the demands of the tasks, so they aren't the weak link. Often a cast masthead fitting (for example) will crack or break before the two rivets holding it in place give way.

    Welding is an options, but it generally weakens the local area.

    Stainless spars would be prohibitively heavy, if not reasonably durable.

    Yes, you need a good marine grade of aluminum, which doesn't need to be additionally protected, though paint or other over coating is a good idea to keep them looking good and provide longer life. Even a few coats of Penatrol will be helpful on raw aluminum.
     
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