Aftmast rigs???

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

  1. brian eiland
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    brian eiland Senior Member

    Orca

    There are 5 different boat photos attached at this posting #118

    The first one is a what I refer to as a wishbone mast that is placed in the aft part of the vessel. The owner of that vessel "Orca"contacted me just the other day with a new photo and this short note:

    A few days after this photo we got into some good stiff breezes and hove to (just to see what she would do) in a good rolling Puget Sound
    SEaster. She sat so nice and quietly over on her side about 10 degrees in 25 knots with just the staysail backed a bit and rudder up a bit. Very comfortable. That big keel and 50% ballast did the trick.
    On one long stretch with all sail up (both of them), in about 10-12 knots of wind, close hauled she sailed herself in perfect balance with the tide slack or near. I had been steering her, enjoying the ride, when I realized it didn't need me so I sat back and watched her work her way up, keeping the mark very nicely. What a joy!

    Regards, Owner of Orca



    BTW, for more on this Wishbone Rig subject there is a separate subject thread over HERE
     

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  2. brian eiland
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    brian eiland Senior Member

    Additional Backstays

    In a off-forum email recently I was asked the following;
    Actually I have given thought to having two backstays (either side) at this 'backspreader' tip location, but not really to help with the forestay loads. Rather these might be some sort of fixed or running backstays that might assist with athwartships movement of the outer tip of that aft jumper strut (backspreader) due to excess power developed by the mizzen sail. However extra backstays could also be detrimental as they might lessen the tension in the primary masthead backstay that I'm using in a double fashion as the forestay for my mizzen. This could allow excess sag in the mizzen forestay, and more tendency for back spreader to bend off sideways.

    I don't see a big decrease in compression loads to the mast tube with these extra backstays, so I've chosen to leave them off initially. And besides this, my primary backstay is anchored into the primary structure of the vessel that accepts the mast compression loading. I think that is a plus factor.
     
  3. brian eiland
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    brian eiland Senior Member

    Aft Mast Alternative on Big Trimaran

    Aft Mast Alternative on Big Trimaran​

    Several years ago I was asked by a gentleman to come have a look at a big Kurt Hughes trimaran he was building, a 63 footer he had stretched to 65, if I remember correctly. He was VERY concerned about handling that big mainsail on that big sloop rig.

    I spent a week as his guess at his home, and in the huge building shed outback he had erected to build this ultimate retirement vessel for himself, his wife and his kids. He is of Romanian decent, and has that Italian flare for home wine making (and drinking). Needless to say we had LOTS of fun drinking all sorts of home brewed wines and talking boats. :)

    Here is what I came up with as a solution for him in the context of my mast aft theme. Please realize I was restricted by the existing structure of the vessel that was already well under way in construction...i.e., bulkhead placements, crossbeams, etc, etc.

    There was one other nagging question he had, that needed to be taken into account....what if the fwd leaning mast idea would not work?? This persisted to be such a big question in his mind that I had to give considerations as to how my mast aft design could be converted back to a more standard rig configuration without a great deal of expense, and/or trouble to him.

    I still chose an 'all-3 sails-furling' arrangement....my single-masted ketch concept. But I made the mast rake almost half (6 degrees) of the original design. And the mizzen sail was made a bit larger in proportion. Thus this rig could be converted to a straight standing cutter rig with the mizzen becoming a more traditional mainsail attached to the aft edge of the mast. Or a new larger mainsail could be constructed for the mast that could be extended upward (taller), but still stepped in same location. The cutter jibs would then both be fractional, but would not require modification.

    Over all I sought to give him the same total sail area as the original design by Kurt Hughes. This was approx 1900 sq ft. BUT notice what happened to the mast height on these two versions !! The original sloop rig carried a 92' high rig. My rig carries that same sail area on a 73.5 foot mast !

    With this significant reduction in the rig height and the 'all-furling' feature, I imagine this gentleman will be able to handle this rig without a lot of additional assistance, and he may well be able to SAFELY sail into higher wind conditions without as much fear of being overpowered....his sailing efficiency should be improved.

    I had two options for the lower backstays here...1) anchored to the aft crossbeam ends (at a 13 degree angle to the mast), or 2) anchored to the ama hull ends (at a 20 degree angle). Both of these backstay angles are better than my original design, thus even less loading reqired of these backstays to offset the inner forestay loads.

    I felt this shorter rig could justify a slightly shorter daggerboard, and we opted to move that daggerboard slightly forward to both be in a better balance with the new sail plan, and to better fit in with a saloon modification he had already planned.

