Controlling draft on flat cut sail?

Discussion in 'Boat Design' started by dustman, Aug 27, 2020.

  1. CT249
    Joined: May 2003
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    CT249 Senior Member

    Well, one of your points alone shows how far off base you are. Laser masts do NOT need 30 knots of wind to bend - they bend with monotonous regularity in 15-20 knots. It's a recognised problem, which is why they measure the straightness of your masts at world titles. You have apparently failed to consider wave impact and momentum, the lateral force induced by the boom and the leach tension, and the downhaul tension. The Laser masts also bends under the weight of a 52kg hull and a 60-85kg sailor on a slender hull. The righting moment (and therefore rig forces) on a cat are dramatically higher.

    Since you've said so often that unstayed masts are easier, may I ask how often you've fitted a mast base for an unstayed mast or fitted a stayed mast? Having done both, albeit in boats of very different size, I'd find it very hard to make such a call. There are, however, good reasons why unstayed masts are not used on the centrelines of small open-bridgedeck cats. The point about the article is that it contains many claims that are factually untrue and therefore the claims about reliability may also not be true.

    I have had considerable experience with varying aspect ratios, going from the high aspect rigs of Formula 18 and 16 cats and Tornadoes, down to the low aspect rigs of Laser Radials, Int. 12s, Windsurfers etc. Which one is better tends to depend on an extremely complicated mixture of hull drag, sail area, basic boatspeed, sailor's ability, point of sail, quality of sail, rig weight, etc. High aspect sails require high leach loads to prevent excessive twist. High leach loads make the mainsheet hard to control unless you spend a lot of money on very high quality roller-bearing mainsheet blocks that will allow the sheet to be eased off quickly and pulled in against very high pressure.

    If you hang a line off a solid gaff and then run two lines off that first line at an angle, the two lines will NOT stay apart and run at angles of 45 degrees to the line of force that runs from the clew up to the halyard. The line going to the foot of the sail about 70% aft will pull the sail down in that area with a huge crease if loaded, and the leach will "collapse" behind that and destroy the sail shape.

    No one said that people "cling to tradition". What they do is to respect earlier designs and learn from them and use them as a base for further progress. If, for example, you think that a Laser mast will not bend in winds of less than 30 knots then you're showing that your belief that current designs are too complicated rests on a lack of knowledge.

    That last paragraph shows why you're not wasting time on. Instead of actually trying to discuss the way designers develop their designs, you just decided to tell me I know f-all about the way boat design has progressed, which is actually a matter I studied for a PhD and learned about during a decade in the industry. A reasonable approach would not have been to tell me "the reason we have what we have", but to ask for more of the information I've been lucky enough to get because I respected many of the top minds in sailing design enough to learn from them rather than to tell them I knew more than they did.
     
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  2. gonzo
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    gonzo Senior Member

    However, the unstayed mast has less failure points, but also a lot less redundancy.
     
  3. dustman
    Joined: Jun 2019
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    dustman Junior Member

    CT249

    The figures I arrived at was using a calculator. I understand that there are a lot more factors. It was just to illustrate the strength of the 4", .125" wall mast relative to the laser mast supporting a similar sail area. I see I must be more verbose in my responses.

    I have built numerous "unstayed" and "stayed" structures that have to stand up to wind loads, such as solar structures, shade structures, fences, etc. I understand and appreciate the difference between using form to deal with loads rather than brute strength. Each situation has a solution depending on the design goals and needs. My desire is to eliminate the complications and operational limitations of the stayed mast.

    Gonzo

    The redundancy will be in the overbuilding of the mast and mast base, as well as being able to quickly and easily reef or drop the sails. I must quantify all the loads and add a large margin. I am far from a final design and will be doing a lot of number crunching and a lot more learning before finalizing. But what I have settled on is unstayed masts, cat-ketch, and gaff rigs on a catamaran. My hope here is to have all your inputs on how to successfully implement such a design.
     
  4. Blueknarr
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    Blueknarr Senior Member

    Dustman, you are 180° off of reality. An un-stayed mast is severely limited vs all those stays which allow the mast's flexation to be adjusted and controlled. I adjust my mast bend with every sail change. (Bigger or smaller jib or reefing and unreefing the main.). Also upwind and downwind get adjustment.

    Un-stayed masts have a single flex caricaturistic. If it doesn't match the sail's needs, then you have the choice of changing the mast or changing the sail.

    Gaff DO NOT reef or drop easily.

    Sails on multiple masts increase the reefing or dropping difficulty and time.

