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  #1  
Old 10-05-2003
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Jeff_H & others?

Came upon this question, which suites your multi-discipline talents (performance, rig, structure, etc). Can you help?

Question:
Keel Fairing?

Has anyone out there had any experiance with keel fairing. I have done repairs to my keel and may have upset the balance, meaning, it seems to sail better with breeze on the port side. Could I have caused with the repairs? Thanks.
Ron

My partial reply: Asymmetric performance Better performance on Port Tack:

You indicate that you’ve made keel repairs, and specify fairing. I wouldn’t expect that merely uneven keel fairing would lead to asymmetrical performance. Did you have the keel off the hull (completely, or partially - just suspended some)? If so, perhaps the keel is not hanging plumb, which could cause the asymmetric performance you describe.

The other likely “bogey” might have nothing to do with the keel, but perhaps the standing rig. Is your mast standing plumb?

“Plumb” can be a difficult condition to achieve on a boat. Using any gravity device (plumb-bob, level, etc) requires that the boat be sitting (floating) level. Many boats are imperfect shapes, with (perhaps) one coaming toe rail, etc. just slightly higher than the other. This can be a result of manufacturing defects, racking in the cradle, or even a permanent rack (twist) to in the structure.

Unless you have compelling reasons to suspect the keel; I’d start with re-tuning the mast - merely because it should be easier to adjust, test, and re-adjust etc (as required).

Good luck, and regards,
Gord
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Jeff_H & others?

First of all, most assymetrial performance results from an assymetrical installation of the knotmeter and/or wind instruments). Differences of as much as .3-.5 knots can occur simply from the placement of the knot meter and from different heel angles.

Really badly done keel and rudder fairing can in fact result in some assymetrical performance on the upwind tack but as Gordon notes, it would not be very big and it would not show up once the boat was off of the wind. To check your keel fairing symmetry you can wrap a horizontal section of your keel in Seran wrap (taping the seran wrap to the keel) and then make a rough corrigated carboard or thin plywood template pattern of your keel on one side marking the height of that pattern on both sides of the keel. This is a job best done by two or three people. Using an inexpensive bondo type automotive body filler, goop up the seran wrap in the horizonatal area where you plan to take your measurement and goop up the edge of the pattern as well. Then place the rough template against the hull, and while holdng the template in place so that it is close to horizontal, squigee the body filler so that the filler fills the void between the rough template and the seran wrap on the keel and is roughly the tickness of the template. Body putty cures pretty quickly so that you might only hold the template in place for 5 or so minutes since much of the cure time is used up applying the bondo to the template and seranwrap. When the bondo has started to gel, carefully untape the seran wrap and carefully remove the template and seran wrap from the keel. You will have a rough pattern of your keel on one side that you can compare with the other side. It will not be perfect but it will give you a sense of the fairness, and shape of that side. If you place the template against your hull on the other side it will give you a pretty reasonable idea of how similar the sections are from side to side. One word of caution about this, because of the keel taper, the top of the template from the side that was measured will be larger than the bottom. To check the profile the bottom of the template should be placed on your horizonatal marks and then sighted from below. Differences as large as a 1/4" are not all that uncommon, expecially on production boats with encapsulated keels.

But of course there are a lot of things that can cause assymetry in speed readings. Some are real differences in speed or pointing angles, and some are not. As Gordon notes, boats are not perfectly symetrical. Some items that can affect symmetrical performance would include:

-A mast or keel that is not vertical,
-Tanks (or gear storage) that are on one side of the boat or the other (When that tank is full that side of the boat would have more stability),
-Jib lead tracks, shrouds, spreader length and angle, and even maststeps or partners that are not symetrical in the boat.
-Differences in shroud tension or even the amount of use a shroud has been subjected to. (with use shrouds stretchiness can change and it not all that attypical for a boat (especially an offshore cruiser)to spent more of its life on one track. For example living here on the western shore of the Chesapeake it is more common for me to close reach on starboard to the Eastern Shore for the weekend and deep reach on port coming home),
-Right handed and left handed skippers (on many boats you end up steering each tack with a different hand, for most of us, this is no big deal as we quickly get used to doing that without thinking. But for some people this is much harder to do, so they always steer with the same hand (which means reaching across your body differently), or steer more poorly on one tack, as was the case of a skipper I raced with who would jerk the boat around more on starboard tack)
-On any given day, assymetry between tacks can result from the affects of waves and current. (Yesterday, I sailed a very interesting and tricky race in a pretty steep chop that was not square to the wind. The otherwise favored tack was squarer into the chop which meant a much harder slog against the waves. The speed difference between one tack and the other were nearly 3/4 of a knot.Interestingly, it was on the slower speed legs where we made our biggest gains against the fleet by playing the traveler (rather than steering as much)with almost every wave, finishing over 4 minutes ahead of the second boat finishing in our class (who owed us time) on a 12.5 mile couse. This was an extremely challenging race from a decision making point of view. Inshore there was a little less adverse current running but bigger waves. We chose to stay in the current rather than to deal with the waves and that seemed to pay off. As mainsail trimmer and helping with tactics I felt like I earned my keep yesterday.)

Is that what you had in mind when you asked the question?

Regards,
Jeff


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Thanks Jeff - your reply is every I''d hoped it to be!
Why didn''t I see the obviouse first/easiest choice - "measurement of" as as opposed to "actual" performance?
That''s why I ask you.
Gord
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