Sailing the angles

[Alan]

Sailing uphill, although it does require keeping boat speed up, is pretty much a case of sailing on the wind and in the grove.Sailing down is a different issue. How do you guys figure your sailing angles down wind? Do you have polars or do you use VMG or do you just sail DDW??

 

[Grizz]

Mind if I piggyback?

Alan, I've been meaning to ask a similar set of questions which for me now boils down to "if you use 90 degrees for a tacking angle upwind, do you use the same/more/less when off the wind"? We try to avoid DDW whenever possible (slow) but are never sure how deep/shallow to run on a downwind leg with 2 or more jibes required. Somewhere I remember reading that jibes downwind are less detrimental than tacks upwind, but I'd love to understand the rationale (if it's true).I'll hang up and listen to your answer. Oops, not a radio call-in show...

 

[Bill Roosa]

Just like beating

compare the ratio of the speed on two different downwind runs to the corresponding distances on those two courses. The ratio of the course distances is the given and if you can get a "better" speed ratio on one course then that is the one to take. Then is is just making sure you tack/jibe at the right spot if needed.Example: My destination is at 180 deg and 10 nmSpeed on course 180 deg = 4 knots wind over the port quarterdistance to travel 10 nmSpeed on course 170 deb = 5 knots wind forward of the port quarterdistance to travel to reach the destination: (make the assumption that when you are even with the 10 nm mark on the 180 degree course you are there. This is not how you would do it in reality but it gets you fairly close mathmatically)The course is 10 deg off the original the base is 10 nm. The Cosine of 10 deg = 10nm/x where x is the distance I want. ---> x = 10/cosine(10) = 10.1 nmThe ratio of the speeds is 5/4 = 1.25 The ratio of the distances is 10/10.1 = 1.02 MEMORIZE THIS NUMBERSo while you only increase the distance by 2% you increase your speed by 25%. The 170 degree course is better.Practically speaking I use a pilots E6B circular slide rule. Just set the distances and all the different possible combinations of equivalent speed ratios are right there. Then you just need to find one that is better, pick a course 10 further in the direction you just changed to and check the speed ratios on the E6B. While it sounds complicated in practice you only compare your last course and speed with a new one 10 degrees off so it becomes a routine.

 

[Bill Roosa]

Grizz, Same answer I gave Alan

Since you can sail ANY course down wind AND you can expect to make the same speed on the opposiit point of sail you should jibe through twice your relative wind angle asten of the boat.So if DDW is 180 deg and you are sailing on a course that puts the relative wind at 10 off the stern then to keep the same speed you would jibe through 20 degrees. You have to know the true wind to make this calculation or jibe once and pay real close attention to the wind WHILE you jibe. The true and relative wind are the same when sailing DDW.

 

[Alan]

Bill

Let me take it a step further. OK, I've just turned the weather mark and headed for the leeward mark 10 miles DDW. Should I sail 168? ..170? ..172? When would you make your determination for the next leg? How do you figure your new optimum course angle?Is this your question Grizz?

 

[Grizz]

Yep, that's the question...

...and Bill's answer helped a lot. It appears (using the 180 degree heading w/ 10 nm leg example) that if we pick up 1+ knots sailing @ 170 and 1.2 knots @ 165, our next jibe would have us on a heading of 190/195. This avoids that DDW crawl to the leeward mark and creates a jibing angle (in this example) of 20-30 degrees, which is much less than the 90 degrees when tacking to weather. This also helps explain how jibes won't affect time to the mark as much as tacks...I think.Thanks Bill and Alan. Very helpfull. Unfortunately, 2+ months before I'll be able to put it into effect (Lake Michigan still blessed with hard water on the shoreline!

 

[Boydaclies]

Just finished reading, "Getting Started in ...

Sailboat Racing," by Adam Cort and Richard Stearns which I picked up from West M. and highly recommend. This may be a gross over simplification, but basically to take advantage of the benefits of apparent wind one should sail at around 40 degrees higher than dead downwind in less than 8 knots of true wind. At 10 knots of wind, bring the angle down to about 20-30 degrees. With 13 knots of wind, it closes to 10-20 degrees. And at 15 knots, you can steer DDW to the leeward mark. I'm sure the exact angles vary from boat to boat, but that's the gist of it. Also, with respect to a jibe vs. a tack - during a jibe, especially if you're flying a spinnaker, the rig never becomes totally depowered like it does during a tack. Mr. Court and Mr. Stearns spend a considerable amount of time discussing these topics - it's a good read!!I like Bill's E6B idea. I've got one of those laying around from years ago when I was taking flying lessons - I think I'll dig it out and take it out to the boat.

 

[Alan]

Boydaclies

That's a pretty good summing up of how you handle it. You are correct that these numbers vary from boat to boat. Also keep in mind that your sails ONLY know about apparent wind. They do not 'see' nor are they ever sailing with true wind.

 

[PHitchcox]

VMG?

I presume with the faster course, be it at 168, 170 or 172, I would want to choose the one that produces the highest VMG. No one responded about VMG, but hopefully that is the goal - the highest VMG gets me there the fastest.But am I right?Paul

 

[Alan]

VMG for sure!

Sorry but I didn't see your earlier post about this.Absolutely VMG is king. If you have VMG capability on your GPS then by all means use it for your best course to the mark. Understand that as you approach the mark and its laylines your VMG will begin to approach 0. There comes a point where VMG is ignored in favor of tactics to approach the mark. In theory, any time you cross the rumb line your VMG starts to diminish.