Gaff Masts - replace timber with aluminium

Discussion in 'Boat Design' started by Two Bob Peter, Sep 16, 2021.

  1. Two Bob Peter
    Joined: Jul 2017
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    Location: Australia

    Two Bob Peter Junior Member

    I am looking at building 2 x new aluminium masts to replace missing timber masts on a 50' gaff schooner.
    Looking for technical assistance to confirm suitability of aluminium sections and actual design specifications.
    Cheers
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    G'day TBP,

    I see that you now have a Daniel Bombigher designed gaff schooner - that is quite a change from the aluminium trimaran you wanted to build a few years ago!

    Have a look at this post by TANSL on here re mast scantlings
    Mast and Rigging calculations https://www.boatdesign.net/threads/mast-and-rigging-calculations.55096/

    It is now 0500 hrs in Spain as I write this, but I am sure that @TANSL will be along soon to give you some good advice.

    I googled Bombigher's Shpountz (as mentioned on your home page) and found this sister ship for sale - she does look very fine indeed.
    I see that she has varnished wooden masts - they must need a lot of maintenance, hence I can see why you want to now go for aluminium masts.

    Bombigher 50 ft Schooner 1982 https://www.sandemanyachtcompany.co.uk/yacht/191/bombigher-50-ft-schooner-1982

    Bombigher 50' gaff schooner.jpg
     
  3. Two Bob Peter
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    Two Bob Peter Junior Member

    Hi Bajansailor
    I was building on the trimaran idea until this came along and I couldn't resist. Currently not quite like "Perseverance", the one pictured, at the moment, but work is well underway.

    Cheers
     
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  4. Two Bob Peter
    Joined: Jul 2017
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    Location: Australia

    Two Bob Peter Junior Member

    Oops - the picture is "Hollander" - now has light blue hull colour
     
  5. Old Stoker
    Joined: Feb 2020
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    Old Stoker Junior Member

    Tom Colvins Steel boat building,vol ,,2 has all the comparison, nice boat I was aboard,Nathaniel, a 55 Sitting Bull,with drop centerboards,she is built in steel,twin 60 HP Peugeot Diesels. One of Daniels finest.
     
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  6. The Q
    Joined: Feb 2014
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    The Q Senior Member

    Carbon fibre might also be worth considering, there are several carbon fibre builders who could do it
     
  7. Two Bob Peter
    Joined: Jul 2017
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    Location: Australia

    Two Bob Peter Junior Member

    Thanks, I will chase up that Tom Colvin info. I saw Sitting Bull in Darwin around twenty years ago, nice boat.
     
  8. Two Bob Peter
    Joined: Jul 2017
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    Location: Australia

    Two Bob Peter Junior Member

    Thanks,
    I did chase up carbon fibre, expensive option compared to aluminium or even timber. Has advantages, but for my project a bit too expensive.
    Cheers
     
  9. Two Bob Peter
    Joined: Jul 2017
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    Location: Australia

    Two Bob Peter Junior Member

    Further to first comment.
    The design and currently sailing vessels have 220mm OD timber masts - and there have been no reported failures of the design, so I feel no need to re-create all the stability / strength data - just use the assumption that if the specifications of a typical timber mast can be met or exceeded with aluminium - it is only the actual engineering of the masts to be confirmed.
    I did have an engineer do a comparison between the strength of timber / aluminium with the same OD and the above aluminium thickness, the aluminium is OK (6082 T6)
    and I have done a weight assessment - aluminium works out approximately 80Kg lighter over the two masts
    My idea is;
    Use 220mm / 8.2 thickness aluminium 13m (6082 T6 - DNV ) long section upto the hounds
    Sleeve a join with a smaller diameter tube and taper this down to 120mm over 4m length.
    What I am looking for;
    Double check on strength characteristics of the selected aluminium section/grade in lieu of timber.
    Method of joining
    Method of attachment of cheeks etc

    I am not expecting anyone to volunteer the information, It will take time to put this together / communicate and I expect to have to remunerate accordingly.
    Cheers,
    Peter
     
  10. Two Bob Peter
    Joined: Jul 2017
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    Two Bob Peter Junior Member

    Update.
    Thankyou for the input Bajansailor, Old Stoker, The Q.
    I have engaged an experienced Naval Architect to provide the specs. Looking forward to receiving the package so I can start the mast building process.
    Cheers, Peter
     
  11. gonzo
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    gonzo Senior Member

    Taking weight from the topsides will make the boat roll faster and may become uncomfortable.
     
  12. Two Bob Peter
    Joined: Jul 2017
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    Two Bob Peter Junior Member

    Hi Gonzo, thanks for the comment. I have considered that. Not know exactly what the previous masts were made from but using a median of the range of densities of timber that may have been used, I didn’t think that a possible 80kg would make a lot of difference. My thought was to prefer to go lighter rather than heavier, perhaps a wrong assumption?
    Cheers, Peter
     
  13. gonzo
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    gonzo Senior Member

    80 kg times the length of the mast create a huge moment force. Have you ever been onboard a sailboat with the mast off? It is surprising how fast they roll. However the amplitude of the roll is shorter.
     
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    Can allways build a crows nest up to make it equal..
     

  15. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    Masts off is a huge difference -this is not. This is the equivalent of a large crew member going two thirds up the bare mast. Would you note a significant change in comfort from that?
     
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