Aluminum outboard mount design

Discussion in 'Boatbuilding' started by Blether, Jan 24, 2014.

  1. Blether
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    Blether Junior Member

    So, here's the mount fitted up with a plywood block (that I'll finish with epoxy), and out in action.

    At this stage the metal is 6063 aluminium, unhardened. What's the downside of leaving it in this state, as opposed to hardening to T6?
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    The basic difference is strength. Which temper did you buy and why?

    You should also be aware that 6000 series has a lower fatigue strength than 5000 series. Have you taken that into account when designing your mount?
     
  3. SukiSolo
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    SukiSolo Senior Member

    With respect AdHoc, the better 6000 series are pretty good for strength. In particular the 6082 and 6005 alloys in T6 condition. 6063 is a pretty standard commercial alloy widely used and in several different tempers. For dinghy masts generally 6082 is used, generally it is extruded, treated to T4 condition, any cuts and welds, rolling etc carried out and then re tempered to T6 condition.

    Whilst I suspect 6063 is adequate for this application, hardening to T6 would significantly improve the tensile strength, approximately doubling it. Note that welding it will automatically change the condition to untempered. Might even be better having a 'softer' alloy (6036 over 6082)to gain in toughness against any work fracture or fatigue from the pulses of the motor. Still worth getting hardened to T6 in my opinion. You can 'feel' the difference if you drill it or work it with a file. Or if you really want to quantify it, try a beam loading test, which will prove beyond all doubt how much stiffer the treated alloy is. Even more fun, try a beam of 6063 against one in 6082 both in T6, there is quite a difference.....

    I suspect the 6000 series was used, as much more widely available than the 5000 series alloys. Certainly most non ferous stockholders here in the UK do not have off the shelf stocks of 5000 series box sections. Agreed, fatigue strength is a bit lower in the 6000 series but not usually too much of a problem for this type of application, a workboat used all the time would be different.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I was referring to its fatigue strength only.

    Exactly ;)

    Without knowing which temper has been used, it could be a real mixed bag.

    The main issue with the 6000 series is being exposed to sea spray water once welded. It is even more deleterious than 5000 series, owing to its poor weld porosity which is difficult to control.

    Thus unless the design of the mount has taken into account the temper, the as-welded strength and the fatigue and the fatigue in a seawater environment...its rather difficult to suggest whether "best to leave it". Even then, it's not cheap to heat-treat such a bit of structure either!

    Opps...almost forgot.

    However i would not have selected the 6063 simply because its strength is poor when welded, almost half that of as-welded 6082 for example.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Looking at that dinky little motor, it is probably more academic than practical to worry about it, but just paint the bloody thing anyhow !
     
  6. SukiSolo
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    SukiSolo Senior Member

    Agreed Ad Hoc, my own preference would have been the 6082 treated to T6. Partly because that material is reasonably widely available and the superior qualities you mention.

    Still reckon it is worth getting the mount treated to T6, for longevity and the ability to take some of life's unexpected surprises. Maybe hitting a semi submerged tea chest, large plastic bag etc. Ideally, if it could be loaded in to an oven with a large batch of other parts or extrusions for heat treatment, it might not be too expensive. Even better with 20-25 microns anodising but that is the icing on the cake, and a good protective painting (2 pack epoxy?) would certainly be better than leaving it bare.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    No need to wonder too much about hitting submerged objects, other than the destruction of the outboard lower unit, the leg will just kick up fairly gently at the sedate speeds that "monster" Merc will reach.
     
  8. Blether
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    Blether Junior Member

    Hey hey! Who you callin' a Merc? :)

    Thanks, all. The design took into account Sukisolo's knowledgable advice that gave me the confidence to take the plunge in the first place, precedent (the maker's standard mount), guesswork & expedient. Google helped me assess a pounds-force / kilogram-force equivalent for the whole 3.5hp, that suggested the sections selected are adequate for the forces involved, even untreated.

    A first fabricator ran me around and lost interest; the second I tried isn't a marine specialist but showed willing: out of a choice of stock extrusions in 3mm, 50 x 50 or 3mm., 70 x 30, for the main load-bearing beam, I went with the 70 x 30.

    I believe the welding is some of the best you'll get anywhere. No tempering post-welding means I assume it's all untempered.

    Funnily enough the fabricator was all ready to anodise it. I just wanted to get out on the water.
     

  9. SukiSolo
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    SukiSolo Senior Member

    Thanks Blether, the welding looks good. What happens is even if the stock was originally T6 (which is a standard grade), the welding anneals it and allows the metal to return to an unhardened state. If the heat treat cycle is then carried out afterwards you get the original properties of the T6 condition. The correct wire choice is important and needs to suit the material grade you have. Sounds like your welder and fabricator knows plenty about that.

    A longish time ago when travelling by m/cycle in France, I was unlucky enough to break an aluminium brake lever on a fence when the machine fell after a rainstorm which softened the ground. They did not sell that Italian machine in France so I got it welded. Some time later and after I had bought a spare lever in Italy, I fell off on the Cannes to Frejus N7, courtesy of a load of unmarked roadworks and lots of loose gravel. The lever snapped at the weld point and there was a large hole right in the middle. This is why Ad Hoc was a bit concerned about porosity. Aluminium can be prone to it when welded if not correctly dealt with. Hence TIG welding or inert atmospheres as the metal has such an oxygen affinity.
     
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