How to Get the Most Speed Out of a Sweptback Spreader-Rigged Cat in Light Airs

[tsmwebb]

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!

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)

[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.

[smj]

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.

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.

[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.

[webejammin]

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.

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.

[smj]

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.

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.

[webejammin]

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.

[tsmwebb]

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.

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.

[Joli]

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?

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?

[Joli]

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

[tsmwebb]

Originally Posted by Joli

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

Interesting. Thanks. What true wind speed and sails is that for?

Tom.

[Dragon Lady]

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.

[Joli]

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

Originally Posted by tsmwebb

Interesting. Thanks. What true wind speed and sails is that for?

Tom.

[daddle]

Originally Posted by tsmwebb

Interesting. Thanks. What true wind speed and sails is that for?

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.

[Wayward Wind]

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