Downwind vmg

[europe172]

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

 

[dunedin]

What type of boat are you sailing, and importantly what instrumentation/ plotters etc on board?
Many modern chart plotters (with wind instruments connected) can give VMG to Wind, so just try to get the biggest (negative) number.
Otherwise it is a calibrated log, a notebook and some basic maths / trigonometry to work it out.
PS It is generally heading up, not bearing away, that increases speed. And a full extra knot would usually be worthwhile for just 15 degrees. It is 45 degrees that is more marginal based on boat type.

 

[anoccasionalyachtsman]

I don't think that there are any sailing boats that don't gain from gybing, but by how much varies. As @dundedin asks, what boat? If you're lucky someone will have done the hard work and published the polar diagram - which is absolutely key to getting downwind efficiently. If not we can probably suggest something suitable.

 

[europe172]

I don't think that there are any sailing boats that don't gain from gybing, but by how much varies. As @dundedin asks, what boat? If you're lucky someone will have done the hard work and published the polar diagram - which is absolutely key to getting downwind efficiently. If not we can probably suggest something suitable.

It’s a bilge keel fulmar , occasional club racing not the serious stuff

 

[Laminar Flow]

It’s a bilge keel fulmar , occasional club racing not the serious stuff

At 15degr. you are looking at a 4% (rounded up) increase in distance but, if you were to go from making, say, 7kts to 8 kts, a 1kt increase would be a 14% gain in speed and a 10% net advantage. The slower your initial speed, the greater your proportional gain, obviously.

 

[europe172]

At 15degr. you are looking at a 4% (rounded up) increase in distance but, if you were to go from making, say, 7kts to 8 kts, a 1kt increase would be a 14% gain in speed and a 10% net advantage. The slower your initial speed, the greater your proportional gain, obviously.

I guess it would be possible to make a small device which set a green or red led from a datum speed, so pressing a button pointing at the mark would record datum speed and direction then changing heading could indicate higher or lower vmg with the green /red led. All it would need would be a small gps chip

 

[anoccasionalyachtsman]

It’s a bilge keel fulmar , occasional club racing not the serious stuff

This shouldn't be far wrong, even if the boatspeed is slightly optimistic . Several plots for different windspeeds, the plot lines break on the transition from jib to spinnaker.


For upwind you want to sail at the angle that coincides with the bit of curve nearest the top of the diagram. Downwind is the reverse, and nearest the bottom of the diagram. The dotted lines are the optimal angles for all windspeeds.

nb. I'm assuming that you're using a spinnaker. If not then what I said about all boats gybing is wrong. For white sails I think straight downwind may be best.

 

[anoccasionalyachtsman]

At 15degr. you are looking at a 4% (rounded up) increase in distance but, if you were to go from making, say, 7kts to 8 kts, a 1kt increase would be a 14% gain in speed and a 10% net advantage. The slower your initial speed, the greater your proportional gain, obviously.

I don't understand why you give such precise figures when they're of no use without knowing what the speed gain might actually be?

 

[europe172]

This shouldn't be far wrong. Several plots for different windspeeds, the plot lines break on the transition from jib to spinnaker.

View attachment 121955

For upwind you want to sail at the angle that coincides with the bit of curve nearest the top of the diagram. Downwind is the reverse, and nearest the bottom of the diagram. The dotted lines are the optimal angles for all windspeeds.

nb. I'm assuming that you're using a spinnaker. If not then what I said about all boats gybing is wrong. For white sails I think straight downwind may be best.

Thank you for posting that , I must admit I am little unsure how to read the polar chart , I guess the dotted line is optimum vmg for the solid lines at 6,8,10 and 20 apparent wind speeds

 

[anoccasionalyachtsman]

Thank you for posting that , I must admit I am little unsure how to read the polar chart , I guess the dotted line is optimum vmg for the solid lines at 6,8,10 and 20 apparent wind speeds

Sort of. You'll see that the dotted line crosses the solid ones at those speeds - and are at the lowest (and highest) points on the curves as I mentioned, but the dotted line also provides you with the best angle at the intermediate wind speeds.

 

[Laminar Flow]

I don't understand why you give such precise figures when they're of no use without knowing what the speed gain might actually be?

It was my understanding that the OP mentioned a gain of 1 knot. To be fair, he did not say how fast he was going, so I picked a plausible speed to illustrate the point. I was basing my calculations on the input of the OP.

By your diagram of course, and at 15 degr. the gains would be considerably more marginal.

