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Board » General Discussion » VMC

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Viva Rod.

Thank you for your comments and my sincere apologies if I wasn’t clear on statements.

Ideal Polar and the butterfly.

Let me start with your ideal boat and so, the corresponding “ideal” Polar.

It can’t simply be a “circle”, Rod.
Because even in a so called “ideal boat” there are some basic limitations from the physics.

For a quick understanding I’ll give just two:

1 - With TWA = 0,00º the BS is plain zero (forget now any water induced effects).
2 - With TWA = 0,00º or TWA = 180,00º the BS can’t be the maximum (the first one is immediately evident).

Polar’s shapes (generally speaking) are similar to “butterflies” with more “wing” area in the down side of it (we agree that we go faster downwind) - in the attached pdf the “ideal” Polar is in orange color and the general “butterfly” Polar in red color (you can also turn on/off the layers clicking in the "eyes" inside the layers window).

The 3D.

Let’s see if you won’t be more puzzled now.

In reality boat’s Polars are contained inside 3D surfaces. Using 3 axes for representing (TWA; TWS; BS) you get it.
In your ideal Polar, a sphere, in a general view, a 3D shape like the one shown on the attached AVI file (it is part of my initial study for the recent SOL Ker40 Polar).

Still puzzled my dear Rod?
Hope not.

A Big Hug
Sail Fair.
Viva Rod and all.

The second part of Rod’s reply.

1 - VMG and VMC.

VMG and VMC are equal in the sense that, they are calculated from the same vector (BS vector) in specific directions.

VMG (Velocity Made Good on Wind”) is calculated with:

VMG = BS x Cos (TWA),

VMC (Velocity Made Good on Course) is calculated with:

VMC = BS x Cos (Angle between BS (vector) and boat Course direction),


“Cos” is the Cosine of an angle;
“TWA” is the True Wind Angle.

They can only be the same in value, in two particular occasions: when your boat course direction coincides with the VMG,Up or VMG,Down angles (assuming they are both unique, which normally they aren’t).

2 - VMC calculations with variation on (TWD; TWS) pair.

In a simple course from “A to B” if the pair (TWD; TWS) changes during race time - which is what normally Nature do - to get the most of your boat performance inside the (TWA; TWS) pair, or boat Polar, you probably won’t go for a straight line course. Most probably you will “curve” your boat’s path/course.

Mind you that when I speak about a “curve” in reality it’s not a “curve” but, lots of small straight consecutive lines circumscribed by the so mentioned “curve”.
At each vertice of your projected path, you can calculate the corresponding VMC value.

The best course will be the one that optimizes (minimizes) the SUM (D,i / VMC,i), where:

“SUM” is the math symbol for sum;
“D,i” = Distance of each straight line “i”;
“VMC,i” = Velocity Made Good on Course during the Distance “i”.

If you have in your projected path, for instance, 100 straight lines, “i” goes from 1 to 100 and you have 99 sums.

Translating now to a simple figure: Total course time, which is nothing more than the result of the above SUM. The less the better.

Rod, you can use your graphical expeditious tools. No problem with that but, for short paths is my advice.
The issue is on long paths with several days to do it, which takes us to the router talk.

In relation to your statement: “This has nothing to do with Orthodromic, Loxodromic, Rhumb or Starting lines, and the precision of the calculation is whatever you desire”, I suppose it would be better if you think a little more on it using now the recent input.

A Big Hug.
Sorry all, I was logged in as "SOL".
Sail Fair.
My comment about loxodromic etc courses was made with the assumption that it was dealing with a "featureless" ocean, where any arbitrary "destination point" might be chosen. Where specific points must be chosen that are more than 'six hours' away, then naturally one is dealing with land, wind speed and direction in a much more complex calculation, as in the use of a router. I do not use a router, just the "little grey cells", which does simplify the calculation--(and increases the "finishing position number" at the end of the race!!)
Re-reading the above columns, I feel I should expand slightly on my "ideal boat" and its polar--it can be represented by an Ocean Liner, or a battleship, on a calm, windless ocean, going equally well in any direction. In high winds, it probably goes a little bit slower up wind, and faster downwind.

--- Last Edited by Rod at 2016-11-03 18:02:25 ---
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Viva Rod.

Good morning.

In general I agree with you, but particularly with your insightful notion of a “futureless” ocean.

At the rate Earth climate is changing, we are all going to be sailors.
I was wondering what will be a global “Venice”.
Sail Fair.
You and I will be long dead, and pushing daisies up from below, before we have a "futureless" ocean, JB!
What I was trying to emphasise was that the 'general' case must be understood by 'everybody' before the 'special' cases make sense. I am not a user of routers, and I suspect the majority of SOLers are not either, but the advice we give to others should not be predicated upon the assumption that they already know more than we do.
If it breaks, it's not strong enough--if it doesn't, it's too heavy.
Viva Rod.

You have a point and it is preety fair your thought.

Sail Fair.
Is anyone having problems downloading Agage's Optimum Angles tool? I get an error message from the link http://agage.sol.hmm.iki.fi/OptimumAngles.html
Viva everyone who has contributed to this thread. You have managed to both confuse me and educate me at the same time - haha. But I thank you for it. I now have a good understanding of the mechanics of VMC - thanks to all of you but particularly to the SPINNAKER; the visual VMC representation there I think was the final piece of the puzzle.
I appreciate that you have taken them time to educate me with a little fun along the way, I still have a long way to go. Thankyou to Outlaw for your new VMC post, I know the practical answers are in there, I think I'll just have to read it a few more times and do some practical training with Kippers CAT for things to finally drop.

Cheers AND Beers from downunder

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