Best rig for small catamaran circumnavigator?

Discussion in 'Multihulls' started by randy quimpo, Jan 23, 2006.

  1. Pericles
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    Pericles Senior Member

  2. masalai
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    masalai masalai

    Hey Pericles, This thread was started over 2 years ago - I hope he is cruising by this??? still can't beat John Hitch for simplicity and ease of everything for multihulls - see "X-IT"
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    MAINSTAY Junior Member

    Half A-frame mast

    Since the mainluff stay is shorter than the forestay, the "enormous" tension can be less than the normal tension on the forestay to result in the same sag. And the mainluff stay tension may be less than the tension of a normal backstay, which is absent in this rig.

    In addition, yes, the rake of the mast will put more tension on the forestay, and this is desirable. The forestay is longer and requires more tension to limit sag to the same number of inchers (or centimeters) as the mainluff. But these tensile values are within normal ranges used by riggers. They are not "enormous" nor do they make this rig "unworkable" (quoted from below).

    You're going to have to show me the numbers. My numbers show less compression of the mast with the mainluff stay rig than with an aftmast rig or a sloop rig.

    Homework assignment:
    Get a sailplan of any Aftmast rig. Get two pens with different colored ink. Pencils, crayons or markers also work.
    A1) Draw a vector from the masthead along the foremost stay about an eighth of the stay length. If much longer than that, you may need to attach another sheet to complete the vector diagrams.
    A2) Draw a vector parallel to the masthead backstay from the lower end of the first vector to the mast centerline. This is the backstay tension.
    A3) From the lower end of the backstay tension vector draw a vector the same length as the first, but parallel to the inner stay. This is valid for
    masthead and fractional stays, as in the DynaRig (TM).
    A4) Draw a vector parallel to the backstay (or inner backstay as in DynaRig(TM)) from the current vectors' end to the mast centerline. This is the tension in the inner backstay or the additional tension in the backstay. All other complexities in the rigging increase mast compression, but are to be ignored in this assignment.
    A5) Draw 2 vectors down along the mast. The number is the same as the number of halyards, and represent the tension in the halyards. For this assignment, the length is exactly half the length of the first vector. There is no vector for the tension of the sail luff, because the stay tension is relaxed by the tenstion in the halyard. When the halyard tension exceeds the stay tension there is no tension in the stay. While this conditions seldom exists in a rig, but it illustrates the principle.

    Use the second color to:
    M1) Draw a vertical line (for the mainluff stay) from masthead to deck.
    M2) Draw a mast centerline from the existing masthead to a point 2/3 of the distance from forestay to mainluff stay.
    M3) Draw a vector parallel to the mainluff stay (vertical) from the lower end of the first vector (drawn in A1 above) to the mast centerline. This is the mainluff stay tension.
    M4) Draw 2 vectors down along the mast. These are the tension in a) the jib halyard, and b) the main halyard. For this assignment the length is exactly half the length of the A1 vector.

    A vector from the masthead to the end of the vector set represents the magnitude of the compression in the mast. This is a two dimensional analysis which is valid for monohulls where the maststep remains on the hull centerline. It is indicative, as far as it goes, of the relative magnitudes of the compression resulting from a 3D analysis. The vector diagrams should show that compression in the half-A-frame mast is NOT increased "by a large degree", but is less than the aftmast.

    Again, show me the numbers that the tensions make this unworkable. What in this rig causes bending moment in the mast? There is no sailtrack on the mast. There is no sail to induce bending or torsion in the mast. If you mean one of the modes of compressive failure, then yes, it would be calculated as part of the design. But there is not a great deal of compression as you wrote, and as I've shown if you did the homework. The compression is not abnormally high.

    If the mast is closer to the luff the advantage of clear air is lost. If the mast is separated from the luff by 10% of the chord of the sail, 20% of the lift is lost; at 0%, 30% is lost. That is exactly why we are considering aftmast rigs.

    Your presumption is wrong. Since there is no backstay, there is no reason to keep the induced drag losses at a maximum with a triangular mainsail, and every reason to use a more efficient planform.

    You are correct that the CE moves forward with roller reefing, and what you suggest is a proven way to adjust for it. But every rig with roller reefing has this problem. There are other ways also.

