Aftmast rigs???

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

  1. brian eiland
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    brian eiland Senior Member

    How about revisiting this early idea,..

    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]

    Attached Files:
    RunningTideYachts.com
    Distinctive Expedition Yachts
     
  2. brian eiland
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    brian eiland Senior Member

    Maybe we could take the next step and place that trimaran's mast straight up while attaching that mizzen sail right behind the mast, but still roller furling.
     
  3. brian eiland
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    brian eiland Senior Member

    Rating Rules Designing Our Boats

    There have been MANY postings in these discussions over the years that reject the idea that rating rules have so influenced our sailing designs, and in many cases hampered their development. In fact the opening salvo of my website suggest this,...


    Just today I was looking up a NZ gentleman rig designer Chris Mitchell of Advanced Engineering Services AES to see if he was still with us. aes home page https://www.aes.net.nz/

    I ran across this excerpted quote of his,..

     
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  4. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Hi Peter,
    Thanks for your attention to my rigging analysis errors. With age I just decided to drop this whole subject. But something clicked the other day as I looked at my model sitting on the counter, and I began to think of how I might make some improvements to fix at least the major concerns that have been presented over the years.......perhaps there is still life in this old rig idea?
     
  5. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Rigging Forces Review

    Looks like we have a big difference of figures here :eek:.

    I first took your analysis using 'moments of the forces about the base of the mast',.... and came up with some figures that are way different than what you arrived at.

    Illustration
    DSCF9587.JPG

    I used a 2-ton force in the forestay at a lever arm of 9"...total (2x 9 =18).
    I then measure my lever arm to that inner backstay at 2". For the moments to be offsetting the force in the backstay would have to be 9-tons (2x9=18)

    (Please note that I measured to than same inner backstay that you did in your original posting. The other two backstays penciled in there are proposed ones if the masthead backstays are lead down to the sterns rather than the base of the mast itself.)
    ...my method calculation next.....
     
    Last edited: Oct 1, 2023
  6. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    PS: I need to come back and edit this whole posting as i believe i have made some errors, and need to recalculate things,...
    UPDATE: I'm just going to leave this posting as it was,...then correct things below in another posting,..



    Rigging Forces Review


    I went back to my older method of determining the forces involved. Basically I look at what it takes to keep the mast head solid in its position from being pulled either forward by the forestay, nor reward by the backstay.

    In order to make the vector forces a little larger for clarity i chose a 2" long vector representing 2 tons of force (I also considered a 3 ton force so that's the reason for the extra marks,..disregard those). At the top of the mast note that I have 2 equal length, horizontal vectors representing the horizonal components of forestay and the backstay to offset one another. When we drop those lines down to the individual stays we can read their values,....2 tons for the forestay,...2.625 tons for the masthead backstay,... that adds up to 4.125 tons . Now if we project those individual vector loads back onto the raked mast, we will end up with a little bit less than that total imparted to the mast itself.
    DSCF9586.JPG DSCF9585.JPG

    You might note that these figures for forces to the raked mast are not that much different than if the mast were straight standing under that combo of fore & aft stays, & with the mast head at the same location.
     
    Last edited: Oct 1, 2023
  7. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    deleted this posting as not needed
     
    Last edited: Sep 29, 2023
  8. brian eiland
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    brian eiland Senior Member

    I intend to match their windward performance!
     
  9. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Hi Nick,
    I have some questions for you about 'torque analysis' verses 'vector analysis', but the forum keeps telling me my posting is TOO LONG.
    I guess I know longer know how to post on this form,...and/or add illustrations ??
     
    Last edited: Sep 29, 2023
  10. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Hi Nick,
    If I do your torque analysis on my lastest version of my rig I arrive at a figure of 5.64 tons of force in that masthead backstay

    BUT if I choose to look at the loads via vector analysis where the forward horizontal force is offset by an equal aft force at the masthead, I come up with something like 2.187 to 2.25 tons of force in that backstay.

    Could you (or someone else) explain that big difference??
    DSCF9589.JPG DSCF9590.JPG

    Note that I have eliminated that back-spreader at the hounds, and put the leading edge of the mizzen sail right behind the mast.
     
  11. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Vector Analysis Sources?

    It turns out that this fellow Nick that submitted the 'torque analysis' previously only made a single contribution to the forums,... back several years ago,...so is likely no longer available.

    Where might I find a site where I could ask my question about the questionable differences in the 2 analysis? I tried googling 'vector analysis questions' ?...one that maybe looks possible ?,..
    Quizizz — The world’s most engaging learning platform https://quizizz.com/admin/quiz/5f10cad2c8d64d001fc695fc/vector-analysis
    (PS: Doesn't appear to have been active in the past few years?)
     
