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

  • 11 Jun 2013 04:50
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    Reply # 1314269 on 1314255
    Anonymous member
    Pete Rasmussen wrote:Yes steel may be the only way to get enough weight aloft.  Thanks Paul, Gary.  I've no experience with working steel or the current technology but I'm sure interested.  Steel tube sounds like a really simple and easy way to go.  Are tubes available that will fit into one another so that it can be tapered?

    Forget steel tube in NZ. You'll not find the sections that you are looking for. Been there done that. I've designed and had fabricated seven steel masts to date. All have been good. Heavier than hollow wood but lighter than sold wood.

    Your very cheapest and most convenient option is a steel light pole. Most ones here in Auckland are made by CSP Pacific and are 8 sided but you can get 16 sided. The taper is more than you really want but it's not really an issue. The standard section can be made to work. The masts that I designed were custom fabricated for me but at six times the price of the standard sections.

    Come to think of it, I may have a mast that you could use. What LAP and bury do you need?
  • 11 Jun 2013 04:29
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    Reply # 1314255 on 1313268
    Deleted user
    Yes steel may be the only way to get enough weight aloft.  Thanks Paul, Gary.  I've no experience with working steel or the current technology but I'm sure interested.  Steel tube sounds like a really simple and easy way to go.  Are tubes available that will fit into one another so that it can be tapered?
  • 11 Jun 2013 03:15
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    Reply # 1314213 on 1313914
    Deleted user
    Pete Rasmussen wrote:There must be something suitable thats durable as well Gary.  I'm thinking the weight is not an issue in regards to being too heavy.  20,000 lbs displacement so mast can be 600lbs  !  I think its going to be difficult to make a large difference from the standard aluminium mast and rigging.
    Maybe the flexibility of an unstayed mast might just be the difference that matters in regards to comfort.   I understand the effects of weight aloft would be much more noticeable than weight below because of the leverage. 
    Dont know whats available in NZ, I stick to the stuff thats easily available in WAust. I'm happy with non marine wood if its going to be sheaved anyway, use 2 or 3 layers of glass if worried. 

    A good option too is steel tube, that'll give you your 250+kg, and with the new coatings available now days (epinamel epoxy based) which you can't even scratch off, it will last for ever.
  • 11 Jun 2013 03:01
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    Reply # 1314197 on 1313268
    Anonymous member
    Pete, you will find a heavy (relatively) mast will be a good thing. My La Chica, (LWL 27'9") with a displacement of 8.5 tons (loaded) handles her 200kg main and 100kg foremast without any trouble. As a general rule, you can safely have a rig that is between 3 and 5% of you designed  displacement.

    If you are looking for a cheap and robust mast, take a look at steel. If you want more info, let me know.
    Last modified: 11 Jun 2013 03:02 | Anonymous member
  • 11 Jun 2013 00:44
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    Reply # 1314089 on 1313268
    Deleted user
    The mast database project is live again.
    Last modified: 11 Jun 2013 00:47 | Deleted user
  • 10 Jun 2013 21:22
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    Reply # 1313914 on 1313268
    Deleted user
    There must be something suitable thats durable as well Gary.  I'm thinking the weight is not an issue in regards to being too heavy.  20,000 lbs displacement so mast can be 600lbs  !  I think its going to be difficult to make a large difference from the standard aluminium mast and rigging.
    Maybe the flexibility of an unstayed mast might just be the difference that matters in regards to comfort.   I understand the effects of weight aloft would be much more noticeable than weight below because of the leverage. 
    Last modified: 10 Jun 2013 21:36 | Deleted user
  • 10 Jun 2013 18:01
    Quote
    Reply # 1313767 on 1313376
    Anonymous member
    Arne Kverneland wrote:

    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.
    Could you guess (or calculate) what a good percentage for a schooner (with a lower centre of gravity) might be?
  • 10 Jun 2013 17:21
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    Reply # 1313692 on 1313456
    Anonymous member
    Mark Thomasson wrote:

    I have never been convinced that a heavy mast make a boat safer.  A heavy mast adds greatly to the inertia as it has a centre of gravity far above the centre of roll.  Now condsider, a large wave hits and the boat starts to roll. The increased inertia causes the boat to continue rolling, and over you go.  (Did some traditional sea going junks not take down masts in a big storm?)

    I would agree that, for a heavy boat, it may give a better motion is some sea conditions.  One thing I have noticed is how a heavy boat acentuates the pitching into a sea, whilst the light boat stick to the sea with an easier motion.

    Sorry, Mark, you've got that the wrong way around. The increased roll moment of inertia with a heavy mast means that the roll is slower to start when the breaking wave sprovides the impetus. Marchaj's tank tests were very conclusive in this respect, showing that a boat with no mast at all was much more easily rolled, and that a deep hull with much more roll moment of inertia of its own, combined with a lot of weight aloft, was the hardest to capsize.
    Also, the thing that accentuates pitching into a sea is weight in the ends of the boat, not weight of the hull per se. Of course, that is an argument for not putting a heavy mast way up forward. 
  • 10 Jun 2013 14:21
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    Reply # 1313484 on 1313418
    Deleted user
    Pete Rasmussen wrote: 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.   
    Wouldnt use Jarrah, too brittle - good for beams in the hull, where it doesnt need to flex. A hardwood like Australian oak (or Victorian Ash, same thing) would be excellent, only 25-30% heavier than Doug Fir & super strong. Not a marine timber so needs sheaving. I was going to use it if I didnt find a source of DF (which I found in the end).
  • 10 Jun 2013 13:55
    Quote
    Reply # 1313456 on 1313268
    Anonymous member

    I have never been convinced that a heavy mast make a boat safer.  A heavy mast adds greatly to the inertia as it has a centre of gravity far above the centre of roll.  Now condsider, a large wave hits and the boat starts to roll. The increased inertia causes the boat to continue rolling, and over you go.  (Did some traditional sea going junks not take down masts in a big storm?)

    I would agree that, for a heavy boat, it may give a better motion is some sea conditions.  One thing I have noticed is how a heavy boat acentuates the pitching into a sea, whilst the light boat stick to the sea with an easier motion.

    Last modified: 10 Jun 2013 13:58 | Anonymous member
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