Sail performance metrics

Discussion in 'Hydrodynamics and Aerodynamics' started by misanthropicexplore, Jul 25, 2018.

  1. CT249
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    CT249 Senior Member

    It's also the lift coefficient at a certain height, isn't it? It's not hard to have a tall rig with the top of the sail completely bladed out so it is developing very little heeling force. Such a rig could have a much lower heeling moment than a rig that is squatter, but where it's hard to flatten the head of the sails out. I'd be willing to bet that a tall, modern fractional rig with good sails that are flattened and twisted would have lower heeling moment than a lower-aspect rig on a stick that could not be bend to flatten the sails, and with a forestay that is sagging and making the headsail deeper.

    The issue of rig heights in working sail and the lessons we can draw appear to be quite complex, of course. For a start, working sailboats were often quite tender, had a very high displacement or had to drag heavy trawls etc. And the structural issues faced in the 19th c were very different to those we face today. Finally, didn't many working sailboats develop their home port around their working needs; for example, they would work out of a port that gave them a reach to the fishing grounds and back. These days we don't tend to have the luxury of putting our home town with a nice convenient reach of our favourite sailing spot.

    With respect, it's not really a case that racing sailboats ignore factors 2 to 5 at all. The popular classes like the Laser, Optimist, Hobie 14, 420, Sunfish etc all use rigs that are compromised in many ways to reduce cost, increase durability, and ease rigging time. Balance is also a consideration, but arguably if you're sailing hard and well it's much less important than some cruisers make out. Even modern classes like the RS Aero don't ignore factors like durability - that's why it uses dacron, I think. Similarly, one of the fastest-growing classes of the moment is the reborn Windsurfer which uses very simple rigging and a mylar sail for economy, durability and easy rigging.
     
  2. sharpii2
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    sharpii2 Senior Member

    The JR's biggest advantage is ease in reefing. The typically un-stayed mast is somewhat vulnerable. I have no experience with membrane sails. Do they last long? Or do they simply out perform everything else?
     
  3. DCockey
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    DCockey Senior Member

    What is a the unstayed mast vulnerable to? How does that compare to a typical stayed mast where the failure of any of the numerous components of the staying system can lead to catastrophic failure of the mast?
     
  4. Mike Inman
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    Mike Inman Junior Member

    Or, that one test is testing for what's most efficient for a fisherman, more in terms of resources to build and maintain and effort to operate while fishing. As mentioned above: if the fisherman using a crab claw instead of a sprit rig can get 70% of the sail-power performance for 40% of the construction and maintenance cost and 60% of the handling effort, which is the better design?
     
  5. Mike Inman
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    Mike Inman Junior Member

    That depends entirely upon how you go about it. Before I started sailing I thought about the places I'd like to sail. Now that I'm doing it, my favorite sailing spot is the bay that the marina closest to my house is located in. Like it or not, I travel that bay every time I go out, I choose to like it.

    I also thought about picking and choosing from the different marinas around town, but with one located 10 minutes from my house and the rest at 30+ minutes away, that choice was also pretty much made for me.
     
  6. CT249
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    CT249 Senior Member

    Valid points, BUT that was not what was being tested as far as I can recall. From my recollection, the crab claw test, like the test for fishermen that claimed that a sprit rig was the fastest (and which had ludicrous claims that run counter to decades of experience and theory) was about speed and pointing NOT about speed compared to the resources required to run the rig.
     
  7. CT249
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    CT249 Senior Member

    Okay, but that's your own experience and there is no evidence that it relates to the mass of modern sailors. In contrast, let's look at my situations. My favourite day anchorages are directly upwind of the convenient mooring spots. That's the third time in a row that has happened for me, in three completely different points. So personal experiences, whether mine or yours, cannot always be translated to the wider world.

