Main Sail Bubble

[RichH]

If Im reading and interpreting correctly ....
Sorry but from your description, thats how one STOPS a boat with sails by true BACKWIND. ....

By simple intuition or complex areodynamics if the sail is anyway positioned so that the high pressure side is now the low pressure side ... the flow across the sail will be operating 'in reverse' ... and you are now STOPPING or at least drastically slowing the boat. Jet aircraft can also do this by applying 'reverse thrusters'.

It would be better to completely drop the main and operate solely on the jib/genoa as the destructive flow and turbulance created from 'reverse flow' can be quite substantial.

Simple rule of thumb .... NEVER put or allow the boom to be on the windward side of the boats centerline when above a beam reach, unless you are using the mainsail as a parachute-brake.

 

[Joe]

Traveller car was all the way to weather (as far as possible away from the genoa. I may have had the main slightly over centerline to weather. My traveller is inside the cockpit in front of the companionway so it only extends about 2.5 ft either side of centerline. I had the main sheeted as tight as I could get it to reduce luffing, and even then had 50% or more of a bubble. My traveller doesn't have pullys for the car - just pins and is a pain to adjust, so I use the sheet to adjust AOA in puffs to reduce excessive rudder and heel.

I'm in agreement with Rich's comments... I can't imagine any situation where you would want the boom above centerline unless you're trying to stop the boat. You can have the traveller car pulled up to windward, but this is usually when you are imparting a lot of twist in the main...i.e. an open leech. (I pull the car to windward quite a bit) But if you sheet down the main, and cinch the vang tightly you end up with a very closed leech... which is not especially desirable in your stated conditions... and it would also result in the sail being grossly overtrimmed and above centerline to boot. That's pretty slow... So.. as was previously recommended.... let the thing out and trust your leech tell tales.

Check out Don's trim guide on how to set the twist in your mainsail...

 

[tomgrigsby]

I appreciate the help. Not sure I understand though, or perhaps haven't conveyed the conditions correctly. Given that I had multiple settings wrong, I'm looking for what I needed to do to get the main powering with only a small bubble vice the huge one that I had. A number of comments said I should have eased the main, but I had done that. Easing the main only resulted in a total luff. I was closehauled and had so much breeze in the slot that the main had a huge backwind (bubble) from the luff to about midpoint on the sail. The leech telltales were streaming nicely, but I know I had too much bubble. I had lots of weather helm indicating a lot of power aft of pivot point, and was able to correct weather helm by easing the mainsheet and depowering what little area of the main that was flowing. If I understand all the comments correctly, either I had the genoa car too far forward for the conditions which gave great genoa power but the wind in the slot was trashing the main, or as noted above, I could have basically doused the main for all the good it was doing. Or I could have used a smaller head sail for the conditions and had two properly trimmed sails AND KEEP THAT DANG BOOM AT CENTERLINE OR BELOW! (smile) And no need to apologize RichH. I won't learn things if people beat around the bush. I can't thank you and all the others enough for taking the time to weigh in. I'm sure I haven't properly credited everyone's comments, but I'm tilting my head so they all rattle around in there before going out my ears, and hopefully I'll become a better sailor for everyone's efforts!

 

[timvg]

Ok, here's my two cents. If you have a lot of weather helm, then I believe you have up too much sail area for the wind conditions. Most boats are going to need to reef when sailing upwing in 17-18 knots or so. You can keep up more sail area if you wish, but you won't go any faster, as the weather helm slows you down.

I have a fractional rig and smaller jib than you do in relationship to my main. If it's windy enough to give me more weather helm than I desire, I let out the main, and yes, it will start luffing. In your case, reefing the main may help, or using a smaller jib may help. Just depends on the balance of your boat. If you move the jib leads aft, you will spill more wind from the jib, which may help. I also know that is common with a 150 genoa to have some luffing in the main when close hauled.

Question: How much wind are you in when you get your bubble with full sail area?

 

[sailaway78]

One thing no one asked

Including yours truly. Where was the apparent wind. Were you trying to sail upwind with the apparent wind at 45 degrees, or at 10 degrees. We all know that 10 degrees is basically a non factor, and I once sailed on a full keel ketch that anything below 50degrees apparent would only result is decreased boat speed. If you have a GPS that shows VMG, this is probably the easiest wiay to figure out you best angles. But, putting the rail down and just enjoying the day is good. And for some good enough.

 

[Scott B]

Don, hasn't seen this one yet so I'll move it over to the Sail Trim Forum for his comments....

 

[RichH]

Why dont you move it back to where it was .... as probably most who view the 'main forum' dont even know this 'sail trim' forum even exists. There is vastly 'more traffic' on the 'main forum'.
Most sail trim and other 'postings' involving "anything to do with sails" that are moved from the main forum to this one .... seem to quickly 'peter-out' and soon die.

