Wind Indicator: True or Apparent ?

[bewitched]

Select the information that's most useful to you

If you are going upwind by feel, it doesn't really matter if you use AWA or TWA If you're using polars, use TWA.

Sailing deep downwind in wind I'd always use TWA as the AWA will be flipping 30 deg or so at the bottom of a wave if you've got a reasonably performing boat. If you follow the AWA down the wave, you'll run a greater risk of gybing.

[Dockhead]

Originally Posted by pete33458

I'm just trying to figure out roughly how much higher I can actually sail, or more accurately, how much higher to the apparent wind other people find they can actually sail (if this is the case). It just makes me nervous seeing that I'm sailing at 30 degrees to the apparent wind, you know, pinching and all that. pete

Don't look at your wind gauge to see if you're pinching. That's your first mistake. Look at your sails and telltales, trim, steer. Then when everything looks right, look at your wind gauge.

Pointing ability varies by rig type, sail condition, and keel and underbody, but a decent modern fin-keeled sloop will usually sail without pinching at around 30 degrees from the apparent wind, plus or minus a little, if well trimmed and well helmed. As you said, the apparent angle to the wind is narrower than the true one.

But besides that, you've got the effect of leeway. When beating, aerodynamic drag will be pushing your boat over to leeward, so your real course will not be precisely where the boat is pointing.

To see what true angle you're sailing, forget the wind gauge which is unlikely to be accurate to better than 5 or even 10 degrees when set on "true". Sail where there's no tide or current running, trim perfectly, tack, sail some more. Then calculate the angle between your two courses on your GPS. Anything less than 100 degrees is good, which means a real angle to the wind including the effect of leeway of about 50 degrees. If you can approach 90 degrees, which means a true course of 45 degrees off the wind, while keeping up your speed, then you are doing great. That's about as good as it gets on a cruising boat even with high performance sails and a crack helmsman.

One more thing: don't forget the purpose of the game -- making miles to windward. So the real measure of how well you are handling the boat on this point of sail is VMG to windward -- velocity made good. NOT angle to the wind, true or apparent. The optimum VMG to windward will not occur at the narrowest apparent wind angle you can sail. That is because you lose speed, first of all, and secondly, because leeway increases. Optimum VMG to windward will occur a little further off the wind, where the boat is humming along, not heeling too much, with lift and draft in balance. On our boat it's about 35 degrees to the apparent wind, and depending on the wind force (you can't sail as close to the wind in very light or very heavy air) we usually tack through about 95 degrees on the GPS when we are making optimum VMG to windward. YMMV.


If you really want to figure all this out, then do some racing. There's not enough incentive while cruising, in my experience, to really develop all of this.

[MarkJ]

Originally Posted by Dockhead

If you really want to figure all this out, then do some racing. There's not enough incentive while cruising, in my experience, to really develop all of this.

yes, I gotta say racing does help. And now and again when you think you know everything!

I certainly would like to do some casual races against people with the same type of boat. I know I'm not getting the best from this boat yet - hows that after 25,000 miles on it lol!

Racing works well as a learning thing because, unlike school exams you can look at the oposition and cheat But only steal the ideas from the boats in front


Mark

[bewitched]

Apparent wind is what the boat and sails see, much more useful than true wind.

I think I may be going against the general concensus here but I'd say the exact opposite is true

For me the true wind is far more important information than the apparent.

If all I wanted was apparent wind, I'd just look up at the windex and not bother buying those expensive wind instruments.

[MarkJ]

Originally Posted by bewitched

.

If all I wanted was apparent wind, I'd just look up at the windex and not bother buying those expensive wind instruments.

If I want the true wind I look at the ripples.

Apparent is wind on the cheek.

For lee way I use the auto pilot readout compared to COG on the plotter.

I have a dome glass thingy on a post in front of the wheel.. glows red at night. I'm stuffed if I know what its for but its cute when I can make it swing right around. Its good for hanging hats.

[bewitched]

'Tis true, the wind instrument is probably not the most important instrument for the cruising sailor, but if we're going to buy these things, we should know their value.

True wind speed and direction lets you know exactly what the wind is doing, apparent wind doesn't.

A fairly obvious statement, but why is this important?

Imagine sailing down wind at say 150° AWA in say 25kts in a fairly responsive boat – say something like a Beneteau in the 40ft range. Now you start sliding down a wave, your boatspeed increases, your apparent wind clocks forward to about 100* AWA, the boat feels overpowered and like it might start to go into a broach so you turn down wind. But how far do you go? 150° sounds safe doesn’t it? No chance of an accidental gybe at 150°.

So you sail down the wave at 150° AWA, you get to the bottom and the boat slows rapidly, the AWA jumps back 50° - Remember that turn from 100° AWA to 150° AWA? Well now it’s payback time. 150° AWA plus 50° = 200° AWA = accidental gybe.

If we had the TWA on the instruments this wouldn’t have happened because we would always have known where the true wind was. We could have simply looked at the TWA and decided not to go deeper than 170° TWA (or 160° or whatever we felt comfortable with) and we wouldn’t have gybed.

One of the many advantages of true wind information over apparent wind information is that it allows you to sail downwind more safely.

[bewitched]

Originally Posted by pete33458

Do you keep your instruments set to indicate true or apparent wind (especially when trying to make maximum windward progress), and how does the size of your "no-go" vector change in one versus the other? Been trying to deduce this while on the boat, as well as on paper, but can't seem to come to a conclusion on my own. thanks, pete

I'd have true wind on the instruments if I was trying to maximise my performance to windward - simply so that I could quickly confirm the quantum of any wind shifts or any trending that was going on.

