Balance Lug Design Ideas.

Discussion in 'Sailboats' started by LP, Jul 20, 2013.

  1. PAR
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    PAR Yacht Designer/Builder

    Knowing what I know about your design, don't worry about "centering" the rig. It'll make very little difference in sailing abilities on that hull, with the performance potential it has. Unless you have a "fouled" tack, because of a spar, there will be insignificant differences, with the mizzen offset.
     
  2. LP
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    LP Flying Boatman

    Hi, Paul. It's purely esthetics and probably a fools errand. And it's an opportunity to complicate and confuse things. Isn't that enough for you? :D I'm keen on trying the stayed rig with an offset rig. In this case, it would throw things (the mizzen) back in line and meet some strange, probably mistaken, sense of balance. I'm guessing I have a couple of years to change my mind. The A-frame mast is still lurking about.:p
     
  3. PAR
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    PAR Yacht Designer/Builder

    Ouch, an A frame, really? I need to get some of your medication, my friend.
     
  4. woodeneye
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    woodeneye Junior Member

    Mmm, over complication of what is a such a simple rig is to its detriment I think. Let's face it, the balanced lug will never be a "performance rig", although it does a very nice job, very simply and performance is quite surprising. There is so little force on the mast that to introduce stays for any reason would not improve things too much. My GIS has a simple box mast that is lighter than my Laser mast and it holds the full 105 sq ft rig with two up in 25 kts with hardly a bend. To counteract windage and the so-called effects of the bad tack, just do what Storer did and increase the size of the sail by 5-10%. Problem solved :)
     
  5. sharpii2
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    sharpii2 Senior Member

    I agree.

    The main reason to make a BL stand off the mast is chafe. That's the main reason the one designed for my 'Lola Boat' is split. It can stand day after day, week after week, without the mast wearing a hole into it.

    I share your suspicion that just about any other gain, such as efficiency, is likely purely imaginary and likely to never pan out.

    This vaunted extra efficiency comes at the cost of extra complexity and often more weight.

    My split BL (an idea Phil Bolger utterly detested), is likely to be more difficult to raise and set, as it is more likely to hang up and bind, and certainly more difficult to reef properly.

    I consider it worthwhile only because it's intended to be a blue water rig.
     

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  6. LP
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    LP Flying Boatman

    I think that Mr.Storrer has done a great job in getting performance out of the BL rig. His website is extremely informative and was influential in my rig choice. I don't disagree with over complication being a detriment to the rig. On the other hand, a stayed Bermudian rig could be seen as an over complication to the unstayed version of the same rig, but we all know there are definite advantages to the stayed version. I'm not making any claims about increasing performance to the BL rig. Freeing the sail from mast interference certainly can't hurt. I'm also looking for other advantages that would outweigh the disadvantages of the stayed version. I think a reduction in weigh aloft and a reduction in chafe on the sail are factors worth considering. I feel that weigh savings is real in the stayed rig as the mast dimension is cut by 25% laterally with the stayed rig and the added components will not weigh more than the mast reduction. The mast section I'm looking at builds on the birds mouth concept, but uses various sized strakes to get the proper shape.

    Mastsection.JPG

    The rig on my design is never going to be a performer as it's a split rig, but I'll take any performance gains I can get if they are real.

    BlackRock24nCBTSElpSailplan.jpg
     
  7. LP
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    LP Flying Boatman

    What are the dimensions of the box. I'm curious how close my mast dimensions are to them. My mizzen is carrying 110 s.f.
     
  8. sharpii2
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    sharpii2 Senior Member

    LP

    The Bermuda rig lent itself to still staying even more than the Gaff rig it replaced.

    It went through an interesting evolution.

    First, as mostly jibless rig, it had no stays at all.

    Then a very small jib was added, which didn't go very high up on the mast, and was probably struck as soon as the wind picked up.

    Later, it was found the jib really improved the effectiveness of the main, so there was a desire to fly the jib almost all the time. Then stays were added to keep the luff of the jib tight.

    From that point on, the head of the jib crept higher and higher up the mast, until the 1960's, when it finally reached the top.

    There were lateral evolutions as well.

    My favorite, which is the 3/4 rig, came with bendy and stiff spars.
    the bendy sparred one was for the benefit of the main. By tightening the boom vang, you could introduce fore and aft curve in both the mast and the boom. This would flatten the mainsail for stronger winds.

    The stiff sparred variety provided a very handy way of quick reefing. Simply drop the jib. I used this option so often, my boat was seen more often then not sailing without a jib.

    Another lateral evolution was three stays and no jib. This allowed for a lighter mast and allowed the bendy spar option for stronger winds.

