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Why try for a light mast (stability, mast weight and rolling)

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  • 10 Jun 2013 12:28
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    Reply # 1313418 on 1313268
    Deleted user
     So durable hardwood should be fine.  I wonder if I could make the mast thinner using something like Jarrah or maybe Pin Oak  rather than Douglas Fir.   
  • 10 Jun 2013 11:43
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    Reply # 1313408 on 1313268
    Deleted user
    Hi Gary,
    Yes mine has a really stiff hull, with her current rig I can carry full main, staysail and large jib to 30 knots easy.  I've only used one reef!  She's very well ballasted also.  We usually have a cup of tea while every one around is on their ear.  This is great for going to windward but in a chop its awful.  We can get a very sharp motion in a cross chop, that is coming onto the beam.  So she's fast, very fast for her length because she can carry lots of sail but we pay for this when anchored or on a reach.  I actually recorded 18.7 knots last april on the way home from trying to get to the Junk Rig "Junket at Waiheke Island.  Rather large swell and 40 knots of wind from behind and whoopee.  18.7 serious.
  • 10 Jun 2013 11:32
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    Reply # 1313400 on 1313268
    Deleted user
    I was thinking of taking some lead aloft to see what effect it had on her motion just need to wear hard hats!
    So on a ten ton vessel I really don't need to be concerned about the weight of the mast.  I could use hardwood and it would still be less than 3%.  It may even be hard to get enough weight to make a significant difference to her motion.

    Really reassuring to have your experienced opinions on this matter Kurt , Graham and Arne.  Thanks kindly.
  • 10 Jun 2013 11:19
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    Reply # 1313391 on 1313268
    Deleted user
    Don't think the alu masts are that light, if they are strong enough for off shore that is. Being junk rig these boats are going to have more weight aloft compared to bermudian anyway. My two hybrid masts are similar to what birdsmouth timber masts would have been, but after the extra tubes are sleeved in I wouldnt call them light at all. Though still lighter than solid timber or steel.

    However, this all depends on the hull right? A stiff hull, or high initial stability can benefit from heavy masts to slow the roll down. But Ashiki is a dory with its inherent low initial stability and doesn't need too much weight aloft.
  • 10 Jun 2013 09:53
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    Reply # 1313376 on 1313268
    Anonymous member (Administrator)

                                                                        Stavanger, Monday

    I guess our unstayed masts, at least our wooden masts, would be called rather heavy, even by Marchaj. The first JR mast on my 23’, 1400kg Malena was 9.7m long, around 21cm diameter, solid wood and weighed about 80kg. This was a bit too much, with the mast adding 5.7% weight to the boat. We clearly had to reef rather early because the mast made the boat tender. I first cut off one panel height (95cm) of the mast and shaved off one cm of the diameter. This was clearly an improvement. When a new mast was made, it was made hollow and just 9.4m tall. The reduced weight (about 70kg - 5% of boat) definitely helped her, but the mast is still on the heavy side - and way overstrong.

    On my present boat Johanna (29’, 3ton) her 25cm hollow mast is around 90kg which adds only 3% to the displacement. From a strength point of view even this is a bit big, but Johanna seems totally happy with it. I guess wind drag is a bigger problem than weight. The mast gives the boat a nice and slow roll rate, so the boat is totally unaffected by swell from motorboats and ships. Only with the waves coming in on the quarter, the boat may roll in resonance with the waves. In all other conditions the inertia of the mast seems to be a good thing.

    When I now have gone for a hybrid wood-aluminium mast for my big dinghy (21’, 750kg), it is not so much to save weight, but rather because it was a quick and simple method of making it.

    If I dare suggest a rule of thumb, it must be that a mast weight up to 3% of the boat’s displacement is no problem from a static stability point of view (..on a cruiser...). The inertia of a "3% mast" mast ensures a nice and slow roll, and as Marchaj (and Kurt) points out; it also provides a barrier against being knocked down.

    Cheers, Arne

    PS: Racing is a different business. Then a superthin and superlight and tall mast is what one wants.

    Last modified: 10 Jun 2013 10:59 | Anonymous member (Administrator)
  • 10 Jun 2013 09:06
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    Reply # 1313361 on 1313268
    Anonymous member
    I belong squarely in the heavy mast camp.  Before I switched to junk rig I had a tall bermudian rigged mast on Arion that was intended for a boat twice the displacement.  The rigging supplier and many others shook their heads but the boat stood up to the weight aloft without a problem and had a delightful slow roll.  Besides the question of stability, there are also the engineering issues.  Masts and rigging with specs that are  2 or 3 X  the minimum have a large redundancy factor built in.  They flex less and fatigue much more slowly.  This is even more useful with an unstayed mast intended for sailing in the ocean.  My new alloy mast is massive by some standards that have recently been discussed in these fora, 200mm dia at the partners, 100mm at the truck, 5mm wall, sleeved to 10mm for the first 1.5m.  Light masts on light displacement racing boats make sense but the tendency of modern cruising yacht design to copy this trend is not one that I admire.
  • 10 Jun 2013 08:24
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    Reply # 1313339 on 1313268
    Deleted user
    Hi Kurt,
    Marchaj says heavier masts add to stability and he's going to be hard to disagree with.
    It seems to me to be a forgotten factor in safety, stability though it seems counter intuitive to me also.  Surely there's a limit to how much weight is effective in increasing stability beyond which the weight causes instability.
    The reason for my interest is that my 32 foot by 11 foot beam double ender is rather llively.  Slowing her motion would make her much more comfortable.  I'm thinking she was designed with a wooden mast in mind but the builders used aluminium and being lighter makes her livelier.  She's a cutter rigged masthead sloop.
    Last modified: 10 Jun 2013 08:53 | Deleted user
  • 10 Jun 2013 06:37
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    Reply # 1313300 on 1313268
    Anonymous member
    Hello Pete,

    Maybe because we're blessed with fairly heavy masts, I was easily convinced years ago that a slow roll rate is an advantage if a wave is trying to capsize you. In the only instance where this happened to us, (the wave trying and the capsize failing, that is) I was further convinced. The wave was gone and stopped trying before the boat had rolled dangerously far.

    I don't worry about the weight of our masts, but then mehitabel has shown herself to be stiff and stable, as well as slow to be rolled. I'm even less concerned about the weight of yards and battens that are usually brought lower when it's rough. 

    There's no argument, though, against the fact that weight aloft hurts the quantity called 'stability.' It's just not the only quantity that matters.

    Cheers,
    Kurt
  • 10 Jun 2013 05:15
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    Message # 1313268
    Deleted user
    I,m reading posts here and they seem keen on lighter masts.  I've read various authors such as Marchaj who state heavier masts add to seaworthiness.  Seems the inertia helps stability in rough weather.
    If these Authors are correct there must be another reason for wanting light weight masts even on cruising yachts and I wonder what these are.  Is it simply for racing? To keep the overall weight down.

    Is there an ideal weight for the mast in relation to the righting moment?
    I understand its a lot easier to ask a question than to give an answer so thanks in advance for considering this problem.
    Last modified: 11 Jun 2013 23:05 | Deleted user
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