69397RE: [bolger] Center of Effort - Balanced Lug
- Feb 28, 2013
I’ve always figures Center of Effort as center of area in sail-plan view was just a natural convenient useful fiction. A more “real” center of effort of a sail might be the effective center of the pressure-differential between windward and leeward sides of the sail, the point from which you could draw a force vector that accurately represented the lift on that sail at the moment.
But I have been truly confused about where and how that lift is transferred to the hull. Phil sometimes talks about a boat being pulled along by its sheet. And if you look at the mast hoops which might be thought to be applying the sail’s force to the mast, they’re pulling in the wrong direction. So I have come to simplify all that stuff, sheets, shrouds, hoops, and so on by considering the hull with its spars and sails, moment by moment, as a rigid unit, so that all I care about is the vector mentioned above, and how much of it can be induced to serve the interests of forward movement.
The rules of thumb for location of a centerboard always make nonsense of CE, do they not? If CE meant what it says, it would obvious how to get a slight weather helm, or a balanced one.
Cruzer’s many links re lugs were a terrific gift to us all! What fun to follow them. Great to learn why the dipping lug, tacked by wearing, with the whole shooting match going around forward of the mast and so never getting in the way of the work aboard, made such good sense! What a scholar we have among us.
Was it Marcaj who did test several kinds of simple single sail, of the same area, on the same hull, on all points of sail, and who came up showing that the best overall was the crab-claw, second best standing lug, third spritsail, and so on? One value of that study for me was the testing all around the compass. That’s the way we actually sail, or I do.
A basic rule of scientific inquiry is to limit the number of variables in the experiment. Alas, sail boats don’t work like that. If a sail boat is upright, it is reasonable to assume that it will go straight and it is easy to calculate the area/centers of underwater foils. But as the boat heels, the hull shape becomes asymmetrical (tending to turn it) and the foils lose area and efficiency. Sails are even worse. So long as they are sheeted close to the center line (where they don’t work), the center of effort is probably pretty close to the geometric center of sail area. As the sail goes out, the location of center of effort probably moves quite some distance from the geometric center of area. To further complicate the issue, add multiple sails and/or multiple masts. For an extreme example of possible variables, see what happens when you sail a Cape Cod Catboat off the wind in a rising wind (hint, it helps to have a gorilla on the tiller—a spare tiller might also be useful).
Possibly a computer could factor in the effect of many variables. I’m not that smart and my approach is to copy what demonstrably works. I figure that generation of sailors before me have learned what does and doesn’t work and I have no urge to reinvent the wheel (or, to keep metaphors straight, reinvent the sail).
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of sweetthug
Sent: Thursday, February 28, 2013 10:37 AM
Subject: [bolger] Center of Effort - Balanced Lug
There has been some discussion about lugsails, so I thought would float a question out there that I have had for a while. I'm wondering how relevant the center of the sail area is in determining the center of effort (CE). I've always seen these diagrams where the center of areas of the various sails (usually a sloop rig) and a little line drawn between them, with the CE being placed magically somewhere in between. To my mind it is not the center of the sails that matter, but the location of the mast, and how the force placed on the mast is distributed. So a standard sloop rig, with both fore and back stays will distribute the force provided by the sails fore and aft. Here the tension of the stay could change the CE, since more of the force would be placed on the stay by the sail (via the mast). There is another point where the force is distributed, and that's at the sheet, and this force will be much more important on a boomless sail, like a jib (or a dipping lug). Here the location of sheet (meaning where it is first connected to the boat) matters a good deal in terms of CE.
But now what of the case of an unstayed, boomed rig like a balanced lug (which is the sail I will be using on my boat build). Tension on the sheet is low, so I assume that the sheet location is not very significant. So the rest of the force is focused on the mast, specifically where the yard and the boom are attached. So in this case, it appears the location and shape of the sail isn't really relevant for determining CE (imagine a kite sail attached to the boom and yard; where is the CE if the sail is a 50 ft in the air?). Without standing rigging, the mast is what makes the boat move by applying the sail force to the mast/boat connection (step and partners in my case). Is this correct? This would place the CE at or just aft of the mast on all points of sail. The reason I ask is I'm wondering how to determine where centerboard should be located.
I have some theories as to why it appears that the CE would change as you change your point of sail. As you approach windward, it may feel like the CE is moving back and increasing weather helm. But I think the weather helm is being caused by the heeling angle of the boat increasing as you go upwind, not the changing location of the sail (which is what seems to be implied by the practise of placing the centerboard directly underneath the center of the sail area). In fact, heeling angle can be used to steer a small boat by itself, or in conjuction with sheet (maybe true on a large boat too, I have less experience there). So am I right in thinking the location of the CE fore-and-aft is essentially at the mast, and doesn't change much?
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