Aftmast rigs???

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

  1. gggGuest
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    gggGuest ...

    You may find that Marchaj has been superceded on some of the finer points by new CFD based work. I can't find the reference right now, but, for instance, I came across something quite recently which suggested the drag of a spar with a sail behind is very substantially less - I think it was between a third and a half - than one without a sail.
     
  2. Alik
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    Alik Senior Member

    I would not rely on CFD only in such comparisons; CFD is something that needs verification first of all.
     
  3. yipster
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    yipster designer

    its hard not to theorise astro physics, quantum mechanics and this rig
    got C Marchaj out to follow performance and wonder if srouding
    is with sprits and a bok this aft mast on a tabernacle can be lowered
    Fanie is sitting in front of his and eventhough its in dutch he keeps that secret
     
  4. peterAustralia
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    peterAustralia Senior Member

    before I call it a night, and go to work tomorrow....

    firstly, it would be nice if we kept it civil. Even it a person is verbally attacked in this forum, it is up to the person attacked as to how they choose to reply. It is possible to make a point whilst remaining polite. Example someone can attack me, saying I have only built one crappy little outrigger sailing canoe, I will try and reply civily all the same.

    As to professionalism and all that, if the rules of this forum were professionals only, then most of us would gladly disappear, but this forum is open to the public, as such people with constructive ideas should be able to be heard.

    The standard marconi rig cant be all that bad. It is so common. A couple of downsides is that it is relatively expensive, and difficult to reef single handed. I can speak from first hand experience in just a few boats, my sailing experience is pretty limited, but when I have had to do reefing, it has not been a fun easy process. Sailing on my cousins 27ft steel yacht, reefing was not fun whilst he steered. Sailing my outrigger canoe on Port Phillp Bay alone at 10pm at night, 5km from shore and trying to get the sail down when the southerly front came through was not fun at all!

    The points made on difficulty in tacking in aft mast rigs may have some merit, but perhpas they can be worked on. The idea of having an inner jib kept taught, whilst loosening off the larger forward jib/genoa makes a lot of sense. It should bring the center of effort aft and assist with tacking. Trying to attach a mizzen to the rear stay seems difficult, with forward rake and all.

    A fellow called Kris Seluga tried a crab claw sail suspended from an 'A frame' bipod on a 20ft catamaran. He commented he found it hard to tack. Most aft placed rid seen in this thread seem to have a much migher aspect ratio, and that combined with a smaller sail further aft, should assist with tacking.

    Does Fanie have any problem with tacking?

    An inner jib would make the boom that Fanie uses on his trimaran more difficult. I personally am a little confused as to why the boom is required. Perhaps Fanie can explain why he added it, I assume there must be some benefits otherwise he would not have done so.

    Say there was no boom, then for a given tack, the clew of the sail could be controlled with 2 lines, one from the centerline, and one from out towards the outrigger, thus the positition of the clew in 3 dimensions could be controlled. I am just a bit puzzled by why a boom was used.

    As to the maths. is a little complicated, depends on wind angle, wind speed etc etc. Say for a trimaran of Fanie's size, what would the typical forestay tension be? 500kg - 700kg - 900kg? (just a guess). If someone can tell me a typical forestay tension for a boat teh size that Fanie sails, then this week I will attempt a description of loads, that everyone and anyone is able to comment and on for good or otherwise. Loads at both no wind and at moderate wind. I will just to static loads as taught in high school engineering science, nothing too high tech.

    What I meant to write eariler was that I assume that forestay tension is the same as backstay tension.

    As to loads on the hull, a backstay would have a horizontal component, putting the rear deck in compression. If the backstay angle was increased so that it was more horizontal, then the horizontal compression loads on the rear deck would be greater, additionally they would be delivered over a longer area, thus a bit more weight would be needed to ensure the deck was strong enough to withstand these compressive loads.