    To my knowledge at this time he is still thinking about his final rig design options, and his boat building processes have been put into a holding pattern while he attends to converting that original building shop into a multiple house dwelling. He then intends to build an even bigger shed to complete final assembly of this very big beamy trimaran. I'm not sure if he will chose my mast aft option, but I would be willing to place bets on it being a very good one for his needs.

    [NOTE: The second aftmast sketch represents a more current modification of the masthead attachment of its backstay, and the ama anchoring of the lower backstay(s). These are just suggested options to the original sketch, and certainly not final ideas]
     

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  4. Spiv
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    Spiv Ancient Mariner

    Brian,
    Could you explain why you prefer a wishbone boom rather than a simple pole as it is normally used on sel-tacking jibs?
     
  5. brian eiland
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    brian eiland Senior Member

    Are you speaking about the boom on the mizzen, rather than the jib?....as I do not have a boom on the self tacking jib (although it could make use of a modified club boom arrangement if desired).

    If that's the case then look how that mizzen roller furls around the backstay...that could not be accomplished with a traditional boom so easily. Besides I sought to keep any boom in this area off of the heads of the helmsman that might be found in this zone.
     
  6. Spiv
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    Spiv Ancient Mariner

    Yes, I am referring to the Mizzen-main.
    I am contemplating replacing the main of my Salina with a roller furler sail, similarly to the one in the first two pics pic below.

    The head-banging is of course to be considered, but at the moment I am swinging a 300mm boom and is well cleas of any standing person.
    I could of course swing a lower sail if I had a wishbone boom wich would compensate for the loss of sqm of the square top sail that is now on my boat (last two pics).
     

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  7. brian eiland
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    brian eiland Senior Member

    Ah-ha, another cruising sailor who is a little disenchanted with handling the tradtional mainsail. I'll have to think about that one.

    Certainly you might lose that nice fat head shape with the roller furling main....maybe even more productive area than any you would gain at the bottom.

    Might be interesting to see if the solution could be found using the vertical batten arrangement by Selden:
    http://www.riggingandsails.com/pdf/selden-masts.pdf
    scroll down to 'in mast furling'

    NOTE: I just googled 'vertical battens' and found a lot more references and suppliers than I was aware of previuosly.
     
  8. brian eiland
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    brian eiland Senior Member

    Power-Sailing Cat with Aft Mast Rig

    Back in posting #254 I made reference to and included a few photos of a 40 foot cat being built over in Thailand, and making use of the aft-mast rig.

    Here is a referece website page of that builder with more info and photos:

    http://tinyurl.com/catamaran-power-sailing-HK40
     

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    Last edited: Nov 17, 2010
  9. Spiv
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    Spiv Ancient Mariner

    A little disenchanted??
    You know that it's at least 6y that I have been saying "I Hate" them....

    In Mast furling would be nice if you don't already have a new mast.... I do and I am not going to buy a new mast.
    However I am quite willing to explore the furler system.

    Vertical Battens? Did not think of them before.
    Do you think they are useful?
    If that was the case we would see them in many headsails, or is it just the heard menthality that refuses them?
     
  10. michael pierzga
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    michael pierzga Senior Member

    Those verticle battens do a good job ..but they make it very difficult to drop the main and hoist the main. Helped a boat out this summer. Took us 4 days to find a suitable windless afternoon to drop the main at the dock. Might be more trouble than they are worth
     
  11. brian eiland
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    brian eiland Senior Member

    I do not know enough about them, and how successful or unsuccessful thay have been. I would go looking for all I could find on the subject.

    I had given thought to them in order to add roach onto my mizzen sail...which might only be an appearance thing. I had thought that if they were there to be furled up, and my furl would be external to a mast tube, then a 'sock' like affair could be used to cover them at rest or stored....like the ATN spinnaker sleeve,.... or maybe just the good old std sewn-on sail cover.

    No need to have roach on your headsails, so no need for vertical battens (at least full length ones). Besides they would be a real headache up there. I believe there are some short verticals for roller furling headsails.
     
  12. pbmaise
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    pbmaise Senior Member

    Philip Maise's Aft-Mast Rig With Crab Claw Sails - First Sea Trial Results

    Well guys, exactly one year after buying a demasted rig, I, with the help of all of you, am pleased to report I have an aft-mast rig with crab claw sails that sails upwind!!! I focused on eliminating negative aspects of the Bermuda rig. I married what I thought was the best of all the ideas floating around, used some high-tech materials and some low tech, and came up with a few creative ideas on my own. It works great!

    More importantly, I built a rig with fewer potential failure points, I eliminated most items that corrode, I can repair from the deck level most problems, and I have redundant lines that allow me to disconnect major support lines while working on them. I have all this on a rig that is lighter and less expensive to build and maintain.