    Though it is true that stays allowed for a lighter tall mast, that doesn't seem like their primary purpose.

    I looked at your drawings again.

    What you have will almost certainly work as a method to stand the sail clear of the mast, especially when the sail is fully set.

    Once reefing starts, the matter may get iffier and iffier.

    This is what I think will happen:

    As the yard gets farther and farther below the spreader, it will start creeping closer and closer to the mast (when up wind of the mast).

    Your shroud arrangement will do little to stop it.

    Your rig has only cap shrouds and no lowers. For this reason, the mast top and top spreaders will tend to sag off to leeward. This will put a compound, athwartship bend in the mast, as the top of the mast will want to stay vertical, as the end of the windward top spreader is held down, with either a shroud or the sail itself.

    There is nothing, however, other than mast stiffness, to keep the mast straight, below the top spreader.

    My paired mast, or 'gallows frame' idea, shares the same fault. That is why, if I ever used it, I would go for a longer yard and a shorter mast.

    This stiffness can come from a larger mast diameter.

    For this reason, I am skeptical on whether you will be able to experience much weight savings at all.

    I do hope you try it, though.
     
  9. LP
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    LP Flying Boatman

    Sharpii,

    Thanks for the input and the history as well. I'm going to approach this fairly conservatively. My thoughts right now are to go traditional with the main and experiment with the mizzen. My rig has the mizzen offset from centerline for tiller clearance and so the temptation to play with that mast is strong. I also think that the greater beam at the stern will lend itself to a better purchase for the stays. I don't know if you saw it, but there is a mid span spreader on the free side of the mast. I'm relying(planning?) on this and the wider footing aft it assist in keeping the mast in column. I was trying to figure if the was a way to preload the mast before the stay tensions to arc the mast to sail-side for additional clearance and maybe to have the mid-span spreader exert a side-load on the mast to maintain the desired amount of lateral curvature. I haven't convinced myself yet that this would be desirable. I see the wider footing aft for spreader attachment as a definite plus. The main mast up in the eyes of the boat doesn't promote the idea quite as strongly.

    One of the basic concepts in my design is to have a rig that is easily and quickly, stepped and unstepped. Any type of staying goes against this fundamental concept for the boat. But, another concept with the boat is ease of trailering and to be ballasted on the lighter side. A lighter rig works the that end and so I come back to looking at the advantages of stays. It's all very circular isn't it. If the boat was to be permanently rigged, I would see that as another plus in the stayed rig column.
     
  10. sharpii2
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    sharpii2 Senior Member

    LP.

    I did see it.

    I finally noticed that the drawing was only a 6% size. I enlarged it and was able to see more detail.

    I didn't mention the lower spreader because it would be next to ineffective.

    It needs some diagonal bracing. It needs a shroud coming up from the chain plate, at the side of the hull, and ending at the mast right under it.

    A second such shroud needs to run from the tip of the lower spreader to the base of the upper on, at the mast. These two shrouds will do a great deal to keep the mast in column, when the sail is set to the lee of the mast.

    They will do nothing, however, when the sail is to windward of it. To make them effective then, you need to exchange the lower one for rigid strut. If you want the upper one to be a rigid strut, you have to run a vertical strut from the side of the boat up to tip of the lower spreader. These three struts would work like 'drag struts' on the inside of a fabric covered airplane wing.

    I must confess, I'm hearing a real conflict of values here.

    Complicated staying and quick raising and lowering of sailing rigs do not go together. Even if the shrouds stay mostly in place, when the mast is lowered, they have a bad habit on fouling on things.

    And they are really only effective when they are tightened down just so, so every time you raise your rig, you can look forward to having to adjust them.

    I've looked at your hull design and think it very appropriate for a trailer sailor. I has a very wide Beam Waterline, which ought to make the boat quite stiff, giving it good initial stability for good sail carrying capability.

    The modest ballast can be augmented with stores and gear, if it is stowed low, and sand bags or water jugs, when the boat is being sailed light. In order for this to work, seat covers and lower locker doors wil have to be firmly latchable.

    The 'dry strip planking' idea reminded me of an idea I have long had.
    (I got it from Alan Waitse's method of salvaging old wooden boats with GRP re enforcement covering)

    That idea is laying the strips, as you stated, but, instead of filling in between them, covering them with a layer of mat, roving, then mat again, using mechanical fasteners to hold it all to the strips. The strips could even be epoxy saturated or just painted, before hand, as a chemical bond between the the GRP and the strips is not being relied upon.

    The theory is the wood will have some freedom to shrink and swell without upsetting the GRP shell.

    The strips would not be encapsulated in either polyester or glass.

