Trimming sails for downwind (confirmation)

[Stu Jackson]

Bill,

I think it's a combination of all three: VMG to wind, VMG to waypoint (mark) and crew and boat characteristics.

F'rinstance, set the favored tack (longest) with the first, KNOW the position of the upwind mark from the second, and make the decision to tack to the "final" layline the third. Remember that with currents the final VMG to the mark could be misleading (pun intended ).

 

[RichH]

There is some natural confusion when using VMG for a downwind course.
VMG 'to the mark' is always erroneous as the angle to the mark from the boat's changing position is constantly changing as a boat follows a straight line course on the 'leg'.

Instead, we use the term "VMG downwind" (or "VMG upwind") which constantly recalculates the "velocity component" (as VMG) as that boat to 'mark' angle constantly changes; ....... or, measures with GPS to some far distant (many many miles) away object or waypoint and which is directly 'behind' the mark.
When VMG is at its optimum/maximum you WILL arrive at the mark (or tacking/gybing point) in the least amount of time and no matter the extra distance sailed.

Since speed is a function of time and VMG is a combination of *boat speed 'times' the cosine of the angle to the mark*, you can (some of us) can then back-calculate to optimize the minimum TOTAL time for the entire course, etc. to windward or leeward and then calculate WHEN to tack/gybe (optimizing the distance of each 'leg' when tacking or gybing). With GPS coupled to 'computers', coupled to the speedo and a masthead wind instrument, all this becomes automatic calculations for which is the proper optimum sailing angle / direction and to optimize - how long to be on each tack/gybe.

If you know 'trig', such mental calculations become quite easy.

Here's a VERY good discussion on VMG ... but you need a math/trigonometry background to get the most out of the discussion. http://knol.google.com/k/why-vmg-matters#

 

[Todd Smith]

OK, I see what you are getting at. Using the VMG to the mark will sometimes not give you any useful info as you have drawn your diagrams. And using VMG to the wind would. However, I'd never take course 1-to-end point cause "you always do the longest tack first" and I'd never do 3-to-end point because I would not want to be that far off the start line or around the mark starting that leg which increases the distance I have to cover. So while I agree with you mathmatically I don't think 1 and 3 are "smart" racing.

I can certianly see the usefulness in general sailing though.

I was always under the impression that as I near the mark i should stop looking at VMG and make a decision to tack or hold on which really depends on how good my crew is at sail handling and now we are off topic AGAIN.

Bill, the diagrams are to illustrate the concept of using VMG to the wind, instead of VMG to the mark. They are not an attempt display race strategy or tactics.

As far as where to start, you start where the RC sets the line. It might be anywhere And you tack at the layline, try not to overstand.

 

[Joe]

I do have a polar diagram from Beneteau, but if I read it correctly, the downwind curves assume the use of a spinnaker. That is, for each wind speed it has discontinuous curves for up and downwind. I assume there's no reasonable way to infer non-spin downwind targets from that, correct?

That's an interesting point... but doesn't mean you revert back to the old way. It means you'll have to modify the polar for your boat.

You can make a curve for each sail configuration.... spin, reacher, genny ... what ever.

Pay close attention to Rich's very wise comment:

Make your own polar plots by careful recording of the angles/windspeeds (correct to True wind, ugh) ............etc.....

(hah,hah.. I just noticed that little expression Rich, cracked me up....)

As earlier recommended, try heading up to 135, maxing your speed before trying to sail lower... remembering to record your boat speed for each change in direction. It will all come together in a short while and you will become the SMOKE-OR instead of the SMOKE-EE.

 

[DancingDays]

We had a regatta in conjunction with another club on our lake yesterday. We ended up (as usual for around here) with an upwind/downwind course, with no reach legs.

We tried reaching off for the downwind legs, and did better than we have in the past--but not enough better to gain on the boats ahead of us that went DDW wing-a-wing. Now, most of those (in the non spinnaker class) ran whisker poles, which we don't have.

We are working on recording wind and speed. But we find it very hard due to the variable conditions on the lake. The wind changes continuously, and it's very hard to experiment with trim, as you can almost never tell if your trim change had an effect, or if the wind just changed.

 

[Joe]

We had a regatta in conjunction with another club on our lake yesterday. We ended up (as usual for around here) with an upwind/downwind course, with no reach legs.

We tried reaching off for the downwind legs, and did better than we have in the past--but not enough better to gain on the boats ahead of us that went DDW wing-a-wing. Now, most of those (in the non spinnaker class) ran whisker poles, which we don't have.

We are working on recording wind and speed. But we find it very hard due to the variable conditions on the lake. The wind changes continuously, and it's very hard to experiment with trim, as you can almost never tell if your trim change had an effect, or if the wind just changed.

I admire your perserverence, lad. It's insane.. unless you change what you're doing the results will not change. You did improve.... because you didn't lose ground, as you had before. So now... get a whisker pole! and get that genny poled out out on the broad reach. You're getting better!!!!