    Larry Modes
  4. sharpii2
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    sharpii2 Senior Member


    Hi, Larry.

    I guess the word 'unworkable' may have been a bit extreme.

    But what I do know is that mast aft rigs have had considerable problems with fore stay sag.

    Phil Bolger and Gary Hoyt both had one built recently.

    The Hoyt one had so much sag that it performed much lower than expected and was, IFIRC, inferior to a conventional fractional sloop.

    The Bolger one did much better.

    He knew exactly what he was up against and designed accordingly. His boat was a vertical sided sharpie with massive chines and sheer clamps, making the hull a very rigid box beam.
    the fore stay sag was correspondingly smaller and the boat was definitely superior to a fractional sloop up wind and, to a lesser extent, reaching.

    This extra performance came at a price. The price was constant vigilance on the set of the sail. A degree or two off, one side or the other, knocked the performance level back down to the more normal category.

    The rig you are proposing, as far as I can surmise, is going to be more complex.

    The buckling loads I mentioned in my previous post are more from luff tension than from halyard tension. There is actually going to be a downward component somewhere between the halyard and the luff, but since I can only guess what this rig is going to look like, I am not clever enough to say where.

    This, in itself, causes the mast to act more as a bow than a straight compression strut.

    With the mast aft rig, this is dealt with with large, aft extending spreader, which extends at least to the transom or even beyond. Your rig could have a similar spreader opposite the luff, off to the side. This could keep the mast from bowing away from the luff. Having the mast curve slightly away from the luff can keep it from bowing the other way, just in case.

    This extra structure, complete with trussing wires, will be beside the sail rather than behind it, as in the aft mast rig. This could make a difference in performance, as the drag might be higher.

    As far as your sail shape goes, I don't have a clue. If you are going to roller reef the sail around the fore stay, I don't see how you can escape a triangular plan form. Perhaps you intend to use vertical battens which run parallel to the luff. That's about the only way I can see getting any kind of a roach or shoulder into a sail that has to wrap around a luff wire.

    Or do you plan to slab reef?

    All in all, I don't see how this proposed rig would be any better, after all the engineering necessities are added, than a conventional fractional sloop. And I see it being much more complicated and expensive and harder to repair.

    A conventional fractional sloop can work with quite a bit of slop in the rigging because the jib is usually the last sail up and the first to come down. And, besides, it is usually the much smaller sail.

    I know this because I owned a fractional sloop that had such slop, due partly to inadequate turnbuckle tightness (due to the laziness of the skipper) and partly to the give of the light fiberglass hull. And it never failed to go to windward, even when pressed, with the saggy jib up. It just went to windward better with out it.

    Simple fractional sloop rigs can arguably work better on multihulls than on monos, because the wideness of the multi's beam. This allows the the shroud/stays to be further apart, greatly reducing the compression loads on the mast, especially when the jib is furled, than on a usually much narrower mono. This often allows spreaders to be dispensed with entirely. The mast can then be made lighter or taller as well.

    With a boat that is supposed to circa navigate the globe and sail in very remote waters, I would vote first for simplicity and reliability, even at the cost of some considerable efficiency.

    I have a feeling that, even if your proposed rig was able to produce the hoped for efficiency, it would not be able to do so untended, as it would be most of the time on a long voyage.

    I would encourage you to build your proposed rig and try it yourself in real life conditions.

    I could be wrong.

    Other innovative designs have been poo pooed in the past and have ended up working. Perhaps yours is one of them.

    Best wishes.

  5. sharpii2
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    sharpii2 Senior Member

  6. Paul B

    Paul B Previous Member

    Can you explain how this was tested against a well-designed modern fractional sloop rig?
  7. sharpii2
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    sharpii2 Senior Member

    Mast aft vs fractional sloop

    Hi, Paul.

    Actually, I don't remember.

    I think the Hoyt boat was actually raced against a fractional sloop of roughly the same proportions.

    I think Bolger just went by feel, using himself and other sailors as judges. But even then, there are some absolutes. Like how close it can sail to the wind and how fast it can reach with a given wind strength, compared to a fractional sloop of roughly the same size and D/L ratio.