    I had limited wind and sea range on my trip down to Singapore from Thailand, however, it was enough to demonstrate that even in a light breeze my smaller crab claw sail powered my boat 3 knots at a 45 degree angle to the wind. Further, tacking, involved nothing more then pushing the buttons on my autopilot. Not a single line was touched and we were again sailing 45 degrees to the wind on the opposite tack. With stronger winds, or by removing all the stuff aboard, I expect a lower number.

    Attached is preliminary report.

    Philip Maise
    Hot Buoys Trimaran
    Volcano Hawaii

    p.s. If you want to copy my rig, please call it by my name and I would gladly take paypal for a fraction of the thousands of dollars of reduced costs you gain from my hard work. No major research facility sponsored my work and now I need to recover some costs. I have no mast cars, no mast track, need only 3 winches to control two sails, no furlers, and reduced the number of sails needed to operate a boat. If you copy my rig, and eliminate these items too, you will save tens of thousands of dollars. 10% of your savings sound fair? Use pbmaise AT yahoo DOT com
     

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    Last edited: Nov 20, 2010
  13. brian eiland
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    brian eiland Senior Member

    I didn’t exactly understand how they dealt with drag of the rig either, but I did find these two quotes that were easy to miss…did you see them??

    "At the end, some runs were performed on the bare hull and rigging (without sails) for both yacht models at different apparent wind angles and in different heeling conditions in order to measure windage. These values are subtracted from each of the measured data points in order to produce the sail force coefficients"

    "Bare hull and rigging coefficients obtained by means of wind tunnel windage tests have been used in order to provide windage aerodynamic model input."
     
  14. brian eiland
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    brian eiland Senior Member

    Sagging Forestays

    I am as much of a fan of tight forestays and great sail shapes as anyone. I just hate saggy rigging. To me it represents a sloppiness in someone’s approach to sailing.
    Over the years there have been a number of postings suggesting that tight forestays are a MUST requirement, and that this aftmast rig will fail because it will be unable to maintain them.

    That being said, there are a number of alternative views. Some interesting examples occurred over HERE where there were some discussions of utilizing a headsail on an unstayed-mast/free standing mast. Super taught headstays are not necessarily the only option,…’the saving grace’ so to say;

    "One of the problems often cited with an unstayed mast is that jib luff tension is low. But why is this a problem? If the sailed is designed to have a large amount of luff sag, why should it perform any worse than a jib with a tight luff? Due to their very high speed, the asymmetric kite on a 49er operates at an angle of attack similar to most conventional yacht and dinghy jibs, so why aren't there any jibs that look like a 49er kite? Just curious."

    "Perhaps I can be a bit parochial and suggest you take a look at a Sonata that is doing well at the moment in the class at a national level.

    Originally David Thomas designed them with both cap and lower shrouds close to being in line with the mast, but with a fairly stiff top mast so the backstay would transmit some tension to the fractional forestay.

    Then Proctor started to make masts with a bendy tapered top that allowed the backstay to depower the main but forestay tension was lost. So the measurement rules were 'exploited' to move the masts to their extreme allowable forward position, and the shrouds moved aft to theirs. Due to the vagaries of Hunter's building quality, some boats were better able to do this than others as their main bulkheads varied in their position. Cap shroud tension was then wound on (à la J24) to try and get forestay tension.

    Well, this attempt to worship the 'god' of headsail luff tension actually only resulted in boats being pulled apart. The aft angles were so marginal that massive shround tension only ever gave marginal results. So ten years or so ago, Goacher Sails stopped fighting the problem and returned the masts to their 'inline' positions and cut genoas to match the sagging forestay. And they have been unbeaten since. It take a while to get used to the different interaction between the sail controls, but works in a wide range of sea states and windspeeds.

    I think the modern Code Zeros set on their soft furlers probably have similar properties."
     

  15. brian eiland
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    brian eiland Senior Member

    Ease of Use...important as well

    I’m not so sure that we are going to be able to put a definitive, quantitative numbers to this issue. As Tom Speer said, “I can understand the desire to simplify sail handling with roller furling staysails”… (all roller furling sails in Brian’s aftmast). These trade offs in terms of handling ease may trump some of the small loses (if any) in performance.

    We will probably have to be content with incremental indicators like this Italian wind tunnel study that show a superiority of the twin overlapping headsail arrangement to that of the single headsail sail, or non-overlapping configurations.

    For the cruising sailor we don’t have to be necessarily better in performance than the conventional sloop, if we offer other attributes such as ease of use, etc.
     
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