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05-08-2011, 00:29
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#16
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Registered User
Join Date: Apr 2007
Location: Hawaii
Boat: Atlantic 42 Catamaran
Posts: 285
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by Kapena
The big chute is fun to fly but if you want to go fast....You need to draw everything in and make it flat, using the sails just like you were going to weather. ...Try it. You can do this in a cat easily (requires gybing)....
Just like that except I can't go near as fast!
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Just like that
In our case I'm pretty sure the big chute (asymmetrical) is also fastest for downwind VMG in most conditions. Even pretty quick beach cats use chutes to good avail for downwind sailing. The "typical" cruising cat is comparatively a bit draggy and a bit underpowered and a big, deep, powerful spinnaker despite its drag and wide sheeting angle is probably the fastest choice for downwind VMG for most most of the time. Of course, typically boats will not be sailing DDW when making their best downwind VMG and will have to jibe. But I think few cruising cats will be able to bring the wind forward to where they can trim their sails like they would going to weather (say AWA <40) and still be making best VMG downwind. YMMV, of course.
Tom.
PS. the TLAs:
VMG = velocity made good w/respect to the wind either upwind or downwind (by convention vmg = speed through the water * cos(angle to true wind wrt the water))
DDW = dead down wind (wind directly aft)
AWA = apparent wind angle (0 = directly forward, 180 = ddw)
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05-08-2011, 01:08
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#17
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Registered User
Join Date: Mar 2011
Location: Pacific NW, sailing the Columbia River, USA
Boat: Gemini 105MC 34 ft hull#753
Posts: 951
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
I'm all for using a screecher sail , our Gemini has one on a curved track so adjusting it is a breeze and it is good in up to 15 kt breezes and max gusts to 25 kt's achording to the sailmaker.
__________________
Wind in my hair and a nice catamaran
Phil & Elaine
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05-08-2011, 06:11
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#18
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Registered User
Join Date: Nov 2007
Boat: Seawind 1000xl
Posts: 7,462
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by webejammin
I'm all for using a screecher sail , our Gemini has one on a curved track so adjusting it is a breeze and it is good in up to 15 kt breezes and max gusts to 25 kt's achording to the sailmaker. 
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Your sail may be able to take 25kts of wind but with full main and screecher up in that kind of wind I would think you would be very close to flipping a Gemini.
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05-08-2011, 06:22
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#19
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Registered User
Join Date: Jan 2007
Posts: 223
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
tsm...Have you tried this or just discounting it relating to your own limited sailing skills of sticking a parachute out when going downwind? I suspect you may also pinch in your cat when going to weather. I'd love to race you.
Keep in mind Dragon is already only at 130 degrees.
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05-08-2011, 10:54
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#20
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Registered User
Join Date: Mar 2011
Location: Pacific NW, sailing the Columbia River, USA
Boat: Gemini 105MC 34 ft hull#753
Posts: 951
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by smj
Your sail may be able to take 25kts of wind but with full main and screecher up in that kind of wind I would think you would be very close to flipping a Gemini.
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We've only sailed the screecher sail in 15 to 18 kt breezes and that is with full tanks and the dingy on the davits which adds lots of weight to the aft, We have had no problems and have reached speeds around 12.5 kt's with no problems, but I'm told that in ocean swells we could surf faster. Our screecher sail was designed to lift the bow and not drag it down.
__________________
Wind in my hair and a nice catamaran
Phil & Elaine
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05-08-2011, 10:59
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#21
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Registered User
Join Date: Nov 2007
Boat: Seawind 1000xl
Posts: 7,462
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by webejammin
We've only sailed the screecher sail in 15 to 18 kt breezes and that is with full tanks and the dingy on the davits which adds lots of weight to the aft, We have had no problems and have reached speeds around 12.5 kt's with no problems, but I'm told that in ocean swells we could surf faster. Our screecher sail was designed to lift the bow and not drag it down.
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That's great speed for a Gemini. My experience with our old Gemini was in 25-30 apparent with full main and 90% jib. I had to let the main sheet fly a couple of times as the windward hull was feeling mighty light. We finally put a reef in and all was fine.
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05-08-2011, 11:40
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#22
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Registered User
Join Date: Mar 2011
Location: Pacific NW, sailing the Columbia River, USA
Boat: Gemini 105MC 34 ft hull#753
Posts: 951
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Sence this is about light air sailing I'll say we think light air is about 10 to 12 kt's true. In the PNW we have some strong currents from rivers and our 10 to 12 ft tides so to keep crusing we need more breeze. I'm sure there's some places where light breezes are 5 to 10 kt's and keeping a screecher or spinnaker sailing is easier.
