sail and boom design

Discussion in 'Boat Design' started by johnholland4, Jul 4, 2009.

  1. johnholland4
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    johnholland4 Junior Member

    I have a 12' aluminum boom that is set up for a loose-footed sail. What are the pros and cons of loose-footedness? Also, this boom has 4 sheaves built in behind the gooseneck and a matching four at the tack end, supposedly to allow all lines to be operated from the cockpit. The sheaves at the forward end have cam cleats, but these only snatch the lines in the direction of pull. Wouldn't it make more sense to have them hold the sails aloft? Far as I can see, there's no way to alter this. The boom came from my wrecked Searunner Tri, which had a rotating mast. I don't have a computer so will pick up replies in the library when I can. Thanks all. John.
     
  2. Gilbert
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    Gilbert Senior Member

    I think most of us would need a diagram of some sort to be of much help.
    The goose neck is at the tack end of the boom.
    A loose footed sail is less likely to hold a bag of water in a knockdown. It is theorized that having the foot fitted to the boom reduces the amount of wind that spills off that edge of the sail a little.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Loose footed mainsails can have a better sail shape in light to moderate wind strengths, but "shiver" the wind out, in heavier air. They are shaped differently then attached foot sails and you can't make one style into the other, without very careful recutting and stitching of the lower panels. A loose footed sail places more strain on the boom then an attached foot sail.

    On small craft, particularly those interesting in good performance, you'll find loose foot is common. These boat will not see much heavy air use, so are at an advantage.

    The theory folks will tell you that an attached foot, acts as an end plate of sorts, preventing pressure bleed off. In reality, the sail shape in the lower panels is so distorted to make it land on the boom, that most gains are lost, except in heavy air.

    You sheaves in the boom sounds like a built in sheet and/or outhaul/vang tackle arrangement. Who knows without a photo or two which more accurately describe what's there.

    Sails are "held aloft" with halyards, which typically are alongside the mast on small craft, though it's possible they could be internal (inside the mast) which is much better for windage.
     
  4. alan white
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    alan white Senior Member

    Regarding cam cleats, they will eventually damage the lines if used for halyards due to the same short section being repeatedly held in the teeth. Cam cleats are better suited to lighter loads and adjustable lines such as were probably set up on the Searunner for outhaul, topping lift, lazy jacks, etc..
    What PAR said about boom strain is a very real reason to be careful when the boom is set up loose-footed. In the case of a loose-footed boom, the boom doesn't bend. All of the boom load is in compression, or should be. That means sheeting should be at the boom-end, something you have to check is actually possible. Otherwise, you are taking a spar engineered for compression loads only and subjecting it to lateral loads.
    If you can manage to make use of boom-end sheeting, the loose-footed setup will work, though as PAR said, the sail's cut may not be ideal.
    If you have to sheet the boom somewhere else besides directly (or nearly so) adjacent to the clew, you ought to consult someone who knows a lot about spars, and have them analyze your setup using the extrusion you are using.
    The other alternative would be a bridle (like a spannaker pole) from clew to gooseneck with the mainsheet somewhere along the length. Not for long term, but it would get you by for the time being until you could either relocate the boat-end sheet attachment point or purchase a new slotted boom.
     
  5. johnholland4
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    johnholland4 Junior Member

    Loose-footed/boom

    Thanks all for your helpful responses. Thank you Gilbert for clearing up my tack/clew confusion. Basically I have a choice between two booms so the loose-footed one can now gather dust in the boathouse. I had not realized that it called for a specially cut sail, so that was a valuable piece of information. I now have a 30' Norm Cross tri,, the other boom is wood with a bolt rope channel set up for the sails that came with the boat. Regards. John.
     
  6. Paul B

    Paul B Previous Member

    Congratulations, you have made a decision based on dis-information.

    Every racing mainsail that I have seen built in the past 15+ years or so has been a loose footed design. Most of those sails were replacing sails that were not loose footed, on spars that were origninally designed with a captured foot in mind. I have never seen one "shiver" the wind out. That is nonsense.

    I've sailed on probably 50+ boats, and know of hundreds, if not thousands, of others that started life using mainsails with a captive shelf foot, that have all changed over to loose footed designs. None of the boats that have changed over have had any issues with broken or bending booms.

    The boom does not know if the sail is loose footed or not. The load line for the outhaul tension is between the tack fitting (above the boom) and the clew attachment (also above the boom). This is true if the foot is captured or loose. Pull on the outhaul of a captive foot sail and you can grab the excess sail material at mid boom between your thumb and forefinger and move it up and down. There is no vertical tension on the material below the load line.

    To use a sail designed with a captive foot and shelf as a loose footed sail would require the foot rope or slides to be removed, along with the excess material in the shelf. If you didn't want to do that you could simply roll the excess material up and velcro it, as was done on many early loose footed designs.
     