    If the backstay angle is less, then a very high upward force would be imparted where the backstay meets the hull, and this area would need to be reinforced, but horizontal compressive deck loads would be less.

    One little idea that i had. Say for example that Fanie wanted to lower his mast at sea, how would he do it? My guess is that he would have a spar that lays on the rear deck, from mast base, to the rear stay. It would lay flush with the deck, possibly recessed and out of the way.

    When a cyclone comes along, Fanie starts to tilt his rig forward, the short spar which was laying horizontal is slowly raised until it becomes vertical. In this way the spar would support the mast as it was being lowered down. In normal sailing, this spar could take the horizontal compressive loads forward to the mast base. By having this spar at deck level, possibly recessed, then windage is eliminated, and center of gravity is kept low.

    If Fanie had a bipod say with a 10ft wide base, then I assume that instead of a single spar, a short horizontal laying bipod would take its place. In my own mind, I am trying to work out what otehr uses such a short horizontal bipod could be used for. Trailing fishing lines... hmmm, no that is silly...

    The ability to lay a rig horizontal whilst in a big storm might be a big plus. Assuming of course that the rig is not so high as to overlap teh bow too much and move the center of gravity too far forward, and upset the trim

    Laying the rig down would reduce windage and center of gravity, probably both pretty good things in a bad storm or cyclone.

    all these words will be better with diagrams...

    is 10.30pm here, and i have to go to work tomorrow...give me 4 or 5 days...
     
  5. brian eiland
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    brian eiland Senior Member

    Peter this rig idea started out as just a twin headsail arrangement in my mind with the headsails separated by a greater distance than usual at the time (1973). And I was looking for some manner to attach a mainsail, but avoid putting it directly attached to the mast (the thing I most wanted to avoid). I hit upon the idea of putting a mizzen on that one backstay, then I tried to arrange the whole sail plan over the vessel such that it would be balanced. Here is one of the original published dwgs:
    http://www.runningtideyachts.com/archives/OrgMagArt.php

    The rake in the mast just happened as a result of the geometry, and my attempt to not make the rig look too radically different than normal sailing vessels. Over the years I have NOT found any need to put more rake in it, and besides that would be more problematic. I have played with a version of less rake (about 6 degrees) along with a bigger mizzen. I've not found that this would be any less of an engineering problem that the 9-10 degree rake. I am convinced I would not care to go more than 10 degrees rake...besides it would not represent any additional gain.

    You might note on that original dwg I had no forward jumper struts at the hounds. All the dwgs on my website at present do show such a jumper arrangement. I have other newer ones that are unpublished, and that have an even newer arrangement than that, both in geometry, and a variation that takes advantage of the newer rigging materials we have available today.

    It was not long after I showed this rig idea to a professional sailing captain friend of mine that he came back from a trip to Central America with this aft rig version here (check out the mast rake, almost same as mine)
    http://www.runningtideyachts.com/archives/othermansboat1.php

    This idea has gone into hibernation on a number of occasions when I got too busy with other projects to pursue it. At several points in time I considered just dropping the whole thing, as it was going to be an even tougher sales effort than that I spent selling multihull vessels to the American public in those early years. But the idea keeps emerging.
    I wrote about that here on this subject thread..
    …..reference posting #98, Aft-mast Origination & Justification
    ...and posting #104, Sails in Combination


    I believe you might find that the most of the aftmast boats have their mast verticle rather than raked.


    The static situation is relatively easy to do with force vector analysis. I'm presently re-looking at this this static situation as I am considering some new variations in the rigging....some more modern materials and methods of attaching them. I will private message you about this. Perhaps you can review my work?

    I repeat, “it sure would be nice to do a three dimensional space frame computer modeling of the rig where lots of variables could be changed around to arrive at the most ideal form of this rig.”

    The dynamic analysis is more difficult. It was my hope that an adventurous client with a few extra dollars would come forth and we might employ a real professional rigging analysis such as Chris Michell at AES
     
  6. Fanie
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    Fanie Fanie

    How many tie points are there then ? :D

    And what difference would a few kg either way make ?

