Generate your own polars

[Will Gilmore]

"In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point is called the pole, and the ray from the pole in the reference direction is the polar axis."Wikipedia​

For this application, it is like taking an overhead view of your sailing performance for angles to the wind. The wind speed and direction should be true wind, so data points are hard to measure just from your boat. You really should have a series of anemometers and wind vanes setup along your course to get accurate local readings. Best guess based upon weather reports and initial and final readings will probably be good enough. Boat speed needs to be measured thru-the-water, not over ground.
I would think the way to be completely accurate, performance wise, is to take wind speed and direction relative to water speed and direction. That's what your boat will see as "true wind" anyhow.

-Will (Dragonfly)

 

[Davidasailor26]

The wind speed and direction should be true wind, so data points are hard to measure just from your boat

As long as you have an accurate anemometer and speed transducer it's not the hard. The instruments can do the math themselves to figure out the true wind speed and direction based on their apparent readings.

 

[Kermit]

And I was concerned whether I should use cassette tape or VCR tape. PLUS I was afraid of using a tape that I might have wanted to watch or listen to again.

 

[Jackdaw]

Try this @Kermit http://www.mysailing.com.au/cruising/what-use-are-a-yacht-s-polar-plot-for-cruising

I was going to write a long reply when I saw you posted this. Nice find. That was VERY well written.

 

[Jackdaw]

For this application, it is like taking an overhead view of your sailing performance for angles to the wind. The wind speed and direction should be true wind, so data points are hard to measure just from your boat. You really should have a series of anemometers and wind vanes setup along your course to get accurate local readings. Best guess based upon weather reports and initial and final readings will probably be good enough. Boat speed needs to be measured thru-the-water, not over ground.
I would think the way to be completely accurate, performance wise, is to take wind speed and direction relative to water speed and direction. That's what your boat will see as "true wind" anyhow.

-Will (Dragonfly)

As David points out, all modern instruments can (and do) easily calculate TWA and TWA from apparent wind and boat speed.

Apparent wind is NOT True Wind. Don't say that please. Apparent wind polars are very hard to use, because they factor in how fast your boat is going. As your boat speeds up and slows down, the AW changes. This makes AW-based polars useless.

 

[Kermit]

I’m reading the link now. It’s a whole lot more informative than I expected. It’ll be a while before I’m finished reading, though. I’m at the technical part and have to look away from it. Thanks to @Jackdaw for the thread and to @LeslieTroyer for the link.

 

[Scott T-Bird]

Propellers is another section that leads to at least one question … I have "free fixed blade" but is that based on 2 blade or 3 blade prop? I have been led to believe there is a big difference. I suppose that no self-respecting racer would have either a free or locked 3 blade prop! I will have to calculate based on folding prop, for no other reason than to justify that acquisition!

 

[Scott T-Bird]

I think I know the answer to this next question but I'll bring it up anyway. I'm pretty sure my true wind direction and speed is based on the DST paddlewheel transducer, which I have to keep clean in order for it to work reasonably well. It would be nice if the true wind was determined based on GPS SOG. Afterall, doesn't current affect the boat speed relative to the wind?

We don't have a lot of current but I think it can be up to about 1 knot on Barnegat Bay, depending upon where you are. I've noticed on some days motoring past the entrance to Oyster Creek Channel with an outgoing tide, I seem to have greater SOG going toward the channel entrance and once I'm past it (heading south) the SOG seems to slow down significantly. It could be my imagination.

 

[Jackdaw]

I’m reading the link now. It’s a whole lot more informative than I expected. It’ll be a while before I’m finished reading, though. I’m at the technical part and have to look away from it. Thanks to @Jackdaw for the thread and to @LeslieTroyer for the link.

The value of polars for racers and cruisers alike really come in to their own when combined with weather routing. Combined, the system can calculate the best course to steer to keep you in best wind, and get to your destination faster and safer.

You can compare the rhumb line course with the suggested track based on wind over a 36 hour period (GRIB file). Leaving from the east, the (green) boat is instructed to steer way above the (red boat) rhumb line, making a sharp break to West in new breeze. This type of software can shave days off of passages. Or races!

 

[Davidasailor26]

I think I know the answer to this next question but I'll bring it up anyway. I'm pretty sure my true wind direction and speed is based on the DST paddlewheel transducer, which I have to keep clean in order for it to work reasonably well. It would be nice if the true wind was determined based on GPS SOG. Afterall, doesn't current affect the boat speed relative to the wind?

