Over breakfast I was reading an article in Sail Magazine in which it was mentioned that a particular autopilot could steer a magnetic course, a course at a particular angle to the wind, or it could sail "by the polars." I like to think of myself as pretty knowledgeable on sailing terminology, but this term has eluded me. Can someone clew me in? What does this mean and what is the advantage of sailing by the polars? Thanks. Gary Wyngarden S/V Shibumi H335
- Status
I'd guess...
They are talking about sailing based on the theoretical performance off the boat's polar diagram. The polar diagram shows expected performance at various wind speeds and angles. If the boat is faster on a broad reach than on a dead run it may be faster to gybe downwind to the mark rather than run. Even though you're going farther you're also going faster. It's like VMG upwind. I suppose an Autopilot could take the wind speed and direction and then (based on the boats polar performance diagram) calculate the fastest way to get to the mark. It would be pretty sophisticated stuff though... Sounds like a nice late Xmas present!
They are talking about sailing based on the theoretical performance off the boat's polar diagram. The polar diagram shows expected performance at various wind speeds and angles. If the boat is faster on a broad reach than on a dead run it may be faster to gybe downwind to the mark rather than run. Even though you're going farther you're also going faster. It's like VMG upwind. I suppose an Autopilot could take the wind speed and direction and then (based on the boats polar performance diagram) calculate the fastest way to get to the mark. It would be pretty sophisticated stuff though... Sounds like a nice late Xmas present!
What autopilot vendor?
Gary, What autopilot vendor and model was the add for? I'd like to read the add...
Gary, What autopilot vendor and model was the add for? I'd like to read the add...
Autopilot Vendor
The reference is from an article, not an ad. Page 93 of the January 2002 issue. "Illusion carries an Alpha Spectra autopilot interfaced through a laptop to the wind instruments. It can be set to steer to the compass, a wind angle, or the polars." The article is about Stan Honey who is a big-time racer-navigator and techy. This may be beyond us mere mortals, but I'd at least like to understand conceptually what he doing. Gary Wyngarden S/V Shibumi H335
The reference is from an article, not an ad. Page 93 of the January 2002 issue. "Illusion carries an Alpha Spectra autopilot interfaced through a laptop to the wind instruments. It can be set to steer to the compass, a wind angle, or the polars." The article is about Stan Honey who is a big-time racer-navigator and techy. This may be beyond us mere mortals, but I'd at least like to understand conceptually what he doing. Gary Wyngarden S/V Shibumi H335
Distant Uncle?
Gary, I've been reading the same article and have been hoping to find out that the Stan Honey mentioned was a distant uncle! There are not too many of us out here! Sounds like he is quite the expert in navigation. Thanks for all your great postings and questions, we all continue to benefit from the HOW site together.
Gary, I've been reading the same article and have been hoping to find out that the Stan Honey mentioned was a distant uncle! There are not too many of us out here! Sounds like he is quite the expert in navigation. Thanks for all your great postings and questions, we all continue to benefit from the HOW site together.
Two comments
IMHO determining your vessel's polars (after you have finished making all changes to hull, appendages, rig and above-deck windage items and have put all necessary on-board gear and tankage in it's usual place) is worthwhile and there are books and articles that describe how you can experimentally determine these for your own vessel. This is very time-consuming but pretty much the only way if you are not sailing a tricked-out one-design with only minimal variation from boat to boat. Even then, though, you need to realize that cruisers, offshore sailors, etc are far more concerned about seastate, current (e.g. Gulf Stream) and weather forecast than about current wind speed and direction when planning their route. Not because current wind parameters are not important but simply because these other factors are far more difficult to include in the overall decision making process. Essentially, polars are just a bare bones starting point, since they are completely oblivious to these other factors; i.e. they will show you the fastest route provided you have a flat sea, no current (let alone eddies, etc.) and a stable forecast. The second comment is that letting any instrument take control of your autopilot in such a way that you cannot precisely predict the actual course over the ground which will be followed can only be a good idea if there are no significant obstacles near the anticipated route. Although this also applies to windvanes (or autopilots under wind control) since the wind may clock back and forth, thereby drastically changing your intended course most windvanes are being used on cruising vessels that adhere to around-the-clock watch schedules. The dangers of letting electronic instruments take over the helm without continuous supervision or fully understanding the maximum possible course changes cannot be overstated, as the annecdotal reports of superyachts running straight onto a crowded beach (usually near a large marina entrance) may perhaps confirm. Flying Dutchman
