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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > Velocity Predictions Polar Diagram
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Thread: Velocity Predictions Polar Diagram Reply to Thread
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Topic Review (Newest First)
07-17-2011 12:26 PM
CowbaySailor I realized yesterday while out on the water that when I took a good look at what my requirements for speed really is, the answer was this.

Velocity Made Good (VMG)vs Distance Made Good (DMG). As a cruiser with destinations I need to shortest route between to points, and the fastest way to travel on each track to get there. My average travel distance per day is 16 to 25 kt miles per day. In the inside of Vancouver Island the Gulf Islands is where I sail. The landscape controlling the winds, tides running fast through narrows and pushing you out of bays when its filling. Add the crazy weather coming in from the open sea or over the mainland mountains. I just need to get to are destination and enjoy all the things we do on the water.

Do I like to see how fast I can get her to sail absolutely, when the conditions are right I will push the limits and grin all the time while doing it but. When I need to make head way I will sail, motor sail, use the motor what ever it is that will give me the best Distance Made Good. This is what I need to learn the most how to travel fast and comfortable in all condition and weather.

VMG will just make me learn how to get the most out of my sailing. Knowing what speeds to expect from winds of the day on sailing legs makes plotting a trip easier and more fun for everyone on the trip. A check sum for my sail trimming. A way to teach the kids how to catch the wind then harness it. VMG is something all Skippers should know about there boats. Just another tool in the sailors bags.
07-16-2011 12:36 PM
blt2ski I would plot, not that I have as of yet, ALL the jibs in inventory with different reef options too. You may find a 130 and one reef in some wind conditions is faster than say a full main and 110 if the SA numbers about the same area. Then also look at the wave conditions too. I remember reading and article in Sailing World interviewing a skipper in a V70. They had three sail options for winds in the 40-50 range?!?!?! depending upon what the wave sizes were, and where the wind was in relation to the water level. Granted in our little and way under info' we have for wind speeds, waves, where the max wind is etc. still plotting the different sails in even the 20-35 range could be worth doing. Yes under 20 maybe the BIGGEST sail(s), but at 30mph, I know I do not have a 155 and full main up. More like a 110 and a single reef, Or if I get an 85% FT jib, then that and a full main. So which is quicker better in what conditions would be worth plotting IMHO!

marty
07-16-2011 11:11 AM
CowbaySailor Thanks for the article on the pdq speed polars that gives me all the info I need. The wing on wing was interesting also. All good stuff.
07-15-2011 12:26 PM
pdqaltair
Yes, for every sail combination and reef...

Quote:
Originally Posted by CowbaySailor View Post
I found and old book called championship tactics with Gary Jobson and Tom Whidden. In it they have Jim Marshall talking and explaining polar charts. As a radar plotter in the navy I'm pretty sure with a rel vel chart I can figure it out.

Now my question is will I have to plot are each jib in my inventory?
... but most only plot the largest sails in the inventory.
07-15-2011 10:31 AM
CowbaySailor I found and old book called championship tactics with Gary Jobson and Tom Whidden. In it they have Jim Marshall talking and explaining polar charts. As a radar plotter in the navy I'm pretty sure with a rel vel chart I can figure it out.

Now my question is will I have to plot are each jib in my inventory?
07-15-2011 10:19 AM
pdqaltair
They are not too difficult to create, but it does take time.

Simply take many data points and plot with Excel as a circle plot.

Sail Delmarva: Speed Polars

You have to play some games with interpolation and smothing, since wave conditions and load will vary over time. In someways the targets of others are better (depending on how they were developed) and in some ways your own are more valid.

I found I mostly worked on this during long, dull watches.
07-15-2011 09:54 AM
jackdale During the server crash, I send a message that US Sailing sells polars, but they are expensive.
07-15-2011 09:51 AM
CowbaySailor I got a message saying you sent one but didn't receive it. Thanks for the info I will plot my collected readings.
07-15-2011 01:27 AM
MikeWhy CB, did you get my earlier from before the server crash? Just draw horizontal lines tangent to each curve to find the upwind or downwind VMG. It works for any wind angle, but straight upwind VMG is most directly meaningful to your questions, and incidentally the simplest to find.
07-14-2011 10:09 AM
CowbaySailor I have started to do that Pedro. That doesn't give me the best velocity made good (VMG) for my boat.
In your mind which is better. you sail into a puff and the apparent wind angle increases, do you chase the tell tails and heat up immediately or steer straight or bear off slightly ease the sails at the same time. When the boat comes up to speed then head up slightly at close-hauled sail trim ?
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