    Looks to me like whatever pull I put on the clew will be devided by the mast head and furling roller.
     
  7. RHough
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    RHough Retro Dude

    For a *very* rough estimate. Use your 105kg and apply it to the middle of the tensioned stay. See how much the stay deflects and how that effects the tension.

    R
     
  8. Alik
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    Alik Senior Member

    If the jib is fixed to stay, and stay is reasonably tight it is not 3-point attachment. Stay will take distributed load. Any engineer will tell You the same.
     
  9. Fanie
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    Fanie Fanie

    Yipster,

    I have no secrets, should I have ? Ask what you want to know. You all have helped out so many times...

    Thanks Peter,

    Hope you can sleep tonight :D

    At times I had problems tacking, but considering the hull it is not surprising, and it was no where near fast either. When I sat at the back the stern made a lot of drag because it was submerged, moving foreward had a significant increase in speed and tacking bacame easier, but made steering and playing with the sail difficult.

    Although a pesky thing on any boat, the boom had a few bonusses, I think it made a big difference to how close to the wind you can sail. Of course the closer to the wind the slower you would go to where you finally stop. What I suspect is that the foot of the sail becomes another leading edge. My arms was too thin to keep the foot taunt for more than a few minutes, especially when the wind was up, so I added the boom.

    While pointing more and more, the sail kept it's shape almost right to the point where it feels like you are going into the wind, which is what Manie referred to. I didn't think he would notice, but he's a lot sharper than meets the eye :D

    While turning gradually into the wind, right to the point where the sail form would collapse there is foreward movement. When the sail collapses however, there seems to be an offset you have to overcome before the sail would catch and function again. To compensate for this you can of course just push the boom sideways and reverse steer, keep in mind you'd be in irons here by now.

    When sailing at some speed, tacking wasn't that big a problem. I don't think it was more difficult than tacking with the windrider tri. I never fitted the auto tacking thingy.

    You cannot sail directly downwind with the aft mast without a boom, simply because the sail would fold in. In theory the sail should fly open and stay open but in practice the clew support will have to be taken too far sideways. While sailing downwind I had to tack without a boom.

    You will notice that the ball joint I had on the boom is a little ways in front of the furling roller - this made the foot taunt when reaching but gave it slack when running so the sail would dump downwards. It worked very well. The boom made it possible to sail any direction downwind without any problems and I could decide how flat or ball the sail should be by lengthening or shortening the clew line... dumping the wind downwards to sustain the lift I like. Made me feel thinner :D

    In a close haul the sail would look like an arrow from the rear, and would dump the wind towards the stern. The foot of the sail looked like a leading edge as much as the stayed leading egde. By moving the boom up and down I could also change the shape of the sail, ie how flat/ round it was.

    Re the forces on the mast and the tie point dispute :D
    Clipping the furling roller in place I suspect there was about 10 maybe 15 kg's of load on the stay.

    At this point if one consider this as a zero load on the stay because it is in rest then if the sail is open and stretched out flat then the load on the stay would also be zero.

    Bringing the clew down so that the sail can begin to fill, would increase the load. If the pull on the clew is say 20kg's, I cannot see that this can be exceeded anywhere else, there has to be distribution. What further lessens the tension in the stay imo is the fact that the sail is flying upwards and against the direction of the sail pulling the stay, but at an angle.

    Sitting right against the mast looking up there was some slight bent in the mast, but it never became much more than that. The mast was a 100mm x 80 x 3mm Alu one and six meters long, the sail was 12m^2. The clew once lifted me out of the hull and dragged me onto the trampoline in a bit of wind. Unfortunately I wasn't watching the mast or it's bend, I was trying to stay out of the water :D

    Mostly the times when the mast bent a bit, I got the impression it was bending more sideways than foreward, but probably more at an angle between the two.

