Masts for a large rig

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  • 23 May 2011 21:22
    Reply # 599211 on 595819
    Anonymous member (Administrator)

    Stavanger, Mon

    My problem with just calculating a mast’s strength from the boats righting moment is that the boat is also to be sailed downwind.

    On my first sailing trip in Johanna this spring there was blowing a F3, rising to F4-F5. The first leg was upwind with full sail up. On the downwind leg the wind picked up, but since Johanna steers so well with the sail fully squared out, I hardly noticed – that is until I noticed that we were touching 7.5knots! Normally 7.0kts is reckoned to be Johanna’s "sound barrier" so the forces involved were very high indeed. I couldn’t see any bend in the mast because you can’t detect it from the helm when going downwind. I still dropped a panel and as we headed up on a beam reach we at once needed another one.

    My (vague) conclusion is that for a serious cruiser, I would let the mast strength more or less follow the displacement of the boat (and height of the mast), almost regardless of the boat's beam or ballast position. The different boat types would anyway have to take sudden squalls from behind from time to time and none of them would tip over – forwards.

    Arne

    Last modified: 23 May 2011 21:24 | Anonymous member (Administrator)
  • 22 May 2011 23:21
    Reply # 598307 on 598022
    Arne Kverneland wrote:

    Stavanger, Sunday

        Scaling up or down a mast section using the cube rule

    Peter

    Finding the right scantlings for a JR mast, sloop or schooner will always involve some guesswork. One method is to start with a known boat with a mast from the same material as you want. The thing to remember is that the breaking strength of a mast varies with the cube of the masts diameter. If you want a mast that is twice as strong as another one, just multiply it with the cube root of 2 (=1.260). Likewise, if you want a 3 times as strong mast, the cube root scale factor gets 1.442.

    Now if we start with David’s "Tystie" at 8tons, your 20tons boat is likely to need a main mast that is 2-3 times stronger than Tystie’s main mast, at 8 5/8" (=21.9cm)

    Then your mast should be between...

    21.9cm x 1.260=27.6cm (=10.86") and...

    21.9cm x 1.442=31.6cm (=12.43")

    Note: I don’t know the wall thickness of Tystie’s mast. This must be scaled up too if the cube rule shall be correct. The 12" pole you mention sounds therefore reasonable if the wall thickness is not too thin.

    Arne

    Yes, I'd agree with that, Arne, so long as the "stiffness" of the boats compared is not too different. Tystie is of shoal draft, and not particularly stiff.

    The strength of a tube is dependent on its section modulus, which is given by:
    0.098(D4 - d4)÷D

    In inches, the section modulus of Tystie's mainmast tube is 10.24.
    The section modulus of a 12" x 1/4" tube is 26.5

    Thus, the 12" tube is 2.6 times stronger than my mast, and ought to be suitable for a boat whose righting moment is 2.6 times greater than Tystie's. I'd still like to measure that righting moment, though. It's quite easy to do, and if a new unstayed mast was  found to be not strong enough it would be an expensive failure.
  • 22 May 2011 14:18
    Reply # 598022 on 595819
    Anonymous member (Administrator)

    Stavanger, Sunday

        Scaling up or down a mast section using the cube rule

    Peter

    Finding the right scantlings for a JR mast, sloop or schooner will always involve some guesswork. One method is to start with a known boat with a mast from the same material as you want. The thing to remember is that the breaking strength of a mast varies with the cube of the masts diameter. If you want a mast that is twice as strong as another one, just multiply it with the cube root of 2 (=1.260). Likewise, if you want a 3 times as strong mast, the cube root scale factor gets 1.442.

    Now if we start with David’s "Tystie" at 8tons, your 20tons boat is likely to need a main mast that is 2-3 times stronger than Tystie’s main mast, at 8 5/8" (=21.9cm)

    Then your mast should be between...

    21.9cm x 1.260=27.6cm (=10.86") and...

    21.9cm x 1.442=31.6cm (=12.43")

    Note: I don’t know the wall thickness of Tystie’s mast. This must be scaled up too if the cube rule shall be correct. The 12" pole you mention sounds therefore reasonable if the wall thickness is not too thin.

    Arne

  • 22 May 2011 11:10
    Reply # 597891 on 595819
    Anonymous member (Administrator)

    Stavanger, Sunday

                                 About Samson

    Samson is a stretched version of the Samson C-Deuce/C-Strutter design.Originally it was to be 45’, but was stretched to 49’ by increasing the spacing of the frames and by adding a second main frame. The hull is of ferrocement, epoxy-coated and the deck is simply a planked fir deck, coated with Coelan. Displacement is about 23 tons. The main mast is 35cm at the partners and the fore-mast just below 30cm if I remember correctly. They are of solid wood, fir (pine) I think. They seem to hold very well. The masts are also coated with Coelan now and look great.