IMHO determining your vessel's polars (after you have finished making all changes to hull, appendages, rig and above-deck windage items and have put all necessary on-board gear and tankage in it's usual place) is worthwhile and there are books and articles that describe how you can experimentally determine these for your own vessel. This is very time-consuming but pretty much the only way if you are not sailing a tricked-out one-design with only minimal variation from boat to boat. Even then, though, you need to realize that cruisers, offshore sailors, etc are far more concerned about seastate, current (e.g. Gulf Stream) and weather forecast than about current wind speed and direction when planning their route. Not because current wind parameters are not important but simply because these other factors are far more difficult to include in the overall decision making process. Essentially, polars are just a bare bones starting point, since they are completely oblivious to these other factors; i.e. they will show you the fastest route provided you have a flat sea, no current (let alone eddies, etc.) and a stable forecast. The second comment is that letting any instrument take control of your autopilot in such a way that you cannot precisely predict the actual course over the ground which will be followed can only be a good idea if there are no significant obstacles near the anticipated route. Although this also applies to windvanes (or autopilots under wind control) since the wind may clock back and forth, thereby drastically changing your intended course most windvanes are being used on cruising vessels that adhere to around-the-clock watch schedules. The dangers of letting electronic instruments take over the helm without continuous supervision or fully understanding the maximum possible course changes cannot be overstated, as the annecdotal reports of superyachts running straight onto a crowded beach (usually near a large marina entrance) may perhaps confirm. Flying Dutchman
I'm pretty sure there is software out there
that will do the polars for your particular boat.....all you need to do is have a GPS, Windmeter, knotmeter, electronic compass wired up...(kind of like a Raytheon setup) and have it feed the PC with NMEA info..... then go out sailing on all sorts of different points of sail and under differening wind speeds the software will plot the speed vs. point of sail vs. Wind speed....obviuosly your instruments have to be calibrated...
that will do the polars for your particular boat.....all you need to do is have a GPS, Windmeter, knotmeter, electronic compass wired up...(kind of like a Raytheon setup) and have it feed the PC with NMEA info..... then go out sailing on all sorts of different points of sail and under differening wind speeds the software will plot the speed vs. point of sail vs. Wind speed....obviuosly your instruments have to be calibrated...
Gary's Polar Hypothesis
Thanks for all your responses to my question. I THINK maybe I understand now. Let me try to put it into words and let you critique it for errors. For any given boat, there is a theoretical best speed through the water for a given wind velocity and a given angle to the apparent wind assuming the sails are optimally set. These speeds can be calculated and expressed in a polar chart for the boat which can also be put into a set of electronic data and fed into your computer. Since boat speed through the water can be faster on one course than another, a direct route to a mark in a race or to a cruising destination may not be the fastest course. Sailing "by the polars" would combine your current location, the location of your mark or destination, the wind speed and direction, and the polars for your boat. Based on all of the above, the computer and the autopilot would steer an optimal course designed to get you to your mark or destination in the fastest possible time. Is that it? I agree with Henk. No way do I want to turn this over to a computer and an autopilot. There's too much stuff going on out there. An intriguing theory though. Gary Wyngarden S/V Shibumi H335
Thanks for all your responses to my question. I THINK maybe I understand now. Let me try to put it into words and let you critique it for errors. For any given boat, there is a theoretical best speed through the water for a given wind velocity and a given angle to the apparent wind assuming the sails are optimally set. These speeds can be calculated and expressed in a polar chart for the boat which can also be put into a set of electronic data and fed into your computer. Since boat speed through the water can be faster on one course than another, a direct route to a mark in a race or to a cruising destination may not be the fastest course. Sailing "by the polars" would combine your current location, the location of your mark or destination, the wind speed and direction, and the polars for your boat. Based on all of the above, the computer and the autopilot would steer an optimal course designed to get you to your mark or destination in the fastest possible time. Is that it? I agree with Henk. No way do I want to turn this over to a computer and an autopilot. There's too much stuff going on out there. An intriguing theory though. Gary Wyngarden S/V Shibumi H335
Bingo!
Now they just need to interface the polar driven autopilot with some electric winches to do the actual sail trimming and then there would be no need to even go out on that tippy old boat. It could sail itself!
Now they just need to interface the polar driven autopilot with some electric winches to do the actual sail trimming and then there would be no need to even go out on that tippy old boat. It could sail itself!
Autopilot control
Henk, I agree with your autopilot and windvane issues. My Autopilot will sail on the wind but alarms with a wind shift. What I see as a bigger problem is continual minor shifts that do not meet the or - alarm thresholds. If you don't watch what your boat is doing you could end up WAY off corse and never get an alarm. Thanks for all the info. Regards, Chris
Henk, I agree with your autopilot and windvane issues. My Autopilot will sail on the wind but alarms with a wind shift. What I see as a bigger problem is continual minor shifts that do not meet the or - alarm thresholds. If you don't watch what your boat is doing you could end up WAY off corse and never get an alarm. Thanks for all the info. Regards, Chris
- Status