leeward angle

Bobg · February 26, 2008, 12:02:51 PM

Just got back from asa sailing school, they talked about leeward angle when chart plotting for various wind conditions, they said it should be listed in our boat data, (catalina 34, 1988)I can't find it, any body got any ideas?

Tom Soko · February 26, 2008, 02:21:30 PM

Bob,
Do you mean the difference between your steered course and actual course? The amount of "slippage" sideways when sailing upwind? If so, I've never seen any published data on any boat. It depends on so many variables that you might not be able to quantify it. Boat speed, wave height, relative wind direction, wind speed, sail set, and probably a bunch more.

Stu Jackson · February 26, 2008, 03:16:04 PM

Bob, it's called leeway, which results in the leeward "angle" when plotting courses. Tom's right, there are a tremendous amount of variables that it's NOT boat specific, like, say, polars. A good navigation book, like Chapman's or better yet Dutton's will give you a good overview of the effects of leeway on course actually sailed compared to compass heading. I forget the actual figures usually used for estimating purposes when close hauled, but in general, the further off the wind you go, the less leeway affects the course sailed.

There are also, undoubtedly, good courses on the web. One is here: http://www.boatsafe.com/ The Navigation Course itself is overpriced, but do a search on "leeway" on that website.

Ron Hill · February 26, 2008, 06:21:33 PM

Bob : You're in the sailing school. Think it's best to ask the sailing instructors the question at the time they first mention an unfamiliar term. Don't wait with a question till later. We all learn from questions.

Bobg · February 27, 2008, 07:39:32 AM

Ron, the sailing school is over and I am at home, of course I asked the sailing instructor, he told me to check my book when I got home, he said some manufactors list it for varying conditions. The salesman at US Marine store said the same thing. In the Annapolis book of seamanship,third addition, on page 258 they list a generic table of boat types for wind speed conditions and "leeway" (thanks Stu) angle for if you are on a Beat or Reach. You guys are the experts in my opinion and if it isn't published for our boats (88 Catalina with a wing keel) you guys would know it. FYI, the course was out of Ft Lauderdale Coastal navigation (105) and (106), Thanks for your time, Bob

Stu Jackson · February 27, 2008, 07:44:22 AM

Bob, the listing for boat "types" would be more appropriate than for boat "models" and I think there may have been some confusion about the meaning of the two words on your instructor's part. I don't have the Annapolis book (gee, how come, must be the "one" book I don't have!

) but let me guess: small monohull, mid-sized monohull, large monohull (perhaps with subsets of deep fin, fin or shoal draft keels), catamaran, trimaran. If that's the case, it's "type" of boat, not manufacturer or specific "model," and, if so, why we have never heard of that "measurement."

Ron Hill · February 27, 2008, 05:29:08 PM

Bob : As other have said what you are looking for in "leeward angle"??, I believe are effects of wind and current.
I don't think you'll find that in any "boat data". If you do it'll probably be in a formula involving the area of the wetted surface, the vector velocity of the current and the same factors for the wind !?!? with the resultant being "leeward angle".

tstrand · February 28, 2008, 03:23:09 PM

Hi Bob,

I have "The Annapolis Book of Seamanship" in front of me. Here's what Rousmaniere has to say about leeway angles (in degrees):

3-10kts 11-21kts 22-33kts 34-55kts
B R B R B R B R
------------------------------------------------------------------------------------------
Shallow-keel cruising sailboat 10 5 8 4 12 10 20 12
Deep-keel racer-cruiser sailboat 6 4 4 2 6 4 12 6

"B" and "R" stand for beat and reach. I'm not sure whether a wing-keel C34 qualifies as shallow-keel cruising or deep-keel racer-cruiser. There was a third category I didn't include called "large cruising sailboat," which I would assume means a full-keel style.

Hope this helps!
Tim

Jon Schneider · February 28, 2008, 04:00:31 PM

I love that he calculates the leeway angle for a beat in 55 kts. That would be some ride.

tommyt · February 28, 2008, 08:52:11 PM

I have talked to a lot of ASA instructors. Some are top of the line, accurate, and would check before making a statment. However, some would make up &^%$# before admitting that they did not have an answer. I think that yours made up some data instead of admitting that it really depended on the boat, the captain, and the envioronment.