Boat speed in relation to apparent wind speed and point of sail.

[lpdsn]

And also, no it is not apparent wind that powers the boat... If that was the case you could motor fast in light winds, get a good apparent wind going, then switch off and sail all day in that apparent wind.

To be fair, Flaming, when beating in light airs racing you try to get the boat going anywhere in order to get enough apparent wind to get going to windward. You must know that.

Works well until someone sheets in too quickly on the helmsman f's up.

Similar principles apply when sailing the angles downwind to get enough flow over the spinnaker.

 

[flaming]

To be fair, Flaming, when beating in light airs racing you try to get the boat going anywhere in order to get enough apparent wind to get going to windward. You must know that.

Works well until someone sheets in too quickly on the helmsman f's up.

Similar principles apply when sailing the angles downwind to get enough flow over the spinnaker.

Yes absolutely, but what I'm getting at is that the energy to move the boat can only come from the true wind. So to describe how fast you can go relative to a measurement that changes with your speed is nonsensical.

 

[lpdsn]

Yes absolutely, but what I'm getting at is that the energy to move the boat can only come from the true wind. So to describe how fast you can go relative to a measurement that changes with your speed is nonsensical.

I agree there. If you think about it, relating the boat speed to apparently wind & AWA is really an overly complex and inaccurate way of relating it to true wind speed & TWA. Even if they don't know they're doing that. Far easier to let the instruments give the true wind info and compare that to boat speed directly.

 

[bedouin]

Pretty sure I don't...

Yes, Apparent wind is what is actually moves the boat. But the measure of the available power in the system is the difference in velocity between the wind and the water. So I'm going to continue to use that to judge how fast I should be going.

As you accelerate the apparent wind will change. So at what point do you know you're up to speed? It just can't work as an arbitrator of speed to come back to.

Yes - when most people talk about TWS of course they really mean speed of wind relative to the water, rather than relative to the land.

For all practical purposes the arbiter of "up to speed" depends on practical experience with the boat, or an equivalent. That does not depend on whether you choose to use TWS and TWA, or AWS and AWA - the conversion between them is a matter of simple trigonometry. So given a set of polars expressed as TWS, TWA I can easily convert them to AWS, AWA and the provide exactly the same information.

So why prefer AWS to TWS and so on? Well there are two reasons.

Firstly because my wind instrument gives me AWS, AWA out of the box - whereas to use TWS it needs to combine that with the log - that is something that has only been easy to do fairly recently, and depends on the accuracy of calibration of the log (and mine is not that good).

Secondly the relationship between AWS and STW is actually a less variable than that between TWS and STW, so it is much easier to remember target speed when expressed in apparent terms than true. (And if fact if you understand the physics you will see why that is the case, at least to first order).

By all means use true if you prefer - but don't say that my method doesn't work just because you don't understand it. In the context of knowing if you are up to speed then mathematically the two are equivalent.

 

[bedouin]

I agree there. If you think about it, relating the boat speed to apparently wind & AWA is really an overly complex and inaccurate way of relating it to true wind speed & TWA. Even if they don't know they're doing that. Far easier to let the instruments give the true wind info and compare that to boat speed directly.

Not so - as explained in my reply to flaming the two approaches are mathematically identical so they are both equally accurate. However using true is the more complex as the instruments on your boat read apparent and that has to be converted to true by the instruments. That conversion is of course exactly the same as the conversion of the "Polars" - so how is my method more complex????

 

[lpdsn]

Not so - as explained in my reply to flaming the two approaches are mathematically identical so they are both equally accurate. However using true is the more complex as the instruments on your boat read apparent and that has to be converted to true by the instruments. That conversion is of course exactly the same as the conversion of the "Polars" - so how is my method more complex????

They are both valid frames of reference. A system based on true wind is the established convention for boat polars, but it is not impossible to do one based on apparent wind. I think that would be a poor tool I think. I'll give you an example that I think makes polar diagrams in your system unworkable, although it is obviously something that everyone has to consider in the real world.

