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 svzephyr44 |
Upwind Sweet Spot |
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Posts: 138 03/30/06 13:44:38 |
Since I live on Lake Michigan, where by definition the wind is always blowing directly from your destination, I am curious as to what tack angle most people utilize for upwind work to maximize velocity made good to waypoint. BTW I have a wing keel and a 155% jib.
Roger |
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470catalina42 |
Tacking angle for VMG | #1 | ||
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Posts: 59 03/30/06 18:14:08 |
The best angle for VMG varies a little with wind speed. I have a Mark I with a Fin Keel, and can make about 45 degrees to the wind in about 15 kts windspeed. Lighter winds require a wider angle to get and keep the boat moving. I can only get about 55-60 degrees to the wind if it is less than 10 kts. I had posted some polar charts that I got from Catalina on the old association website. They show the boat speed and wind angle for wind velocities from 6 to 20 kts.
Norman Duford Fortune Cookie Mark I, 3-cabin Fin |
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melelana |
Use of Polars | #2 | ||
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Posts: 391 03/30/06 19:34:26 |
Roger,
As a hard core racer in a one design class and C-42 owner/cruiser, the best way to use the polars upwind is to draw a horizontal line that's tangent to the highest point on the curve. That will tell you a "target upwind boatspeed" For the 155/fin keel posted on our site, that varies from 4.5 knots in 6 knots of true wind to 6.6-6.8 in 20 knots. The cluster of values in the 12-20 knot range show you where the boat is fully powered up. The somewhat abrupt cutoff as you sail closer to the wind indicates that pinching up doesn't work well for our heavy boats. As most polars are calculated for flat water, sailing this target speed keys you to head up a little when there's no chop to slow you down and gives you an idea of how much to foot off for power in choppy conditions while maintaining the best VMG. Even on a well sorted out race boat with high $ sails, the calulated polars are just a guide. Each boat/sail combination is different. Good sail shape and trim will help you get the best performance out of your boat. With my boat and sails (WK with full battened main and 140% genoa) I see about 34 degrees apparant in 10 knots of wind and smooth water. 31-32 in 16 knots of true wind and 35-36 in 5-7 knots of wind. That translates into a tacking angle of about 85 degrees in the higher wind and 100-110 in light air. With a wing keel, it's very important to keep boat speed up as you lose some efficiency over the fin keel and it gets worse as speed drops. Probably more data that you need, but the message from our polars is "don't try to pinch" and sail to a target speed. For what it's worth, the interrupted curve as you approach a beam reach is where the polars assume a shift to a spinnaker. Good Sailing Larry Melelana Catalina 42 #175 1990 3cbn, wk Solomons, MD (where the wind blows either up the bay or down the bay. Always from the direction you're headed!) |
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svzephyr44 |
Re: Use of Polars | #3 | ||
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Posts: 138 03/30/06 21:40:18 |
Your answer was very helpful. Let me add some more background and see if it engenders additional comment.
Like the Chesapeake Bay, the wind on Lake Michigan frequently comes either up or down the lake. Since the Lake is about 66 miles across a good day of sailing is normally up or down the lake. For these trips we are most interested in setting the sails for best VMG and only making one tack. I am trying to predict the total amount of time on the upwind leg. The simplest solution is to presume a 45 degree angle to the wind and use the old 1-1-sqrt2 triange to assume that the total track will be 1.414 times the direct distance (really 1/2 the direct distance times 2.818 since you go out and come back in.) From your answer this sounds like a conservative estimate of total track since I will be able in most of the prevailing conditions to point a little bit higher. Would you agree that my above estimate would be a good working estimate? New question - since you are a hard core racer - do you tack downwind or just put the stern to the wind and do a wing on wing run? Roger |
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melelana |
Tacking downwind | #4 | ||
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Posts: 391 03/31/06 05:10:14 |
I set the assymetrical and tack downwind. The stronger the wind, the narrower the gybing angle. You can see this on the polars as the "bulge" in the curve around 130-135 true. At the wind increases, the "rise" in the VMG curve gets flatter so you can drive further downwind.
Main and genoa, the gybing angles are pretty wide (best VMG comes at about 100-110 degrees relative which works out to around 130 true). Above 10 knots of true wind, the difference between DDW and gybing downwind is less than .5 knots. It doesn't take much of an error in gybing angle to take that away. For best downwind VMG and least hassle, I'd tack downwind in light air and pole out the genoa in wind over 10-12 knots. Upwind, the VMG is the cosine of the angle off the true wind - including any leeway. .707 is the cosine of 45, so .7 times your speed through the water is a good rule of thumb. If you use a target speed of 6.5 knots upwind, your VMG is approx 4.5 knots. Most of all- have fun sailing! Larry Melelana Catalina 42 # 175 1990 3cbn, wk Solomons, MD |
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