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Polar Diagrams for beach catamarans? #12933
11/12/02 08:39 AM
11/12/02 08:39 AM
Joined: May 2002
Posts: 217
J
jcasto1 Offline OP
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J

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Many keelboats have polar diagrams, e.g. http://www.c34.org/faq-pages/images/polarDiag.gif
Here's a description of how they are sometimes used.
http://www.sailnet.com/collections/articles/index.cfm?articleid=leonar0021

Has anyone ever seen, or generated, "polar diagrams" for beach catamarans?

Mostly they are used on large keelboats as a guide for whether the crew is properly trimming the sails for the given wind speed, in order to maximize "VMG".

But what got me thinking about this was the recent thread on reaching vs W-L race courses.
If we had polar diagrams for a couple different beach cats, could we use them to define the real difference between a W-L course vs a course with a triangle shape? e.g. is a reaching leg a real "penalty" for a particular design?
Does a spinnaker cat get really get an advantage on a W-L course compared to a sloop-rigged or unirig cat?

Also, in another thread, there was some excellent data collection, using GPS and downloading digital maps, and such, that showed effects of wave angle on port vs starboard tack.
Perhaps, we are at the point where the instrumentation is compact enough, and sophisticated enough, we could actually develop polar diagrams from the data collection.

Does it even make sense to have a polar diagram for a beach cat, or do we have too many variables relative to its performance - (e.g. crew weight & position), compared to a large keelboat?

What about larger offshore multihulls, do they use polars? Farrier Trimarans? Stilettos?


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
-- Have You Seen This? --
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12934
11/12/02 03:22 PM
11/12/02 03:22 PM
Joined: Jun 2001
Posts: 292
Long Island, NY
Ed Norris Offline
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HI Jim
I'd be very interested in the polar diagram for "your boat or one very much like it" I'm considering calling US Sailing, as the article in your post suggests. Unless you called them and got no where?
Let me know, I'd hate to wase their time if you just called


Sail Fast, Ed Norris
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12935
11/12/02 05:23 PM
11/12/02 05:23 PM
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DHO Offline
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I seem to remember that there were some polar diagrams in Bethwaite's book "High Perfomance Sailing" or something like that. Bethwaite thinks that planing monohulls such as the Aussie 18 ft skiffs are the way to go. I heard such boats are so unstable that they're better swimming platforms than anything else. Any advantage in straight line speed would be more than cancelled out by time spent capsized. Where's Bill Roberts? He probably knows where to find some diagrams.
David Ho
TheMightyHobie18 1067

Re: Polar Diagrams for beach catamarans? [Re: Ed Norris] #12936
11/12/02 08:45 PM
11/12/02 08:45 PM
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jcasto1 Offline OP
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I did not call US Sailing, I expect I would get the answer "we don't have polars for multihulls". But, why don't you try & let us know how it works out.


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
Re: Polar Diagrams for beach catamarans? [Re: DHO] #12937
11/12/02 09:02 PM
11/12/02 09:02 PM
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J
jcasto1 Offline OP
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Thank you, DHO, for the suggestion. I actually have the book, and pulled it out. It has a bunch of polar diagrams for some keelboats and a few dinghies (not cats), and overall, the book is incredibly instructive.

But, Bethwaite does kind of dismiss multihulls early on in the book. His summary of their "disadvantages" - total wetted surface area is too high, hulls cannot plane, can't heel in light air for proper sail shape, high speed downwind buries the bows, they tack too slow to be tactical race boats - so their racing crews will be frustrated.

I think he wrote his book before the advent of asymmetrical spinnakers on cats with wave-piercing hull shapes.


Jim Casto
NACRA 5.5 & NACRA 5.7
Austin TX
Lake Travis
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12938
11/13/02 08:46 AM
11/13/02 08:46 AM
Joined: Jun 2001
Posts: 292
Long Island, NY
Ed Norris Offline
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Ed Norris  Offline
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You were right in your expectations. Closest to a beach cat they have is a Hobie 33.

Think they offer discounted membership for discounted members?


Sail Fast, Ed Norris
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12939
11/13/02 09:00 AM
11/13/02 09:00 AM
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sail6000 Offline
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Hi J

Closest thing we catsailors have readily available is a handheld GPS on board , often distance racing good crews will check vmg directly from the GPS -

distance traveled vs speed to the finish -VELOCITY MADE GOOD ,-A boat may travel at higher speed by heating it up w spin set but sail a longer distance and longer time than a boat sailing more directly towards the finish point at a slightly slower speed , that often a surprised crew finds have finished in front.--tHE KICKER IS THERE ARE TIMES YOU MAY NOT WANT THE BEST vmg --if you know your setting yourself up to take advantage of an offshore current sooner ,-expected shift ,or other course factor ,--this is where distance racing varies greatly from short term gain oriented buoys racing .

--Beyond playing ocilating shifts as noted in the article Variables of tides and current ,-wave size and pattern effect on speed and angle sailed ,-wind speed variables ,-gusts , persistant shifts ,-ocilating wind directions that play such an important part of racing , then add crew and skippers inconsistancies along with tactics ,-other boats partially blanketing or effecting wind ,-being washed off the side temporarily by large waves taking out your feet ,-recovery time ,-gybing time , often very difficult to get a boat going full speed again in large seas after a 6 hour straight high speed spin run .
All the intangable aspects make any polar diagram that can not factor these almost infinate variables in combination just a basic guideline for settings and angle of that particular design.
If you are going at full max theoretical speed ,--but in the wrong direction into a persistent shift ,-or away from better wind velocity near shoreline ,-or ignoring major ocilating shifts , or heading into adverse current offshore or one that may be very favorable like the Gulf Stream . the end result and latter finish time will be very confusing .

