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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Polars

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KYD516
Deckhand

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USA
24 Posts

Initially Posted - 03/28/2005 : 09:46:11

Hello to everyone and thanks for making this site so useful with all of your input.

I am a new owner of a 1985 25' with a standard rig and fin keel. Does anyone have polars? Am I lucky enough to get 'em with a 135 foresail, too?

thanks

Aquila, rhymes w/ Tequila

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 03/28/2005 : 11:44:10

Polars would be fun to have, in lieu of some just remember you did not buy a racing boat and when trying to point crack it off a little for speed.

Edited by - on

Steve Milby
Past Commodore

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USA
5913 Posts

Response Posted - 03/28/2005 : 11:50:42

Welcome to the forum, and congratulations on buying the boat. I’ve never seen polars for the C25, but I see on your user profile that you "love to race." If that's so, I think you're going to love your C25. IMHO, if you prep and sail it well, the C25 can easily sail better than its rating. If you’re really serious about racing, rig it like a racing boat. Lead all your control lines to the cockpit, and use them to constantly power her up and down with every puff and lull, all around the race course.

Edited by - on

thaind
Navigator

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Canada
145 Posts

Response Posted - 03/28/2005 : 12:01:52

I used to sail Star class in the distant past. I bought the C25 simply as a family cruiser and haven't sailed it yet, but all this talk of racing in it is awfuly stimulating.

Edited by - on

Buzz Maring
Master Marine Consultant

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USA
1773 Posts

Response Posted - 03/28/2005 : 12:13:33

Hi Keith,

Congratulations on your new boat, and welcome to the group!

OK ... I'll bite ... someone please tell me what "polars" are ...

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 03/28/2005 : 12:18:52

The squares show the fastest point of sail, note neither dead down nor pinching is optimal.

http://www.ockam.com/ocku1.htm

Edited by - Frank Hopper on 03/28/2005 12:20:29

John G-
Admiral

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793 Posts

Response Posted - 03/28/2005 : 12:38:50

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Buzz Maring
 Hi Keith,

Congratulations on your new boat, and welcome to the group!

OK ... I'll bite ... someone please tell me what "polars" are ...

<hr height="1" noshade id="quote"></blockquote id="quote">

 
Buzz,
I believe Frank’s illustration came from[url="http://www.ockam.com/ocku1.htm"] Ockam U’s online manual [/url] on true wind, polars’ and other topics.

 In the figure above,(Franks illustration) the curved lines represent how fast a boat can sail at a given true wind speed for any true wind angle. The squares mark the fastest point of sail upwind and downwind. These points are the so-called maximum Vmg (speed made good) points. An arrow has been drawn at the port tack upwind sailing point, showing that that boat can be expected to make 8.3 knots at 35 degrees off the true wind. The boatspeed at that point is called the Upwind Target Speed.
Polars can be used to determine the best point of sail not only up and down wind, but in any direction. In addition, the shape of the polar provides further information. For example, the upwind sailing point shown above is the maximum Vmg point upwind. However, the curve is fairly flat there, indicating that one could sail 8.1 knots at 30 degrees or 8.4 knots at 38 degrees without sacrificing much Vmg.

Underlined words are a HOT link.

Edited by - on

takokichi
Captain

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USA
321 Posts

Response Posted - 03/28/2005 : 18:58:25

I have hunted high and low for a set of polars for the C25 and I'm satisfied that they don't exist. So I plan to do some VMG work this spring. Bear in mind that polars are vaild only for the boat configuration they were taken from - Hull, rudder, keel, rig, sails, full or partial battens, even load. But they'll be close among similar boats.

Last summer I discovered that my best upwind VMG in winds over 15 was at ~45 degrees TWA which became ~40AWA. In light wind I bore off to 55 degrees until up to speed then came up. Off the wind over at 12 we see hull speed dead down wind, so that's where I point the boat. Under 12 it depends on wave state, etc. If I can attain hull speed, I head up until I get there, then head down as far as I can and maintain it. Not scientific but it more or less works. Its a harder call if I can't get hull speed on any point, or if I have to come up more than 20 degrees or so.

You can do pretty simple VMG math on the fly if you have a scientific calculator on board, or if you take the time to plot some charts. Point the bow briefly at your target and note the GPS heading. GPS, not compass, it matters. Then you fall off to whatever heading feels right. Note the track (not the heading) on the GPS. Now you can calculate the angle between the target and your track, and use that and you SOG from the GPS to calculate VMG pretty easily (SOHCAHTOA). You then change the course a little bit, retrim (essential) and recalculate. You can do the math in a few seconds once you get the hang of it. If the vmg goes up, go 5 degrees further in that direction. If it goes down, turn back. Try 5 degrees the other way. You soon get a sense of what angles to sail when without doing the math.

Remember, though, that this is a pale imitation of true Polars. With true polars you know what the boat is capable of and so if you're slow, you tweak until you reach target boat speed. With my version, you get best VMG for however the boat is set up, which may not be optimum.