    Even the fact that you have lots of different marinas within such a small area may indicate that you are not in the usual situation of sailors around the world. Most of the major yachting places I can think of (ie Sydney, Newport Rhode Island, San Francisco Bay, the Solent, much of the French Atlantic coast, Auckland etc) don't have so many available moorings and so many good local anchorages that you can choose to reach back and forth from the mooring to the best anchorage.
     
  8. CT249
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    CT249 Senior Member

    Well, from my recollection one well known designer used to claim that the structural issues of unstayed masts in the days of alloy sticks, including IIRC buckling of the lee side under compression, were greater than the problems of stays. The really interesting point appears to be that we have seen many claims from unstayed mast proponents that unstayed masts are more reliable, but actual facts such as statistical analysis of rigs lost as a percentage of boats afloat, appear to be missing.

    A while ago an unstayed rig proponent here mentioned that there had only been on rig loss in a certain unstayed type. A quick Google found that there had been at least one more. There were reasons for the two losses, but there are reasons for the loss of stayed rigs as well. What was significant, arguably, was that the proponents of unstayed rigs were not aware of all the losses of such rigs, even in a type that they had used as an example of reliable unstayed masts. When they are not aware of all the losses in a type they use as an example it can be said that their research is not completely reliable.
     
  9. rwatson
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    rwatson Senior Member

    Only 2 unstayed failures ? There must have been hundreds. 100 years of unstayed masts on British traditional craft alone will have had plenty of mixed results.

    I could dig out dozens of stayed reported failures, but there must have been thousands over the years. Sydney to Hobart races alone over the last 40 years for example.

    The idea that there is any reliable statistics on stayed and unstayed mast failures over the last say, 10 years, is pretty improbable.

    From the Maltese Falcon to Tom Colvin designs , unstayed masts are as successful as the design and the
    engineering that's gone into them, just like stayed masts.
     
    Doug Lord likes this.
  10. CT249
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    CT249 Senior Member

    No one said that stayed masts were 100% reliable. There'd probably be some large pleasure boat insurers who have stats on comparative rig failures, but such things are not released publicly.

    From what I can find out there has been one mast failure in the last three Hobarts, out of something like 280 entries and 35 retirements. That shows the enormous improvement in stayed mast reliability since earlier eras. But even that is pointless as a comparison because no one has put a significant number of unstayed rigs out there, at the same time, and pushing so hard to win the race.

    Nothing I have ever said indicates that unstayed masts are not as successful as the design and engineering that has gone into them, just like stayed masts. The issue is that we have not been given any evidence to back up the claims of some proponents of unstayed masts that they are more reliable. Claims and opinions are not evidence.
     
  11. rwatson
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    rwatson Senior Member

    That's my point - there CANNOT be any claim of 100% reliability, nor can there be any reliable evidence.

    You expecting to get some sort of numeric support for either point of view is unrealistic and totally impossible.
     
  12. CT249
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    CT249 Senior Member

    Really? Why? I didn't say I wanted to get something utterly reliable. All that would be needed was, for example, someone to say "I was the production manager at (insert spar company here and date) and we built X masts for Brand Z and in all those years only Y of them broke". If a bit of Googling seemed to support that claim (rather that contradict it, which has happened) we'd have more information than the unsupported claims we get.

    What's the alternative? Simply take what those who try to sell rigs tell us at face value? Assume we know more than them about their rigs? Neither of those seem to be as good as trying to get more information. And perhaps most importantly, if the information is not available then the various claims of rig advocates are shown to be merely differing professional opinions. In that case, not falling for either side's claims is perfectly reasonable.

    Sorry, but it's hard to see what's wrong with merely asking for evidence of claims that are made by vendors of a product.
     
