Aftmast rigs???

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

  1. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Not sure you would get any better head stay leverage than you would get with a far smaller mast crane. 2:1 is 2:1, no matter how long the lever.

    The argument that you might get away with a smaller mast section might be true, if you are looking for the same sail area as a more conventional rig. But the more conventional rig can just add another set of spreaders and shrouds, and have the VCG of all this new gear far from the mast head.

    Another thing to consider is that the back stay has two jobs:

    One, to counteract the downward pull of the fore stay, and
    Two, to keep the top of the mast from moving forward.

    With the conventional set up, the back stay does both.

    With what you propose, the jobs get divided.

    Keep in mind, if the mast head moves forward, the fore stay will go slack.

    For this reason, you may need a set of additional back stays, leading directly to the mast head, to hold the mast head aft, if you plan to go too long with your mast crane.

    These additional back stays will be more than happy to foul your nice flat top main. They will also put more compression on the mast. But, for the same sail area, the mast will be shorter.

    So it may actually come out to be a wash, with neither option getting a real advantage over the other.

    IMHO, though, the conventional approach holds at least a slight advantage.
     
    Last edited: May 7, 2013
  2. sharpii2
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    sharpii2 Senior Member

    Can't believe I said this.

    The back stay keeps the mast head from moving forward. Period.

    It has no other job.

    It only comes down to the deck because there is nowhere else to attach it.

    The more vertical it ends up, the less effective it becomes.
     
  3. sharpii2
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    sharpii2 Senior Member

    Brian.

    I checked out your post (#345) and looked at your drawings.

    Just measuring where the mast sits on the deck, in relation to where the fore stays and back stays attach, I can see a pretty convincing case for the aggregate back stays taking up at close to three times the the load of the aggregate fore stays.

    The jumper strut merely allows the back stays to be shorter (a very good idea to limit stretch).

    Now that I think of it, this may not be so bad. Especially if windward performance is not at the top of the list.

    If the fore stays sag, and the jibs bag a bit, they will still draw (especially if they are cut to anticipate such sag). Even to windward.

    The fore stay tensions would also add to the back stay loads, and probably more so than if the mast were vertical.

    There was once a pdracer that had an aft mast, with no back stays at all, but was reported to go to windward reasonably well (by box shaped scow standards).

    I suppose there is at least argument that less than perfect fore stay tensions can be more than made up for by having a cleaner leading edge for the sail.

    Performance wise, there is probably a real limit to this (hence racing boats almost all have fractional sloop rigs), but with a cruising rig, where comfort, convenience, and safety may trump all out performance, this limit may come far later.
     
  4. pbmaise
    Joined: Jul 2010
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    pbmaise Senior Member

    I'm back. A bit nervous yes. Round two here we are.

    Hi guys.
    Made it to Cebu. Incredible trip. Really cheap too as I fueled in Brunei. I was doing the opposite of most cruisers. Since I had to motor up here I had to wait till dark and the winds died down. I then traveled through the night. With 6 crew members taking shifts I got to sleep 10 pm till 5 am. Joy!

    Each day we awoke just like we were on a cruise ship at a new destination. We just looked for spots along the coast that were we could anchor at about 15 meters. Our moto became if it is listed in the guidebooks you won't find us there. Well that is not entirely true. We did go swimming with the whale sharks.

    Well enough on the social.

    The new mast is 19 meters instead of the previous 24 meters. It is a single spreader aluminum wing mast made by Proctor.

    While in Miri I met two multi-hull owners. One was a trimaran owner like myself who regretted that he was talked into a big Bermuda rig. He hated it. Just like the builder and first owner of my boat he only sails his heavily reefed.

    The other owned a James Wharram cat. He installed a mast in the forward position. However, it is way shorter than a Bermuda rig. He had an overlapping head sail, and a rear crab claw sail. He said he loved his rig. He claimed the forces on his mast were small owing to the crab claw being a lifting sail.

    His crab claw sail had two spars sewn into the sail. To reduce sail area, he would lower the upper spar down.

    In researching crab claw sails I kept hearing that the lower spar gets lifted up to reduce sail area. That is the way I went the first time. I've decided to repeat this again.

    The Proctor mast has a halyard located almost at the very top. I suspect this was for a spinnaker/screecher. I plan to reuse this point to lift the lower spar of the sail and reduce sail area.