     
  5. dustman
    Joined: Jun 2019
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    dustman Junior Member

    "An un-stayed mast is severely limited vs all those stays which allow the mast's flexation to be adjusted and controlled. I adjust my mast bend with every sail change. (Bigger or smaller jib or reefing and unreefing the main.). Also upwind and downwind get adjustment. Un-stayed masts have a single flex caricaturistic. If it doesn't match the sail's needs, then you have the choice of changing the mast or changing the sail."

    ---So a tiny sail on a relatively short and thick mast is going to turn into a bow with a relatively small amount of tension close to the mast? Ok... You adjust your mast bend with every sail change? I sincerely hope I never have to do that. If the numbers work out that way I will simply increase the diameter of the mast. Yes I know that will cause a little more drag and a little more weight.

    "Gaff DO NOT reef or drop easily."

    ---Why do so many gaffers say that is one of the advantages of the gaff rig?

    "Sails on multiple masts increase the reefing or dropping difficulty and time"

    ---I understand and accept what comes along with multiple sails. My goal is to simplify the operation of each individual sail as much as possible to account for that. I hear too many stories of people not being able to handle their sails simply because they are too large. Seems safer to me to have 2 small easily managed sails. And there are other advantages to the cat-ketch rig. One of which is the mast placement with my design aligning with preexisting structural members(as well as making my cabin layout unobstructed, and I won't have to climb on top of my cabin to deal with sails). There are so many reasons for me to go with an unstayed cat-ketch. If I had more time and money and engineering expertise maybe I would do something else. Probably not, because it seems like a good solution to many of the problems people have while at sea.
     
  6. Rumars
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    Rumars Senior Member

    1. Masts and their partners are not engineered on sail area. They are dimensioned by the boats rightning moment.
    2. Unstayed masts behave like trees, they bend in a curve along their whole length. It's a whip, not a bow. This is continuous and changes the sailshape without any control on your part. Automatic reefing by twisting the top and spilling the wind is guaranteed. Keeping a good shape in prevailing conditions while still effective in other ranges involves some fancy sailmaking art.
    3. Gaff rigs reefing advantage is not what you think, it refers to the ability to reef downwind and only applies if compared to similarly old tech bermudian sails. Yes the sails easily come down on some gaffers with the right fittings, but you have a swinging gaff to keep under control (not always pleasant).

    I don't know why you became convinced that a cat ketch gaffer is the way to go, but it is actually more complicated to design and build than a normal three stay rig, especially on a catamaran. This is the reason why you see so few in this configuration on the water.

    Can you detail some of the "problems people have at sea" that you are trying to solve? "Sails to big to handle" is a question of the propper equipment and tehnique. If you want to do it all without winches there is an upper limit on the individual sails weight, wich is only indirectly related to size and more related to your fitness.
     
  7. gonzo
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    gonzo Senior Member

    There is a huge difference between redundancy and a safety margin. Overbuilding is bad engineering. It will make the mast heavy and reduce stability. Redundancy is a system where if one part fails, there are others that can take the load.
     
  8. Blueknarr
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    Blueknarr Senior Member

    Dustman
    You are the only person who claims gatfers are easily handled performance rigs. Yes, when compared to the square riggers they replaced two centuries ago. But, a century ago they where repaced by better performing and easier to handle Bermuda rigs.

    I crewed on a trimastet with a small gaff on the mizzen.three deck hands performed sail changes. A single hand changed both of the bermuda sails then went aft to assist the other two hands working on the much smaller gaff.

    The only advantage to un-stayed masts is there ability to spill wind automatically. But this takes precise engineering of the mast and sail. You plan to defeat this advantage twice. Once with an inflexible overweight telephone pole and secondly by adding a gaff to the top

    Masts need to be flexible. They ought to be designed with finesse not brute force.

    Splitting sails in to smaller packages is an issue for large boats. If you are have an infermoty that prevents you from handling the size of sails typically found on your less than 30 footer, them you have no business on your proposed voyage.

    However, I understand the need to arrange masts around accommodation requirements.

    By the way, since the sail is directly attached to the mast and boom. Every time the sail's shape is changed it will also change the shape of the mast. They can't be altered independent of each other. Easing or pulling in on the main sheet will slightly alter the mast. Reefing or changing head sails changes the loads on a mast and thus it's shape
     
  9. messabout
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    messabout Senior Member

    Four inch diameter with one eighth wall 6061 tubing weighs 1.79 pounds per foot. It has a section modulus of 1.429 and a moment of inertia of 2.859. (Alcoa Structural Handbook) That is both a heavy and stiff stick for the amount of sail that the OP has specified. Mast bend is not likely to be a problem with a tubular section of that size. The more relevant issue is how the mast base will be supported. It will need a most robust structure to endure the cantilever load when the wind pipes up......This is to be on centerline of a catamaran? ??? Dismasting will not be a fault of the mast itself but of the structure that was to hold it in its assigned position. ...............A more practical and sensible solution is to use shrouds and stays. In that case the section size and weight of the mast can be reduced. The pitch and roll factors of the boat will also be less severe.
     
