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jandrus
1st Mate

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USA
79 Posts

Initially Posted - 09/24/2002 : 12:48:37

I know this subject has been hit on many times but I can't seem to find a search string that gets me what I want. I am looking for best velocity angles for upwind and downwind sailing. Pursuant to the last thread I started assume the boat is rigged as follows:

Fixed keel, standard rig boat. Head sail is 2 year old 135% Catalina loft gennoa. Excellent condition. Foam luff pad. CDI-FF4 furling unit so no adjusting halyard tension. Mainsail is 2 year old Dolye full-batten mainsail. Perfect condition. Slightly extended roach. Just clips the backstay (as designed). No chute. Adjustable whisker pole. Old style rudder for the moment. Rig set up per standard and then dynamic tuned. No adjustable backstay.

So - has anyone figured out the optimum wind angle for VMG on a beat?
How about on a run?

Justin

c25 #1942 s/v tako kichi

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/24/2002 : 22:49:03

Depends.

...mostly on wind speed
...also on sail shape (vang, genoa leads, etc.)
...somewhat on keel configuration
...probably on some other things
...probably not on which rudder you have (original or ballanced)

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - on

Brooke Willson
Admiral

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USA
983 Posts

Response Posted - 09/25/2002 : 01:15:08

You'll have a lot more fun actually going out in your boat and using a GPS to see your fastest point of sail, instead of sitting at your computer waiting for someone else to tell you.

Brooke

Edited by - on

jandrus
1st Mate

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USA
79 Posts

Response Posted - 09/25/2002 : 10:49:17

I understand that all boats are different, and that conditions vary. Still though, there is a theoretical best. Witness the polar diagrams available for most boats.

I know the angles my particular boat makes best VMG. What I what to do is sail the theoretical best courses and then see if I can find a sail control setting that results in a VMG higher than the courses I sail now.

Justin

c25 #1942 s/v tako kichi

Edited by - on

Steve Milby
Past Commodore

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USA
5932 Posts

Response Posted - 09/25/2002 : 19:11:15

Justin,

In “Sail Power,”1st ed., by Wallace Ross, there is a good explanation of VMG, and how to calculate it for a specific boat, starting at pg. 436.

Ross doesn’t discuss the availability of theoretical polar diagrams for one-design boats. My guess, however, is that there is no such thing. I suspect that the polar diagrams you have seen were based on speeds actually measured by the manufacturer, while test-sailing a prototype of the one-design boat. In order to theoretically calculate the potential speed, at different wind angles, of a boat that exists on the drawing board, but that has never been built or sailed, there are many variable factors that would have to be hypothesized. For example, you could only guess at the extent to which a concept boat of a particular design would drift to leeward at any given boat speed, wind angle and wind speed. You would have to assume that the sails were of a certain design and cut to enable the boat to sail at a certain wind angle and speed. In addition, look at all the specifics that you have included in your original message:

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Fixed keel, standard rig boat. Head sail is 2 year old 135% Catalina loft gennoa. Excellent condition. Foam luff pad. CDI-FF4 furling unit so no adjusting halyard tension. Mainsail is 2 year old Dolye full-batten mainsail. Perfect condition. Slightly extended roach. Just clips the backstay (as designed). No chute. Adjustable whisker pole. Old style rudder for the moment. Rig set up per standard and then dynamic tuned. No adjustable backstay.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

I can’t imagine what would be gained by calculating theoretical VMG, when it would have to be based on so many fudge factors. It couldn’t have much validity when applied to a boat whose equipment and maintenance varies significantly from the hypothetical standard. The extent to which guesswork would necessarily be involved would seem to make the theoretical calculations of doubtful value.

But, if Catalina has calculated polar diagrams for the C-25, based upon either theoretical assumptions or on performance measurements, it might give you a chance to compare your own plots against Catalina’s, and to experiment with sail control settings, as you have suggested. Why don’t you write Catalina and ask if they have any kind of polar diagrams for the C-25? If Catalina doesn’t have them, I doubt that anyone else has them.

Personally, I think that you will find your own calculations to be of the greatest real value to you, because they are specific to your boat. I don’t know of anyone else who has made them for a Catalina 25.

Steve Milby "Captiva Wind" C-25 T/FK #2554

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/25/2002 : 21:07:11

"I am looking for best velocity angles for upwind and downwind sailing." - Justin

Justin,

You might try contacting U.S. Sailing to see if they have a Performance Package for the Catalina 25. According to their website,...