  7. johnholland4
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    johnholland4 Junior Member

    Thanks Paul, Well, that's all very interesting and perhaps puts me back to a choice between the two booms. I think I prefer the wooden one, but am still intrigued by the four sheaves built in at the ends of the aluminum boom. Each has a 1/2" line, about 25' long (!!!) fed from forward to aft. The mystery that was never quite answered was why the cam cleats that snatch each line at the tack end of the boom snatch in the direction of pull, i.e. when I'd be hauling a sail aloft. What good would this do? John.
     
  8. alan white
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    alan white Senior Member

    Regardless of whether you understand what the four lines and cam cleats are for, ask what you want to achieve and work forward from there. You may need a set of lazy jacks, an outhaul, a topping lift, or a vang. You may want a fast reefing system.
    See if the existing system would accomplish any of the above things.
    Of paramount importance is the placement and type of sheeting you already have on the boat itself. The boom with a foot groove can generally be bailed from any number of positions depending on sail area, attachment points, and ratios (purchases). The other, the one meant for loose-footed sail attachment, must be either bailed at the very end where the clew is, or analyzed to see how far forward the bail could be safely located (and this shouldn't be too far forward).
    in other words, unless you are able to locate the entire sheeting system at the end of the boom, you'd better stick with the grooved boom, and match up boom and boat attachment points with much more flexibility.
     
  9. Paul B

    Paul B Previous Member


    Complete and utter nonsense. I can't for the life of me understand why people who don't understand basics in an area feel compelled to give others incorrect advice.

    I am sure this guy Alan will not be able to provide any calculation to support his silly claim. I would love to see the reference that he draws from to "learn" these types of things.


    If a boom is strong enough to have the sheet bails at mid boom using an attached shelf foot sail it is strong enough for the same setup using a loose footed main.

    If you do go with a loose foot on the sail you will need to use something to hold the clew down to the boom. Whether that is a velcro strap (like most people use) or a length of low stretch line, you will need something.
     
  10. alan white
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    alan white Senior Member

     
  11. johnholland4
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    johnholland4 Junior Member

    Well, it wasn't my intention to generate so much heat but clearly passions run high when discussing these things. I've benefitted greatly from all the replies and have had much to chew over. I've appreciated your outspoken responses, Paul, but agree with Allan that a less angry tone might be more convivial. Anyway, as I said, I'll stick with the wood for now. The aluminum boom was clearly fabricated for a special system. There's no chance of it breaking; it's 6-sided and might double as a crane's boom. Also the wood is much more beautiful, but that's another area entirely. Fresh breezes all, and thanks again. John.
     
  12. messabout
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    messabout Senior Member

    John: Blessed are the peace makers. Thank you for that last post. Participants on this forum often disagree but we usually do so in a respectful manner. I believe it was Jimmy Carter who famously said that "we can disagree without being disagreeable".

    In this case, I do suspect that the protagonist is himself the victim of limited knowledge. From a structural engineering point of view, a boom is a complex piece of gear. It can be subjected to loads that are purely compressive, partially compressive, cantilevered, uniformly distributed beam loads, non uniformly distributed loads, static and dynamic loading, complicated by eccentric loads such as when a boom vang or lazy jacks or topping lifts are used, etc.... In the real world we rarely find it necessary to analyze boom structures on a lofty academic level. We simply use past experience to select something that we are pretty sure will work.
     
  13. Submarine Tom

    Submarine Tom Previous Member

    Paul B,

    "dis-information" accuses the author of intentionally misleading the

    recipient. Do you really think this is the case?

    Tom
     
  14. alan white
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    alan white Senior Member

    Thanks for understanding, Messabout. Now that I've heard the wooden boom is massive enough at least in the owner's estimation, I would be more inclined to the possability of treating the boom as more than simply a compressive member. What I was trying to say was that the wood boom may have started life designed to only handle compressive loads. Then, if bailed at points along its length that were simply chosen for the convenience of cockpit sheet attachments, failure might result due to inadaquate boom stiffness. The aluminum (grooved) boom could work either with loose-footed or shelf-type sail attachment methods, since such a boom would have been built to withstand lateral loads to begin with.
     

  15. Paul B

    Paul B Previous Member

    When you are in a deep hole you should just stop digging.

    Please refer to post #5 where Mr. Holland clearly states the wooden boom is the one set up for the captive foot, and the aluminum boom is the one set up for a loose footed sail.

    You know virtually nothing about the aluminum boom, yet in your quote above you comment that it is "grooved" (it may not be) and "would have been built to withstand lateral loads" (how would you know?).


    The problem with the statements you make (not only here, but I have noted on other threads) is people with little knowledge will take your answer at face value as fact. They have no idea that you haven't a clue. Then some other person reads what you wrote and they go off to another thread or forum and quote your incorrect message as fact.

    I'll bet that is how you come up with some of the things you believe.
     
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