    Svein Magnusb Ueland, the owner, told me that once he was running before down the coast in a Force 6, he just dropped the main and sailed on the foresail (full, 37sqm). No problem; steering and handling were very easy and Samson went fast too.

    Arne

  • 22 May 2011 10:35
    Reply # 597845 on 597762
    Annie Hill wrote:Come on, David.  That's hardly encouraging!  Is there such a huge difference between 'Leto' and, say, 'Pacific Spray'?  They are both big, heavy, beamy boats, but if the rig is working on the Spray - and latest reports imply that your input has made it into a great success - then it would surely work on a Venus.

    And Arne - what about 'Samson'?  She's a 49ft heavy displacement boat.  Surely comparable with 'Leto'.  Peter, why don't you go to 'Junk Rig Association - Public Pages/Pages of information generously placed ....'/Arne Kvernland's Page/Samson, A 49ft Schooner'?  That will not only give you information, but inspiration, too.  If my memory serves, 'Samson' is ferro cement, so whatever else she is, she won't be a lightweight flier!
    Annie,  "Pacific Spray" has wooden masts, as does "Samson". The question was about alloy masts on a rig that big, and I can't think of any. Alloy can certainly be used, if a big enough size is available. I was only saying that I don't know what that size ought to be, and the best way to find out is to measure the stiffness of the boat.
    Last modified: 22 May 2011 10:57 | Anonymous member
  • 22 May 2011 10:00
    Reply # 597762 on 595819
    Come on, David.  That's hardly encouraging!  Is there such a huge difference between 'Leto' and, say, 'Pacific Spray'?  They are both big, heavy, beamy boats, but if the rig is working on the Spray - and latest reports imply that your input has made it into a great success - then it would surely work on a Venus.

    And Arne - what about 'Samson'?  She's a 49ft heavy displacement boat.  Surely comparable with 'Leto'.  Peter, why don't you go to 'Junk Rig Association - Public Pages/Pages of information generously placed ....'/Arne Kvernland's Page/Samson, A 49ft Schooner'?  That will not only give you information, but inspiration, too.  If my memory serves, 'Samson' is ferro cement, so whatever else she is, she won't be a lightweight flier!
  • 20 May 2011 09:02
    Reply # 595825 on 595819
    Peter Hosford wrote:A few years ago I designed A JR conversion (based on Derek Van Loan's book ) for my 21 ton gaff ketch but shelved the idea because of the difficulty and expense of finding or making masts. I am considering the idea again as the gaff rig seems to be getting heavier each year and I wonder whether any members know where to acquire masts of 50 to 55 ft.? I saw tapered aluminium lamp poles in Florida USA whilst cruising there about 12 years ago . Presumably hurricane proof, the poles appeared about 12ins. dia. at the base tapering to about half of that  at around 50 ft. An open wiring hatch revealed a wall thickness of about 1/4''.  I do remember thinking at the time that they may be good for a large JR but  I had several years of cruising and  oceans to cross before thinking of converting Leto to JR . Does anyone have experience using large tapered alloy poles  for masts on an ocean going JR ?
    My mainmast is 8 5/8" on a boat of about 8 tons, and you're going way above that, into realms where there is very little practical experience.
    I think you should go through the process of measuring the righting moment of "Leto", as described  in "Ask the Junkmaster", "Masts". Hire a load cell, and haul her down to the dock to thirty degrees of heel, and get some figures to base your mast calculations on. A 12" diameter x 1/4" wall mast is certainly going to be good for quite a big boat, but whether it's big enough for "Leto" I wouldn't want to guess at.
  • 20 May 2011 08:19
    Message # 595819
    Anonymous
    A few years ago I designed A JR conversion (based on Derek Van Loan's book ) for my 21 ton gaff ketch but shelved the idea because of the difficulty and expense of finding or making masts. I am considering the idea again as the gaff rig seems to be getting heavier each year and I wonder whether any members know where to acquire masts of 50 to 55 ft.? I saw tapered aluminium lamp poles in Florida USA whilst cruising there about 12 years ago . Presumably hurricane proof, the poles appeared about 12ins. dia. at the base tapering to about half of that  at around 50 ft. An open wiring hatch revealed a wall thickness of about 1/4''.  I do remember thinking at the time that they may be good for a large JR but  I had several years of cruising and  oceans to cross before thinking of converting Leto to JR . Does anyone have experience using large tapered alloy poles  for masts on an ocean going JR ?
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