Our chartplotter has a setting for whether to base the true wind on the GPS SOG or the paddlewheel Speed Through Water. We keep it on paddlewheel unless that gets fouled and inaccurate. What you really want for the purpose of analyzing sailing performance is to look at speed through the water, and calculate true wind speed based on that. If there is a knot of current helping you, you don't want to think that youre doing great if you're actually below your target speed.

 

[Jackdaw]

I think I know the answer to this next question but I'll bring it up anyway. I'm pretty sure my true wind direction and speed is based on the DST paddlewheel transducer, which I have to keep clean in order for it to work reasonably well. It would be nice if the true wind was determined based on GPS SOG. Afterall, doesn't current affect the boat speed relative to the wind?

We don't have a lot of current but I think it can be up to about 1 knot on Barnegat Bay, depending upon where you are. I've noticed on some days motoring past the entrance to Oyster Creek Channel with an outgoing tide, I seem to have greater SOG going toward the channel entrance and once I'm past it (heading south) the SOG seems to slow down significantly. It could be my imagination.

There ARE two 'types' of True wind; Ground Relative and Water relative. Water relative is not perfect but always used for these calculations.

Boats don't know about current. They act in the water relative to them, so measure from that. If you are in 4 knots of current and your polars say you should be doing 6 but the instruments say 10, what good is that?

If you use land relative wind when there is current, your true wind angles will be different on each tack!

Paddle wheels are 10x more sensitive to changes in speed than GPS

 

[Scott T-Bird]

Our chartplotter has a setting for whether to base the true wind on the GPS SOG or the paddlewheel Speed Through Water. We keep it on paddlewheel unless that gets fouled and inaccurate. What you really want for the purpose of analyzing sailing performance is to look at speed through the water, and calculate true wind speed based on that. If there is a knot of current helping you, you don't want to think that youre doing great if you're actually below your target speed.

I will have to read my manual more carefully. I have to assume that my new B&G Vulcan chartplotter would also provide that option. This is where I'm thrown … true wind speed is the speed of wind over ground, so why would it be best calculated based on boat speed in water? I would think that water current would have an adverse affect on the true wind calculation. If you are sailing with the current, adding a knot to SOG, isn't the true wind speed over-calculated and the true wind vector mis-represented? Not that a knot really matters at all, but isn't the true wind speed more accurately calculated by the chartplotter when the boat SOG is utilized instead of speed thru water?

 

[Scott T-Bird]

Paddle wheels are 10x more sensitive to changes in speed than GPS

Agreed, until they are fouled, which is a problem when they sit in our water!

 

[Scott T-Bird]

If you use land relative wind when there is current, your true wind angles will be different on each tack!

There was some overlap in our responses …. so, what we want is the true wind relative to our boat in current, not relative to unmoving land. Is that right?

 

[Davidasailor26]

I will have to read my manual more carefully. I have to assume that my new B&G Vulcan chartplotter would also provide that option.

Yeah, it's probably in the settings somewhere. We have a B&G Zeus 3 and it's in one of the menus.

This is where I'm thrown … true wind speed is the speed of wind over ground, so why would it be best calculated based on boat speed in water? I would think that water current would have an adverse affect on the true wind calculation. If you are sailing with the current, adding a knot to SOG, isn't the true wind speed over-calculated and the true wind vector mis-represented? Not that a knot really matters at all, but isn't the true wind speed more accurately calculated by the chartplotter when the boat SOG is utilized instead of speed thru water?

Uh-oh, now we're bordering on the whole "sailing upstream on the Amazon" discussion.

Like Jackdaw said, there are two ways to consider true wind - relative to the ground or to the water. Since the boat's motion is going to be limited by the water that it's floating in, you want true wind over water to be measured.

If you really wanted to know the true wind relative to a spot on land then yes you'd want to use GPS SOG for that calculation. But that won't be nearly as valuable in measuring the boat's performance relative to its theoretical capabilities.

 

[Scott T-Bird]

I find that if I dig deep enough, I can finally learn something I didn't know!

 

[Jackdaw]

There was some overlap in our responses …. so, what we want is the true wind relative to our boat in current, not relative to unmoving land. Is that right?

100% correct.

 

[Jackdaw]

Agreed, until they are fouled, which is a problem when they sit in our water!

Yes. But here is a point for everyone.

You wanna be fast, you have to work for it. Keeping stuff clean is part of the deal. Once a week, once a month, whatever.

I had a coach who said; "maintain it and don't whine about it, OK. Don't maintain it and don't whine about it, OK still. Don't maintain it and whine about it, NOT OK".