Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. masalai
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    masalai masalai

    Thanks Brian, That was a nice read... One question for me is relating to the stresses on the mast of a "hitch-hiker rig" - does the removal of the main reduce the loads?
     
  2. brian eiland
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    brian eiland Senior Member

    Prout Rig sailors

    The Prout catamaran rig had considerable influence on me when I first began to develop my aftmast concept.

    So it was interesting to have just this evening run across a forum discussion group that includes a number of Prout owners who seem to be quite happy with their aftmast rigs for cruising, albeit not raked rigs.

    http://www.cruisersforum.com/forums/f48/prout-rig-3362.html
     
  3. brian eiland
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    brian eiland Senior Member

    Sailing off Mooring or Dock without engine assistance

    ...this subject came up here...and I felt it better to add my response here, as it concerns this rig concept

    I imagined being able to do that as well with my 65 catamaran with a single masted ketch rig;
    You know I can still imagine sailing a big 65-foot catamaran with this rig right off the mooring, and back to the mooring, without the engine, by myself, with so little effort that I might take it out having only a few spare hours to kill, or a day sail. I could simply unfurl my 'central mainstaysail' and back it over if neccesary to go backwards or fall off the wind...all very tame and controllable in and around a crowded harbor area.

    And I would rig mine with tiller steering rather than a wheel and get a really good balanced helm. I would be less concerned with reefing by myself if the wind were to really come up. If I were short-handed at sea, I would have many of the benefits of a ketch rig, without the necessity of slab reefing the main and mizzen sails of the traditional ketch rig. And I wouldn't have to uncover any sails, nor recover them when I returned to port....I could be at the club's bar while everyone else was putting their boats to bed.


    Interestingly the 290' Maltese Falcon with its modern square-rigged "Dynarig" has manage to sail off, and back to its mooring.
     
  4. Pericles
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    Pericles Senior Member

  5. brian eiland
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    brian eiland Senior Member

    the Ketch Rigged Cat

    ...this comes from another forum, and I post it here to make reference to it in the near future

    I'm uncertain that he is still with us? Certainly he has not made any recent contributions to the subjects he so adamantly contributed to. Besides his contribution would likely be primarily aerodynamic in nature. I think I've shown that the aerodynamics of the situation conform with his findings and will be just fine. It's the stress loading that will be tricky.


    I don't know exactly where his Norseman fitting was located in his rigging plan. If he had only one single backstay on his rig that was clearly not enough. And I am unsure as to what size rigging wire and fitting were utilized. I have a feeling that his fitting was faulty, and/or his installation of it. The reason I say this is in reading his log I can find nothing really heavy or extreme weather-wise that should have brought the rig down unless it was greatly underdesigned by his own design calculations.



    First let me clear up this point about 'less stress'. I'm sorry if I've given the impression that my rig would be less stressful. Rather in fact it will be more stressful, both within itself, and in transmitting its forces to the hull structure. I only meant to say that because it has a lower CE, it will have less overturning moment on the vessel than the corresponding sail area of the taller sloop rig. So the side shroud loading could be less from a geometric point of view. BUT, on multihull vessels, this shroud loading can be SIGNIFICANTLY higher than on a monohull due to the huge stability factor** of the beamy hull form...yin and yang.

    Here is the gentleman & firm I want involved in the final stress analysis of this rig concept, Chris Mitchell of AES. I'm currently in discussions with him about this consultation work, and hoping to have a definitive client willing to pick up part of that expense.
    **(have a look at this paper, Design Notes, and particularly under "Cruising Catamaran Rigs", )
    (while you are in this paper have a look at "Geometry:Cap Shroud/Chainplate". I will reference that in a minute)

    Please understand that there are many designers and naval architects that could not do adequate justice to rigging analysis that falls outside the 'norms'. Free-standing mast systems are another of these 'outside-the-norm' design projects that need a specialist involved. Eric Sponberg is another choice of mine to evaluate unusual rig designs as he has had to do on many free-standing projects before. I'm the concept guy, no longer an engineering specialist.

    Having said that let me point out a couple of critical items I see in my aftmast rig design:
    1) Masthead Loading: Some folks have noted that I would not be able to maintain a reasonable tight forestay as a result of the mast leaning forward. But look closely at my masthead. The single backstay at that point is making a greater angle with the mast than many traditional sloop rigs....more reward pulling advantage...about 27 degree 'back-capstay' angle...pretty good.

    2) Hounds Loading: Here is were I will experience some problems. As drawn at present There is only a 10 degree angle between the lower backstay and the mast....thus significantly higher compression loads to the mast. But if I attach this backstay to the front of the mast and run it to the very sterns of the vessel I get a 15 degree angle...much better. And if I chose a 6 degree rake for the mast rather than the 10 degrees shown, things change again...likely for the better. This is a portion of the 'stress mapping' I am seeking, and this is Chris' forte.

