backdraft

[EGregerson]

I tried using the inner track today for the 1st time when closehauled . Seems to point better; but i noticed the upper part of the main appeared to be getting air off the genoa, and not properly filled. The telltales on the genoa were fine all the way up. Is there a way of correcting this? or is it a price to be paid?

 

[Frank Langer]

I have found that the inner track makes my E30+ point higher but go slower than if I use the outer track. I have tested this several times in the hopes of using the inner track (it gets the jib sheet out of the way as well), but I keep going to the outer track for the additional speed. I don't think there is any way to avoid this problem on the E30+. Maybe its the same on your boat.

Frank

 

[Slick470]

The effect you are describing is commonly referred to as a "speed bubble" and is caused by the air moving through the slot between the two sails. It's more common with larger overlapping headsails.

You can reduce it some by raising the traveller above center some, but keep a careful eye on boat speed, because you'll end up stalling the air flow over the main if you go too far and end up slowing the boat down.

I'm sure others will have opinions, but I've always been told by sailmakers that if it's fast with it there, and anything you do to get rid of it makes the boat go slower, then leave it alone.

 

[windjunkee]

I have to agree. The majority of the upwind drive you get is from the flow over the head sail. The trailing edge of the main should show laminar flow, with your leech telltales flowing, but when you're hard on the wind, you're going to have some "bubbling" in the luff. Its that weird vertical metal thing that the mainsail is attached to that interrupts the air flow initially. Keep the leeward telltales streaming on the headsail and head up until you have a slight upward flutter on the windward telltales. That's the sweetspot.

While your boatspeed may be slightly reduced from a close reach to close hauled, your VMG should be just as good or better and that is what really matters when you're beating to a windward mark.

On Voice of Reason, our fastest point of sail in virtually any wind is somewhere between a beam reach and close reach, when we've got better AWS but aren't beating into wind chop. VOR has inside tracks for a #3 but we have to go to the rail for anything over a 100 or 120.


Jim McCone
Voice of Reason E-32 Hull #134
Redondo Beach, CA

 

[mherrcat]

I have noticed that same effect on my main sail even when using the outer track and sailing close to the wind with the jib sheeted in tight. Adjusting the traveler helps a little but, as noted, at the risk of stalling. Could also have something to do with the age and shape of the main sail?

 

[Slick470]

I would imagine that the effect would be more noticable with an older more stretched out sail. However, I've noticed the effect with a brand new crisp main on a boat I race on.

I think that there is so much air pressure coming through the slot that the sail material at the luff of the main just gets pushed out of the way.

 

[CaptDan]

I would imagine that the effect would be more noticable with an older more stretched out sail. However, I've noticed the effect with a brand new crisp main on a boat I race on.

I think that there is so much air pressure coming through the slot that the sail material at the luff of the main just gets pushed out of the way.

The size, cut and set of the jib really matters here. When I fly the 140% (Kevlar, radial full cut) genoa, I have to ease out to below a close reach to avoid main back draft and stalling. Even with the traveler above center, the jib fair leads adjusted, in true winds over <>8kts, close reaching isn't an option without degrading the sails' interaction. Conversely, with the 90% blade, (flat cut) close reaching's a piece of cake.

If I were to add another headsail, it would be a flat cut 100% with lower foot. In that scenario, perhaps adding an inner track would be an effective upgrade. Otherwise, I leave well enough alone and enjoy the boat's performance as is.

YMMV

Capt Dan G>E35II "Kunu"

 

[e38 owner]

back draft

I have found that back draft is fine and fast. The keys we use are the telltales on the back of the main and to never have the main above centerline.

The size of the back draft bubble can be manuputed to see how much is right for each wind and sea condition by making sure your draft and mast bend are right for the condtions. More downhaul will move the draft forward and reduce the bubble. Make sure your draft is in the right position for the conditions. Also a little mast bend will flatten the front of the sail and reduce the bubble. Again based on the conditions mast bend make take to much curve out of the headstay. We have found that with a little practice the size of the bubble is a good indicator if the boat is set up right for the conditions

 

[Seth]

Mainsail backwinding

If the sails are set up right you should only see this at the very upper end of the genoa range. First thing is to make sure the genoa lead is not too far forward. I like to see the top telltales on the genoa break just a bit before the lower ones as I head up from a close hauled course. For sure, having the leads too far forward can make this worse. If the genoa halyard is not set correctly, the draft will be too far aft and make things worse.

The biggest problem is that your mainsail is probably too full for the conditions. Max out the outhaul and make sure you have proper halyard/cunningham to keep the draft around 40-45% of the way back.
Even more importantly, either increase backstay tension and/or set up the rig with more prebend. You do this by moving the mast step to the aft end of the track/slot on the keel (base of the mast), blocking the mast as far forward as you can in the partners (coming through the deck), and then setting the rig up with the aft lowers looser than the fwd lowers (so they pull the mast forward, or bend it, where the lowers are attached to the mast). Make sure you have proper load on the cap shrouds (uppers) for the conditions. The mast should be in column with no fall off to leeward as you sight up the windward edge of the mainsail track. The lee shrouds should be just unloaded, but not swinging in the wind.

If you can't get the main significantly flatter with backstay tension, prebend and rig tune, it is time to have some luff curve removed by the sailmaker.

Yes, there are times when you will see a small bubble, but if you see it all the time while you are trimmed in you have to make some adjustments:

1). Genoa car back a few inches (and make sure the genoa halyard is properly tensioned for draft no more than 40% of the way back!). If the genoa is draft aft or blown out, this will make it that much worse.

2). Use the tools at hand (described above) to flatten the mainsail. What is happening is that the excessive draft of the main is poking out into the exhaust of the genoa.

3). Got it?

Oh, and Frank-same for you. I know your main is too deep! I have seen it!