Instruments set to True or Apparent wind

[flaming]

Mmmm

In defence of ground wind...

1) it tells you reliably if the (atmospheric) wind is changing over the course of the day - that's quite useful/interesting ? (can't do this with apparent or true)

2) allows you to compare wind on different days/outings - useful to decide choice of sails etc

3) means you never have to clean a paddlewheel

4) it's very accurate (true wind at best pretty approximate)

5) where/when I sail (SW UK) tides are usually <1 knot and ground wind 10-25knot - so tide wind fairly small effect (and I know pretty well what the tide is doing from the tidal arrows on the chart plotter)

1. The only use of Ground wind. But can be flicked to / checked periodically, does not need to be a constant display.

2. Absolutely incorrect. Ground wind does NOT power your boat. The difference between velocity of the air and water does. It matters not whether you have a 10kt ground wind over a lake or a 0 knot ground wind over a 10 knot current, both result in your boat being able to make the same headway through the water. If you want to use the data coming from your wind instruments for any type of sail selection, tuning, performance measuring etc, then True is the ONLY measurement that is repeatable.

3. Paddewheel removal is just a standard thing you do at the end of every trip.

4. True is only approx if you don't take care of your paddlwheel.

5. You I think massively underestimate the difference the tidal wind can make. For example, using your numbers a 1 knot tide with a 10 knot wind can mean the difference between 9 knots true and 11 knots true. And on most polar diagrams that can be at least a knot, if not more, in boat speed. So if you go out and your instruments are telling you 10knots, but you only have 9 true, you'll wonder where your boat speed went, because 2 hours ago, still in 10 knots ground wind, you were going a knot faster. Except of course then you had 11 knots True and a lot more power in your sails.

 

[bbg]

1. The only use of Ground wind. But can be flicked to / checked periodically, does not need to be a constant display.

2. Absolutely incorrect. Ground wind does NOT power your boat. The difference between velocity of the air and water does. It matters not whether you have a 10kt ground wind over a lake or a 0 knot ground wind over a 10 knot current, both result in your boat being able to make the same headway through the water. If you want to use the data coming from your wind instruments for any type of sail selection, tuning, performance measuring etc, then True is the ONLY measurement that is repeatable.

3. Paddewheel removal is just a standard thing you do at the end of every trip.

4. True is only approx if you don't take care of your paddlwheel.

5. You I think massively underestimate the difference the tidal wind can make. For example, using your numbers a 1 knot tide with a 10 knot wind can mean the difference between 9 knots true and 11 knots true. And on most polar diagrams that can be at least a knot, if not more, in boat speed. So if you go out and your instruments are telling you 10knots, but you only have 9 true, you'll wonder where your boat speed went, because 2 hours ago, still in 10 knots ground wind, you were going a knot faster. Except of course then you had 11 knots True and a lot more power in your sails.

^^^^^^^^^^^^
What he said.

 

[lw395]

A good apparent wind indicator is helpful in monitoring sail setting. It's hard to read a windex to the nearest degree, and sometimes it's not convenient to be looking at the windex at all.
Most of the time upwind, what matters is true wind, as indicated by the compass. On a beat the true wind is about 40 degrees above heading on my boat. The number varies a little with sea state and wind strength, but 90% of sailing a good beat, as far as instruments are concerned, is using the compass to make the best of shifts and bends.
Wind speed?
If you can't convert true to apparent speed and v/v 'near enough' in your head, you need to worry about your basic understanding of vectors rather than thinking about instruments. Try switching from one to the other. Is the answer a surprise? It should not be.
If it is a surprise, it is worth get your head around it, because it could be something is out of cal.
The biggest use of a windspeed reading is to know what sails to set on the next leg. That's easiest thought of in true.

It's worth having some understanding of what errors do to the true wind indication.
Particulary when the true wind and boat speed/direction are similar.
I.e downhill with a big kite.
The apparent wind indication becomes a small difference between two bigger vectors, small errors in log, masthead unit, compass suddenly make it hard to tell the difference between a change in TWS and TWA. Especially when there are waves and there is averaging going on.

 

[Iain C]

OK, here's one I never quite got my head around. At large angles of heel, how much are wind instruments affected?

Imagine you are facing the wind, stood up. You hold your arm out horizontally, with a dinghy burgee in your hand, with the "mast" bit of the burgee also horizontal and in line with your arm. In theory the burgee will be "flying" horizontally, with the flag bit out behind you, and parallel to the ground. If you then experienced wind shift of 45 degrees one way, then 90 the other, then back to your original bearing, the burgee will in theory not move, as all it will have "seen" is the wind coming from above it, and below it.

