Downwind vmg

[Birdseye]

That is not at all what I'm saying....

I'll have one more go....

For any boat, from lightweight 30ft tris to bilge keelers, there is an optimum downwind angle to get downwind at the best VMG. These angles widen as the breeze drops. If your next leg is within the cone formed by these angles, such that you would have to sail lower than your optimum downwind VMG to get to it, then you will be faster sailing those angles (assuming good gybes etc) than you will be pointing straight at the mark, always. The slower the boat and the stronger the wind, the narrower those angles are. To the point where for more traditional boats in 15kts plus they are effectively 180. But even in traditional boats those angles will be wide in 6 knots of breeze. Look at those polars up thread, that's a 60 degree "cone" in 6kts of wind.

If you choose your downwind angle using VMG to waypoint, you are going to get it wrong on occasion. And those will be mostly when the mark is NOT dead downwind, but is off to one side, whilst still being within that cone. Under those circumstances is might look better on VMG to waypoint to sail just a bit lower and point straight at the mark.
BUT it isn't the instantaneous VMG to the mark that is the important measure, it's the total VMG to the mark over the course of the leg. If I sail a bit higher and faster than you, then my VMG to the waypoint might be a little slower initially. However, when I then gybe on the layline for the mark my entire boatspeed is now at the mark, and my boatspeed is higher than yours because I'm sailing higher, so my VMG is a lot higher. If my average VMG over the whole leg is higher, then I will arrive first. This is easy to see on the water, if I sail a hotter angle but am not going forwards relative to you such that a gybe will just take me back to you, or behind you, then your angle is better. If however I inch forward and get to a point where a gybe would take me clear across your bow, then I've sailed a better downwind angle, and despite your VMG to waypoint being initially higher than mine, I'm now ahead..
Golden rule of thumb is if your mark is within the cone, you always, always sail your best downwind angles. It is extremely rare to gain an advantage by sailing lower than your best downwind VMG over a leg of any length. Noting of course, as per my 1st post in this thread, that in reasonable breeze for the type of boat the OP is sailing, 180 degrees may well be the best angle.... But it certainly won't be in light winds.

This is simply about how to get downwind the fastest way possible. The application of tides, and which side of the course you should favour etc is a completely different topic. You are confusing how to sail the boat fast with tactics. Best upwind and downwind VMG angles are just about getting the most performance out of the boat. Deciding how to use that performance to position the boat on the race track to take advantage of tides or windshifts is tactics.

Was a bit surprised to see this thread still going so I popped in depite the earlier offensive and childish comments of some posters.

VMG is a vector so its meaningless to say VMG without saying what the velocity is towards. But as I read your post, I entirely agree with what you have said above. I am no more proposing that you should ignore speed through the water than you are saying I should ignore the waypoint. After all, the objective is to round the waypoint after sailing the fastest possible course, which in my case sailing in the Bristol channel has to include tides .

Out of interest I have done a few calculations using a polar, as it happens for my last boat which was a fin, and some maths. What that suggests is that the breakeven point for sailing at an angle rather than directly at a dead down wind mark is around the 10kn wind speed. Below you benefit as far off wind as 150deg, but once you get up to14kn wind then straight at the mark assuming its dead down wind, works better. The issue of course is the weight of the boat in relation to its sail area and keel drag - boat design is a key factor in the decision. Or to put it another way, how much faster your particular boat goes off the wind than it does dead down wind.

Bye!

 

[anoccasionalyachtsman]

Was a bit surprised to see this thread still going so I popped in depite the earlier offensive and childish comments of some posters.

VMG is a vector so its meaningless to say VMG without saying what the velocity is towards. But as I read your post, I entirely agree with what you have said above. I am no more proposing that you should ignore speed through the water than you are saying I should ignore the waypoint. After all, the objective is to round the waypoint after sailing the fastest possible course, which in my case sailing in the Bristol channel has to include tides .

Out of interest I have done a few calculations using a polar, as it happens for my last boat which was a fin, and some maths. What that suggests is that the breakeven point for sailing at an angle rather than directly at a dead down wind mark is around the 10kn wind speed. Below you benefit as far off wind as 150deg, but once you get up to14kn wind then straight at the mark assuming its dead down wind, works better. The issue of course is the weight of the boat in relation to its sail area and keel drag - boat design is a key factor in the decision. Or to put it another way, how much faster your particular boat goes off the wind than it does dead down wind.

Bye!

I'll try not to be childish.

If you're going to lecture us in maths you'd better get your facts right. Velocity is a vector quantity. You're confusing it with speed, which is scalar.

If you look at the diagram I posted you'll see that it's already done the sums on different windspeeds for you for you - no need for further calculation. That's kinda the point really.

 

[Roberto]

In a race yesterday there was a long down wind leg . What is the best way to decide if it is quicker to jibe or sail directly, if I bear away 15 deg for an extra knot it it worth it ? Thanks for any advice

For a quick evaluation you may use a graph like this (supposing all simplifying factors no current perfect jibes etc): start from your full downwind speed, say 6kt. Climb up vertically until one deviation angle (here 30°) from the wind direction. Read on the circles the minimum speed you need in order to compensate for the additional distance to be sailed (here 7kt).
It can of course be used in both directions: say you are going at 6.4kt at 20° fromthe wind, if you bear away full downwind your speed must not drop below 6kt; if you luff by 10° (so to 30°) you need to gain at least 0.6kt (from 6.4 to 7).