I'd use this information as one of the factors in my decision to tack or not.

[Bash]

The wind gauge on the forward cluster is set to apparent. This helps the trimmers. The wind arrow on the binnacle chart plotter is set to true. This helps the helm.

[Hydra]

Originally Posted by bewitched

Imagine sailing down wind at say 150° AWA in say 25kts in a fairly responsive boat – say something like a Beneteau in the 40ft range. Now you start sliding down a wave, your boatspeed increases, your apparent wind clocks forward to about 100* AWA, the boat feels overpowered and like it might start to go into a broach so you turn down wind. But how far do you go? 150° sounds safe doesn’t it? No chance of an accidental gybe at 150°.

So you sail down the wave at 150° AWA, you get to the bottom and the boat slows rapidly, the AWA jumps back 50° - Remember that turn from 100° AWA to 150° AWA? Well now it’s payback time. 150° AWA plus 50° = 200° AWA = accidental gybe.

Bewitched
I think that your reasoning is wrong.

When the boat slows down in the wave trough, the apparent wind cannot pass on the other side. If you sail on a constant course and the true wind doesn’t change direction, a simple change in boat speed cannot result in a gybe.

You could get into trouble if the wind were light and the swell high: when running downslope, the boat can exceed the wind speed and become close hauled. This would backwind the sails and cause a gybe.

But I agree with you about the interest of TWA when sailing downwind, because it is then that the difference between apparent and true wind is the largest and the most difficult to estimate, if only for deciding the right moment for gybing.

Of course, for TWA information to be accurate, you need a precisely calibrated speed log (don't use GPS for this, because you want to know the wind relative to water, not to ground).

Another issue results from the fact that the vane at the masthead doesn't see the same wind as the sails, for at least three reasons:
1) The atmospheric boundary layer: the true wind speed isn't the same at the mast head and at the middle of the sails. In fact, the true wind direction isn't exactly the same, either, but this is less significant.
2) The sails modify the airflow over the masthead: if you were motoring on the same course and speed, the windvane wouldn't be at the same angle.
3) The pitching and rolling motions of the boat disturb the apparent wind felt by the vane. You would need an inertial navigation system to feed the right information to the computer.

Alain

[Jim D]

The answer is Apparent.

Apparent wind moves forward relative to true, thus the no-go cone gets directionally smaller than it would be for true. Nes't ce pas?

~

[bewitched]

Originally Posted by Hydra

When the boat slows down in the wave trough, the apparent wind cannot pass on the other side. If you sail on a constant course and the true wind doesn’t change direction, a simple change in boat speed cannot result in a gybe.

If you sail on a constant course as an autopilot would do when steering to a compass - yes you are correct, the TWA doesn't change (much) and a gybe is unlikely.

However, my example was about actively sailing downwind - steering the waves to get downwind as efficienly as possible, rather than trying to maintain a particular course - My Heading will change and so my TWA will change.

When you are actively sailing down wind, you do so on AWA (because that's what your sails feel-ish), but TWA is the critical information as this is what keeps you safe.

As the boat travels faster (going down a wave), the apparent moves forward and so you can turn down wind. I am not maintaining a constant course, I am actively turning downwind when the apparent wind allows me to do so. What I'm trying to achieve is to keep the roughly the same AWA throughout the wave cycle and therefore keep my trim optimised.

If you do this you will sail lower and faster and arrive at your downwind destination sooner.

The problem is that if I use AWA I don't know how far is too far, because I don't know where the true wind is, relative to the boat. This becomes a problem when the boat comes to the bottom of a wave and slows down and the apparent wind comes aft again. If I have turned too far downwind, I will gybe

So what I do is use the windex as my primary indication of AWA and use my wind instrument to give me TWA so that I don't turn too far downwind.

Or have two wind instrument readouts - one set to AWA one to TWA - I personally find this confusing - especially if they are both digital readout - as it's very easy to look at the wrong one.

[bewitched]

sailing like this

[Hydra]

Originally Posted by bewitched

When you are actively sailing down wind, you do so on AWA (because that's what your sails feel-ish), but TWA is the critical information as this is what keeps you safe.

As the boat travels faster (going down a wave), the apparent moves forward and so you can turn down wind. I am not maintaining a constant course, I am actively turning downwind when the apparent wind allows me to do so. What I'm trying to achieve is to keep the roughly the same AWA throughout the wave cycle and therefore keep my trim optimised.

If you do this you will sail lower and faster and arrive at your downwind destination sooner.

The problem is that if I use AWA I don't know how far is too far, because I don't know where the true wind is, relative to the boat. This becomes a problem when the boat comes to the bottom of a wave and slows down and the apparent wind comes aft again. If I have turned too far downwind, I will gybe

Bewitched,
I use the same strategy as you when sailing downwind, trying to maintain approximately the same AWA (I have no autopilot). Most often, I do this by watching the spinnaker luff, which is the most sensitive indicator that I have.

But only when running straight downwind, AWA=TWA=180°, whatever the boat speed. So, as long as you keep AWA less than 180°, TWA is also less than 180° and you don't risk gybing.

Alain

[bewitched]

Originally Posted by Hydra

So, as long as you keep AWA less than 180°, TWA is also less than 180° and you don't risk gybing.

Too risky for me

Draw it out with vectors

[Bash]

Originally Posted by Hydra

So, as long as you keep AWA less than 180°, TWA is also less than 180° and you don't risk gybing.

If you need the gauge to tell you when a gybe is immanent, you've already lost the battle. Especially in a light breeze when a swell is running.

When running, or even on a deep broad reach, TWA is more helpful for figuring gybe angles than AWA.