Soooo...in a yacht being driven hard upwind with 35 degrees of heel on, how much are the instruments affected? I don't know of any systems that have a clinometer built in to compensate? Or is it a moot point as the sails are "seeing" the same wind?

 

[flaming]

OK, here's one I never quite got my head around. At large angles of heel, how much are wind instruments affected?

Imagine you are facing the wind, stood up. You hold your arm out horizontally, with a dinghy burgee in your hand, with the "mast" bit of the burgee also horizontal and in line with your arm. In theory the burgee will be "flying" horizontally, with the flag bit out behind you, and parallel to the ground. If you then experienced wind shift of 45 degrees one way, then 90 the other, then back to your original bearing, the burgee will in theory not move, as all it will have "seen" is the wind coming from above it, and below it.

Soooo...in a yacht being driven hard upwind with 35 degrees of heel on, how much are the instruments affected? I don't know of any systems that have a clinometer built in to compensate? Or is it a moot point as the sails are "seeing" the same wind?

For example..

https://www.bandg.com/bg/type/instr...sducers/motion-sensors/h50003d-motion-sensor/

Other instrument manufacturers either ignore it or have a simpler solution. I'm pretty sure that most have an inclinometer built into the base station.

 

[Iain C]

Interesting...thanks for that.

I do wonder if you could do a budget version using a damped pendulum system...I know I've seen self levelling radar scanners...shouldn't be that hard to get an anemometer to self-level...

 

[flaming]

Interesting...thanks for that.

I do wonder if you could do a budget version using a damped pendulum system...I know I've seen self levelling radar scanners...shouldn't be that hard to get an anemometer to self-level...

I'd check to find out if your system isn't doing something clever itself first... If you went to all that bother but the electrickery was already taking into account it would be somewhat self defeating....

 

[johnalison]

I'd check to find out if your system isn't doing something clever itself first... If you went to all that bother but the electrickery was already taking into account it would be somewhat self defeating....

Years ago I worked out a formula for correcting the wind reading for angles of heel. At first this seemed simple enough, just multiplying by 1/ the cosine of 90- the heeling angle. Unfortunately, this only works on a beam reach and you have to correct for the wind angle, since the meter will read correctly if sailing directly into the wind at any angle of heel (if it were possible). We need gimballed wind vanes perhaps.

 

[Laser310]

The main reason to look at ground wind is to compare your observed wind with a forecast - as the forecasts are always with respect to a fixed (non-moving) reference.

When racing, you really want to keep track of TWD and TWA - that's how you know which sail to have ready to hoist on the next leg. Obviously, if it's windward/leeward it's easy, but if the course is a variety of TWA angles, you need to know what the TWA and TWS will be for the next leg. figuring out the next leg TWA is quite easy if you know the TWD and the bearing to the mar on the next leg. In contrast, knowing the AWA or AWS for the next leg is quite difficult - so even if you have a sailchart in AWA / AWS.., you won't be able to use it...
The other thing that is easier with TWA and TWS, is knowing when to _not_sail the rhumb line on a reaching leg.., because, for example.., sailing 5deg lower might mean you can hoist the C0 rather than the J1 - sure you might have to clean it up at the end if the wind doesn't shift.., but at least you can think quickly through the choices.

 

[lw395]

The main reason to look at ground wind is to compare your observed wind with a forecast - as the forecasts are always with respect to a fixed (non-moving) reference.

When racing, you really want to keep track of TWD and TWA - that's how you know which sail to have ready to hoist on the next leg. Obviously, if it's windward/leeward it's easy, but if the course is a variety of TWA angles, you need to know what the TWA and TWS will be for the next leg. figuring out the next leg TWA is quite easy if you know the TWD and the bearing to the mar on the next leg. In contrast, knowing the AWA or AWS for the next leg is quite difficult - so even if you have a sailchart in AWA / AWS.., you won't be able to use it...
The other thing that is easier with TWA and TWS, is knowing when to _not_sail the rhumb line on a reaching leg.., because, for example.., sailing 5deg lower might mean you can hoist the C0 rather than the J1 - sure you might have to clean it up at the end if the wind doesn't shift.., but at least you can think quickly through the choices.

But you really only need a general idea of the average TW that's going to apply for the start of the next leg, not a second-by-second digital readout.
Sailing dinghies, we do that with just a compass.

 

[thinwater]

OK, here's one I never quite got my head around. At large angles of heel, how much are wind instruments affected?

Excellent point and good comments.

There is also the whole business of the effect of sails on flow. With the wind dead astern, with wind coming up off the main, I promise the data is non-sense. It's pretty obvious, wing and wing.

But since most boats build their targets based on sailing experience, it all corrects out.