    3) Capshroud Loading: As Chris points out in his paper you neither want too small a capshroud angle (big compression loads), nor too big a capshroud angle. But why not bigger....because on a traditional rig with a mainsail the head of the mainsail bends off to leeward and tries to twist the top of the mast excessively. I don't have this 'head-of-mainsail' load as I have no mainsail...so I might well make use of very wide spreaders and big capshroud angles to lessen the significant compression loads to the mast imparted by the stability loading
     
    Last edited: Aug 22, 2008
  6. brian eiland
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    brian eiland Senior Member

    Aft of Amidships Mast

    Interesting multihull site just brought to my attention by Sailing Anarchy.

    As I paged thru the site, this feature jumped out at me,
    "Several features give the M61 the most powerful and versatile rig possible for this sized yacht: a racing standard all-carbon rig, with an aft-of-amidships raked wing mast"

    Moxie Yachts http://www.moxieyachts.com/
    Click on their 'M61' model and then under 'Innovations', have a look at the 'Rig'

    ...(BTW it's not raked fwd, and it's not a mastheaded rig)
     
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  7. Pericles
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    Pericles Senior Member

  8. brian eiland
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    brian eiland Senior Member

    Attached Files:

  9. Spiv
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    Spiv Ancient Mariner

    55 ft Cat sail plan

    Well, it has taken a long time, but my new boat design is nearly complete.
    I engaged Richard Woods as my consultant in the design of the hull.
    He has patiently guided me while I designed the sailing rig and the accommodation and he has been exceptionally professional in his approach to the hull design.

    Brian, you can see a lot of your thinking in this rig with a mixture of other ideas of mine and from pictures. I hope you will give me your blessing, but please feel free to comment on any aspect of it.

    Here are the drawing of the rig.
    While I know that they will not suit every taste, they are just what I want: a mainless rig that is easy to handle single handed and that I can safely lower in a minute to go under bridges.

    The two fore sails will be on 'Stay Furlers', since the forestay are Dianeema rope, they can be bent over the trampoline when I lower the mast. They do not reef, so they will be either in or out according to the wind.
    The mizzen is on a conventional Furler that can be reefed.
    Lastly there is a storm sail that can be hanked in the unfortunate case I get caught in a gale.

    There you go, I reopen the subject for all to participate in the discussion...
     

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  10. Fanie
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    Fanie Fanie

    Eh Spif,

    It looks very nice. I like the width, although some think it just takes you longer to get to the head. You can't go too wide either else you won't fit in any harbour :D

    I like your sail setup as well. For your purpose I can certainly see why you have it as it is. That 128m^2 main sail is going to be a nice and powerfull bugger, better add a proper grab rail and some gloves... or get a decent winch. You may want to add a detachable boom on it's foot, especially for when you run.
     
  11. Spiv
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    Spiv Ancient Mariner

    Thanks Fanie,
    The actual size of the sails might still change a little, but the foremost sail will be a Yankee of ~120mq, the Genoa is ~60, while the Mizzen ~40.

    I don't think i will keep all the 220mq up in more that ~10kn of true wind.
    Then I will furl the Genoa or the Yankee depending of what the wind is doing.

    At 25kn, perhaps just the Genoa and furl the mizzen, then swap and just sail with the mizzen.
    With furlers it is going to be easy to reduce sail even going downwind!
     
  12. Alik
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    Alik Senior Member

    aft mast - dream or reality?

    I see Spiv's version of aft mast rather practical. Mast is close to vertical, frame stucture for transverse stiffness. It will be good to test it, but I do not expect excellent windward performance.

    Actually I do not belive in aft mast rig so much advertised by Brian - it has never been built and never proved working. Unfortunately some people are mislead by this nice but unproven concept.

    I have serious doubts if aft mast rigged boat is capable to tack due to sail area distribution. Aslo there are serious concerns about windward performance. Anyway soon we will have chance to test it on one of our designed cats.
     
  13. Spiv
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    Spiv Ancient Mariner

    Hi Alik,
    you are right that Brian's mast has not been tested yet, however the concept is aerodynamically excellent.
    You are mistaken if you say that aft masts have not been built as we already know of several, I repost their pics here.
    I have spoken to the owner of boat No3 and he very happy with his rig and assures me that the she goes to windward exceptionally well.

    My personal experience is that i could tack my 42' cat without problem when sailing with jib only.
    Anyway, who wants to beat to windward when cruising???
     

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  14. Fanie
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    Fanie Fanie

    On a smaller scale, haven't you guys seen what I did with the little trimaran I built ? Aft mast...

    I've slowly started on the 10m cat and have decided to do a 8m to 9m one as well, both with double masts and sails.
     

  15. Alik
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    Alik Senior Member

    I never said 'aft mast has never been built'. There are few boats built most known is 'Prout rig'.

    I just stated that Brian's aft mast rig has never been built and tested. It is aerodynamicaly excellent (if we neglect slack of forestay :) ) but structurally weak, and mainsail does not seem practical.
     
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