    Putting the mast up or taking it down was not difficult. I made a slot where the mast foot would push into and you can right it. On a larger rig the same setup should work well but with the use of a winch and a gin pole, one person can do. The sail stay would keep the mast from going up and would keep it snug in place. Lowering it you would clip the furler loose and the mast can hinge backwards and down.
     
  10. yipster
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    yipster designer

    sorry, ginpole is the word not "bok" as i wrote
    mast up from a slot is almost a tabernacle
    and setting and strinking 6m goes easy you say
    apart from possible advantages in the sail plan
    my filosofy on these systems is they can perhaps
    better transfer a boat from motor to sail or vicaversa
    as Spiv's aft mast drawings show here
    for the mathemathicians bipod mast calcs
     
  11. brian eiland
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    brian eiland Senior Member

    Attached Files:

  12. brian eiland
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    brian eiland Senior Member

    Reducing Drag of Mast and Standing Rigging

    In general this is true. There are more indepth discussions over HERE:
    http://www.boatdesign.net/forums/sailboats/sail-aerodynamics-457-24.html

    BTW I have an idea about reducing the drag on my spar that will be relatively cheap and simple. At one time I was also working on a idea to reduce standing rigging drag via some replacable 'shroud covers'. Forgot where I put that info.
     
  13. pbmaise
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    pbmaise Senior Member

    My this thread is active.

    Brian..Thanks for posting your storm experience. I've not had to run or stand in front of waves the size you described. One difficulty I foresee is managing the change from running to having to deploying the para-anchor. If you kept running to the point it was no longer feasible..the situation would not be conducive to doing a lot of activity on deck to deploy the para-anchor. Therefore, I'm almost thinking it would be best not do any running and just deploy the para-anchor BEFORE seas are too big.

    Yes, I did have some contact with Brian before I made up my mind to go with the aft-mast configuration on my boat. Yes I kind of came up with the approach of putting a "crab claw" sail in front of the mast. I put in those quotation marks as some say I really have a lanteen.

    I most certainly looked very carefully at my boat before making the change. From the start..the rear box wall of my boat was designed to support the two ama, and it was designed to be a lifting point to lift the boat by crane. I have a photograph of that.

    I also took care to reinforce the box wall just in case.

    We started painting the decks today. I'm leaving Thailand on November 7th.

    Cheers Phil

    p.s. Yes I invested big money in this ...however..it would have been far bigger money if I went with a full baton main. A solution to the problem I know I didn't want.
     
  14. MikeJohns
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    MikeJohns Senior Member


    There are myriads of anecdotal tales but the tales are largley eclipsed by the work of people like C.Marchaj, A.Claughton and M.Renilson and others who have looked in detail at boat behaviour in large waves.

    Just be careful, Brian's obseravtion is for one particular event on one vessel ( a wooden ketch). On another vessel with something as simple as a better rudder arrangement and the experience may have been quite different. In Naval Architecture you cannot extrapolate form very limited observation unfortunately.

    In a cat it's probably not such an Achilles heel to have so much windage stacked aft under bare poles. Since they do seem to survive better when lying to a drogue. The real danger for a cat is being picked up under the bridge deck by a breaker and surging out of control.

    Upwind a mast will have a significantly lower drag with a sail attached (it prevents vortex shedding ). It seems a poor choice to have a non-attached luff on the sail abaft the mast for windward work. You lose both on the sail operating in the masts turbulence and a high drag factor from the mast itself.
     

  15. brian eiland
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    brian eiland Senior Member

    Forestay Material ??

    I was going back over this subject thread and ran across that video you posted Manie.

    WOW, I wonder what they are using as a forestay material. Everyone of those boats appear to have a very rigid, straight forestay. And it must be reasonable light weight material considering the motion as they swing it around the mast....very interesting. :idea:
    http://www.youtube.com/watch?v=8TtcZjAuF4A

    Does anyone know this class of sailing boat, and where additional info could be found?
     
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