    The important thing was that neither boat was much copied.

    Another important thing to note is that Bolger was not so much looking improvements in speed performance as he was better sea keeping qualities.

    With the mast aft, the boat rides much better at anchor and rises to the sea better.

    These two qualities alone, even if the performance slacked a little, were in his eyes, worth it.

    The balance while reefed problem was quite real, though. But it could be solved by using twin jibs and reefing them differentially.

    All in all, It was looked at, at least by Bolger, as a very expensive way to build a sail boat for a given amount of sail. And Bolger thinks that a sailboat should be built as inexpensively as possible.

  8. RHough
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    RHough Retro Dude

    I think you need to plug some real world numbers into your rig. If it comes out lighter and more efficient than a conventional rig or unstayed rig I'd be very surprised.
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    MAINSTAY Junior Member

    Sharpii2 wrote:

    I agree. The half-A-frame rig I propose avoids sag problems by keeping the mast between the main and foresails, rather than aft of both.

    I agree. The bowing of a mast is due to the component of the halyard tension perpendicular to the mast centerline, the force opposing it, and the leverarm between them. I believe a solution to be within the current materials and mathematics.

    I recognize your point. A mast must be designed to resist the bowing caused by halyards and other forces. In the fractional rig example you give, the bowing force would have a leverarm of some feet, and in a masthead rig of inches. So, even though the halyard tension may be larger in the masthead rig, it will not be 12x as large, and the bowing would be less than in the fractional rig where it seems to be no problem.

    I agree that aft extending spreaders are needed on some aftmasts, and also that shroud spreaders are needed on many boats. I recognize that there are needed to reduce the shroud or stay tension, and therefor the mast compression when there is a narrow base. These spreaders induce bending forces in the mast, which the mast or additional shrouds (etc.) must be designed to resist. I recognize that jumper struts are needed on some fractional rigs where they are used to reduce bowing of the mast when the mast itself can not handle it.

    While the aftmast may have a base of 10% to 15% of the LOA, the half-A-frame mast may have 35% to 45%. So spreaders, with their drag and weight aloft, are not needed

    Reefing with the half-A-frame rig can depend on the use.

    A coastal crusing single or couple can use roller reefing, with electrical help if senior or they wish. Roller reefing the main is without large mast or boom extrusions (or the need to quiet them), and as simple as jib reefing.

    A racer, regardless of crew size, or blue water or not, could use slab reefing, to minimize the forward shift of COE, to reduce the overturning leverarm, and to keep more SA with efficient shape driving toward the finish.

    A circumnavigator (and the others) have the option of setting smaller sails on stays placed in the plane-of-sails between the mainluff stay and the forestay. These stays would allow balancing the boat like a yawl or ketch. 123 up in light weather, 1-3 as it builds, and -2- when it rages, and a stormtrysail in survival mode.

    The half-A-frame rig gives a designer and a sailor reefing options, not limitations.

    I hope I have shown that the problems you raised, which are real problems, have solution are easily within the current materials and mathematics, and that there are rigs where these problems are of greater magniture and have been successfully solved.

    I hope I've shown how the half-A-frame rig is not more complicated, and hopefully less expensive and easier to repair.

    Larry Modes
  10. RHough
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    RHough Retro Dude

    Some simple numbers for the Half A rig:

    Assume transverse load at the masthead is X.

    Assume that a shroud to mast angle of 15deg on a 4ft lever balances that load. The compression load on the mast will be about 15X.

    Now move the mast so the shroud has a 2ft lever. To balance the same load X, the mast compression is now 30X.

    Just to keep the same mast up when you cut the shroud base in half, you double the compression load.

    Now look at the reduction in fore/aft stay base for the forestay and backstay. It looks like another doubling of the compression load.

    Now you have at least 4x the compression load on the mast that you had before you started. And you haven't provided for more than normal sailing loads. Now try to keep the mainluff wire under tension when it is on the leeward side ...

