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esoxproblem

Downwind main trim in light to moderate

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I am guilty of  giving a lot less thought to downwind sail trim than upwind.

I was looking at a pic of our J33 behind a 35.  The 35 had a little twist in their main.  Ours was off on the halyard, way out on sheet and vang on.  Kinda barndoor style.

In the pic, the 35 trim looked nicer.  I had always thought that the slightly twisted leech was just an air dump, but maybe there is more to it.?

Any thoughts to downwind angle against the J35?  Right now, if we match angle, we probably loose a little ground.  And, if we heat up a little, we still loose a little ground. 

Thanks

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I would not go barndoor with it unless it is breeze-on, soften it a little but don't drop the halyard, just take any Cunningham off and a little loose on the outhaul. If you can get the telltales on the leech to flow some that is good. All dependent on if you are DDW or up a little, which you follow the basics, up in the lull - down in the puff.

Settings also depend on the sail type you have, Dacron or Kevlar/carbon/mylar, etc. More shape with the Dacron to play with, the others already have the shape built-in so less to adjust. Talk to your sailmaker too. You should not be loosing ground to the 35 downwind.

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As to main trim, can you test varying sail shape/twist against just  yourself, using the knotmeter in a steady breeze?  Would seem the simplest way.     

I can't tell what the diff is in your last para, so will leave that alone.

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Note that the masthead J's, 33, 35, 29, etc., in anything more than about 7-8 knots, will want to sail nearly DDW according to their polars.  Pretty much calls for the barn door approach.

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On 1/26/2018 at 12:35 PM, condor said:

Note that the masthead J's, 33, 35, 29, etc., in anything more than about 7-8 knots, will want to sail nearly DDW according to their polars.  Pretty much calls for the barn door approach.

DDW does not equate barn door. Play with the shape a bit and try to attach some flow.

 

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I think a little twist is good and easing the vang some allows the head to be a little fuller instead of dead flat.

Maybe try heeling to weather a little and get weight forward in the light stuff to get the wetted surface down?

Is boat as light as possible (empty tanks etc.)?

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On 26/1/2018 at 3:40 PM, esoxproblem said:

I had always thought that the slightly twisted leech was just an air dump, but maybe there is more to it.?

In waves (or if there's a little rocking...) you'll want the top of the sail to be able to move a little to help propel the boat.

Other than that, I guess top batten parallel to the boom is the textbook approach.

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On 1/29/2018 at 10:55 AM, Plumber said:

DDW does not equate barn door. Play with the shape a bit and try to attach some flow.

 

How is flow attached when DDW? I would have thought maximum surface area and let it push you DDW.

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13 hours ago, SOSOS said:

How is flow attached when DDW? I would have thought maximum surface area and let it push you DDW.

No... the sail does not push you, it creates lift and propels you forward.  The only way to increase lift is to maximize the velocity of the wind on the back of the sail.  You do not do that by locking everything down.  You do that by allowing the sail to breath, that is the leach needs to flow and open up.  In drifting conditions, that's a deep sail with little or no vang.  As the breeze picks up you bring on the vang a little to stop the leach twisting too much

Too little vang and yes you are spilling power out of the top.  Too much and the sail stalls.  Think and air plane wing when it stalls: air-planey-no-flyey.     There is a magic tension where the leach opens and closes nicely with every gust.  As the leach opens, it creates flow and propels you forward.  As it closes, the leach acts like a fan creating more flow and propelling forward.  Leach fanning is a good thing.

IMHO, assuming you're not sailing by the lee, you want your leach tell tails flowing most of the time or at least having the leach open and close consistently with each puff.   If the leach doesn't open ever, it's too tight.  if it doesn't close ever, then it's too loose.

Just looking at the knot meter isn't enough unless you've got really good data logging or are fantastic at doing mental maths.  how can you differentiate between a change in settings to a change in wind velocity/direction?   Ideally, two boat tune with someone who's roughly the same speed and try a few things over the course of the leg.  repeat as necessary.

 

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My favourite all time sail trim video is below:

Mostly because I can't watch it without going "how's the helm feel Tom?..." with a north american acscent afterwards...

Not really much discussion on main trim downwind, but at around 52mins they show a nice picture with a nice little curve in the leach and importantly, the Leach tell tails streaming.   Not sure what boat it is.  Maybe a J? about 40ish foot?  *shrug*  

I googled briefly for sail trim guides for J33's.  I couldn't find one but the North J35 trim guide is available (below) On page 17 it says this

http://www.j35-fleet7.com/training/training_tuning_NorthSails_guide.pdf

"Play vang, keeping telltales near top batten flying on both sides of mainsail. Do not stall upper leech. The top batten should be parallel to the boom."

