Untapered aluminium masts....really that bad?

  • 22 May 2019 19:22
    Reply # 7356714 on 7356468
    Anonymous wrote:Hi Darren thanks for sharing this.  Good info.  53 foot mast on a 40 foot schooner seems lofty as well.
    Yes, a little long, she came that way.  She was a staysail schooner with freestanding masts that we're converting to junk.  The masts are keel stepped which leaves about 46' height above deck for the main mast.  We also have a pilot house and aluminum bimini which forces the foot of the sail a bit higher up the mast.
  • 22 May 2019 17:18
    Reply # 7356468 on 7356454
    Anonymous wrote:

    As another data point, our 40' schooner has all-aluminum masts assembled in the fashion Graeme describes.  Our 53', 10" diameter mainmast is parallel walled tube for the first 24', at this point the upper section of the mast is inserted 3' into the lower section.  The annular gap is filled with a aluminum rings at the top and bottom of the overlap with bolts joining the lower and upper mast sections through the rings.  There is some fairing that extends 4.5' above the join to smooth the transition between the two diameters.  The top section of the mast tapers down to 4.5"  Although the boat has only been a coastal sailor it seems to have held up well (masts were made in 1989).  Unfortunately, I have no idea where they got the tapered top sections from.

    Hi Darren thanks for sharing this.  Good info.  53 foot mast on a 40 foot schooner seems lofty as well.
  • 22 May 2019 17:05
    Reply # 7356454 on 7352044

    As another data point, our 40' schooner has all-aluminum masts assembled in the fashion Graeme describes.  Our 53', 10" diameter mainmast is parallel walled tube for the first 24', at this point the upper section of the mast is inserted 3' into the lower section.  The annular gap is filled with a aluminum rings at the top and bottom of the overlap with bolts joining the lower and upper mast sections through the rings.  There is some fairing that extends 4.5' above the join to smooth the transition between the two diameters.  The top section of the mast tapers down to 4.5"  Although the boat has only been a coastal sailor it seems to have held up well (masts were made in 1989).  Unfortunately, I have no idea where they got the tapered top sections from.

  • 21 May 2019 06:45
    Reply # 7353075 on 7352044

    David, 

    In the USA, the larger sizes of tube are available in 20ft lengths, and for your proposed schooner rig, you’ll be wanting lengths of up to double that. The tube is likely to be 8in or 10in diameter.

     The hybrid mast remains the practical pragmatic way to get masts of this size. I first made a hybrid mast, of alloy tube with a douglas fir topmast, in about 1976, and it’s still in service as the foremast of  Ivory Gull. Maybe I’ll be able to see how it’s holding up  this weekend, at Brixham.

  • 21 May 2019 01:23
    Reply # 7352647 on 7352044

    OK great replies.....sorry I forgot/ didn't realize that tube has a length limit.


    These rigs would be for a schooner rig for a boat up to 33 to 37 feet.


    Not sure what max length tube is available.  Would 2 Annie/ Raven style ali with wood top masts work?

  • 20 May 2019 21:33
    Reply # 7352379 on 7352044

    I am planning my next mast (for a fairly heavy vessel) to be a solid timber pole for the bottom part, and a large tapered aluminium tube (recently acquired) for the top part. I suppose the solid timber pole will be made to fit inside the aluminium tube, at the join, but I have not yet worked out a satisfactory detail for that, and not yet quite sure how to do it.

  • 20 May 2019 21:13
    Reply # 7352359 on 7352044

    Since we are discussing untapered masts here why not consider making the bottom portion of the mast out of timber and the top part alloy. Weight in not such a big issue at deck level and there should be no issue with strength at the partners.Or if a tabernacle is your thing I would guess that timber would be easier to fit. I hope that some of our more qualified engineering members can assist with the numbers here.

  • 20 May 2019 21:11
    Reply # 7352355 on 7352044

    Arne didn't call it a "split" mast, he called it a "hybrid mast" which is better terminology. ("Split" gives immediately a quite different image.)

