Polar Curves

[Juniperskip]

VMC = best speed for the required (arbitrary) course.


I know quite a bit about the use of polar diagrams for racing, but I was wondering if you had a wrinkle about using separate polars for each sail to optimise VMG and VMC.

It sounds as if you are using the procedure in a different way from me, and I was looking for enlightenment.

thanks anyway.

Apologies for not appreciating yr knowledge level. The Raytech software (and Navman I think) refer to this as VMG on Course I believe. Thus I am guilty of moving to rely (yet again) on automation!!

The other VMC is a throwback to a previous career!!!

 

[sarabande]

Yes, we are overwhelmed with TLAs !


At least, with aeronautical background and expertise you will be more than familiar with that excellent device, the pilot's flight computer - the knee-shaped one with no need for electrical backup !

 

[Keen_Ed]

just discovered that B&G produce the Deckman program which just about covers most decent tech tools for sailing more efficiently. It includes a polar generator.

http://www.bandg.com/Products/Deckman-Tactical-Software/More-about-Deckman/

You have to be careful about generating polars from observed data. There's a lot of noise there, and working out what's actually going on is not easy. Big problems in working out true wind - calibration is critical, with all sorts of things like leeway, upwash etc having effects.

Far better to go to somebody with a VPP and get a set of polars off them. Then you can compare those to what you're seeing.

 

[Juniperskip]

Yes, we are overwhelmed with TLAs !


At least, with aeronautical background and expertise you will be more than familiar with that excellent device, the pilot's flight computer - the knee-shaped one with no need for electrical backup !

And the 5 mile thumb!!!!!

 

[rwoofer]

When I used to race on big boats, one of the B & G nav displays always used to be on performance against polars expressed as a percentage. Polars were all held on a laptop integrated into the nav system, which allowed you to tweak your own polars if you could be bothered. The most useful thing was just to know at a glance when you way off the prediction ie. something was wrong.

The eeking out of the last few percentages points was not really used. Of course we all had a smile on our faces everytime we got over 100%.

 

[sarabande]

yes, Ed, I have been reading up on Deckman in spare moments today. Given the variables which can be induced in the system from boat motion (surge and roll being the worst ones ?) and wind gusts and directional quirks, etc, I can see that the data logger will need to send a huge amount of info to the program, to reduce the fuzziness of the curves.

Creating a reliable and verifiable polar from empirical data would take ages. Using one derived from a VPP, using aerodynamic and hydrodynamic data from the architect is a much better starting point.

Getting the program to resolve all the variables on a single solution for one particular set of environmental and yacht data is a horrendous but intriguing mathematical challenge. Then when you have a set of polars, it is for an ideal boat at one particular state of loading and trim, hull cleanliness, sail selection and trim, crew position, etc.

Need to look at Expedition tomorrow !

 

[Juniperskip]

I have used the Navman and raytech software in anger. Navman is really aimed at the top end racer and you need lots of setup time and a navigator that is a regular user and ideally a tablet pc in the cockpit with wifi link. The Raytech system is a compromise between navigation and racing but I find it much more user friendly. I am a Raymarine fan so biased. Their displays are more fexible than B&G but B&G have always produced a better mast pod setup.
Interesting last year at Cowes week. We raced class 1 with a B&G / Raymarine combo without a tablet display - many of the 20 something navs in our start spent the whole time on the pc in the cockpit. Don't know who does a better job but it is certainly a sign of the times!!!

 

[Juniperskip]

You have to be careful about generating polars from observed data. There's a lot of noise there, and working out what's actually going on is not easy. Big problems in working out true wind - calibration is critical, with all sorts of things like leeway, upwash etc having effects.

Far better to go to somebody with a VPP and get a set of polars off them. Then you can compare those to what you're seeing.

You can download a number of "popular" boat polars off the B&G website.

 

[Angele]

So, Magnum, I summarise:

If you are prepared to invest a lot of time and effort making sure your instruments are precisely calibrated, then you can spend your time at sea staring at readouts that tell you how well the boat is sailing relative to its theoretical maximum prediction.

If you couldn't be bothered, or you just prefer to spend your time enjoying your surroundings, just sit back, forget about polars and r....e....l....a....x

 

[Magnum]

So, Magnum, I summarise:

If you are prepared to invest a lot of time and effort making sure your instruments are precisely calibrated, then you can spend your time at sea staring at readouts that tell you how well the boat is sailing relative to its theoretical maximum prediction.

If you couldn't be bothered, or you just prefer to spend your time enjoying your surroundings, just sit back, forget about polars and r....e....l....a....x

Good advice

However, I might just like to have a play with all this technology from time to time. I find it interesting, being a bit of a techie myself. However, relaxation will always be my priority

 

[Magnum]

For interest here are our polar curves.

.

Actually these are for a standard Bordeaux 60, so I hope our taller carbon rig and 3DL sails will do a bit better

 

[Angele]

Good advice

However, I might just like to have a play with all this technology from time to time. I find it interesting, being a bit of a techie myself. However, relaxation will always be my priority

There's just no stopping him. He just can't resist the toys .......

 

[sarabande]

I think the first thing that polar tells me is that the boat is not going to give you any sudden frights as you ease round a buoy or mark. No sudden accelerations or surge of power.

That means all the different sails will be relatively forgiving over a range of wind speeds, and you will not be forever changing or reefing if the wind pipes up or down a couple of knots.

Hmm, 60 ft , slippery hull, tall mast and decent sails... I'd be looking at quite a bit more speed with a spinnaker. Wait till you experience a run downhill and the spray from the bows coming straight past your ears in the cockpit with a noise like a 747 poised for takeoff - then you can come back and tell us what 'Relax' means....

 

[Twister_Ken]

My polars here

How do I work out best VMG to windward and downwind (cruisy chute, not spinny as shown in the polars)

 

[BrendanS]

Does anyone use this data in their day to day sailing? If so how?

If you want to understand polars, have a go at the online sailing races in another forum on here. When you get involved in those, polars suddenly start to make real sense.

 

[Pinnacle]

My polars here

How do I work out best VMG to windward and downwind (cruisy chute, not spinny as shown in the polars)

I think they have been done for you......small red diamonds.

 

[Twister_Ken]

I think they have been done for you......small red diamonds.

Doh!

 

[Badgers]

Source of Polar Curves?

Hi, I have been trawling the net.....(to date not in the right places!),
to find a polar curve diagram for my BMW/FARR designed Bavaria 32 Cruiser.

Can anyone point me in the right direction?

Thx

 

[sarabande]

try here?

http://polardiagram.com/node/13

and here

http://www.stentec.com/en/news/navigation/167-nieuwe-schepenlijst-met-polairen-wingps-4-voyager-


If you can find a PD for a similar boat, you won't be too far out for practical porpoises.

 

[Badgers]

Thx... but not quite what I am looking for.

I really want the actual curve for my boat?