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1152Re:: Brass running strip....

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  • olivershaw4229
    Feb 9, 2016
      Brass is not recommended for marine use - it dezincifies - maybe ok for dinghies that live on dry land, but not on boats that live on moorings.  Ever tried removing old brass screws from a boat? They crumble before your very eyes.  I would not use brass for a keel strip, especially when fitted with screws that penetrate the hull.



      Agreed in general principle,  although it depends very much on both the type of brass and the size and nature of the fitting.

      I am not a metallurgist,  but this is my educated layman's understanding.

      There are various different grades of brass,  including Admiralty brass and naval brass  -  which I understand are actually bronzes  -  and aluminium brasses.   To add to the confusion,  a technical article which I read a few years ago but cannot now trace also informs us that certain alloys named as bronzes are actually brasses ....

      To quote Encyclopedia Brittannica,  "A third group of brasses includes those with other elements besides copper and zinc, added to improve physical and mechanical properties, corrosion resistance, or machinability or to modify colour.  Among these are the lead brasses, which are more easily machined; the naval and admiralty brasses, in which a small amount of tin improves resistance to corrosion by seawater;  and the aluminum brasses, which provide strength and corrosion resistance where the naval brasses may fail."

      The preferred metals for use on boats are either a suitable bronze (gunmetal being the most usual choice,  or phosphor bronze for propeller shafts and bearings) or the correct grade of stainless steel;  note that not all grades are of marine quality.    Galvanised steel is a popular low cost option,  but ugly and heavy,  and once the galvanising wears off - which it tends to do in any situation exposed to moving friction (and even with some applications which aren't)  - it can quickly become a rusty mess.

      Classic Marine are one of the top boat suppliers,  within their chosen field of classic boats;   so because of the classic emphasis their stock is overwhelmingly brass or bronze,  supplemented by galvanised,  and a very limited stock in stainless steel.   Their reputation is such that where they stock brass I am sure that it will be a suitable grade,  and fully fit for purpose.   And remember that keelbands are intended from outset to be sacrificial,  and also virtually non-structural;   all they have to do is to protect from rubbing and impacts.
       
      Dezincification is a process that works slowly inwards from the surface,  and the keelband has a moderately substantial amount of metal;   however screws are very much smaller,  which is why brass screws have a notorious problem in this regard.   I think it highly unlikely that the keelband will cease to be functional due to dezincification any earlier than one would wish to replace it anyway because of extended batterings from hitting the bottom.   My guess is a lifetime of years,  perhaps many years,  and that with a suitable grade of brass dezincification is not going to be an issue with it.

      But you are absolutely right that the screws should not be brass;   which is why I used (and mentioned) bronze screws.

      Adding extra layers of GRP is of course a valid alternative,  but one which is very obviously sacrificial,  and which is likely to wear faster.   The plus point is that although the need for repairs is likely to be much more frequent,  they are likely to be comparatively smaller touch-up repairs on each occasion rather than wholesale replacement.   But if a boat regularly dries out on a harsh bottom the repairs may been to be done at least annually.

      Whichever route you decide to take,  inspect from time to time while the boat is out of the water,  and be prepared for either periodic replacement or ongoing maintenance.



      Oliver

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