main flow issue

[DinghySailor]

We took our new-to-us 37 out for sail#1 last weekend. In sea trials, we'd sailed with a 90% headsail that was a bit blown and unimpressive. As the new owner now, I swapped the 90 for the far better-shaped UK 125%. Going to weather and playing with the slot (genoa skirted), I couldn't seem to achieve adequate positive flow on the full-batten main. Traveller just windward of center, vang in & out, the main continued to flag at the luff all the way up. This did not occur with the 90, so I'm curious why a medium-sized genoa would have this effect and what I might try to mitigate it. Would adjusting sheeting angle on the genoa make a difference? Ideas?

 

[shemandr]

I think you may be talking about what we used to call the Ranger Bubble. It is normal and good for the first couple of feet of the main to be backwinded when the jib is sheeted tight. The bubble corresponds with the jib overlap. I'm sure you had the traveler on max to flatten the lower main. It didn't happen with the 90 because it doesn't overlap.
How was your speed and pointing ability?

 

[DinghySailor]

Thanks Shemander, I’m aware it’s good to have some backwinding on the main—though didn’t know it was brand-specific This seems excessive, but to be honest I didn’t have much time (or leeway) to tinker with sailshape. We were doing about 7 in 4.5 kts of wind and pointing about 30 degrees. Tried uploading the brief video but it didn’t work.

 

[Stu Jackson]

though didn’t know it was brand-specific

DS, everybody's got a different name for it, and it's not brand specific. A little bubble at the luff isn't an issue, and is to be expected in many cases, depending on sail trim, angle of attack, etc. One must assume your halyard was tight. Do you have a cunningham?

 

[DinghySailor]

DS, everybody's got a different name for it, and it's not brand specific. A little bubble at the luff isn't an issue, and is to be expected in many cases, depending on sail trim, angle of attack, etc. One must assume your halyard was tight. Do you have a cunningham?

Thanks. Yes, I understand it's to be expected, though in other boats I've not had this much bubble. I did tighten the halyard while out as well as the cunningham. I think I just need to spend more time with this rig to understand the dynamics.

 

[DrJudyB]

We were doing about 7 in 4.5 kts of wind and pointing about 30 degrees. Tried uploading the brief video but it didn’t work.

That sounds a too good to be true. That indicates you’re sailing at 81 degrees to the true wind, at 155% of true wind speed.

VMG to wind	1.09
T Wind Direction	81.1°
T Wind Speed	4.5
Apparent Wind Speed	8.9
Boat Speed	7
Apparent Wind Direction	30
TWS = AWS - BS *	0.63
Boat speed as % of true wind speed	155.6
VMG as % of true wind speed	24.2

 

[DrJudyB]

Please Upload the video somewhere (like Vimeo or YouTube) and then give us the link. Personally, I’m reluctant to comment with seeing it with my own eyes.

 

[DinghySailor]

Ha! Very likely, Judy. I wasn't paying attention to the precise angle into the wind, so yeah, 30 degrees is way off, as you note. The vid is just a few seconds of the main. I'll make better observations next sail.

 

[DrJudyB]

@DinghySailor
Peter, do you havel telltales on the leech of the mainsail, one at each batten?

 

[DinghySailor]

Yes, telltales at each batten: here's the (very) brief video:

 

[PaulK]

Looks like you need more backstay and sheet to flatten out the sail more, but perhaps not if that was only 4.5 knots of wind. (Is the video time lapsed?) The leech seems much looser than it should be. It could also be that the sail, like the 90% jib, is bagged out.
This is the boat, yes? https://sailboatdata.com/sailboat/cc-37

 

[DrJudyB]

my first impression upon viewing the video is that the mainsail is stalled. That’s not what a “speed bubble
look like. So I took a frame grab and zoomed in on the windex at the masthead. It’s really grainy, but I think it confirms that it’s stalled.

How wide is the angle on your windex flag legs? It looks like it’s only about 55 degrees wide at this camera angle. And it looks like the vane is inside the flags. If that’s the case, your apparent wind angle at the mast head is < 28 degrees, and even tighter going towards the foot, with all the twist you have in the mainsail.

I’m curious, what was the angle of heel?

on edit: i changed the measured angle to 55 at the mast head after checking again with a better protractor.

 

[DinghySailor]

Judy, your observations from that tiny grainy video are impressive, and you found the correct boat on sailboatdata. Yes, I was likely pointing too high for the main, though I maintained a smooth luff on the 125, telltales streaming parallel. The main is about 9 yrs old and a pretty stout offshore sail. I agree it's stalled there; playing with the backstay and leech tension should reveal more. It'll be a few weeks before I can sail the boat again, but I'll report back when I can get some more accurate data. Thanks for your interest.

 

[shemandr]

...That’s not what a “speed bubble look like...

I agree and based on the video I retract that post. I can do that can't I?
The main looks under trimmed or the heading is too high for that trim. It is also very deep. Backstay on a C&C 37 won't help with that very much.
Since the OP stated the traveler had the boom above centerline, the twist in the main is consistent with the traveler high and mainsheet eased.

 

[DinghySailor]

Returning to this a couple years late. The issue was mostly due to the sheeting angle on the jib. Moving the cars back opens the leach and improves flow on the lee side of the main. And yep, new to the boat at the time, I was definitely undertrimming the main

 

[jssailem]

Thanks for sharing your observations @DinghySailor .
You are a wiser and smarter sailor now.