 

[RichH]

Now that youve related that the boom was pulled fully towards windward, my impression is this:
1. The angle of attack of the mainsail was probably soooooo 'off', that the airflow from the main was mostly seeking to 'go through the slot' (between the genoa and main). The 'slot' is a place where the total airflow is greatly reduced, almost 'stopped' (mathematically its sometimes 'flowing backwards'!) when sails are trimmed/shaped to perfection. With more flow over the WRONG SIDE of the mainsail and at the WRONG angle of attack the main was 'luffing' (the flow was 'separating' from and destroying whats called a 'boundary layer' ... creating a HUGE zone of ZERO flow close to the surface) on the WRONG SIDE even if your observations of the leech tell tales said otherwise.

The combined flow through the 'slot' should be greatly SLOWED, not 'speeded up' for maximum efficiency of jib/main to occur. MOST 'sailing books', high school 'science classes' have been WRONG since the time of the Wright Brothers. The airflow doesnt 'speed -up' or slow down on the 'backside' of a sail or wing or foil, such shapes only change the **AMOUNT** of air flowing on one side of the foil or the other ..... even the WRIGHT BROTHERS knew this.
Sails, wings, foils, 'work' because the air across them have viscosity or friction and this friction imparts energy more to one side of the foil than the other. The air flow on one side of a sail/wing isnt FASTER than the opposite side !!!!!! yet there is MORE (amount) flowing on one side than the other.

Aerodynamics is NOT an intuitive science. Simply stated, the use or observation of correctly placed TELL TALES is *all one needs* to approach 'maximum aerodynamic efficiency'. If ALL the tell tales are not 'streaming straight back' one will not have 'good aerodynamic efficiency'.

Are you a 'science teacher'?

 

[tomgrigsby]

Funny you should ask - I'm not a science teacher but I do have a BS in Professional Aeronautics and spent 24 years in Naval Aviation, so I hear what you're saying regarding airflow on the windward and leeward sides of the sails and the slot effect. The flow was wrecked at the main luff but had reattached at the leech. I think we are in violent agreement. The excess flow in the slot put far to much air on the wrong side of the main. No matter how the main was trimmed, it wasn't going to work. Easing simply made it luff more. Given the conditions of 20 kts + gusts, I am more convinced now than at the beginning of this thread that I should have shortened sail forward by dousing the genoa in favor of the jib. The wind then would have been seeing all four conditions as they should be: 2 properly flowing sails, 1 properly flowing slot and 1 properly flowing overall combination of jib & main.
The trouble with my aviation experience is that it was mostly helicopters. The only adjustment in helicopters is angle of attack (via collective and cyclic). Never had the opportunity to change the belly position, depth of belly, or any significant interaction with another airfoil. On the fixed wing aircraft there were flaps and slats to increase lift, so there's some belly depth experience, but the cg wasn't significantly impacted by the degree of flaps/slats settings, so still no one gave it a thought (except designers, I suppose) Sailing - what a great sport!
I'm sure many of you can tell how green I am at sailing. I didn't know I had a boat that was so forgiving that I could carry too much sail and still be relatively safe.
By the way Scott B, I really like the sail trim forum, but as mentioned above, my experience is that I don't get much help and suspect readership is relatively low. I've asked questions before and didn't get much response. Maybe on your note in the main forum where you say it has been moved, perhaps you could add where you moved it so that it can be easily found.

 

[Stu Jackson]

By the way Scott B, I really like the sail trim forum, but as mentioned above, my experience is that I don't get much help and suspect readership is relatively low. I've asked questions before and didn't get much response. Maybe on your note in the main forum where you say it has been moved, perhaps you could add where you moved it so that it can be easily found.

Tom,

Glad we could help.

As far as "readership" goes, you have two choices: enter on any given forum and monitor that forum, ! OR ! I do believe you can set you viewing pleasure to All Forums, which is what I do and catch everything that gets posted in All Forums. I keep learning new things just by reading as much as I can.

Welcome aboard.

 

[RichH]

Tom -
With an aerodynamics background you will certainly find these summary articles useful:

http://www.arvelgentry.com/techs/The Aerodynamics of Sail Interaction.pdf

http://www.arvelgentry.com/techs/Origins_of_Lift.pdf .... remembering that sails are 2 dimensional foils - essentially NO 'thickness'.

http://www.arvelgentry.com/techs.htm

http://www.arvelgentry.com/magaz/How_Sails_Really_Work.pdf !!!!!!!!!!!!!!!!!

http://www.arvelgentry.com/magaz/How_a_Sail_Gives_Lift.pdf

http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf

http://www.arvelgentry.com/magaz/More_on_the_Slot_Effect.pdf

.... even those of us deeply involved in aerodynamics, gas mechanics, etc. find these articles somewhat 'mind blowing' when compared to 'classical aerodynamincs' of only a few years ago.