    No matter how hard you try, the mainluff will sag on one tack and be straighter on the other tack. That will require a complete re-trim of the main after each tack ... is that what you want on a low effort cruiser?
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    MAINSTAY Junior Member

    half-A-frame mast

    RHough, thank you for your comments.
    I agree that the relationship of the forestay tension and mainluff stay depend on the angles. But, my statement was not that there would be equal tension in the stays, but that it would not take an enormous amount of tension to prevent sag; and that since the mainluffstay is shorter, it would take less tension to in the mainluffstay to get the same sag as in the forestay.
    I agree that using the main sheet is an unacceptable way to get forestay tension. Why are you making that comment about this rig? I’ve never said anything about the mainsheet.

    A raked mast creates forestay tension because centroid of the mast is aft the step which causes a moment force. That force is counteracted either by tension in the forestay in a mast stepped on desk, or by added forces in the partners if stepped on the keel. With near vertical masts the additional tension is negligible or zero. But with a rake of about 20 degrees the additional is about 5% of the weight of the mast. It’s not much, but now you know it’s there. There is no bending of the mast to get the 5%, just gravity.

    Did you do the vector diagrams I suggested on 3/14? You’re a rigger. It’ll take you minutes. Until you do, you will be guessing that the tensions will be “enormous”, the compression “abnormally high”, and “much heavier” sections will be needed. Your 2x and 2x are good rules of thumb, but don’t apply here. Look, it took me a year plus half a dozen methodologies to get past that rule of thumb and finally believe my own math. If nothing else, do the diagrams just to show me where I went wrong. Or to see where the benefits come from. It’s all there in the arrows.

    I’ve started modifying my Hobie 16 (not quite 18) with a raked mast. It will NOT be a half-A-frame, since the mast will be on centerline. There are some practical limitations on where steps, chainplates, etc can be placed without major changes to the hull. But it will be a good test platform, and may show that some hull changes may be justified. The current design is to keep the same step, but rake the masthead 23”aft and use a bridle between the existing shroud chainplates to support the lower end of the mainluff stay, similar to the forestay bridle, with a tie to the fore crossarm. It’s looking like I may have to move the shrouds to the ends of the crossarm to keep the bridle forces within the hull’s strength at that point.

    I’m looking at two options for the jib: 1) to keep the stay attachment to the mast where it is, with or without a masthead stay to counteract the mainluff stay forces (oops, that’s 2 options already) and 3) to fly it on a masthead stay but keep the clew the same, which coincidentally keeps the sail in almost the same position as before the changes. In addition, the 23” gap is really minimal. The theoretical benefits will lost in the top fifth of the sail, and may be negative in the next lower fifth. But, 100% drive over a fifth, plus 85% over a fifth and 115% over three-fifths which averages 107% drive, which is better than 100%.

    On the Hobie modifications, I will not have the one problem you mention about the half-A-frame rig. One for which I have no solution, yet. Yes, the sag is different on different tacks. So, should I put a backstay tensioner on the shroud? That addresses it directly, but a what cost? Would replacing the plastic parts of the rig (the hull) make it negligible? Maybe not. I don’t have an answer. Yet!
    Larry Modes
  12. RHough
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    RHough Retro Dude

    Okay, I was feeling bad about hammering your mainstay rig so hard, so I'll try to be a clear as possible.

    Let's assume that you know how much tension you need in the forestay to create the desired sag. You are correct in that a shorter wire needs less tension to produce the same sag. Your conclusion that the shorter mainstay tension will be lower in the rig as you have drawn it is in error.

    When the mast is not vertical, you are right. I won't bother with the math, I'll take your 5% of mast weight as an addition to forestay tension. A Hobie 16 mast weighs what? 30 lbs? (I know I can pick one up and carry it around on my shoulder) ... 5% of 30lbs = 1.5lbs of extra forestay tension. For a forestay, 1.5lbs might as well be zero. If the mast weighs 100lbs, you still only get 5lbs, still zero as far as real world rig loads.

    I'll re-read what you posted. From memory, you had vector directions, but not magnitudes. I'll check again.

    I have respect for anyone that thinks outside the box. It seems to me that you are chasing a holy grail of reduced drag and higher efficiency of a wire supported sail compared to one behind a mast.

    There has been some discussion of late in the sail aerodynamics thread. If have not slugged through that thread, it is a good read. In a nutshell, the free standing mast may very well have higher drag w/o a sail attached. The drag from the mast exists on either rig, the total drag of the mainstay rig may be higher than a conventional rig and still have the practical issues I've laid out.