  10. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I have looked at your sail design and have read your posts. I think there are a few thing I would be remiss by not pointing out.

    I'll start with the sail design.

    With a gaff rig, the boom controls the sheeting angle of the gaff. What this means is that the trailing edge of the sail, known as the "leach" holds the sheeting angle of the gaff. If it fails to do so, the gaff sags off to leeward (down wind), and this kills half the lift of the sail. The longer the leach is, in proportion to the length of the boom, the more this is true. Phil Bolger wrote about this in his book 100 Sailboat Rigs. He said this was a huge problem in the days of racing schooners, because making the gaff-headed fore sail as narrow as possible caused its gaff head to sag off to such a degree that it was contributing very little power.

    In his book, Phil recommends that the luff (leading edge) of the gaff sail be no longer than than 2.5 times the length of the foot. Your's seems much longer than that. Another problem with a gaff sail this tall and narrow is that it is more likely to get what is known as a "Chinese jibe". This is when the top of the sail jibes to one side, and the bottom jibes to the other. In short, this sail would be most likely unmanageable.

    As for the boat itself, including the unstayed masts, it is likely to consume your entire weight budget all by itself--and then some.

    The 36 ft hulls and the 16 ft beam is going to be very difficult if not impossible to achieve at anywhere near 1200 lbs. Maybe, with inflatable hulls and a single stayed mast, it may be possible. But there would have to be tension wires and turnbuckle everywhere, not to mention a lot of expensive carbon fiber. And even if that were achieved, the boat wold be so light that a decently strong squall would be able to lift it off the water like an empty shopping bag. And it will probably not land right side up.

    As for the unstayed masts. Even at 1200 lbs total weight, your boat would have an initial righting moment of close to 9600 ftlbs (1200 lbs * 16 ft/2). This must be translated into inlbs, before you can calculate the bending stresses of the mast at the mast step. More likely than not, a sudden squall would dismast you in an instant. And these are utterly unpredictable, so you can not plan on reducing your sail area ahead of their arrival.

    In the early days of multi hull racing, there were a lot of rig failures, because of sudden shock-loads brought on by gusts. They learned to use heavier rigging wire and bigger turnbuckles. A mono, which developed most of its righting moment at about 30 deg. of heel, can roll away from such sudden loads. A multi, which developers most of its heeling moment at less than 10 deg. of heel, can't.

    Overall, I think you need to rethink your whole concept here.

    Almost top to bottom, it doesn't seem at all workable to me.

    Maybe, if the mast height, hull length, and over-all beam were reduced by one third, you might be in more realistic territory.
     
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  11. dustman
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    dustman Junior Member

    I understand that, it was a metaphor. I would argue that large margins are as good as redundancy. It's only bad engineering if your are making a tradeoff that will have a significant negative impact somewhere else. Stability is not going to be much of issue here, I have small sails with a low CE and plenty of stability relative to that. 10 extra pounds per mast seems like a small price to pay for a significant safety margin.
     
  12. gonzo
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    gonzo Senior Member

    Your sails, as you drew them, have a high aspect ratio which means a high CE; the opposite as what you claim. Large margins of safety are only good when they are global. This stiff mast will increase stress on the rest of the structure, possibly causing a major failure. In fact, good engineering is designing for failure. It is a fact that any structure can fail. Good design will plan for a failure that will cause the least damage and/or prevent critical systems from being disabled.
     
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  13. dustman
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    dustman Junior Member

    High CE is relative. Even my "high CE" is relatively low given the beam, small sail area, and splitting the sail area between two shorter masts. Run the numbers... If you can show me that I'm wrong then do it instead of bashing my idea for the sake of it, it would be much more helpful. My supporting structure will be more than strong enough, the masts will fail well ahead of that.

    16' beam, 1200lb displacement, 140ft2 sail area, center of mass approx 36" above waterline, CE of sails approx 11' above waterline. The masts are 6061-t6 aluminum, 4"diameter, 1/8" wall, 21' span from base support.
     
  14. gonzo
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    gonzo Senior Member

    I did prove you are wrong. A low aspect sail will have a ratio of maybe 2.5:1 maximum. Your has a much higher ratio, which makes a high aspect sail; the opposite of what you claim. The CE of the sail will therefore be much higher than if the same sail area was on a low aspect ratio sail.
     

  15. brendan gardam
    Joined: Feb 2020
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    brendan gardam Senior Member

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