A good basic guideline with spin rigged cats in higher winds is once you get beyond 20 degrees course heading it is time to take down the chute and begin to reach .In lighter winds you can often reach typically 90 -some to 75 best vmg without spin ,--from wind direction .

A GPS can be used effectively by for instance placing two on the wire w spin up sailing at higher speed but higher angle to maintain it ,--or one on the wire at lower angle and speed sometimes as mentioned with a better speed to distance traveled ratio.
Direct comparison with other boats in the area ,--their settings and angles chosen is sometimes best source of info on the race course , each is different in each condition .

There in the extreme difficulty of rating different boats fairly or with any consistancy .
It may be best to group boats into similar max -Length beam weight ,sail area catagories , allow crews to modify each for the conditions as they choose , just race in the truer ideal of sport .

The one wind condition that gets many catsailors is the slight persistant shift where the inside cat on a beat gets lifted up inside the outer boats often making them feel they have no boat speed , -but this occurs mainly from the wind shift and the ability of a crew to recognize it and place themselves relative to the fleet to take advantage of it .

Only looking at theory or finish times without the comprehension of all the other factors involved is misleading ,
We always see catsailors after racing asking questions about wind speed near shore , shifts ,-wave size and pattern further out etc . Very honestly I,m very fortunate to race each year around some of the best catsailors that I often ask these types of questions from ,--This is the best way to learn your particular type of cat design raced .

It is a wonderfull complex interesting sport we play ,-never completely mastered by any .

Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12940
11/13/02 09:07 AM
11/13/02 09:07 AM
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Houston
carlbohannon Offline
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Jim

There are polars for a few multihulls, H16, Tornado's some A-class's, and a few others. I have some EXCEL tables but I am not sure what I did them.

Attached is a link to a web article that includes links to the data available on the web.

. On The Wire


Try searching "Tornado Polaris" and "Hobie 16 polars" for more data.

I seem to remember that that some of the data was collected as kinda of "fun sailing day". Different boats, sailed individually through a line to marks set ~10M out while someone recorded wind speed, direction and time. I think the boats were supposed to hit the mark so they could get an accurate time. I do remember the event was called due to light air and heavy drinking

Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12941
11/13/02 09:34 AM
11/13/02 09:34 AM
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Posts: 612
Cape Town, South Africa
Steve_Kwiksilver Offline
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Hmm. I`d like to see Bethwaite on a 49er in 25 knots in a lumpy sea. I`ll bet he tacks on every windshift while trying to remove his underpants from his head.
Contrary to his beliefs, those cat sailors who tack often up the beat always get there before the guys who sail layline to layline.
Time spent upside-down on a skiff does nothing for vmg. Perhaps he can add another chapter to his book while clinging to the centerboard.

Re: Polar Diagrams for beach catamarans? [Re: Steve_Kwiksilver] #12942
11/13/02 08:32 PM
11/13/02 08:32 PM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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AMEN!


Jake Kohl
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12943
11/14/02 01:19 PM
11/14/02 01:19 PM
Joined: Apr 2002
Posts: 805
Gainesville, FL 32607 USA
dacarls Offline
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Then to add to the fun-- how do we get a Wild Thing bump added to the polars? It can really be something. I won a race (A-class) from 100 yards behind last weekend by this valuable technique.


Dacarls:
A-class USA 196, USA 21, H18, H16
"Nothing that's any good works by itself. You got to make the damn thing work"- Thomas Edison
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #12944
11/15/02 08:55 AM
11/15/02 08:55 AM
Joined: Jun 2001
Posts: 778
Houston
carlbohannon Offline
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Taking polars on big boats is easy. Most already have the instruments to do the job. Attach a laptop to the
data bus, sail the required course, reduce the data and plot it out.

With beach cats, the problem is the wind speed and direction.

You could take a GPS (that will record and output the course and speed) and spend an afternoon buzzing around a weather buoy whose data is available online. If you have a laptop that can interface to a NEMA bus, find a monohull with wind and compass, that will anchor out and let you buzz them. Get a couple of boats and swap out the GPS

Polars are useful to beach cats, especially if you convert them to relative wind direction. They give you a starting point on angles. For example, they tell you real fast to sail a Tornado hot and sail deeper and higher on a A-class. They also told me that it sucks to be on a 5.5uni when the wind is over 15, except downwind. I am still looking for my data files.

Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #289204
11/03/19 01:43 PM
11/03/19 01:43 PM
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Frank N Offline
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Those polars disappeared from the original web site. but they are here on the wayback machine.
http://web.archive.org/web/20050101071744/http://www.west.net/~lpm/hobie/archives/v1-i3/feature3.htm

Tony Marchaj got the Tornado polars from a paper by Bradfield published in The Ancient Interface Symposium on Sailing, California, Vol 10 1971.
You may be able to get reprint from AIAA.
This was for the Classic Rig Tornado.


Last edited by Frank N; 11/03/19 01:45 PM.
Re: Polar Diagrams for beach catamarans? [Re: jcasto1] #289205
11/03/19 04:21 PM
11/03/19 04:21 PM
Joined: Dec 2002
Posts: 305
Memphis, Tennessee
Damon Linkous Offline

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Revived this topic from 2002!

FYI, all the issues of Bill Mattson's On The Wire eZine are still available at https://www.thebeachcats.com/OnTheWire/catsail/archives.html

When Bill stopped publishing he was gracious enough to donate all the content so it wouldn't be lost.
https://www.thebeachcats.com/OnTheWire/mattson-fairwell-message.html


https://www.thebeachcats.com/OnTheWire/westnet/_lpm/hobie/archives/v1-i3/feature3.htm


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