Mast materials and Specifications

  • 04 Jun 2013 05:51
    Reply # 1308773 on 1306051
    I'm coming in a bit late to this discussion, but I might just point to the notes I wrote on mast sizing here. I used these principles in the design of Tystie's mast, and 75,000 miles on, I've no cause for concern. 

    Aluminium alloy does have a finite fatigue life, there's no getting away from it. So you have to design your mast so that the life is long enough, which to me means as long as my life, or the life of my boat. The less the stress, the longer the life. That means, in practice, the larger the diameter, the thicker the wall, and the fewer the flaws and holes in the mast (which are "stress-raisers"), the longer the life. There's nothing novel about living with this notion. Aircraft design is based on the principle of calculating, or establishing by testing, the fatigue life of a component, and then writing into the manual for maintaining the aircraft when that component is to be replaced, well ahead of the final fatigue life. We can't do that, with limited resources, so we must be very conservative in our designs. 

    The plain, blunt truth is that the Sunbird 32 foremast is subjected to too high a level of stress for too long, whilst sailing. The fact that Lexia's foremast stood for so long only suggests to me that in her previous ownership, she was not sailed very hard, or for very long. 
  • 03 Jun 2013 17:00
    Reply # 1308225 on 1306051
    Deleted user
    Yep, from the pics the foremast is a noticeably thinner than the main, so it would be 3.5in. Can't get over the fact such a thin mast is good enough for 36 years..  Aluminium must be better than I thought.
  • 03 Jun 2013 15:05
    Reply # 1308153 on 1306051

    When racing around Kvitsøy last year, the passage on the outside of the island was against the wind and seas, with some old waves across thrown in for good measure. The wind was about 20 knots, and I pulled a panel down to ease the forces a bit. The leg became a bashing sail with lots of spray and crashing into the waves, and I think the biggest strain on the rig was from the sudden stops when falling from the crests. I dont think the heeling forces are the culprit, but the whipping of the mast in a seaway.


  • 03 Jun 2013 08:35
    Reply # 1307822 on 1306051
    Anonymous member (Administrator)

                                                                     Stavanger, Monday

    It appears that there is a big gap between Sunbird Marine’s and Hasler/McLeod’s view on mast scantlings.

    When using the SA of Lexia’s foresail (217sqft/20.2sqm) and guessing the LAP of the 30’ foremast to be 8m, fig 8.16 in PJR ends up with suggesting a solid wood mast with a diameter of 20.4cm. That should give a breaking moment of around 3800kpm.

    The aluminium mast section of 5" x 0.25" (127 x 6.35mm) as suggested by Sunbird Marine, should give a breaking moment of only 1690kpm (still 2-3 times stronger than the 3.5" mast which broke). It would take a 7" (x 0.25") section to equal PJR’s wooden mast.

    A 6" x 0.25" section should stand up to about 2500kpm.

    Who is right? Two things are sure. Lexia’s mast lasted for 30 years. That is good. Then it broke. Not so good.

    My hunch is that the thin masts from Needle Spar may serve well enough for ordinary coastal sailing (in motoring distance from harbour). However, Ostar racing is anything but ordinary and calls for stouter gear.

    The righting moment of a 4.6ton Sunbird 32 should be somewhere between 2500 and 3500kpm. If one wants to go dancing Atlantic Rock’n Roll, one must have masts that reflect the conditions and the boat's stability.

    I am still very unsure of how one should distribute the loads on the two masts. Much simpler with sloops...

    Cheers, Arne

    Last modified: 03 Jun 2013 12:46 | Anonymous member (Administrator)
  • 03 Jun 2013 04:01
    Reply # 1307743 on 1307605
    Deleted user
    Thanks for posting Jonathan. It is a positive, at least, that Lexia's skinny mast lasted over 30 years. This has prompted me to sleeve Ashiki's, though I've always thought about it.
    Hope you get back in the race. Eating boat food on dry land doesnt sound too appealing..