    You can make a mast from two different sections of aluminum, and this is probably the quickest way of making a pseudo-tapered mast. There are a number of ways the smaller diameter tube can be built up to fit quite accurately into the larger one. I have tried various different materials and ways to fill the annular gap between the two tubes and they all seemed to work. If the gap is small, a fibreglass bandage is a straightforward way of doing it.

    I made one recently from three tubes (because that is what I had) and it looks quite good. You need sufficient bury of one tube inside the other. The final result (the join) must be robust against compression and also twisting (rotation.) The outside of the smaller diameter tube must also be built up so that the transition between the two diameters is fair, and will allow parrels to slide down easily. This fairing cone can slightly over lap the larger one below, and is easily made by a build-up of epoxy reinforced glass bandage. I am relying on this factor on its own to provide the strength to resist compression and twisting, expecting just a dab of glue inside of the join is sufficient. I I have relied entirely on epoxy glues and fillers, though I believe something like a polyurethane type of slightly flexible glue between the two tubes is probably better practice. For epoxy to stick well to aluminium, a light sanding and a pre-coat of resin on its own seems to be a good preparation and does make a difference.

    There have been a number of threads which touch on this subject, the most recent being "Recommended thickness for partners/mast-support?"

    Last modified: 20 May 2019 21:21 | Anonymous member
  • 20 May 2019 20:37
    Reply # 7352318 on 7352044
    David Dawes wrote:

    Annie did untapered with a wooden top (composite?) for her Raven rig.

    ...

    Has anyone employed an untapered ali spar or one with a small taper just at the top where it won't weaken too much?


    Yes its ugly, and yes its weight and windage up high.  But it is dirt simple.

    If you read Arne's Paper on the split mast you will find the reasoning behind the design. A) aluminum masts should not be welded (or are expensive to get done correctly). B) premade tubes of the right diameter/thickness only come in certain lengths which are generally too short to make a whole mast out of one piece. The idea is to use a non-custom made mast at much lower cost.

    So because we need extra length a top part must be added. Both Annie and Arne (and possibly others) have done this using wood as the top part and because wood is easy to work without unknowingly decreasing the strength, these have been tapered.

    Reading farther in Arne's document, he puts forth the idea of using a second aluminum tube with no tapper for the top part. I think, but am not sure, that someone has actually tried this with success. While neither tube has any taper and thus no welding that can weaken the tubes, the top tube is of smaller diameter so it can fit inside of the lower and may look like it is slightly tapered for this reason.

    Again, the idea of a split mast is cost. If the top is wood, why not make the whole mast wood? Again length of cost effective bit of wood. Yes wood can be scarphed to make longer bits... how much is your time worth and how much do the tools and jig cost to set up, how much does it cost to rent space big enough to fit a full length jig? What is the cost of moving the finished mast? The split mast can legally be moved in two parts from home to water where final assembly can take place. The bits to make it can be taken home on a minivan and possible be bought at local outlets. It is about making a big project smaller and much more cost effective without sacrificing safety or strength.

    For what it's worth, this is the method I would be looking at.

  • 20 May 2019 18:36
    Message # 7352044

    Hi everyone, One of the big expenses and decisions in converting or repairing junk rig is mast replacement or purchase.  Wood technical and takes time, steel strong  and welds but heavy, fiberglass....forget about it, flag pole and light pole ali harder to source and expensive.

    In comparison the cost and quality of what one could get in an untapered ali tube is a lot better.


    The problem with ali is that welding in a taper weakens the tube.

    Annie did untapered with a wooden top (composite?) for her Raven rig.

    Now with considerable wood working skills under her belt from her build I wonder if she will do birds mouth staved tapered wood or a simple composite again.


    I rather lean towards it for spars in my future (I will either buy a junk and have to replace or convert and make new ones.)

    Has anyone employed an untapered ali spar or one with a small taper just at the top where it won't weaken too much?


    Yes its ugly, and yes its weight and windage up high.  But it is dirt simple.


    Finally would a simple tube spar need internal stiffening by way of the partners?

    I look forward to your comments on this.

    Last modified: 20 May 2019 19:39 | Anonymous member
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