Not done an calculation to derive the exact numbers, but based on experience. Say you have a true wind from 45 degress abaft the beam and you're sailing with jib & mainsail. So maybe the apparent wind is 30 deg abaft the beam. You now put up an assymetric and start whizzing along with the wind may 10 deg for'd of the beam.

In a frame of reference based on true wind, you are simply going faster on the same course (which you are). In a frame of reference based on apparent wind, you have a change of speed and a rotation compared to the apparent wind. That would make the polars in an apparent wind reference very difficult to use, despite this being a real world situation that all sailors, especially racers have to consider.

Convention polars will allow you to very quickly check the optimum course to steer to windward for various wind strengths by imagining a horizontal line touching the peak of the curve. You frame of reference wouldn't have this - it could be determined just as accurately, but not by eye in a fraction of a second.

It would be even worse downwind. It is hard enough to explain optimum downwind angles to inexperienced racing skippers by reference to a diagram that a four year old would understand. Using diagrams drawn using your frame of reference it would be virtually impossible.

 

[lotusman]

I went through a similar evolution, starting with a boat that could not displaying true wind speed I produced polar diagrams using apparent wind speed. I found them very frustrating to use.

Imagine sailing at STW 5 knots with AWS 10 knots on the beam 90° relative to heading. My polar diagram, based on apparent wind speed tells me that I should be able to achieve STW of 6 knots under these conditions, so my sail setting etc. must be suboptimal. My notes tell me the settings I used when I achieved 6 knots, I compare these to what I am using and make adjustments to match my notes. My STW increases and I am now doing six knots. However, as a consequence of the increase in STW, AWS increases and moves forward of the beam. I am now at a different place on the polar diagram, and I see that I should now be achieving 6.5 knots. I consult my notes again, adjust my sail trim, STW increases again, etc. I am following an iterative process to achieve target speed – every time my STW changes, my frame of reference changes and my notes tell me to do something else.

Two other outcomes are equally possible, if not equally likely: my STW remains unchanged; my STW decreases and consequently AWS decreases and moves aft of the beam.

Apparent wind speed and direction results from combining boat speed, true wind speed and true wind direction relative to boat’s course. Any one value of apparent wind vector can result from many combinations of different boat speed and true wind vector. It is not possible to define a unique optimal boat speed corresponding to sail setting relative to an apparent wind vector.

I have used ‘sail setting’ as an abbreviation for the many variables that are brought into play in search of best boat speed.

True wind speed and direction are assumed constant above. This is rarely the case, and is another good reason for monitoring true rather than apparent.

I do not understand how a polar diagram based on the apparent wind could be used to select angles and sail setting for tacking downwind to give the fastest course without making this an iterative process requiring guesstimation – during which time competitors using truewind polar diagrams would have disappeared over the horizon

 

[Twister_Ken]

Just to throw more spaniards into the works even the true wind measured by your whirligig may be an apparent wind - the combination of true wind and 'tide' wind. As you alter course relative to the tide, the 'true' wind may change.

Perhaps some software is cute enough to work this out?

And to the OP - a splendid example of Fred Rift

 

[flaming]

I went through a similar evolution, starting with a boat that could not displaying true wind speed I produced polar diagrams using apparent wind speed. I found them very frustrating to use.

Imagine sailing at STW 5 knots with AWS 10 knots on the beam 90° relative to heading. My polar diagram, based on apparent wind speed tells me that I should be able to achieve STW of 6 knots under these conditions, so my sail setting etc. must be suboptimal. My notes tell me the settings I used when I achieved 6 knots, I compare these to what I am using and make adjustments to match my notes. My STW increases and I am now doing six knots. However, as a consequence of the increase in STW, AWS increases and moves forward of the beam. I am now at a different place on the polar diagram, and I see that I should now be achieving 6.5 knots. I consult my notes again, adjust my sail trim, STW increases again, etc. I am following an iterative process to achieve target speed – every time my STW changes, my frame of reference changes and my notes tell me to do something else.