  13. Mike Inman
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    Mike Inman Junior Member

    This problem isn't unique to sailing. I've worked for different companies over the years, some that compete with one another. Two in particular sold treatments for epilepsy. Both had what appeared to be an abundance of "peer reviewed literature" from the best medical journals out there documenting the efficacy of their devices/methods. Problem is, each cherry picked the results they wanted to talk about, so while one set of literature (dozens of articles, thousands of patients studied) quoted a "cure rate" of 33% for one method, with a significant improvement in symptoms or better for 66% (and no change / minimal side effects for the other 1/3), the competition went out and found a similar body of documented medical research which showed a cure rate of less than 3%, and significant improvement in symptoms for no more than 5% for the same method. The truth is, likely, somewhere in the middle of those two extremes, but where exactly is very difficult to say.
     
  14. sharpii2
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    sharpii2 Senior Member

    An unstayed mast is vulnerable to two things:
    1.) work cycle fatigue, and
    2.) fracture due to a sudden massive impact.

    For the first, a boat at sea goes through countless pitches and rolls. A mast has a Center of Gravity (CG) high above the deck. It also has at least some weight. This distance (to the fourth power) multiplied by its weight gives us its inertia. So a taller mast of the same weight is going to have a greater amount of inertia than a shorter one. What inertia is, is the reluctance to move when not moving and a reluctance to stop when moving. This means that when a boat is at sea pitching and rolling, this mast CG is having to change from moving to not moving and from moving in one direction to moving in another. This puts considerable stress on the mast partners and the portion of the mast resting against them even when there is not a square inch of sail being carried. Failure from these repeated stresses occurs when the number of them multiplied by their severity reaches a certain total. This can mean a relatively low number of high stresses or a very high number of relatively low ones. At this point, the mast can fail suddenly at the partners even during relatively mild conditions.

    Some materials handle these stresses better than others. Good old fashioned wood is one of the best. But it is not particularly strong for its weight. the temptation is to move to an aluminum mast. But aluminum is a metal, and it is very prone to repeated stress falure. Carbon re-enforced epoxy is much better mainly because it has a much higher strength to weight ratio. But if we take this hapless aluminum mast and add stays and shrouds, we remove these bending stresses from the mast at the partners and convert them to compression loads. These compression loads are far milder than the bending loads would have been because the CG of the mast is often at or below the shroud and stay attachment points. If these shrouds, stays and their attachment points are strong enough, an aluminum mast, which would have otherwise failed within months or even weeks, may last a lifetime.

    For the second, a sudden massive impact, such as a sudden squall, or even worse, the mast hitting the water during a vicious capsize caused by a wave, can simply overwhelm the bending strength of the mast. At this point, the mast fractures not far above the partners. The mast supported by stays and shrouds converts most of these bending stresses to shear and compression ones. The stays and shrouds, of course, have to deal with tension stresses. If these are all up to the job, and the mast can stand the compression and shear loads, the rig will stay intact. It is much easier to design a mast supported by stays and shrouds and keep it within a reasonable weight than it is to design an unstayed one which can stand the same loads.

    Hull design is a major factor in reducing the severity of repetitive stresses on an unstayed mast. If a hull has a long keel (which dampens rolls) and has relatively low initial stability, it will also likely have a slower pitch motion, especially if it has a short water line. Such a hull puts far less severe repetitive stresses than which has high initial stability, a short keel, and a long water line.

    The mast can be kept relatively short and tapered considerably at the top. These two factors accomplish two things. One, they keep the CG of the mast closer to the partners. And two, they encourage the mast to bend along its entire length rather than just at the partners.

    Overall, a mast supported by stays and shrouds is always going to be lighter than an unstayed one which is made out of the same material, even if we count the weight of the stays, shrouds and their attachment points.
     

  15. rwatson
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    rwatson Senior Member

    Some sort of consensus by opinion isn't worth the screen its typed on. There aren't any VENDORS of Unstayed Masts FFS! There are boat designers who calculate and specify suitable rigs for a particular boat. The whole package goes together for obvious reasons.

    Once again, the ONLY sort of guarantee is the solution I mentioned before :-

    "unstayed masts are as successful as the design and the engineering that's gone into them, just like stayed masts"

    You have to get a qualified person to design and supervise the build.
     
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