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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Polar plots

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indecentseas
1st Mate

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Canada
67 Posts

Initially Posted - 02/23/2014 : 18:22:35

Anyone have polar plots for our beloved Catalina 25? How about with a swing keel?

This product looks kind of interesting.

http://www.sailtimerwindvane.com

It measures wind speed and direction and sends it to an iPad app. The app will then make a polar plot of your boat's individual performance and suggest ideal bearings for sailing to a particular destination. This would not only allow you to make a polar plot for your particular boat but for your particular boat under a variety of different sails

David Hopkins
Vancouver, British Columbia
'78 Swing Keel, Standard Rig, Dinette
#534

Edited by - on

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 02/23/2014 : 18:46:09

I've been a member of the C25 "family" for about 34 years, and have never seen or heard of a polar plot for a C25.

You might look for a polar plot for a C27. C27S are raced very seriously in some places, and you might find one for the C27, and it might give you some guidance. I would think that the performance of C27s would be somewhat similar, even though I have always believed that the C25 is faster, because it's lighter and has a longer waterline length. If you find one for C25s, some folks here would love to see it.

Edited by - on

Kim Luckner
1st Mate

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USA
96 Posts

Response Posted - 02/24/2014 : 07:27:42

So this might work.
http://www.altendorff.co.uk/archives/1309

Article is titled "Getting a Polar Plot for your Boat"

The above link doesn't work but it looks like you can create one here.

http://www.letscreate.dk/letscreate/?q=node/300

Edited by - Kim Luckner on 02/24/2014 07:39:15

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 02/24/2014 : 08:10:04

The Sailtimer Windvane looks promising, if it works, and the price appears to be reasonable for the info that it provides.

But, my friend's boat has an expensive array of B&G instruments that tell you true windspeed and angle, apparent windspeed and angle, boatspeed, target boatspeed (based on polar plot data), and similar info, and personally, I think he would be better off without them. He lets the instruments tell him how to sail the boat, instead of looking at the sails and telltales, and the result is that he pinches for long periods of time, and doesn't realize it, because he is looking at the instruments, instead of the sails.

The reason is because the instruments tell him the <u>optimum</u> wind angle to steer for best vmg, and he tries to steer that optimum angle, but nobody can steer that accurately, and the result is that part of the time, he is pinching. If, as a practical matter, you can't effectively use the accurate info that the instruments provide, then the instruments aren't worth the cost. If you just watch the telltales, and steer the boat to keep the telltales flying, you'll do better. Whenever you pinch, the boat loses speed, and it takes a long time to regain that lost speed. I'm not saying the instruments don't provide some useful information, but that I don't think they provide enough useful info to justify their cost.

Polar plots don't really help you when sailing closehauled. They only help when off the wind. Most racing currently is windward/leeward.

Modern uldb racing boats can often get on a plane fairly easily, and it makes a big difference if you know the angle at which they are more likely to plane when sailing off the wind. But, conventional cruiser/racer designs, like the C25, don't plane easily, so, getting the angle exactly right downwind isn't as critical as it is when the boat is capable of planing easily.

Edited by - Steve Milby on 02/24/2014 08:22:31

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 02/24/2014 : 09:56:58

I would suspect the polars would be somewhat different for the three keel types, and possibly for the two rigs... but particularly the keels. People here have searched for them before, to no avail.

Variations in sail sizes and condition can further complicate things. You might be best off by taking a note pad and a GPS out in a few different wind conditions and recording your own speeds at various angles.

Edited by - on

Boomeroo
Navigator

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Australia
128 Posts

Response Posted - 02/25/2014 : 17:08:18

hi , I have seen the polar for the wing keel 30 and it does give a good guide to the need to sail a little free to windward .I think the same with the 25 . I race an etc hells not my 25 and have a GPS devise called a puck that gives speed and direction in a small battery powered devise that we take off and down load the race track . The software has the ability to convert that into a polar track digram giving direction and speed.
I will take it out on the 25: next time and do a plot.
We do use the polar diagram to give a chart of target speeds at different wind speeds and very important down wind gybe angles/and target speeds on the mount gay 30 I race on Wednesday.
Look up velocitek puck for info. Lots of skiff sailers use them as they are waterproof .

Hope its warming up ,our summer is almost over ,94 degree today

Edited by - on

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 02/25/2014 : 17:12:52

Thanks Graeme! A lot of guys up here would be interested ion seeing it.

Edited by - on

John Russell
Master Marine Consultant

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USA
3444 Posts

Response Posted - 02/26/2014 : 14:48:22

Don't you get those at the Circle K???? Oh, wait, those are Polar Pops. I guess that's different.

Edited by - on

Boomeroo
Navigator

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Australia
128 Posts

Response Posted - 02/26/2014 : 17:54:26

Thanks ,the pressure is on . I will scrub the hull tomorrow if calm and look for a good weather pattern next week to give it a try. Just some info for general reading below, copied by me ....