"Whether you race or cruise, Performance Packages are made-to-order, using measurement data on file to provide you with an accurate method of optimizing your boat's performance.

Includes tabular data for different wind speeds, True Wind and Apparent Wind Polar Diagrams with optimum gybing angles, hydrostatic data and much more. Data is available on over 700 offshore production boats to choose from."

This information isn't cheap though. $210.00 for nonmembers <img src=icon_smile_shock.gif border=0 align=middle>

Here is a link to U.S Sailing's Performance Package page.
http://www.ussailing.org/storedeleteme/performance.asp

Don Lucier, North Star
C25 SR/FK

Edited by - dlucier on 09/25/2002 21:09:31

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/26/2002 : 23:15:54

Justin: I just checked my Garmin 48 GPS and it has a user-selectable VMG measurement. With that diplaying, you could establish a waypoint as precisely upwind as possible, and then watch the VMG as you try various angles on a beat to that waypoint. Then do the same thing with the waypoint precisely downwind. Then try it all over again in different wind conditions. Sail trim is still a factor--you want to try to get maximum speed at each angle you try. Do you have a GPS with VMG? If not, you could get one for less than the cost of those $210 charts.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 09/26/2002 23:17:06

Edited by - on

Steve Milby
Past Commodore

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USA
5932 Posts

Response Posted - 09/27/2002 : 11:59:50

Outstanding idea, Dave! <img src=icon_smile_approve.gif border=0 align=middle>

Steve Milby "Captiva Wind" C-25 T/FK #2554

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/27/2002 : 12:26:17

Thanks... But I just thought of an issue: As you approach a waypoint on a tack away from it, VMG is going to drop steadily (eventually to zero and then negative if you pass the 90-degree course to the waypoint). To get good comparisons, you'll need to repeat from the same starting point for each angle you try, or else reset the waypoint each time you change angles. The waypoint needn't be in the water... I supsect the further away, the better. It may be a little more complicated than I thought. <img src=icon_smile_dissapprove.gif border=0 align=middle>

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - on

Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 09/27/2002 : 14:33:02

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
I know this subject has been hit on many times but I can't seem to find a search string that gets me what I want. I am looking for best velocity angles for upwind and downwind sailing. Pursuant to the last thread I started assume the boat is rigged as follows:

Fixed keel, standard rig boat. Head sail is 2 year old 135% Catalina loft gennoa. Excellent condition. Foam luff pad. CDI-FF4 furling unit so no adjusting halyard tension. Mainsail is 2 year old Dolye full-batten mainsail. Perfect condition. Slightly extended roach. Just clips the backstay (as designed). No chute. Adjustable whisker pole. Old style rudder for the moment. Rig set up per standard and then dynamic tuned. No adjustable backstay.

So - has anyone figured out the optimum wind angle for VMG on a beat?
How about on a run?

Justin

c25 #1942 s/v tako kichi
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Justin,

How fat is the crew....your pointability on a beat is going to be effected by the rail meat you bring along.

Downwind...that's going to depend on the wave angle

Duane Wolff
"The Flying Wasp"
C-25, #401 std,sk
Chief Measurer C-25/250 National Assn.
<img src="http://im1.shutterfly.com/procserv/47b2d603b3127cce9f858f0ef3ef0000001010" border=0>

Edited by - on

arlingva
Navigator

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USA
116 Posts

Response Posted - 09/28/2002 : 11:28:12

Justin, Start racing in a competitive fleet. You'll figuire out what works best that way.

bill c 250 wk, #614

Bill Swanson
C250WK #614

Edited by - on

Derek Crawford
Master Marine Consultant

Members Avatar

USA
3324 Posts

Response Posted - 09/29/2002 : 18:01:41

"Justin, Start racing in a competitive fleet. You'll figuire out what works best that way."
I heartily endorse Bill's idea!
Here's a tip or two:-
Don't try to point high in light air.
When both telltales stream straight back you're going fastest (but not necessarily highest)
When your bow is 45 degrees from the weather mark - tack! (Or sooner!)
Don't sail dead downwind. Sail the long jibe first. When you're 30 degrees off the leeward mark, jibe!
Derek on "This Side Up" (and the Master's Regatta is next weekend!)

Edited by - on

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