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Old 10-24-2009
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Minimlist sail physics

Hi, all:

Background, then a question for those experienced in sailing physics:

I have a lateen sail for use in an upwing sailing rig for a kayak. I can rig the sail as a traditional lateen. However, by attaching a gooseneck to the front tip of the spar I can also rig it is a sliding gunter. The latter has advantages (can shorten the mast yet get the sail higher off of the water, more headroom below the boom, sail can be slackened in higher winds as a substitute for reefing). Is there an appreciable difference in sail efficiency between these two options? If the sliding gunter is as, or more, efficient, it seems like a better alternative.

With some modification, I could also rig it as a crab claw, though without the vortex-reducing, scalloped leech. By removing the boom, I could rig it as a loose-footed delta.

Any comments regarding the efficiency and handling characteristics of the alternatives?

Thanks,

Steve C.
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Old 10-24-2009
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Hi, all:

First a correction...spelling should have been "Minimalist..." in the title.

A second question arises out of the first: What is the effect of moving the center of effort of the sail plan forward? As it stands, with the sail rigged as a sliding gunter, there is a very slight weather helm. Rigging as a lateen should slightly lower the CE, and possibly move it slightly forward. Rigging as a delta will definitely move the CE forward. Adding a jib, which is an option with the gunter rig, also will move the CE forward.

Further comments?

Thanks,

Steve C.
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Old 10-24-2009
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While I have not done the conversion that you are describing myself, it is my belief that the sliding gunter would be a more effective rig. As you mention, it is higher up where the wind is(this also means that you will heal more so if this is an issue, think twice). You will also have a more vertical leading edge which will help a lot with weatherly performance.

As to your question about weather helm, there are a few ways to deal with that. When a boat is sailing completely upright, what causes weather helm is the center of effort (where the wind is pushing) being behind the center of lateral resistance (the hull in the water). You can either move the CE forward by changing your sail plan or the CLR aft by changing how much is in the water back there (bigger rudder, keel further aft etc).

Once you consider the fact that weather helm is also a function of boat trim, it gets a little more complicated. When you heel over, the CE of the sail moves outboard so it is no longer vertically in line with the CLR and you end up with a torque that is weather helm. Also, if you move weight aft, you CLR will move aft (your CE will as well but it isn't as significant) and you will have less weather helm. And then you can talk about changing the hull form which is a whole different discussion and probably not applicable because it would mean chopping up your boat.
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Old 10-24-2009
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With his Kayak, the lee board can be moved with little modification. Thus changing the immersed CE forward or aft
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Old 10-25-2009
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Grateful thanks to both for the replies!
sccid
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