Catamaran Thoughts

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  • 07 Sep 2019 22:16
    Reply # 7869392 on 7869048
    Anonymous member (Administrator)

    What about a stayed mast?

    Howard,

    from time to time, I have also been pondering this subject. One thing is how to support a freestanding mast. That may in many cases be solved, somehow. The other challenge is to find some sort of rule-of thumb as for the strength of the mast.

    A third question is how to make an automatic sheet-release or sheet-brake device to avoid capsizing or overstressing the mast.

    One can quite easily find the max righting moment of a cat by multiplying its displacement with half the beam. However, the resulting righting moment is so far beyond that of a monohull, that it would require an overheavy mast (aluminium), or an over-expensive one (carbon).

    Then there is the question of simply staying the mast. There are two arguments against that on a monohull:

    ·         Since the shrouds has to be from the mast top, the shroud angles will be very fine and the unsupported pole (mast) under compression will be very long. In other words, the shrouds will add stress to, rather than relieve it from the mast.

    ·         Shrouds are in the way when we want to square out the sail for downwind sailing.

    However, on a wide beam catamaran the shroud base will be so wide that the compression stress may be reduced enough to make it work. A bit serious engineering has to be done, but a cat is a result of serious engineering, anyway. A pair of shrouds, set well forward, and a pair of aft shrouds, set as far aft as is practical, is all that is needed. One may even support the mast at the partners and step to reduce the problem with compression loads (reducing the ‘compression length’).

    As for sailing downwind, a cat may just as well be broad-reached instead of running straight before (faster). It may therefore not be such a big disadvantage with those aft shrouds (Btw. forget about running backstays)

    As for the matter of making a sheet-release devise. I think the simplest and most reliable would be to make a sort of sheet brake or elastic devise, set up to ease the sheet as the load exceeds a preset limit. This should be easy enough to make and adjust.

    From the safe position of my armchair  -

    cheers,
    Arne


    Last modified: 07 Sep 2019 22:20 | Anonymous member (Administrator)
  • 07 Sep 2019 20:46
    Message # 7869048
    Deleted user

    For my needs, a catamaran best fills the bill.   For what will mostly be solo sailing, the security of a fairly level platform, and the reduced fatigue of not living on your ear, and constantly worrying about stuff sliding around, etc is significant.   A bridge deck cabin means that I can sleep at the same level as the cockpit, and with all around view, sleeping in a recliner as I do, keeping a night watch is that much simpler.   It is where all my needs converge.  My target being about 30' LOA, but that seems less realistic than it once did due to payload.  


    The junk rig likewise fills the bill from a safety standpoint, as well as low effort, and minimal maintenance / low cost.  No standing rigging is a huge load off the mind in so far as upkeep and inspection.   Virtually instant reefing is a big safety factor in terms of capsize prevention if one is hit by a blast of katabatic winds for example. 

          Ideally I would prefer a single central mast, which is potentially doable using the main beam for the step and cabin top to carry the partner..... with suitable reinforcing to distribute those loads, and one would get about 1.5M more mast height for the same actual length (quick and dirty estimate).   You also have one of everything, not two of everything.

         On the other hand there is the issue of keeping things low, on a boat that doesn't heel until it's about ready to go over, and that points to the biplane rig.  Two low aspect rigs with shorter masts can put out a very nice spread of canvas...... though they do not reach up as high into the wind gradient...... a trade off.  They also, being shorter, can be lighter masts, and have less leverage against the boat to make it heel than a single taller mast would........and that is a safety factor.  With a high balance area junk rig such as a split junk at close to 30%, running wing and wong would cover a lot of the foredeck with both sails outboard.

         My natural concern about the catamaran as a platform for a junk rig is the fact that there is no heeling to absorb gust energy.   The only response can be acceleration or side slip, but as there is very little boat in the water, and the boat itself is far lighter due to lack of any ballast, there is little to resist acceleration.

        The question would be how much stronger would a mast actually need to be....  With a single mast, that should be fairly simple to work out from the stock rigging.  Opinions seem to be all over the map depending on the individual's prejudices, with some monohullers feeing that it need to be built as if it were simply to be planted in the ground, and on the flip side some people I feel overestimate the acceleration and side slip.... there is after all inertia to contend with, and I'm talking here about 3-5 tons, and that is not insignificant.  There may not be a lot of resistance.... It's not a case of the irresistible force and the immovable object, but neither is it a thistle down floating on the wind.  I suspect that the inertial resistance to acceleration forward or sideways is significantly more than heeling resistance on a monohull, but perhaps not nearly as much more as some folks suggest.   The force on the mast required to pull the boat sideways or capsize it with a stead pull is not the same as taking off with a rope tied 2/3 up the mast and enough slack to hit 30kts before the slack was out of the rope.     The same of course applies to a monohull.  A shorter, stouter mast goes a long way toward mitigating this

         Conservative sailing and good seamanship go a good way too, but Murphy is always hanging about looking for an opening...... "what can go wrong will go wrong... at the worst possible time".   A sheet fuse seems like a no brainer... there is a product called a "sailfuse" out there  

         Perhaps I spend too much time thinking about potential problems and issues, but I'd rather have thought things out and prepared than be caught by surprise mid Pacific.


                                                     H.W.


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