As for cruising, if I just look around I'm generally within a few knots. Mostly I never turn the instruments on. For me, sailing is not a video game and I don't feel like having LEDs tell me how to sail. And yes, I'm an engineer and understand the flow and the math very well. I would rather sail based on feel, like riding a bicycle. That is the joy.

And that said, I have used the heck out of wind sensors, not so much for racing, as for supporting research on anchoring sail related topics for articles. The science, for me, is related to but somewhat separate from, sailing.

 

[Laser310]

Excellent point and good comments.

There is also the whole business of the effect of sails on flow. With the wind dead astern, with wind coming up off the main, I promise the data is non-sense. It's pretty obvious, wing and wing.

high-end racing instruments have correction tables for these effects - it takes a while to figure out what values to put in the tables.., but if care is taken do do a good job, the numbers aren't "non-sense". They are not perfect either, but the can be pretty good.

 

[lw395]

high-end racing instruments have correction tables for these effects - it takes a while to figure out what values to put in the tables.., but if care is taken do do a good job, the numbers aren't "non-sense". They are not perfect either, but the can be pretty good.

But when push comes to shove, give me a helm who drives like a bastard over any of this sailing by numbers malarkey.

 

[Laser310]

But when push comes to shove, give me a helm who drives like a bastard over any of this sailing by numbers malarkey.

sure.., but the thread is about instruments.., so...

anyway, i'm sure even you will admit that instruments are a big help on a dark, moonless, night.

 

[Yealm]

Respectfully I disagree - but of course we have different boats/needs/preferences so one choice of wind doesn't suit all. Cleaning the paddlewheel for me is a nightmare chore - third of a bucket in the bilge each time as deep keeled boat - I'd do anything to avoid it !

1. The only use of Ground wind. But can be flicked to / checked periodically, does not need to be a constant display.

2. Absolutely incorrect. Ground wind does NOT power your boat. The difference between velocity of the air and water does. It matters not whether you have a 10kt ground wind over a lake or a 0 knot ground wind over a 10 knot current, both result in your boat being able to make the same headway through the water. If you want to use the data coming from your wind instruments for any type of sail selection, tuning, performance measuring etc, then True is the ONLY measurement that is repeatable.

3. Paddewheel removal is just a standard thing you do at the end of every trip.

4. True is only approx if you don't take care of your paddlwheel.

5. You I think massively underestimate the difference the tidal wind can make. For example, using your numbers a 1 knot tide with a 10 knot wind can mean the difference between 9 knots true and 11 knots true. And on most polar diagrams that can be at least a knot, if not more, in boat speed. So if you go out and your instruments are telling you 10knots, but you only have 9 true, you'll wonder where your boat speed went, because 2 hours ago, still in 10 knots ground wind, you were going a knot faster. Except of course then you had 11 knots True and a lot more power in your sails.

 

[KellysEye]

>Apparent windspeed is almost irrelevant to any decisions about which sail to hoist, whether to reef, or if you can lay that headland. (If indeed you base these decisions on a readout of wind speed at all) True is the measure of how much power is available to your sails.

Apparent wind is what you sail in plus boat speed so it is higher upwind and lower downwind in both cases that affects power and the amount of sail needed, e.g. downwind put up a spinnaker when the apparent wind drops.

 

[Babylon]

sure.., but the thread is about instruments.., so...

I'm thinking of taking up golf, with Bridge for the evenings....

 

[johnalison]

>Apparent windspeed is almost irrelevant to any decisions about which sail to hoist, whether to reef, or if you can lay that headland. (If indeed you base these decisions on a readout of wind speed at all) True is the measure of how much power is available to your sails.

Apparent wind is what you sail in plus boat speed so it is higher upwind and lower downwind in both cases that affects power and the amount of sail needed, e.g. downwind put up a spinnaker when the apparent wind drops.

I'm not clear if the first part is a quote which you are rebutting. If that is the case, I agree with you. Apparent is what you feel when on board and to which you both adjust your sails and reef or deploy accordingly. It is not uncommon to be enjoying a downwind romp in 25kn of true wind under full sail and be faced with the need to reduce sail when turning the corner.

 

[Aja]

Each to their own as far as I am concerned, but my habit is fairly straightforward, Apparent when sailing and True when motoring or motor-sailing.

When sailing, I need to know the wind direction affecting my sails. When motor-sailing, I need to be sensitive to changes in the true wind which might affect my decision perhaps either to reef or to resume sailing. When manoeuvring in and out of harbour, I need to know how the wind will affect my boat when I am going slowly.

That to me is the most sensible settings. +1

Donald

 

[BelleSerene]

Originally Posted by bbgApparently true.

Sorry, .... couldn't resist it.

The art of communication is to make that which is apparently true truly apparent.