Using the VMC polars

27 10 2009

So far we’ve only hinted at ways to use the VMC polars while starting to try to explain the concepts. The printed polars give you a couple of advantages, and there are two main ways to use them for “no math” navigation. Advantages for normal use are that they are bigger, and that there are more windlines to help with planning.

VMCexample1rotated

I suggest two ways that they can be best used to find the best VMC angles for a given target. The first is to lay the polar over the boat (or over your position on a paper chart in rl) and align it to the wind, as in the previous VMC examples. The VMCmax is the point on the polar that is furthest toward your target – TWA 50° in the example we’ve been using, for a target of 30° off the wind.

I find that I can see through the polar well enough even printed on regular paper, so long as the room lights are dim, but printing on thin paper or better yet transparencies would work better. Another option might be to wax the polar… rub with candle wax and iron (don’t use the “good” iron if you know what is good for you). Rubbing in a very dry coat of salad oil works too.

SO76TP-90mono

The other method makes use of those radial “spokes” that are a prominent feature of the polars posted here. The spokes trace lines of maximum VMC for given target angles relative to the wind. The heavy lines are for targets of 0-90-180°, while the solid and dashed lines mark of 30° and 10° intervals respectively. We’re used to finding VMG, so you can see pretty quickly that the 0° spoke just traces those points (remember that VMG is just a special case of VMC). If you rotate the polar, you can see that the spokes always point to the point furthest “up”… max VMC!

In our 30° example, you pick the 30 line (first solid line counting clockwise) and read off the TWA where it intersects your wind line. Like magic! You get the same 50° answer as the overlay method.

Using the VMC spokes is a particularily good way to work out a set of delayed commands for those awkward times when you have to sleep. It is easy to pick a TWA to about a degree of accuracy just by eyeballing the polar. Believe it or not this is better than most routing software will give!


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6 responses

29 10 2009
SWE54

Well done…!
I went through all this a couple of years ago and only started sailing on Sailonline to try and test it out, now years later, I’m still sailing…

2 11 2009
Andrea (ita10267)

Although crucial, VMC sailing is not the golden rule for 100% of the cases.
Quoting Ockam website: “the disadvantage of Vmc sailing is that you go off rhumbline, and will eventually have to get back. In the upwind case, you can tack. But in the offwind case, you cannot. If the mark is days away, the wind will likely change and allow you to consolidate back to the rhumbline. If the mark is minutes away, maybe no Vmc sailing. If the mark is in between, it depends on circumstances.”

How these circumstances have to be chosen, I think this is our space for debating.
In my opinion I think this is something to underline.

2 11 2009
76trombones

ita10267 brings up a good point, and one that needs to be discussed when we start to decide how to choose our targets. As he reminds us, sailing below our VMG angles upwind (or above them down) only works if there is a shift that lets us trade our loss to windward for a gain up-course.
I guess the outlook I’ve been using is that so long as the target chosen reflects the expected wind, it works out. If there is no shift available, VMC automatically=VMG.

16 11 2009
BER_Baggus

Hi 76T – is there a polar diagram (for the Orange 125 specifically) online anywhere, or do we all have to make the effort to produce our own? 🙂

Baggus

23 01 2010
VMG applied to Course A of ACA-SL CUP

[…] first, comes from a discussion as founded on another blog. The author suggests a way to find the best VMC angles (called “VMCmax”) for a given […]

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