 

[Wing Mark]

Thank you for your detailed answer , the forums can be great for picking up hints, it was a light wind leg of about 2 hours and with hindsight I should have sailed further faster, next time I will

On a two hour leg in light winds, it's likely that the 'true' wind will change over time, so achieving the best VMG relative to the 'true' wind at any one moment might not turn out to be optimum a short while later when the wind has changed. The mark on the other hand, hopefully stays put?
Likewise the wind and current often vary from place to place around the course.
Even in a little dinghy race, the question is often whether you think you understand the wind patterns.
Ideally, one might play the shifts and bends to have both flavours of VMG favourable.

Look at your polars and consider if you sail off at say 160deg, for 10 minutes, how does it look if the wind then either backs or veers 5degrees/10 degrees etc??
Or indeed, if the wind picks up or reduces.

If you look at the polars for something like a 49er, it's pretty simple, the polar is so spiky, sailing for speed covers a lot of moving goalposts.

Life is simpler with shorter courses.

Generally I would say being blown along dead down wind is usually not the right answer unless it's quite windy.
But even then it can pay for tidal or tactical reasons.

OTOH I've sailed comedy light airs dinghy races with an asymmetric where the winning tactic is to point the boat at the mark and let the sails do their worst as the wind puffs from random directions. Sometimes it even pays to take the kite down because you keep trying to overtake it in the lulls!

And then there's waves.
Surfing a Laser dead down wind with the apparent wind on the nose is great for the few seconds it lasts!
Not so clever in a Laser 4000.

 

[Birdseye]

I'll try not to be childish.

If you're going to lecture us in maths you'd better get your facts right. Velocity is a vector quantity. You're confusing it with speed, which is scalar.

If you look at the diagram I posted you'll see that it's already done the sums on different windspeeds for you for you - no need for further calculation. That's kinda the point really.

You failed!. VMG = velocity made good = a vector quantity as I said in my post and you confirmed above.

My maths confirms your posted diagram albeit using a different polar but for a non surfing boat.

 

[anoccasionalyachtsman]

You failed!. VMG = velocity made good = a vector quantity as I said in my post and you confirmed above.

My maths confirms your posted diagram albeit using a different polar but for a non surfing boat.

And yet you said that we need to know velocity towards. Velocity already defines direction (eg x Knots at y degrees Magnetic) If you need to know the direction then it's speed.

In the case of VMG on your instruments it's speed to a point directly up or downwind of you.

 

[Wing Mark]

And yet you said that we need to know velocity towards. Velocity already defines direction (eg x Knots at y degrees Magnetic) If you need to know the direction then it's speed.

In the case of VMG on your instruments it's speed to a point directly up or downwind of you.

You feel sufficiently omniscient to assert what other people's instruments call 'VMG'?
There are lots of GPS's on boats which don't know anything about the wind, but they'll still tell you a VMG.
I've just picked up an old Magellan which does exactly that!

And I suspect, a lot which will tell you rubbish, because there's so much error in working out the 'true' wind.
Or is the ground wind?

I think the thing to know about VMG is that it may not mean the same in all situations or to all people.

 

[anoccasionalyachtsman]

You feel sufficiently omniscient to assert what other people's instruments call 'VMG'?
There are lots of GPS's on boats which don't know anything about the wind, but they'll still tell you a VMG.
I've just picked up an old Magellan which does exactly that!

And I suspect, a lot which will tell you rubbish, because there's so much error in working out the 'true' wind.
Or is the ground wind?

I think the thing to know about VMG is that it may not mean the same in all situations or to all people.

Sorry, yes you're quite right, I meant the boatspeed/wind instruments deriving data from the log impeller and masthead wind.

Omniscience? Hardly.

 

[europe172]

For a quick evaluation you may use a graph like this (supposing all simplifying factors no current perfect jibes etc): start from your full downwind speed, say 6kt. Climb up vertically until one deviation angle (here 30°) from the wind direction. Read on the circles the minimum speed you need in order to compensate for the additional distance to be sailed (here 7kt).
It can of course be used in both directions: say you are going at 6.4kt at 20° fromthe wind, if you bear away full downwind your speed must not drop below 6kt; if you luff by 10° (so to 30°) you need to gain at least 0.6kt (from 6.4 to 7).
View attachment 123412

Thanks Roberto
Do you have a digital copy of that I can print and laminate

 

[Roberto]

Thanks Roberto
Do you have a digital copy of that I can print and laminate

Good question, I have the blank laminated one on the boat, this one unfortunately has the red lines on it, I'll try and find the blank one as file.
Otherwise, simply google images "polar graph paper", choose one suitable image as to number of lines, 5° angles, number of circles etc, open it with for example MSPaint, add vertical lines, text labels, crop the unnecessary part and there you go.
I'll look for the file anyway.
r.

ps if you take a wider sector, say up to 50-60° you may use it for upwind sailing as well, the principles are the same.