    Before you go too far, consider the dynamic response of the rig. Also factor in the mainsheet tension and it's effect on forestay tension. To have equal sag in the forestay with the mainstay rig, you have to consider both the load on the windward shroud and the mainsheet tension to estimate tension required.

    If it is alright with you. I'll draw out and post the vectors you described, the vectors in a standard rig, and the estimated vectors in the mainstay rig.

    I think they will show that the mainstay rig will create more challenges, rather than solve problems. I'm always happy to be proven wrong. :)
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    MAINSTAY Junior Member


    Yes the drag on a bare mast is greater than on a mast with a sail (ignoring the drag on the sail, so DO NOT hoist your mainsail in survival conditions to reduce the drag on the mast.):) Tom Speer has said that the drag of the bare mast is 25% higher. That's a big number. But how much does that increase the boat's total D? Perhaps <1%?

    I can think of no worse location for a mast than at the luff of a sail, where it nullifies the most powerful area of the sail. Doesn't one of Marchant's graphs show the difference in lifts between a sail with a mast, without a mast, and with a gap between the mast & luff? There was a 20% higher drive on the sail without a mast (at the point most favorable to my point, of course). If the 15% improvement in speed that has been reported by Procyon, by the aft-mast rigs, and others are true, then 1.15L/1.01D is a mighty attractive goal. Even with an A-frame this would be 1.15L/1.02D, and just as attractive.

    That goal may turn out to be an unattainable grail. Perhaps your vectors will show that. If so, are those giants or windmills I see out on the plains??
    Post away, sir.

    Larry Modes
  14. sharpii2
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    sharpii2 Senior Member

    Mainstay rig

    Hi, Larry.

    I have attached two drawings of your rig.

    Please look at them and tell me if I got it right.

    They are drawn in scale so the proportions are more obvious.

    I went with a 20 ft luff length because I thought that was the least I could get away with. The boom would be about 16 ft and the sail would have quite a shoulder on it. It would have full length battens all the way up that run parallel to the foot, for easy reefing. It's area would be either side of 200 sf.

    A fore shroud is shown because, with two stays lined up with the mast (the luff line and its opposite) and only one left as an aft shroud, the mast would topple backward. Other than that, I drew the rig I think you described.


    PS- What if you moved the mast to the center Beam and moved the luff stay 1/6 of the Beam over from that? Then you would get better staying angles and probably better luff tension.

    Attached Files:

  15. brian eiland
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    brian eiland Senior Member

    Tacking Problems with Bi-Masted Vessel

    In this subject thread I've expressed some concerns about the reaching and running capabilities of a bi-masted rig. But I don't think the subject of tacking problems has come up as anything negative.

    I just happened across this discussion about tacking a bi-masted vessel on another forum. I've not had time to digest it, but there are some follow on discussions.

    "Have now returned from my weekend of racing the Radical Bay 8000
    - 70 miles feeder race on Friday (no wind to Bft2 beam reach)
    - 25 miles double S-shaped bay race with lots of tacking, wind 15 - 40 kts on Saturday.

    What I can now tell you about tacking this boat is:

    1) In wind under 20kts-ish just before commencing the tack release the windward sheet totally and tack on the leeward sail. Wait for the (new) windward sail to fill and push the bows around, then slowly sheet in the leeward sail and off you go. Only in really flat conditions would we play around with the daggerboards to assist the tacking effort.
    Results 10/10

    2) In wind in the 20-25-ish kts we have a problem! Depending on wave action we cannot tack and have to gybe.
    Results 4/10

    3) In winds of approx. 30kts and more we cannot tack nor gybe!!
    When tacking the boat would stall 10 degrees from the turn, even sailing backwards did not work to push the bows through the wind.
    Attempting a gybe, the pressure on the rudders was incredible, could run down-wind but not turn further to the leeward side. Ended up anchoring in 40kts of wind on a lee shore! (Can really recommend the Fortress anchor!). And managed to break the mast / gooseneck boom connection due to a gybe which went wrong.

    This weekend's experience has put a big damper on my otherwise positive aspects of this rig design!
    Regards Roger"
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