The earliest GPSs were poor tools for tracking speed. Fixes were deliberately imprecise (for the sake of national security) and they erratically averaged speed by using the distance/time formula and a breadcrumb-trail of trackpoints. By comparison, a modern GPS receivers has very precise fixes and can almost instantaneously calculate speed by monitoring the frequency shifts (Doppler shift) of the satellite signals it tracks. Just as a train whistle’s rising pitch can be used to calculate how fast a train is approaching the station, the frequency change in signals from GPS satellites can be used to determine how fast a boat is going. Advanced GPS chipsets in two devices reviewed for this article can calculate speed and heading solutions four times per second. This often yields results that are more accurate than can be displayed by typical sailing instruments, since these are usually limited to resolutions of 0.1 knot and 1 degree.
]One of the SpeedPuck’s key features is the way it presents heading data. As soon as you hold a compass heading for 20 seconds, it locks on that course and a single bar appears on the display, centered at the 12 o’clock position. Any significant variation from that heading is indicated by additional bars to the left or right of the 12 o’clock position on the puck; each bar represents three compass degrees.
Off the water, the testers used the free downloadable Velocitek Control Center to modify the SpeedPuck’s default settings. The software lets users specify compass deviation, change the rate of position logging, and alter how frequently speed and heading solutions are updated. A bundled third-party program called GPS Action Replay lets users replay and evaluate logged data, but the track data (a nearly invisible light yellow color at low speeds) is hard to interpret, and the graphics are uninspiring.

The above give a good idea of the devise . I use it as a guide for headers etc while sailing.
We tried for a year to get the mt Gay 30 to perform equally on both tacks with the paddle wheel speedo ( retuning mast resetting jib leads . Speedo is in centre ahead of keel . Then put puck on the boat and found the speed was similar on both tacks and the puck was more responsive down wind ...
ALSO THERE IS MORE .. I have used it in 3 protests to prove our track ( port & starboard ) and to show our distance from the mark ( buoy room ). So payed for its self in insurance claims

Edited by - on

indecentseas
1st Mate

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Canada
67 Posts

Response Posted - 02/27/2014 : 09:12:41

Interesting Graeme. Problem is the speed puck does not know wind speed or direction. Provided the wind is steady the puck should be able to provide some useful information.

http://www.youtube.com/watch?v=TUNr7-Fpetk

Edited by - on

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 03/05/2014 : 16:15:02

I've had Catalinas since 1983. IMHO, Steve's right: polars are a waste of time. Almost all boats of our type (full displacement) have the same information on a polar. Do you really need that information to know a keel boat sails faster on a screaming reach? That's why they call it screamin'. :)

Keeping your head OUTSIDE the boat is much more important and well placed telltales on both main and jib will tell you a LOT more than polars, which need to be setup for different windspeeds anyway.

Edited by - on

Boomeroo
Navigator

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Australia
128 Posts

Response Posted - 03/07/2014 : 05:10:45

Stu, you are correct in the general importance of looking out at the wind and sail set etc . Polars are similar for displacement boats of the cruising type however hull shapes , wing vs swing keel all change the performance . It is academic,but understanding what and why performanse changes is of interest. I haven't bothered to even take the puck on the Catalina , but use it twice a week for racing. Accurate speed , wind variation etc ,post race analysis, protest winning . The etc hells we race on a Saturday uses the above but the poplars did give some data to back up our observations on down wind angles in various winds speeds . For the mt gay 30 it allowed a quick guide to be established for up wind performance in various conditions with water ballast in ,sail settings , crew dittribution etc etc . This allows each of the team to check actual speed against target and work together to make it happen whilst the helmsman focusis on the wind and waves and speed ,not the ten other things to distract him .
But you are correct it probly won'tchange the c25 world ... but it is interesting .

I tried to get one but the wind didn't get above 6 kn this week so not much use .

Edited by - on

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 03/13/2014 : 20:17:19

Graeme, you're right. I suggest, lacking any C25 specific polars, that any skipper who is interested do what Graeme suggests: find a polar for a C30 or C34 and find this:

- screamin' reach vs closehauled is easy

- the further off the wind you go, from broad reach to run, speed drops.

What else do you need?

If you can't find polars, then go sail your boat and check speeds on different angles to the wind.

Good luck.

Edited by - Stu Jackson C34 on 03/16/2014 14:50:28

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 03/16/2014 : 15:21:01

It seems to me that speed is not what you want to know... What you want is VMG to a windward or leeward objective. If I were putting the effort into recording a rough set of polars, I'd use the GPS to measure VMG, using a waypoint many miles away (even if it ends up far inland) to minimize the distortion due to the changes in angles. Otherwise, the further out you are from directly downwind of the point, the lower your VMG becomes--all the way to zero when you reach a the layline to the point.

The resultant comparisons will show you, for a particular wind-speed, what bearing from windward (or leeward) will maximize your progress toward your objective, based on speed and angle. Speed alone doesn't tell you what you really want to know--unless you just want to outrun a nearby boat that's on the same tack. (We all know about that...

)

Edited by - Stinkpotter on 03/16/2014 15:22:38

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 03/17/2014 : 10:34:50

Dave's idea is good. The concept with his "far away" waypoint is to figure out VMG to the true wind. Find the true wind, and set a waypoint 500 to 1,000 miles in that direction, then you'll know which side is favored on any course to windward. Keep the waypoint to the next mark, too. You'll then have two VMGs to deal with - wind and next mark (or your destination). Good practice for learning how to use a GPS and its VMG functions.

Edited by - on

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