    Last edited: Oct 1, 2023
  12. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    It appears to me that rigging loads are a proportional problem. What matters most is the backstay angle to the mast. It really doesn't matter so much where the mast is actually stepped on deck, as long as the stay angle to the mast is the same Rather than using angles, I prefer to use rise over run proportions. This simplifies the math somewhat. You simply divide the rise by the run and multiply that by the forward load of the headstays. But we must keep in mind that the longer the headstays are, the greater tension they are going to need to stay reasonably straight. And the less straight they are, the more compromised your windward performance is going to be. It is likely that for this reason almost all high performance sailboats have fractional rigs. The fact that multihulls are very much like airplanes in that they rely more on resiliance than on absolute rigidity for strength, adds even more to this problem. With a stayed rig, all the forwards and backwards loads are turned into to tension and compression loads on the hull. Without an incredible amount of rigidity in the hull, it is all but impossible to maintain the very high tension and compression loads needed to keep the forstays straight. The loose luff aft-sail, with its head attached to the end of an aft facing spreader, adds even more to the compression and tension loads. Yes, its backwards pull counteracts the forward pull of the headstays. But not by much. This is because the angle is quite acute, making its rise over run proportion rather large. This means that the lion's share of its loads are going to be verical tension and compression loads. This situation could probably be greatly improved by making the mast dead vertical and attaching the aft-sail luff to the mast. You should try a scaled sketch of this and see what your rise over run proportion ends up being to your backstay. I'd expect a significant improvement.There is no rule that says the aft-sail has to go to the top of the mast. Nor is there any rule requiring it to have a short boom, unless it has a backstay right behind it. It appears that your plan is to use back shrouds rather than a backstay, and that you intend to attach them to the aft crossbeam. If so, you should have adequate room for the aft-sail boom to swing. I think that attaching a huge roller-furling headsail to the outer headstay is likely a big mistake. As the wind pressure increases, the headstay will sag more and more under the load. When this happens, it will get harder and harder to get the huge headsail to properly shorten. It may even refuse to furl neatly. What can be done instead is to put a yankee style rooler furling jib there. This would have very limited area, but itts aspect ratio would be attractive. The bigger genoa style jib could be put on the inner forestay. It doesn't go quite so high up the mast, and it has a shorter luff. You can always set larger headsails in front of this. But they had better have their own luff and be able to drop by cutting the halyard if necessary.

    I think it is very useful to point out that absolute windward performance is a diminishing returns game. Once one gets to say 45 Degrees off the wind, at say a five knot speed, additional gains become exponetially more expensive. What really matters is speed made good to windward. So the faster your boat is, the more it can afford to point off the wind. This is good, because the faster a boat goes, the more forward the wind moves in relation to the boat (and sail rig choices much more limited). This is a big reason Multihulls almost all have fracional sloop rigs, with huge mainsails. I think it would be quit ethical to point this out to your client. As long as he is not intending to race, he doesn't need fantastic windward performance. What he neeeds instead is adequate and reliable windward perfromance, especially in a blow. What he can't afford is a rig that will sag dangerously or even collapse when he has to fight off a lee shore in a strong wind. At this point, he should have the yankee style roller furling jib, that I proposed, safely furled. He may even have the aft sail furled too. This would leave him with only the roller-reefable inner jib to contend with. And his beat to windward may look more like a close reach. But his speed may well make up for this enough to get him out of trouble.

    In short, I think it is ethical to tell him that he will be trading atleast some windward performance for safety and greater ease in handling.
     
  13. brian eiland
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    brian eiland Senior Member

    I don't see that method as less complicated than the two simple methods I outlined? And I am not so sure of the justification(s) for using such a method? (they appear to be based on a straight standing mast,...rise and run?)

    I'll come back to the rest of your posting shortly.
     
  14. brian eiland
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    brian eiland Senior Member

    DSCF9591.JPG Let me explain again my reasoning for questioning the torque method of determining the backstay load. I refer back to this updated figure,..

    Using the suggested torque method (rotational forces around the base of the mast) for just the masthead backstay alone yeilds a force of 5.76 tons required of that backstay. BUT if I were to translate that into its vertical and horizontal components, it would be pulling rearward on the masthead with 2 tons of force, rather than the balancing amount needed of 0.813 tons.

    That's why I question this torque analysis !!
     

  15. Nick12345
    Joined: Jun 2020
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    Location: Ireland

    Nick12345 New Member

    Hi Brian,

    True, I haven't posted much on this forum - guilty as charged!
    I mostly just read what people are saying over in the multihull section, as I am building (very slowly) a 35ft cat to my own design, and only spotted your post on Oldmulti's thread.
    The issue I think you are having with your vector force analysis is that you have not lined up the mast compression load with the angle of the mast. See attached image.
    Once the compression of the mast is lined up everything starts to balance a bit better. The vector triangles all add up and the force rotating fore, X, is balanced by an equal force rotating aft.
    Using torque was more about removing the mast compression element of your original design to help visualise the actual forces pulling the mast forward and what forces where actually opposing this.
    This is a very basic approach and there will be plenty of people on here much more qualified than me to advise on this.

    Any how, hope this helps. Keep up the good work.
    Maybe I'll manage another post in another 3 years!

    Nick

    basic vectors.jpg
     
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