    I'm still working on the concept of rolling the lower spar upward instead of lifting it. Rolling didn't work on the first sea trials since the spar in the sleeve was round. It simply turned in the sleeve. Therefore, I'm considering reshaping my spar oblong.

    This time, unlike last, I plan to hang around and spend some time doing more sea trials and testing before heading out on a long trip.

    I'm only working with the smaller of the two sails I had Tasker make in Phuket. The bigger one is was left in Miri in storage. I also left behind several hundred pounds of gear. During the 25 day trip to Cebu we didn't use the big fiberglass dinghy once. So I have it and an engine on my eyes to drop weight. Instead of a dinghy we used the three kayaks. That worked really great. We even took on some fuel with the Kayaks.

    More later.
     
  5. Kojii
    Joined: Oct 2006
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    Kojii All is remodelling

    Semi-crabclaw

    You are correct with regards to crabclaw lifting in a puff and thus spilling excess wind while maintaining forward thrust. Ours does so as well though it is a semi-crabclaw - hanging off the innerstay. At first I thought it a flaw. I have come to regard it as a self-tending sail that rocks.
    Incidentally, running downwind when we set wing and wing we gain a full 2 knots over both set to same side. Still working my way though Marchaj and Gentry with regards to the interaction or perhaps non-interaction of the genoa and staysail (crabclaw-ish) but can vouch for the phenomena empirically.
     

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    Last edited: May 15, 2013
  6. pbmaise
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    pbmaise Senior Member

    Semi-crab claw

    Aloha
    Hang on a second. I thought I coined that term. Like your sail I am using just one spar and using the forestay as the second one.

    I have seen images of your boat before. This is first closeup with the inner semi-crab claw.

    I am going to mount my semi-crab claw at the 50 foot mark, and the mast is overall 62 feet. At the 60 foot mark there is a halyard on my new mast for a screecher or spinnaker. I had two semi-crab claws made with the idea in the back of my mind that somehow I could figure out how to fly both at the same time. However, I see your logic to have a conventional head sail since it can be tacked around.

    Could I get a detail shot of how you handle the attachment point of your spar to the mast. Also can you roll your semi-crab claw around the spar.

    Here are my semi-crab claw measurements
    Foot 36 feet
    Clew 45 feet
    Tack 62 feet

    Phil
     
  7. brian eiland
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    brian eiland Senior Member

    Hi Sharpii,
    Thanks for taking the time to look my rigging force review over.

    But I think you may have missed the figures I come up with for the 'aggregate loads' of my forestays and backstays.
    In my example if I were to add up the aggregate of both forestays I would come up with:
    Aggregate Forestays Loads.........1000+750=1750

    There are two figures I come up with for my 'aggregate backstay loads depending on whether I am rigging the vessel with the more 'shallow' backstays for a more open cockpit to sportfish from, or the broader verson where the backstays go to the extreme ends of the sterns.
    Aggregate Backstay Loads (shallow rigged).....1260+1940=3200

    Aggregate Backstay Loads (stern rigged)........1260+1340=2600

    In both cases here I am still less than 2 times the forestay aggregate,...not 3 times as you suggest by where my mast sits on the deck ….hmmm, interesting.

    And you might even say I have a tight 'forestay' for my mizzen sail without paying an extra penalty for that?



    The jumper strut really allows me to get a MUCH better backstay force angle at the masthead....no other way to do it that I've thought of.


    Would that be Bolgers experiments you are speaking about?
    Some postings about those back in #259, and #266

    Don't have time to address this at the moment.
    Brian
     
  8. Kojii
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    Kojii All is remodelling

    Staysail boom articulation

    Our boom jaws hold a block of aluminum with a SS u-bolt. The deck eye is through the deck into the cast aluminum L brackets underdeck which are bolted through the septum which runs vertically from deck to keel. This has been in place over 20 years. I took it apart and found the aluminum block had fused with the SS bolt but no other issue. Soaked it with aluminum treatment and will replace the block with something non-conductive. In mean time chromated it.
    As for range of motion - this system moves freely, without any thumping or catching. When it lifts, obviously, it is off to one side or other so hitting the stay turnbuckle has not been a problem. This point of stress could be a major failure prone point but for the free floating nature of the ring through ring. I think we have all seen the snapped gooseneck and cracked booms on standard rigs. That kind of torque never develops with this floating arrangement. The sail takes the boom where it will and the boom is free to go.
    The only trouble I have to deal with is the craddling when at rest. I am have to hang it to keep it off the sliding hatch. A UHMW block will work too. Not a serious issue.
     

