Loose footed mains

Discussion in 'Sailboats' started by Polarity, Dec 12, 2003.

  1. gonzo
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    gonzo Senior Member

    Loose footed mains are cut like a head sail. If you run a sail designed to be attached loose it will have an awful shape.
     
  2. Paul B

    Paul B Previous Member

    Please stop posting nonsense

    Do you even sail?

    Loose footed mains are NOT cut like headsails. Where do you get this? Why would you post lies and nonsense like this?

    Go to the North Sails site (biggest sailmaker with the most R&D dollars). Their standard mainsail specs for racing AND cruising sails now includes loose footed design.

    Try showing up a a major regatta like Block Island Race week, or Key West, or Big Boat Series. Count the mainsails that use boltropes for the foot. You'll probably only need one hand.


    Sometimes when I see stuff like this posted on the internet I wonder if someone has suffered a brain injury.
     
  3. shu
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    shu Junior Member

    Take it easy on Gonzo, guys; it may not be the current fashion, or even the fastest solution, but his basic concepts are correct. A captive-foot main with a large shelf may not have much effect, but a flatter cut foot will cause the vertical forces in the sail cloth to provide some opposition to the bending moment caused by the vang, or mid-boom sheeting (granted most of the forces are concentrated at the clue and tack). Several decades ago, bending the boom was used on some boats to flatten the lower portion of the main. Why does this seem so impossible? We flatten the forward portion of mainsails by bending masts all the time.
     
  4. shu
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    shu Junior Member

    One more item in Gonzo's defense:
    The FOOT of a loose footed main is constructed similarly to the foot of a genoa.

    Have you ever tried taking a nap in the foot of a loose-footed main?
     
  5. Paul B

    Paul B Previous Member

    Give me a break. Gonzo is talking pure nonsense and you are trying to defend pure nonsense.

    Read what you wrote. If the sail cloth was helping to make the boom more rigid then the old booms would not have bent to flatten the sail. If you did have a main with a shelf and you did bend the boom all you would be doing would be taking up the loose cloth in the shelf. After that you would begin to take out some fullness, but the sail load is not as strong as the boom so any load difference would be insignificant. That would add zero strength to the boom.

    As far as the foot of one sail being similar to the other, well I guess they both are made of cloth and both would have a foot cord. So you are correct there. But a mainsail is not cut like a headsail. That was the claim. How many headsails have the kind of roach you see in a main? Do they generally have a similar clew positions? What about draft position, etc?

    You could say that a dog is a cat, because they are both mammals that are domesticated. But one or two similarities do not make things the same.
     
  6. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Paul B says:

    "Give me a break. Gonzo is talking pure nonsense and you are trying to defend pure nonsense."

    Paul,
    I think you would have less trouble with people if you learned to reply tactfully, rather than like the playground bully. Even if you were a re-incarnation of one of the Herreshoffs, and gifted with the most amazing insight into the future of design (without the effects of fashion), you could still do us the honour of replying tactfully. There are ways of saying "You're full of it" without taking that antagonistic tone.
    Your opinions are your opinions, and you are entitled to them and their defence, but athere are politer ways of doing it.
    Thanks (I hope),
    Steve
     
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  7. gonzo
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    gonzo Senior Member

    Paul B:
    I really resent you libelling me. They are not lies. Maybe you have a different opinion, but loose footed sails have been cut like head sails for centuries. I do sail and race. Can you show the significant difference in cut between a roller reefing genoa and a roller reefing main? An attached mainsail can be flattened by several methods. One is to have a zippered lower section. Another is to have a shallow reef line.
    Also, my claim is that an attached mainsail foot spreads out the effort. Can you show me how I am wrong?
     
  8. Chris Krumm
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    Chris Krumm Junior Member

    I have a queries for both Gonzo and Steve on this one:

    Gonzo, you mentioned earlier, "The lower part of the sail can be more efficient if it is attached to the boom when going upwind. Observe the direction of the wind on the sail. It goes upwards, with the angle increasing with heel. The boom and mast are the leading edge at high angles of heel."

    Is that really the case? I would think at even 20-30 degrees heel and assuming forward motion of the boat (as opposed to pitching and yawing around in waves), the flow of apparent wind is predominantly chordwise along the sail.