__________________
Wind in my hair and a nice catamaran
Phil & Elaine
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05-08-2011, 11:48
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#23
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Registered User
Join Date: Apr 2007
Location: Hawaii
Boat: Atlantic 42 Catamaran
Posts: 285
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by Kapena
tsm...Have you tried this or just discounting it relating to your own limited sailing skills of sticking a parachute out when going downwind? I suspect you may also pinch in your cat when going to weather. I'd love to race you.
Keep in mind Dragon is already only at 130 degrees.
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Yes, I've tried it. And, I'm always happy to have my limited sailing skills honed by racing. Bring it anytime.
We may be talking around each other. I'm not saying 130 for the OP is VMG. I don't know. That's very boat and true wind specific. Certainly heading up might help his VMG, but heading up until he is trimming his sails like he does upwind would mean to me that he has to be sailing at apparent wind angles less than 40 degrees. Maybe you were just suggesting that the sails need to have some flow around them? In that case I'm with you in anything but the strongest winds. But,the AC boats you used as examples really did sail downwind to best VMG with upwind apparent wind angles. They could do that because they were extraordinarily powerful and had extremely low drag. Those two boats were probably the fastest sail boats in terms of downwind VMG ever made. Nobody is going to get similar performance out of a cruising cat.
Tom.
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05-08-2011, 14:41
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#24
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Registered User
Join Date: Nov 2005
Location: Ohio
Posts: 2,939
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Can you get the polars from FP? That would be a starting point, it should indentify the sails for a given AWA and WS. Is the FP polar flat or heart shaped at the bottom, that will dictate DDW or some angle?
Quote:
Originally Posted by Dragon Lady
Hi guys,
We are currently trying to get the most out of our FP Lavezzi 40 sailing downwind at about 130 deg apparent with about 10 to 12 knots apparent.
We don't seem to be able to get any lower without stalling to genoa and loosing a lot of speed.
The swept back spreaders are stopping us getting the main out very far and we have tried traveller up an down with appropriate main sheet but it seems to make little difference.
Any positive suggestions great fully received as we are still learning how to get the most out of new boat?
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05-08-2011, 14:48
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#25
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Registered User
Join Date: Nov 2005
Location: Ohio
Posts: 2,939
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Here it is but it's incomplete, maybe FP has the complete polar?
Or, you may end up having to develop your own based on WS/WA, sail wardrobe, sea state and load.
http://www.fountainepajot.com.au/lav/lavezzi40polar.pdf
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05-08-2011, 15:51
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#26
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Registered User
Join Date: Apr 2007
Location: Hawaii
Boat: Atlantic 42 Catamaran
Posts: 285
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by Joli
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Interesting. Thanks. What true wind speed and sails is that for?
Tom.
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09-08-2011, 04:07
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#27
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Registered User
Join Date: Mar 2010
Location: Sydney Australia
Boat: Lavezzi 40, Pourpre
Posts: 964
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Thanks, very interesting.
That polar must be for a very light bare bones Lav, we have found once we go any lower than 120 deg the speed drops off considerably.
Mind you the way my girls are using the water on board we are getting a lot lighter LOL.
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09-08-2011, 06:11
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#28
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Registered User
Join Date: Nov 2005
Location: Ohio
Posts: 2,939
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Hi Tom,
Not sure, that's all I found. FP would be the source for the complete set of polars. This could be for 25 knots true and the bump at 90 degree probably represents a sail change? Looks like no deeper then 160 true, not sure what the awa would be with out knowing tws.
FP should be able to provide DL with a set of polars they can sail to. Something like this: Performance curves
Generally you sail to a target speed for a given wind. (ie if it's blowing 10 and you are sailing upwind then you should be sailing 6.7 knots. If you are faster then you are sailing fat, if you are slower you are skinny). The same holds true for downwind sailing. You try to hit the numbers.
Joli
Quote:
Originally Posted by tsmwebb
Interesting. Thanks. What true wind speed and sails is that for?
Tom.

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09-08-2011, 15:41
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#29
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Registered User
Join Date: Dec 2009
Posts: 2,959
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Quote:
Originally Posted by tsmwebb
Interesting. Thanks. What true wind speed and sails is that for?
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Yeah. Useless without more info. Plus polars tend to be computed at some very light weight, perfect sails, empty tanks, perfect bottom, no canvas, no dinghy, friendly seas, etc, etc, almost useless in real life.
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09-08-2011, 16:47
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#30
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Registered User
Join Date: Jul 2006
Location: Pittwater
Boat: Mahe 36 Wayward Wind
Posts: 226
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Re: How to Get the Most Speed Out of a Sweptback Spreader Rigged Cat in Light Airs ?
Has anyone considered using a parachute? See video on
SAILCHUTES HOME
Don't know about speed, but it looks like a more comfortable sail.
Cheers
JohnC
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