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  9. Kojii
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    Kojii All is remodelling

    Mast attachment of inner stay

    Mast terminus of inner stay. Not likely to be of much help to you given the carbon/kevlar with the integrated ss terminus, but hope this helps.
    The forestay terminus is set into the carbon/kevlar spar and then bolted through to the backstay terminus. Two back stays out to deck plates at quarters.
     

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  10. flagg
    Joined: Oct 2011
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    flagg Junior Member

    Hi all,

    Complete novice here that has been following this thread for a long while. I am a fan of simple efficient rigs and appreciate all the the forms - ketch, yawl, catboat, single headsail, etc.

    However I thought I would try and contribute something to the discussion (just dont ask me to explain any of it! :)). I haven't seen this research mentioned in the aft mast or wishbone sailing rig threads and thought it was directly relevant to both (please forgive me if it has been posted before. I have had a good look and searched)

    It is a paper presented at the 2008 High Performance Yacht Design Conference in Auckland - "WIND TUNNEL AND CFD INVESTIGATION OF UNCONVENTIONAL RIGS" on "research activities carried out by the authors to investigate aerodynamic behaviour of several unconventional saiplans in comparison to the sloop traditional solution. In particular an “A- shaped” mast, placed in the stern area of the yacht has been considered in single-jib and double-jib configurations. Wind Tunnel tests and performance prediction analyses have been performed in order to compare different configurations."

    A link to the paper can be found on one of the authors websites Dr Viola who is a lecturer in Naval Architecture at the Newcastle Uni, UK.

    http://ignazioviola.com/ignazio_maria_viola/download_files/Viola_HPYDC2008.pdf

    Anyway I hope it adds something to the discussion as I thought it was a pretty good paper comparing a conventional sail type to aftmast headsail arrangements.

    Cheers.
     
  11. Kojii
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    Kojii All is remodelling

    Thanks for the post. Interesting paper. I am reminded of recent experience with Orca where we were working into 25 kts closehauled and exceeding hull speed (>9 kts) and trying to slow the boat down by reefing the big long-footed genoa. As we rolled it up we did not slow down. Quite the opposite of what we were trying to accomplish, given the amount of water coming in the open transom. We kept rolling it up until we had a very small foresail. Anecdotal, surely. The wind could have intensified, but curiously fitting with the theory as I read it. I also am cognizant of the importance of other aerohydrodynamics in the mix - very slender vessel; low freeboard; low windage; 50% ballast; furcula carbon/kevlar mast (obviously a different modulus from aluminum); and steep leading edge trapezoidal keel. Results will vary by any number of known and unknown relationships among the elements of the vessel. Regards
     
  12. brian eiland
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    brian eiland Senior Member

    Problems Printing Out Referenced Document ??

    Any reason I should have a problem printing out this PDF document?....My same old printer acts like it wants to, then it does not?
     
  13. Kojii
    Joined: Oct 2006
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    Kojii All is remodelling

    try renaming it and saving it with shorter name maybe

    Printer may be reluctant to accept innovative or "radical" concepts. I have seen that before...;)
     
  14. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    :D:D:D:D:p

    I tried that renaming trick, and it still didn't work.

    I've had this happen to several PDF documents recently, and I wonder if it has anything to do with all of these MULTIPLY notices I've received in my email from numerous sources indicating some PDF update bull...t

    I'm wondering if Adobe has come into my computer and updated something that forces me to buy some subscription from them??...if so I'm pissed, and I wouldn't put it past these software jerks that want to keep changing things....like Microsoft, etc (my good old Windows XP is now 'old stuff', and my Explorer 8 browser is now 'incompatible' with my Gmail accts)
     

  15. dskira

    dskira Previous Member

    The downside is when you reef the center of sail is going forward, so much that the boat will be not balanced anymore. The Latin sail will make more sense.
    As for the stay, mentioned by a member already it will be impossible the keep it taught.
    That said, two mast at the aft end make a lot of sense. And this is the paradox: good and sound mast and boat structure, bad sail positioning.
     
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