YMMV,

Justin

Edited by - on

takokichi
Captain

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USA
321 Posts

Response Posted - 03/28/2005 : 21:23:52

This is a chart of VMG for boats speeds on the left, and target headings across the top. Calc VMG: VMG=BoatSpeed(COS(Target heading - boat heading). Excel thinks in radians, so the formula is VMG = Speed(COS(RADIANS(Target heading - boat heading))

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 03/28/2005 : 22:14:45

Wonder if anyone has a C25 set up with the "high-zoot" insturmentation that can give you all the data you need to develop polars... (easily)?

Saw a recent article on some development projects linking wind, wave, current, gps, sail pressure and gyro motion sensors to a computer, which then controls the helm and sail control winches... the computer will sail the boat optimally on any course you desire. I reckon it will probably do better than a human can. Kinda takes the romance out of sailing though.

Edited by - on

takokichi
Captain

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USA
321 Posts

Response Posted - 03/29/2005 : 11:09:24

Developing polars is never done easily. Its so hard, in fact, that Ocean Planet raced without scientific polars in the Vendee Globe. To develop polars you need - perfectly consisent wind on flat water for long enough to plot each angle of sail - that'll get you the Polar for one wind speed. Repeat for each windspeed you want to plot. And even that will only get the Polars for the boat you're on, with the sails you have etc. Any change in sea state etc will throw the numbers out.

More practical, and probably more useful on the race course is to sail to optimum VMG for the condition. A micronet or similar device will figure out where windward is by averaging closehauled angles and will then calculate VMG on the fly - allowing you to make the best speed for the conditions. This assumes your ability to keep the boat moving at highest possible speed for any given angle of sail and for the conditions. If you can't, you should be working on trim and helm skills before worrying much about best VMG. I certainly could work on my trimming skills!

Even the tacktick/micronet process is flawed, however, in so far as it will give you vmg to windward, but NOT to the mark. Since even in windward/leward racing, the mark is never directly up or down wind of you for more than a moment, and because much (most?) racing is point to point or random leg, what you really want to know is VMG to the mark. Enter my goof chart above.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/29/2005 : 11:25:52

US Sailing offers polars for [url="http://www.ussailing.org/offshore/YachtClasses.pdf"]a fair amount of boats[/url], but they don't have them for the C25 (C27, C30,...and up).

Edited by - on

takokichi
Captain

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USA
321 Posts

Response Posted - 03/29/2005 : 11:33:41

LOL - my last few posts look a little in conflict. To clarify: True polars, produced by either a computer or very scientific data aquisition are helpful to the boat they were produced on, but of less help to other boats, even of the same type, unless perfectly identical.

For us, polars are less useful b/c of the varriety within the type and type or racing we tend to do.

Edited by - on

Steve Milby
Past Commodore

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USA
5913 Posts

Response Posted - 03/29/2005 : 13:14:50

IMHO, polars only add a little bit of useful information to your basic sailing knowledge. You don't need polars to tell you that a reach is fast, and sailing off the wind is slow, and to have a pretty good idea of relative boatspeed on all the various points of sail. Even under ideal conditions, polars only add a little specificity that can help you make the most accurate judgment call when you choose your course to the next waypoint. However, as takokichi points out, polars are specific to the boat, wind, waves and other variables, and I think your time would probably be much better spent concentrating on sail trim, helmsmanship and strategy than about what the polars say. The fact that Ocean Planet raced without polars implies that they didn’t consider polars all that useful.

A sailboat race is not a speed contest. It‘s a time-and-distance contest. The winner is the boat that finds the shortest route around the course, and covers that distance in the shortest period of time. Other boats might achieve higher speeds over a longer route, and still be beaten by a slower boat taking shortcuts along the way. If you learn to accurately recognize windshifts, and tack on them to windward and jibe on them downwind, and play the big shifts that are often found along the shore, you’ll gain much more than if you sail according to what the polars tell you.

Edited by - on

takokichi
Captain

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USA
321 Posts

Response Posted - 03/29/2005 : 21:51:43

Steve - you're right on the money, as always. Tacking on the shifts leads to a shorter course which gets you there faster, and then you win. And I agree, polars are far from essential, especially at our level. (Given Ocean Planet's budget I'm not sure you want to assume that if something wasn't aboard it wasn't useful - we're talking the kind of budget that makes a lawyer with zero carbon experience a fabricator when he happens to stop by after work. If you followed the race and noted Bruce's broken radar mount - yeah, I built that

)

On the otherhand, I find the Justin Andrus spare-time VMG calculations very helpful, especially on random leg races. There have been times I want to be 40 degrees above the target heading on downwind legs. I woulnd NEVER have thought I had to go that high to get optimum VMG. The longer the leg, the more it matters. On one race where a fleet of much bigger boats did a horizon job on us on the first 7 miles of the course (reaching, mostly) we caught the fleet over a six mile downwind leg by sailing max angles of about 140 degrees. We were so far off the rhumbline course I would have sworn it would have taken longer even at the hotter speed. I'd have been wrong.

That said - unless your bottom is race prepped, spend the time you'd be doing the prelimanary math sanding. Unless your trimming is spot on, spend your time working on your trim. Etc.

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