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Mast selection questions about steel

  • 31 Dec 2015 05:32
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    Reply # 3733074 on 3722587
    Deleted user

    Thanks for the help. Where I am located, it is actually cheaper to get a 33ft flag pole then to order an extruded pipe, buy the wood, glue and freight on the pipe. Aluminum pipe seems to be in quite scarce supply around here and would need to be trucked in. At $565 for a 20ft 6in sch 40 aluminum pipe plus $245 for freight to have it delivered, So at $810 just for the pipe without even the wood part, paint, and other materials I can have an anodized flag pole delivered for slightly more. I am just going to get a full time job until my boat is finished and ready to take off, so I can gear up properly. I have an interview tomorrow wish me luck :)

    Happy New Years!
  • 30 Dec 2015 19:13
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    Reply # 3732367 on 3722587
    Anonymous member
    Your solution might be to make a hybrid mast, such as I made for Fantail.  You can read about it in JRA magazine Issue 57 e–supplement ii, which can be downloaded from here.  I am going to do the same for SibLim, except with new (rather than second-hand) wood and a hexagonal top mast.

    The 6m x 5mm alloy tube supposedly weights 37 kilos, although it doesn't feel that heavy when you lug it about.  The topmast weight considerably less - I could easily pick it up on my own and carried it from the workshop where I made it to the marina, over my shoulder.  I assembled the mast on the marina (where we weren't allowed to do any work on the floats!)  I doubt the whole mast weighs much over 50 kilos.  Certainly, Fantail floated higher with the new rig than she did with the old sloop.
    Last modified: 30 Dec 2015 19:24 | Anonymous member
  • 29 Dec 2015 22:20
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    Reply # 3724042 on 3722587
    Deleted user
    Thank you for your input. I just came to the same realization. I did not factor in the buoyancy. Darn. I am worried about the weight, but will not have any place to build a 30 ft spar with 5ft 7in of bury anywhere :(


    Here are more accurate numbers for a wood mast for my boat

    Length (overall) 10.97 m 

    Base Dia. 215.8mm

    top dia 88.9mm

    Douglas fir

    112kg (247 lbs) (this is still around 5.6% total bouncy tisk tisk)

    4319kpm

    I feel a solid wood mast will be too heavy, paired with the fact I only have a marina slip to work in. Does anybody know about building a mast in 2 parts? If I can build half the mast at a time, I can probably build a spar on the deck of my boat with some sawhorses after I drop the Bermudian rig. It may just be worth saving up for a flag pole :(

    thanks for all the help!

     
  • 29 Dec 2015 21:32
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    Reply # 3724013 on 3722587
    Anonymous member

    I am not an engineer, but I think, from empirical studies, that a 4 inch steel tube will be unsuitable for an unstayed mast.  It will not have enough stiffness.  The design of your boat will influence how it responds to the weight of the mast.  Obviously, the heavier it is, the less critical the weight of the mast, or the easier it will be to stay within the recommended percentages that Arne suggests.  My vessel, Arion, displaces 5 tonnes and has a 10.5 metre mast (8.5 m above the partners).  The mast is alloy, 200mm at the partners and 110mm at the truck, with a wall thickness of 5mm.  I am not sure of its weight but it is too heavy to pick up alone, something less than 200kg, I'd guess.  The delivery docket gave the weight as 200kg, but that included the delivery pallet and the heavy flange on the base that I cut off.  It has no effect on Arion's stability. Under sail, with the junk rig's lesser heeling forces, the boat is stiffer than it was under bermudian rig.  My sistership, Minke, has a solid fir mast, 10 inches at the partners and 4.5 inches at the truck.  Her skipper said he wished he'd made the diameter of the truck a bit wider, to make the top of the mast a little stiffer.  Arion's mast is stiff enough.  Tom Colvin used 6 inch steel tubes for his junk rigs, I think, but they are loosely stayed.  Other friends have made unstayed masts from tapered steel tubes they bought from China, which are lighter than solid fir but use similar dimensions. 
  • 29 Dec 2015 09:53
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    Reply # 3723229 on 3722587
    Anonymous member (Administrator)

    The heaviest mast I have used added 5.5% to the boat's displacement and I  found that one to be unacceptable. The boat simply felt too tender with it. Later I have been able to build masts which are below 3% of the displacement, and this is much better. Your proposed steel mast will add 6% to your boat, so my guess is that it will reduce the stability and thus sailing power too much. The 12" wooden mast you describe is way too thick. If solid, you should get away with 21 cm , or 8.5" mast (.. I use higher safety factor on wooden masts to guard against imperfections in the wood...)

    Arne
  • 28 Dec 2015 23:48
    Quote
    Reply # 3722609 on 3722587
    Deleted user
    James

    Not sure why you chose a solid wood mast of 12" diameter as a comparison. According to my rough calculation an 8" solid fir mast tapering to 90mm would have a breaking strength in excess of your steel and ally examples, and weigh around 90kg.

    You need to consider stiffness as well as breaking strength in your considerations.

    Chris
  • 28 Dec 2015 23:14
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    Message # 3722587
    Deleted user

    I am searching for a mast most suited to my boat and have been running calculations on the jr calc. 

    I noticed that a steel 4 in (4.5 in od) sch 40 galv pipe was lighter then a solid wood mast. Galv steel pipe is super cheap here so I am thinking about using it. I only need 30ft above the partners.  

    Here is a comparison between 3 similar masts.

    sch40 4in (4.5in od) galv steel

    base diameter 114.3 mm  Mass 161 kg or 354.944lbs

    Breaking strength 2149 kpm (what does this mean and what is the lowest acceptable value?) 

    Aluminum Flag pole

    base dia 6in or 152.4mm

    top dia 3.5 in or 88.9mm

    mass 46.9 kg or 103.395 lbs

    breaking strength 2177 kpm

    Douglasfir solid 

    base dia 12 in

    top dia 4in or 101.6mm

    mass 186 kg or 410.06 lbs

    breaking strength 12153


    The steel would be nice since it would save me over half the cost of aluminum flag pole, and the 4.5 in diameter of the sch40 works really well in my boat and would be out of the way. I just want the boat to be safe. What do you folks think? Thanks for the help.
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
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