 

[anoccasionalyachtsman]

It was my understanding that the OP mentioned a gain of 1 knot. To be fair, he did not say how fast he was going, so I picked a plausible speed to illustrate the point. I was basing my calculations on the input of the OP.

By your diagram of course, and at 15 degr. the gains would be considerably more marginal.

Marginal is the name of the game! Last weekend I missed first place by seven seconds, and second place by two....

 

[flaming]

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

The polars provided are a very good guide for that type of boat. But it will also depend on the skill level of your crew and where the racing is being held.

For example if your crew can be trusted to gybe the kite whilst keeping it full (and just as importantly you can be trusted to drive the boat smoothly through the gybe to give them that chance) then yes it is likely to be faster sailing angles. But if gybes often result in a collapsed kite and some time getting it all sorted, then the leg will need to be pretty long before you gain that loss back again - especially when the wind is higher and the angles narrower.

In addition, polars are always given for flat water. The effect that waves have can be fairly significant. If you are sailing somewhere where the wind is open to the sea, and reasonable waves have built up in a decent wind, then in fairly traditional displacement boats it is often faster to point straight at the mark downwind and surf on the waves, rather than trying to sail angles. This is because even though your baseline speed might be higher whilst sailing angles, you are less likely to catch the wave or hold the surf for as long sailing at an angle to it.
In contrast, if there is a lot of left over slop from a recent blow, but the wind is now light, say 8 knots or less, it can often pay to sail high of your polars to keep the boat powered up and prevent the slop from shaking the wind from your sails which would happen if you were sailing low.

 

[DJE]

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

The extra distance sailed is proportional to 1 divided by the cosine of the angle.
10 degrees off = 1.5% further
15 degrees off = 3.5% further
20 degrees off = 6.4% further
25 degrees off = 10.3% further
30 degrees off = 15.4% further
35 degrees off = 22.1% further

 

[europe172]

The polars provided are a very good guide for that type of boat. But it will also depend on the skill level of your crew and where the racing is being held.

For example if your crew can be trusted to gybe the kite whilst keeping it full (and just as importantly you can be trusted to drive the boat smoothly through the gybe to give them that chance) then yes it is likely to be faster sailing angles. But if gybes often result in a collapsed kite and some time getting it all sorted, then the leg will need to be pretty long before you gain that loss back again - especially when the wind is higher and the angles narrower.

In addition, polars are always given for flat water. The effect that waves have can be fairly significant. If you are sailing somewhere where the wind is open to the sea, and reasonable waves have built up in a decent wind, then in fairly traditional displacement boats it is often faster to point straight at the mark downwind and surf on the waves, rather than trying to sail angles. This is because even though your baseline speed might be higher whilst sailing angles, you are less likely to catch the wave or hold the surf for as long sailing at an angle to it.
In contrast, if there is a lot of left over slop from a recent blow, but the wind is now light, say 8 knots or less, it can often pay to sail high of your polars to keep the boat powered up and prevent the slop from shaking the wind from your sails which would happen if you were sailing low.

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

 

[Birdseye]

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

maximising boat speed through the water isnt the answer. In a race you are normally aiming for a mark, so what you want is the best VMG to the mark not the wind. Your GPS will give you a good indication of what course to sail if you just vary your course a bit and see what happens to the VMG to the mark. A bilge keel Fulmar is never going to plane outside a hurricane and its when you can plane by bringing the wind a bit more forward that the gain in speed outwieghs the loss in direction .

 

[anoccasionalyachtsman]

maximising boat speed through the water isnt the answer. In a race you are normally aiming for a mark, so what you want is the best VMG to the mark not the wind. Your GPS will give you a good indication of what course to sail if you just vary your course a bit and see what happens to the VMG to the mark. A bilge keel Fulmar is never going to plane outside a hurricane and its when you can plane by bringing the wind a bit more forward that the gain in speed outwieghs the loss in direction .

Do you win many races?

 

[europe172]

maximising boat speed through the water isnt the answer. In a race you are normally aiming for a mark, so what you want is the best VMG to the mark not the wind. Your GPS will give you a good indication of what course to sail if you just vary your course a bit and see what happens to the VMG to the mark. A bilge keel Fulmar is never going to plane outside a hurricane and its when you can plane by bringing the wind a bit more forward that the gain in speed outwieghs the loss in direction .

Thanks Birdseye
I don’t have vmg on my plotter, but I am sure i was sailing too directly

 

[flaming]

Do you win many races?

We’ve been there with Birdseye before on the subject of VMG… I gave up.

 

[anoccasionalyachtsman]

We’ve been there with Birdseye before on the subject of VMG… I gave up.

Thought so, I had that déja thing again.