    As you may have guessed, I'm looking for less expensive ways to build boats, for the days when building them out of mostly GRP become prohibitively expensive, and straight grained wood, for either planking or making plywood out of, becomes nearly extinct.
     
  11. PAR
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    PAR Yacht Designer/Builder

    LP, I'm not excited about your birdsmouth tear drop spar. It looks to have a natural stress riser, that happens in the thinnest portion of wall, even though it appears you're using an exceptionally thick (looks to be about 27%) wall. Of course, this type of section has to be stayed. Since it's stayed, why such a thick section?

    I think the section attached will work better. It's a 20% wall and you note the flat spot of the trailing edge, where a sail track can be installed. The wall thickness doesn't pinch down significantly anywhere, so it'll be stable. The sail track stave needed to be made thicker, to prevent too thin a wall in this location. If you insist on a full tear drop section (no flat), the two long staves will need to be thicker, to again prevent this "necking" down situation.
     

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  12. LP
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    LP Flying Boatman

    Hey Paul,

    I was looking for simplest construction possible and saw the thinning section, but tried to justify it through several reasons.

    The rectangular box is the major and minor dimensions of the mast. In this case, 4.25"x3.0". Perhaps this is a misapplication of methods, but chose the unstayed dimension for the major dimension and chose the stayed dimension for the minor dimension as the mast is only stayed laterally. I took 20% of the major dimension as a starting point for mast thickness . With the reduction in lateral dimension, I can also see a drop in moment of inertia in the longitudinal direction. I may have to rethink this. I drew the section shape and eye-balled the tail as the was a definite loss of material there that needed to be supplemented. There was obviously a thinning of side-wall material on the tail, so I left full thickness there as supplemental material of in column support. I guess I was thinking the change in thickness was gradual enough that stress gradients would be acceptable. Maybe not then. I'm also thinking that my simple staying method could use more in column support, based on Sharp's comment, but on my part, I'll confess to it being a bit of a WAG.

    I did notice on your mast section that you have chosen a 20% dimension based on the minor dimension and used that all of the way around. Is this standard and based on the higher I^4 (moment of inertia) in that direction?

    I also notice that you meet the outside corners of your mast staves. Is it more efficient use of material or personal preference? I'll admit to being lazy and 45ing a midline intersection to utilize redundant cuts on the staves.
     
  13. LP
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    LP Flying Boatman

    Absolutely! Not to be a smart arse, but I think it's called boat design. ;)

    I really hope to use this vessel as more than a daysailer. As such, I might forego some simplicity, but still want to keep it as simple as possible to rig. Multiple stays and cross bracing is going beyond my limit. I single stay each side, if it will work, is as far as I'm willing to go. My idea for the shrouds is snug them and run with them. The sail loads will determine shroud tension.

    Attached is a basic mizzen geometry drawing showing multiple staying options. One option is stayed from the red spreader. Another extends a spreader farther to offer more clearance on the lug and the last would be used with an unstayed mast. It would only utilize the stays and spreaders in red to spread the torsion loads over a larger portion of the mast. The last option is nice in that the rig is fixed to the mast and doesn't need to be rigged every time. The draw backs though are added weight and windage as it uses a full section, free standing mast. Perhaps some aerodynamics added to the mast would offset the windage.

    The last one is similar to your idea though it is implemented differently and is not as articulated as yours.
     

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  14. LP
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    LP Flying Boatman

    Paul.

    Here are a couple of mast sections.

    MastSect01.jpg

    The upper utilizes the same section and changes the staves. 20% wall gives me 0.60". Utilizing 0.75" stock conveniently gives me that section at the thinnest point. Do you think I'm still looking at the stress concentration?

    The lower utilizes 45's everywhere for simplicity and gives a flat spot for mounting hardware FWIW.

    I've left off the sail track flat on both as there is no need. I do like the shape of the upper section.
     

  15. sharpii2
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    sharpii2 Senior Member

    LP.

    Attached is a crude sketch of a possible solution.

    Both the mast and compression strut, next to it, can be fitted to a tabernacle.

    A single pair of shrouds would lead up to the top of the compression strut to keep it from twisting fore or aft. The diagonal lines would each require a turnbuckle, but such would only have to be adjusted once. The two shrouds, mentioned above, can have lanyards at their lower ends.

    The main part of the mast could have a pair of shrouds too, to handle the fore and aft loads. These too could have lanyards at their ends. Such lanyards could be used to help raise and lower the structure.

    The compression strut would be expected to take tension loads, as well as compression loads, as when the sail is to the lee of the mast.

    The distance between spreaders ought to be no more than four times their length, so you may need more than the one shown.

    The solution shown in your drawing might work as well, but the shroud shown does not meet mast and, for this reason, it may be looser than you may hope for.
     

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