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Cf Masts: deflection vs strength

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  • 22 Nov 2014 20:23
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    Reply # 3156237 on 3151800
    Anonymous member
    A couple got back to me selling single items but they want to cut to fit in 40' container! Will keep you updated on progress.
  • 22 Nov 2014 15:32
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    Reply # 3156126 on 3151800
    Anonymous member

    The problem with Alibaba is that there is usually a minimum order. In the case of aluminum poles, maybe 5 or 10.
  • 22 Nov 2014 02:26
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    Reply # 3155985 on 3151800
    Anonymous member

    Has anyone experience with Alibaba.com?

    Looks like a good choice of round tapered street light poles, both aluminum and steel, varying thicknesses up to 4.5mm.

    In some cases they can make up to 12 Metres long with no joins.
  • 20 Nov 2014 07:50
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    Reply # 3154850 on 3151800
    Anonymous member
    Thank you so much David. A new one from C-tech is simply TOO dear! Am back to the drawing board.
  • 19 Nov 2014 20:46
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    Reply # 3154610 on 3151800
    Anonymous member

    Hi Roger, I had CF masts in Arcady, a boat I built in California. They were purchased second hand from an early Freedom 40 and supported my initial balanced lugsail sail plan very well. I later changed to a bermudan sail plan with a jib supported from the top of the foremast. This required the installation of a triatic stay and backstay to support the tension in the luff of the jib. I sailed her for twelve years along the Southern California cost and then sailed her across the Pacific to New Zealand and never had any problems with the masts. I saw considerable bend in the masts when hit by gusts under full sail, probably in the order of a meter of bend at the top. The masts were 52'-4" long and 12" diameter at the base, 4" at the head with an entasis taper. The wall section was about a quarter of an inch at the base and head, I don't know how the thickness varied over the length of the masts. They weighed just over 200 pounds each. The masts were made by Tillotson Pearson  and they are located in Maine USA if I remember correctly. Arcady is now for sale in Dunedin and the last time I looked at the ad he was thinking of selling all of her gear separately and wrecking the boat as he had had no interest from anyone wishing to buy her. I will have a look on Trademe and see if that is still the case. If so I will get back to you.

    All the best David
  • 18 Nov 2014 18:38
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    Reply # 3153645 on 3151800
    Anonymous member
    Weight, windage and, not unimportantly, aesthetics :-)
  • 18 Nov 2014 17:58
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    Reply # 3153611 on 3151800
    Anonymous member

    That's interesting.

    I wonder would it not be better to just get an 'off the shelf' straight tube 212mm OD, 6mm thick and add extra thickness at the partners and 1.5M above and below the partners? It would mean having to extend the mast head fitting outwards to allow for the yard halyard and lazy-jacks.

    I assume junk masts were traditionally tapered to save weight aloft, or was it also about windage?  With CF weight doesn't play a role.
  • 17 Nov 2014 10:36
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    Reply # 3152529 on 3151800
    Anonymous member

    Roger,

    Deflection is not only about thickness at partners and at the top of course. Diameter is very important. Ketil's mast is 200mm at the base and 120mm at the top. It could have been stiffer and lighter weight if it had been a straight tube or less tapered. But in the end the curve indicating wall thickness along the lentgh of the tube will show the expected deflection at the top. 8.5mm at the base and 4mm at the top are what Ketil's mast has. Looking at your boat, I would say your mast could do with some extra wall thickness. 

    Rudolf
  • 16 Nov 2014 18:24
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    Reply # 3152281 on 3151800
    Anonymous member

    Thanks Rudolf, thanks Arne. That helps and you're right about not looking up at the mast when you hit a rock. I have also tested the strength of my keel bolts.

    Rudolf, C-Tech recommend, on a spar sticking up 9 metres above the partners an 8mm thickness at the partners reducing to 3mm at the mast head. Do you think is enough to keep deflection to a minimum?

    Their calculations are based on a mast @ 250mm OD to 4 meters below the mast head, narrowing to 162mm OD at the top. Ideally I would like an even taper from about 230-250mm, narrowing to 100mm. They haven't got a straight taper mandrel that length.

    Do think it would be possible to use a lamp post as a mandrel and round off those eight sides? If I get the right taper would 3mm thickness at the

    100mm OD top still be enough? I'm thinking 4mm.

    If I spread my costs and decide against a tabernacle for now, I'll probably go for a 12.5 metre mast (10.5 metres above the partners) to replace the existing. I am still thinking 250mm OD at the base reducing to 100mm at the mast head, 8mm thick at the partners and below, reducing to 4mm at the mast head. Do you think that would be about right?

    Thanks again in advance!

    Roger
  • 15 Nov 2014 14:17
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    Reply # 3151924 on 3151800
    Anonymous member (Administrator)

    Roger

    Under the thread ‘New sail plans for Shoestring’ I suggested a way to guess the righting moment of a boat when the righting arm is not known, only displacement, beam, ballast and vertical placement of ballast. On Shoestring I came up with the righting moment, Mr  around 3000kpm +/-20%  -  that is 2400  -  3600kpm. A suggested CF-mast for offshore use then ended between 6500 and 9000kpm breaking strength.

    The uncertainty of the righting moment, Mr makes the resulting need for mast strength uncertain as well.

    My rather simple method only involves calculations of breaking (or yield) strength, not about bending (deflection) resistance (stiffness). My experience (and from reading about others’ boats) makes me think that if I make the mast 2-3 times stronger than the boat’s righting moment, and use a diameter (of aluminium masts) about 65-75% of that of a wooden equivalent, it will be stiff enough.

    Serious calculation of a tube’s stiffness involves the calculation of its moment of inertia plus the material's modulus of elasticity. I can do that (although I am only an electronic engineer), but with both the hull’s righting moment at max sailing heel, and also our tolerance for mast deflection, being uncertain, I find it easier to use the rule of thumb, above to decide the combination of diameter and wall thickness .

    • (..let’s say we have two tubes of the same bending strength. One has a big diameter and thin walls. The other has smaller diameter and thicker walls. The latter one will have a lower moment of inertia and will be more flimsy than the former one..)

    Conclusion:
    I would not try to teach the professional sparmakers their job, and certainly not when it comes to CF-masts, but I suggest you inform them about this: The boat will at working heel, have a lot lower righting moment than the max one, maybe only half. The stiffness of the mast should be calculated at this stress, not with the max shock-load with a safety factor in mind. Trust me, when you run onto a rock at full speed, you will not look up to study the mast top, but rather make a natural nod forward. On this, I am an expert!

    Cheers, Arne

     

    Last modified: 15 Nov 2014 19:51 | Anonymous member (Administrator)
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