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  1. #1
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    Default Centre of the sail area

    You know, that important index mark that's supposed to go over the centreboard or slightly behind it (for weather helm). It's supposed to the be centre of pressure on that sail.

    But how do you determine it?
    I've been looking at lots of lug sails with it shown and on none of them can I work out where this came from:
    - it's not the centre of the diagonals
    - it's not the intersection of the lines drawn from the centres of the sides
    - it's not the claw mark left by the cat when you throw him at the sail

    So how do you determine it?

    Richard

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  3. #2
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    Default

    The geometric center of the sail or sails can be had a few ways, all with simple geometry. Typically, on a quadrilateral shape, you divide the sail into triangles (2) and calculate the centers. There's also a purely geometric method that doesn't require math. You seem to be looking for something other then the CE (Center of Effort).

    The CE is forward of the CLP in most boats and it's relationship with the centerboard is generally happenstance, depending on underwater configuration. Assuming you have "nothing to hold 'er back" such as a skeg, keel, deadwood, etc. then often the centerboard or central appendage is nearly under the CE. The central appendage (board, dagger, whatever) is just part of the lateral area exposed to hydrodynamic forces. The static CLP will generally be a good bit aft of the static CE.

    So it seems you are looking to determine lead (the difference between CE and CLP as a percentage of LWL). This suggest you're playing with something? As a quick reference, boats of normal proportions will need 15% for a sloop, 13.5% for a yawl, 12.5% for a ketch, 10% for a schooner. There are many things that can alter the "lead" of a sail plan, entry angle, hull form, buttock angles, type and location of appendages, sail plan choices, etc.

    Maybe it would be easier if you gave us a hint as to what you're trying to do.

    The attached file shows the method to determine the geometric center of a quadrilateral shape. It's done in two stages. The first stage is drawn in gray and produces 2 "X's" which are connected (blue line). The opposite corners are used in the next stage and 2 more X's are made and connected. The resulting blue X is the geo center of the shape. No math, just a ruler/straight edge and a pen. Of course this doesn't determine area, just the center. To test the theoretical center, stick it to a wall with a pin at the center and spin it several times. It should stop at different points if it's the center. If it's not, it will always stop at the same point (CG down).

    Every foil that was used in the US space program at NASA, up through the Apollo program, had to pass the spin on the wall test, before being sent to production (no kidding).

  4. #3
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    Default

    Thanks Paul. That's probably the right answer but it's not the 'right' answer if you get my drift.

    Jim Mikalek's latest newsletter has a bit on setting up lugsails, and he talks about putting the centre of the sail area over the centreboard - even draws it in to show what he means, without telling us how he put that little symbol where he did. The inference is that it's not hard.

    The first time Mik saw Redback in sailing trim, he pulled down the sail and adjusted the halyard fitting on the top yard to put the centre of the sail over the centreboard, which improved the way the thing sailed (it actually sails quite well if you're about 2/3 my size and can fit in the thing). He did this by eye (or guess ... or threw a cat at the sail when I wasn't looking).

    And so on. Nearly every diagram of a boat has this funny mark. So where doesn't it come from?

    Richard
    if this is unclear, it's because I don't what I'm talking about (as usual) and am trying to change that

  5. #4
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    Default

    Yep, the funny mark is the CE of the sail or sail plan. In small craft that don't have a lot of stuff under the boat (skegs, keels, rocker, etc.) and a simple sail plan like a lug, then the sail's CE will fall very close to the centerboard location. Mikalek shows how to find the CE in one of his news letters and it is easy.

    What you aren't understanding is it doesn't matter much, if the boat is reasonably designed. If the boat has a natural balance (appreciable amount of lead), then moving your butt (literally) will sort out most all handling ills. This is because the CE is only static on paper or a computer screen, while in real life it's constantly moving around. This is why the good dinghy sailors scootch their butts all over the boat, depending on what point of sail they're on, wind conditions, etc. Since you butt apparently is more endowed then others, you should have an advantage and not have to move it quite as far for the same level of boat trim.

    What Mic did was more likely a matter of sailing experience and he was adjusting the helm balance by moving the sail forward (ease helm pressure) or aft (to increase helm pressure). These are things you feel right away when sailing a small boat. You can tell at an instant if you've got too much helm input for the course. Most rigs don't have the benefit of being able to move the whole sail, so Mic was taking advantage of the rig.

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