    I thought one of the major reasons for loose footed, and fully battened, sails was to maintain a more efficient chordwise foil shape along as much of the sail as possible. A mildly cambered airfoil maintains a better L/D ratio than a flat plate in the Reynolds numbers sails operate in. A disadvantage of the loose foot without a "shelf" might be an increase in induced drag as low pressure air from the windward side leaks around to the leeward side; but then the parasitic drag from that shelf may be just as bad or worse. Anyone, please correct any faulty assumptions I'm making.

    Steve, you previously posted, "I had always worked on the pricipal that a loose-footed sail with end-boom sheeting was the perfect way to ensure that the boom stayed in column. "

    Consider the boom on a loose-footed main with end-boom sheets and no vang vs a very flat, captive-foot main set up the same. Assume the sail is sheeted close to the centerline of the boat, so compression loads on the boom from the mainsheet are nill.

    Compression loads on the loose-foot boom would be induced by the clew of the main and reacted at the gooseneck, giving us a double pin-ended column with most of the off-center compression load along the top of the boom. As Gonzo and Shu have suggested, taking any portion of that load at the clew and distributing it along the boom via the sail reduces the compression load along the top of the boom and works to pull the boom back in column. How much load re-distribution you get from a captive foot is the question. Again, please correct me if my assumptions or conclusions are in error.

    I suppose one could put strain gauges on sails and measure forces and deflection along a boom for equivalent loose-footed vs. captive-footed set-ups. Doing this on water would be tricky because you'd have to duplicate boat speed, wind direction, heel angle, et al for both setups. Setting it up on land with a test rig parallel to the ground might work. Simulate wind loading by distributing weights around the sail. Or set it up as an FEA problem and run it through the PC. Or chuck it all and go sailing with whatever rig your boat has on it...

    Chris "who sails a Holder 20 with both a captive-foot main and a loose-foot main, though not both at the same time" Krumm
     
  9. gonzo
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    gonzo Senior Member

    At 30 degrees of heel the angle of attack would shift by 30 degrees. That means it would have an angle of 70 degrees at the mast instead of 90. Try putting telltales and look at the air flow.
     
  10. PAR
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    PAR Yacht Designer/Builder

    SuperPiper, a main constructed as boom attached will set rather poorly rigged up loose footed. It looks very unprofesional, a bit like having your fenders hanging over the side well away from the dock.
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    You have seen me sail!

    The fenders add bouyancy to the leeward rail.
     
  12. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Chris - if the boom is stationary (relative to the boat, at least) then the mainsheet must be pulling down as hard as the clew is pulling up. So there is no net force on the end of the boom, otherwise it would move. If there is no net force except along the axis, then there can be no moment. hence no compression, etc.
    Add a vang, or move the sheeting inboard, or attach the foot of the main to the boom, of course, and you're in a different ballpark (to mix a few metaphors)

    Steve ;-)
     
  13. gonzo
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    gonzo Senior Member

    An important detail this discussion missed is the mainsheet attachment points. The assumption seems to be that it is at the end. This may not be possible or desirable.
     
  14. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Gonzo, any attachment point but at the end or very close to the end renders the previous assumption (no net force except axial) totally invalid. But then so does a vang, which you would want for "normal" sailing anyway ;-)
    Horses for courses. etc.
    Steve
     

  15. Chris Krumm
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    Chris Krumm Junior Member

    Steve -

    In the example of the loose footed main w/ end boom sheeting I assume primary sail load at the tack would be vertical - mainly due to halyard tension along the luff/boltrope. There is no horizontal component and the vertical force is totally balanced by mast compression.

    If we assume the primary sail load at the clew is along the leech (or from clew to head), there is a large vertical force component, but also a significant horizontal component that results in boom compression. The mainsheet pulling perpendicular to the boom and straight down counters the vertical component of the clew "force vector" but does nothing to balance the horizontal force along the boom.

    The boom in compression against the gooseneck balances the horizontal component and since the compression load is applied off-center along the top of the boom (clew outhaul is usually above the boom section centroid), the boom will want bow down. If the mast is really stiff vertically as opposed to athwartships, it might tend to pop sideways.

    Pulling down and forward with a center vang would only increase the compression and bending moment on the boom - maybe another reason "why many open-class boats have no fixed vang."
     
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