    Jonathan Snodgrass wrote:
    Or maybe it was a failure waiting to happen. 
    Definitely, it is how aluminium works. Design one that lasts longer than the boat's lifetime is best.
    Last modified: 03 Jun 2013 04:01 | Deleted user
  • 03 Jun 2013 01:28
    Reply # 1307688 on 1307605
    Jonathan Snodgrass wrote:  At dawn the wind dropped and we wallowed at around half a knot.  If I had been just cruising I would probably have thought fine, let's wallow whilst I get some rest.  Alas, I thought that I should get some sail on and as it was pretty much dead calm I put both sails up fully.  I went below and set my timer for 15 mins.  As I woke up I noted that we were sailing much faster.  Just as I was getting up to look out the foremast went over the side, breaking at the deck. 

    So, my fault I think.  A bad decision, perhaps made because I was tired and was very sea sick.

    Absolutely not, Jonathan.  Even if you are keeping watch, on a dark night, a black squall can come at you unseen and raise the winds force from 3 to 6 in a few seconds.  Any rig should be able to withstand this.  The boat should look after the crew: not the other way round.

    Or maybe it was a failure waiting to happen.

    I fear that you are correct, here.

    Uninsured as I was technically racing.  Salt in the wounds.

    Simply bears out all my prejudices against insurers ;-)
  • 03 Jun 2013 01:09
    Reply # 1307676 on 1307335
    Graham Cox wrote:
    Apart from Galway Blazer, has anybody else given thought to an inbuilt jury rig system?  GB's bipole jury rig was ingenious but best suited to long light boats with lots of fore and aft deck space. 
    Bob Burns fit one to his Ebbtide 33(?), anything but a long, light design, and subsequently lost his masts in the Falkland Islands. (Too long a story for the forum).  He erected his jury rig and sailed back to the UK.  I believe he got into the Guinness Book of Records for having made the longest passage under jury rig.  Although I would have called it a planned alternative myself.      
  • 03 Jun 2013 00:59
    Reply # 1307668 on 1307261
    Arne Kverneland wrote:

    This was only varnished (2-pot Polyurethane) and I suspect that this has given the epoxy less protection from the sun than the paint on Johanna’s mast does.

    I think you're right here: I re-glassed two masts in Capetown and varnished both.  They lasted for about 7 years of total neglect before the glass started to fall off.  Had they been painted they would have probably have lasted 15 years of equal neglect.

    The strong argument for the hybrid mast is that it is so quick to make. The dug-out spruce mast called for quite some work to build (but not super accuracy) and also long waiting for the log to cure. Building from staves sounds like a BIG job and calls for high quality at every stage. In comparison the recent construction of my new hybrid mast was a walk in the park.

    Not only quick, but quite a lot easier for someone one their own and/or with limited woodworking skils, like me.  I would have been dubious building what is essentially a square mast for a boat the size of Fantail (while it worked just fine on the 1-ton displ Missee Lee), but for the topmast, which only has parrels against it in light airs, I felt quite happy about it.

    Last modified: 03 Jun 2013 01:03 | Anonymous member
  • 03 Jun 2013 00:33
    Reply # 1307662 on 1307204
    Graham Cox wrote:I would HEAVILY sheath it in epoxy and glass.  Lovely stuff, wood.

    I think sheathing a mast in epoxy and glass is a Good Thing, but if it's laminated, I don't believe it's necessary to have a heavy layer of glass.  We only put one layer of boat cloth on Badger's mast and they were still going strong after 20 years.  I've done a similar job on my topmast, but it's still far too early to tell if this will work and, of course, once there are a couple of reefs in the sail, there is nothing chafing the mast either.

    Interesting about the oiled wood.  I have heard of (and seen) parrels sawing ugly grooves into a 'plain' wooden mast; once it happens, bad becomes worse as the parrels roll into the extant groove and deepen it.

    But yes, wood is good: lovely stuff as you say.
  • 02 Jun 2013 23:28
    Reply # 1307634 on 1306051
    Thanks for the report Jonathon.  Blondie would have been proud of you, both for lining up for the OSTAR and for getting yourself back in one piece.

    I cannot comment on the specifications for your spars as I am not much of an engineering buff.  My mast is massive compared to yours, 250mm dia, with a 5mm wall at the partners, tapering to 110mm by 5mm at the truck.  This choice was the closest available section to the one on David Tyler's Tystie, which has proven itself over many ocean miles.  I am more of a maximum strength person rather than a minimum one, not being a racing sailor and also having a bit of an anxious personality.  So no advice, I am sure you will get plenty of that.  Good luck!
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