Vastly different boat, similar principles,  the nice laser video I've used for others

 

IMHO, do not, ever, under any circumstances "Barn door" your main.  It's not fast.  Fast sails are sails with maximum pressure difference between front and back.  you do not get that with super flat, strapped down sails.  Not in light winds DDW, not in any condition. 

Hope that all helps

Cheers
Craig

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I feel like Spoonie might be trolling this thread...Going deep downwind your sails are not really powered via lift.  On a reach or beat, sure, but on a deep run lift is not your focus.  Projected sail area is your focus.

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That North Sails Trim for Speed video is shot on J/35s.

I battle with trying to get a nice shape to the sail, and putting on a big more vang to try and get the boom out a little farther without the sail sitting against the spreaders.  My downwind main trim isn't great, I feel like I have more to learn.  When I see photos of us racing with the kite up I always cringe a bit at how I have the main set.

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Spoonie is on target.  Even DDW a sal will shed alternating vortices off each edge.  A little curvature and softness in the leach will attach flow somewhat and generate a little more differential pressure than a flat plate.  Especially true in a seaway, or if your boat accidentally rolls back and forth a bit.

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3 hours ago, GuessWhat said:

I feel like Spoonie might be trolling this thread...Going deep downwind your sails are not really powered via lift.  On a reach or beat, sure, but on a deep run lift is not your focus.  Projected sail area is your focus.

You would be incorrect on both fronts.  The flat board analogy works because your drag is in the same direction as you are trying to travel (more or less) 

Even if you can argue the flat board is better DDW (which it isn't)  you are never truly DDW so as soon as you have directional flow of any kind, even by the Lee, you are losing efficiency. 

Sailing deep is a highly inefficient point of sail. I can't even grapple with the idea that making it less efficient is faster.  But let's do this.  I could very well be wrong.  If you can find a reputable tuning guide for any boat, or a photo of a leading yacht that suggests a a flat board main is faster in light downwind conditions, I will be both surprised and admit I might be wrong. 

Probably won't change the way I sail or trim because well...   Physics

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2 hours ago, Alex W said:

That North Sails Trim for Speed video is shot on J/35s.

Well there you go.  About as close as the op is going to get to a tailored trim coaching video as just about anyone here.  

As a note, If you are still reading this thread and haven't watched that video you are doing yourself a disservice.  Beyond the cheesy music,  80's computer graphics, and well presented trim information is some excellent behaviours to copy.  Communication, teamwork, constant looking outside the boat, discussion on their tactics and thought processes.

 If your attention span can hang in there for the hour that is... (yay for YouTube) 

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 I love that video.

I sent it to everyone on the boat and begggged them to watch it.

I really appreciate the feedback.  Thanks

 

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4 hours ago, Kenny Dumas said:

Even DDW a sal will shed alternating vortices off each edge

OK last point from me then I will stop, but note this point above from Kenny.  What happens when you start shedding vortices on each edge is the sail wants to start oscillating back and forth.  In light winds you won't notice it so much but as the wind picks up the oscillation forces get higher and the boat will want to start rolling back and forth.  So not only is your rig less efficient but you're now burning energy as you roll (even just a little) down the course.

Maybe it's a tenth of a knot, maybe it's more.  I actually have NFI but surely it ain't fast. 

For the laser guys it can often seem  counter intuitive that to get control of the boat an keep the boat under the rig DDW, you need to fatten the head.   Even with a strapped on vang, a fatter head will at least try and induce some directional flow. 

I think Kenny hinted at that in his earlier post, along with weight forward and to windward. Rolling your rig to windward gives more "projected area" into the good stuff than flat boarding ever will. 

 

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On 1/26/2018 at 8:40 AM, esoxproblem said:

 

In the pic, the 35 trim looked nicer.  I had always thought that the slightly twisted leech was just an air dump, but maybe there is more to it.?

Any thoughts to downwind angle against the J35?  Right now, if we match angle, we probably loose a little ground.  And, if we heat up a little, we still loose a little ground. 

 

Hard to compete with waterline downhill in 2 boats that don't plane. Match angle and get waterlined or heat up, get waterlained and sail farther. Have you attempted to jettison crew at the windward mark?

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