Two other outcomes are equally possible, if not equally likely: my STW remains unchanged; my STW decreases and consequently AWS decreases and moves aft of the beam.

Apparent wind speed and direction results from combining boat speed, true wind speed and true wind direction relative to boat’s course. Any one value of apparent wind vector can result from many combinations of different boat speed and true wind vector. It is not possible to define a unique optimal boat speed corresponding to sail setting relative to an apparent wind vector.

I have used ‘sail setting’ as an abbreviation for the many variables that are brought into play in search of best boat speed.

True wind speed and direction are assumed constant above. This is rarely the case, and is another good reason for monitoring true rather than apparent.

I do not understand how a polar diagram based on the apparent wind could be used to select angles and sail setting for tacking downwind to give the fastest course without making this an iterative process requiring guesstimation – during which time competitors using truewind polar diagrams would have disappeared over the horizon

I think this is a much better explanation of what I was trying to get at.

If you know that you should do 6 knots upwind in 10 knots true (for example) then as you accelerate you always know what speed you should be heading for.

But if you only know what speed you should be doing relative to apparent wind, then the target changes as you accelerate. In this case it would be 6 knots in circa 14. But you're not going to have 14 knots of apparent wind until you get to 6 knots of boat speed. So how are you supposed to know what your target is as you start accelerating?

 

[flaming]

Just to throw more spaniards into the works even the true wind measured by your whirligig may be an apparent wind - the combination of true wind and 'tide' wind. As you alter course relative to the tide, the 'true' wind may change.

Only if the tide changes....

 

[lotusman]

Just to throw more spaniards into the works even the true wind measured by your whirligig may be an apparent wind - the combination of true wind and 'tide' wind. As you alter course relative to the tide, the 'true' wind may change.

Perhaps some software is cute enough to work this out?

And to the OP - a splendid example of Fred Rift

hey! don't give all the secrets away at once

 

[Twister_Ken]

Only if the tide changes....

Quite right. I should perhaps have said "as you sail into or out of areas of slacker or fiercer tide..."

 

[pteron]

I went through a similar evolution, starting with a boat that could not displaying true wind speed I produced polar diagrams using apparent wind speed. I found them very frustrating to use.

Imagine sailing at STW 5 knots with AWS 10 knots on the beam 90° relative to heading. My polar diagram, based on apparent wind speed tells me that I should be able to achieve STW of 6 knots under these conditions, so my sail setting etc. must be suboptimal. My notes tell me the settings I used when I achieved 6 knots, I compare these to what I am using and make adjustments to match my notes. My STW increases and I am now doing six knots. However, as a consequence of the increase in STW, AWS increases and moves forward of the beam. I am now at a different place on the polar diagram, and I see that I should now be achieving 6.5 knots. I consult my notes again, adjust my sail trim, STW increases again, etc. I am following an iterative process to achieve target speed – every time my STW changes, my frame of reference changes and my notes tell me to do something else.

Two other outcomes are equally possible, if not equally likely: my STW remains unchanged; my STW decreases and consequently AWS decreases and moves aft of the beam.

Apparent wind speed and direction results from combining boat speed, true wind speed and true wind direction relative to boat’s course. Any one value of apparent wind vector can result from many combinations of different boat speed and true wind vector. It is not possible to define a unique optimal boat speed corresponding to sail setting relative to an apparent wind vector.

I have used ‘sail setting’ as an abbreviation for the many variables that are brought into play in search of best boat speed.

True wind speed and direction are assumed constant above. This is rarely the case, and is another good reason for monitoring true rather than apparent.

I do not understand how a polar diagram based on the apparent wind could be used to select angles and sail setting for tacking downwind to give the fastest course without making this an iterative process requiring guesstimation – during which time competitors using truewind polar diagrams would have disappeared over the horizon

If you just monitor true wind speed and set your sails to the noted settings you will find you don't reach the optimal speed as quickly as if you run through your iterative procedure above. This is because the boat does actually sail in apparent wind and your optimum settings for the true wind speed will not be the optimum settings until you reach the apparent windspeed associated with that true wind speed.