And lastly, The Wright Brothers used wing-warping - the same method used by sailors to control their 'wings'. Curtiss (flaps, slats, etc.) apparently never sailed a boat.

 

[tomgrigsby]

Thanks Rich. I've downloaded all the articles and am beginning to wade through them. Very helpful and interesting!

 

[Scott B]

By the way Scott B, I really like the sail trim forum, but as mentioned above, my experience is that I don't get much help and suspect readership is relatively low. I've asked questions before and didn't get much response. Maybe on your note in the main forum where you say it has been moved, perhaps you could add where you moved it so that it can be easily found.

The thread still appears on the Ask All Sailors forum, and when you click the Thread title, takes you to this thread. I was hoping Don would jump in, as this seemed like an interesting problem.

I guess it all depends on how you view the forums. I typically click on Show all Posts from All forums, and get a listing of all the recent posts, but then I'm looking for different things than most.

 

[RichH]

I must respectfully, sort of, disagree with your statement about the main on a mast head rig.

.... while you don't just 'throw out' mainsail trim on a masthead rig, you also shouldn't place too much importance on getting mainsail trim right at the expense of setting the genny properly. ....

The following 'aero' articles (written for the 'layman') should help with the understanding of the VERY IMPORTANT 'interaction' relationship between the mainsail and the jib/genoa. The important issue is that each (when correctly trimmed/shaped) ADD AND CONTRIBUTE to aero efficiency to the other - called 'bootstrapping' &/or 'velocity dumping' (... especially discussed in article 3 - below; but, you have to read and understand #2 before you read #3). Especially, look for 'boot-strapping' and 'velocity dumping' in #2 & #3.

1. www.arvelgentry.com/magaz/How_a_Sail_Gives_Lift.pdf

2. www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect.pdf

3. www.arvelgentry.com/magaz/More_on_the_Slot_Effect.pdf

Hope this helps to dispel or lessen the moderately erroneous notion that the JIB is somehow the 'engine' on a masthead rig and that the mainsail somehow 'doesnt count' .... as it take BOTH sails trimmed and shaped to near perfection that makes 'the engine'. To get the 'bootstrapping' right (as seen by increases in speed by the Speedo) one really has to 'trim' and 'shape' each sail several times in quick succession.

 

[Don Guillette]

Tom: The bubble is caused by backwind from the jib. Lot's of guys don't even bother with it or give it a second thought. If I were you I'd just forget about it but if you wanted to remove it the remedy is very simple - if you you had a better traveler - just crank in on the traveler slightly. With a masthead rig you don't want to mess with the jib if it is trimmed to 100% efficiency. On a fractional rig - just ease the jib sheet until the bubble goes away.

 

[tomgrigsby]

Thanks Don. I appreciate the input. As best I could tell, that was about my only option (ignoring the bubble) since I was closehauled and had the genny trimmed where I wanted it. Weather helm was still responding to main sail inputs and heel wasn't excessive.
RichH - you've really given me some homework. I already am beginning to comprehend things I've been seeing like main teltales trailing off to leeward (not so good) and beginning to understand what's happening.

 

[s sauer]

Backstay adjuster?

Tom,
I you have a backsgtay adjuster, I'd use it in conjunction with max outhaul and firm halyard (or cunningham) tension to flaten and de-power the main.
Perhaps the H30's stiffer single spreader rig won't allow much mast bend but any bend will flaten the main and slightly open the leech. The tighter headstay should help as well. Does the Genoa have a leech cord that maybe over tensioned and hooking the leech?

 

[Don Guillette]

Tom: The cause of the bubble in the main is backwinding from the jib. Most sailors just ignore it. It can be removed by simply moving the traveler but in your case that is not easy to do. You could ease the jib but on a masthead rig that is not the best idea but it might be the only one open to you.

 

[tomgrigsby]

Tom,
I you have a backsgtay adjuster, I'd use it in conjunction with max outhaul and firm halyard (or cunningham) tension to flaten and de-power the main.
Perhaps the H30's stiffer single spreader rig won't allow much mast bend but any bend will flaten the main and slightly open the leech. The tighter headstay should help as well. Does the Genoa have a leech cord that maybe over tensioned and hooking the leech?

No backstay adjuster. The Genoa leech DOES have a cord that I haven't messed with. I think vaguely recall seeing a hook in the leech at some point. I'll check on that, Thanks!

 

[Joe]

No backstay adjuster. The Genoa leech DOES have a cord that I haven't messed with. I think vaguely recall seeing a hook in the leech at some point. I'll check on that, Thanks!

The leech cord won't have any effect on the bubble. Its purpose is to quiet a fluttering sail's trailing edge. If the leech is cupped the cord is over tensioned. Sails in newer condition rarely need leech cord adjustment.