This is known as changing gear - you need to adjust your sails as you accelerate.

 

[lotusman]

If you just monitor true wind speed and set your sails to the noted settings you will find you don't reach the optimal speed as quickly as if you run through your iterative procedure above. This is because the boat does actually sail in apparent wind and your optimum settings for the true wind speed will not be the optimum settings until you reach the apparent windspeed associated with that true wind speed.

This is known as changing gear - you need to adjust your sails as you accelerate.

I am trying to understand the difference in our methods.

I agree that the sails are experiencing the apparent wind and that there is a difference between sail trim at optimal boat speed and sail trim adjustment during acceleration. The significance of this varies between boats. It is easy to appreciate the effect of sheet tension and boat’s heading on re-building boat speed coming out of a tack and a skilled crew will be used to manipulating this without reference to the diagrams or tables.

It is not, in my experience, routinely necessary to change sails, or adjust the outhaul, vang, backstay, lead positions etc while accelerating.

As soon as the boat is settled on the wind with all settings at predetermined positions it is the job of the helmsman to stay in the groove.

Off the wind, it is the helmsman job to hold the best course and the crew’s job to adjust the settings’ if the relationship with the true wind changes.

A polar diagram or table of target speed with accompanying notes on ‘settings’ is most useful before a race for selecting sail plan and settings in the light of expected weather and course. It is useful during the race for determining downwind tacking angles or modifying the plan if conditions change etc.

I repeat myself: data based on true wind has data points with unique solution, data based on apparent wind does not. An iteration based on apparent wind has a risk of taking you down a blind alley with non optimal solution.

Iteration or trial and error has its place in preparation and tuning when the performance envelope under a given set of conditions is being defined. Use of this information in a race provides a firm foundation for the skipper and crew to exercise judgment. Basing this information on apparent wind rather than true wind leaves them trying to find a firm footing on quicksand.

 

[pugwash94]

The interesting thing about this conversation is that the instruments and the polar diagrams, SOW, SOG, VMG (to windward) can all be calculated to fractions of a knot and allow you to decide in advance on sail plan, car positions, tensions etc.
But then you throw in sea state and that will change all the calculations. Short steep seas will demand power and speed rather than pointing etc so even with steady wind, a change in tide will demand a change in settings. This means ultimately the only useful figure while sailing is VMG, which as pointed out requires a well calibrated log and even then it will undoubtedly vary in accuracy at different angles of heel, sea state and speed.
So ultimately while the information developed over a period will help judgement it still boils down to just that - judgement and not taking the information from the instruments too seriously.

 

[bedouin]

I agree that if the objective is to pre-set Car positions, traveller and so on then the predictive nature of using True wind has some advantage - but also requires you to have a set of tables of the polars, and associated settings.

My AWS approach is much simpler - it answers the question "am I sailing near optimally now" and all I need to do is hold a couple of numbers in my head. It also provides a useful indication (for SOG) even when the log is inaccurate to TWA is not available (as in the OPs case)

 

[lpdsn]

This is known as changing gear - you need to adjust your sails as you accelerate.

Absolutely. You just have to watch to see how much less ground a boat loses after a tack when the helmsman bears off slightly and the crew trim for acceleration before progressively adjusting the trim for speed as the boat approaches target speed on a close hauled course compared to one that just tacks, sheets in the sails and goes sideways whilst painfully and labourously building up speed.

 

[Twister_Ken]

This is known as changing gear - you need to adjust your sails as you accelerate.

There was an Aussie Admiral's Cupper - Police Car I think - which did spectacularly well on downwind legs. The boat was good at surfing, and every time it did the crew would sheet in to keep the sails perfectly trimmed, then ease away as she came off the wave and slowed, then sheet in as she picked up the next one, then ease away, etc... Must have been a very fit (and exhausted) crew but she took loads of time off of other boats.