Jib Physics & "the slot effect"

The topic of jib physics came up in another post so I thought it would make an interesting discussion since there is some debate on this. First, I am no expert, just fascinated by the topic.

Conventional thought has been that the jib creates a slot between the main and jib and that this slot feeds air to the back of the main. The idea being that the air is speed up on the backside of the main and the mainsails lift is enhanced.

The other theory I have read is that air in the slot actually slows down and this slower air creates greater pressure on the windward side of the jib. This actually enhances the Jibs lift. On the other hand the slower air in the slot will degrade the lift of the main. The overall effect is has to be more lift, thus the greater boat speed. The jib flies at a higher angel of attack than the main and redirects the apparent wind to the main by about 10 degrees. This redirected air allows for the main to function without luffing when the boat is pointed higher. Recent research supports this theory.

I like the second theory if for no other reason than what I am seeing when sailing the jib on the AI. I have noticed on my boat, (and others when viewing YouTube) that when the sails are working hard, a dimple forms in the luff of the mainsail. This occurs about half way up the luff and the main, in this area, is slightly directed windward vs the normal leeward. The rest of the main looks normal in shape. It doesn't happen at low speeds, only high. I don't have video of this in my boat but you can see this in this YouTube video at about :20.



https://youtu.be/R3K_MQsaYvE

I think this is air slowing down and pushing the luff of the main windward. Could be wrong but that's my take. If the air speed were increasing I would expect to see a very tight sail in this area. Eventually the air speeds up on the remaining main and the main is getting some lift, just not what it would if the jib were not there.

Do you agree?

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Dear Vetgam,
I am following your posts with a lot of interest, I have no enough sailing knowlegde or experience to answer to your question.
I just tried to google the words "Slot effect", and I found out that the "other theory" explanation is currently considered as the right one.
I have no idea about what this could affect in terms of sail design but it is very interesting to know.

I am planning to make a jib for my AI, but I would like to follow a different design approach. I don't like so much the jib furling togheter with the mainsheet, but I like very much the self tacking option.

I am just thinking about an articulated joint to put on the top of the mast.

In the following video you can watch at my first prototype:



Any suggestion is welcome.

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Biagio

OK, guys. First, if you really want to understand how your boat moves through the water-get a book. You won't learn much, if any, truth on the internet. Even books, though, will offer various interpretations and explanations of how lift occurs.

Slot effect - neither explanation you provided is very accurate.

On a boat designed for a single sail (ie, the boat is balanced with a single sail) such as the AI/TI, a jib isn't very useful. You add one because you want to either go faster or point higher.

You go faster with a jib because you have more sail area. The cost of adding a jib to a boat that is well balanced without one (ie, a cat rigged boat) is that you will introduce lee helm--this will force you to use more rudder...so you don't get the whole benefit of the additional sail area since more rudder=more drag.

You will probably point a few degrees higher with a jib than without one. Why? Couple things...predominantly, it is the fact that the jib luff has a fine entry (compared with the mast (blunt round) entry into the mainsail). The other reason you can point a little higher is because you are going faster through the water because of additional sail area, you probably have all noticed by now that the faster you go, the higher you can point (up to a point, that is). Another reason you can point a tiny bit higher is because the jib's camber is "placed" further forward than the mainsail camber-this give more forward drive to the resultant vector than you get by the mainsail whose design is usually with the draft further aft.

SO, bottom line. Forget slot effect-doesn't exist (some boats have tiny jibs that do not overlap the mainsail--ie, no slot). Oh, folks will keep saying stuff like bernoulli effect (but air is compressible, not like water). Another favorite is "choke the slot" by which someone is explaining why his crew gets the jib too tight. The ONLY time that the slot is useful for sailing in a sloop rigged boat is when the boat is overpowered. You can watch the jib flow collapse the mainsail luff to reduce effective sail area when you traveler down and help keep the boat on her feet without having to reef.

Sail physics is a complicated topic and I find it fascinating-part of the reason I started designing and making sails.

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R/Thom
SeaRail 19
Triak
BMW C600
Formerly Getaway with Custom Spinnakers
Formerly raced F24 Mk II

I have been testing different tensions on the self tacking jib traveler by using a block on one end and then cleating the line. I was supprised to fine that I seemed pick up speed when the line is relaxed and I have a larger slot. I might be minimizing the backwinding of the main minimizing the collasping of the main's luff. The added efficency of the main may be why the speed increases with the wider slot.

I plan on keeping the adjustable tensioning capability for the jib traveler because i like to be able to adjust the jib for different winds and sailing angles yet maintain the self tacking feature.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Actually, what you are doing is finding the correct sheeting point for your jib's design (assuming you are actually trying to point during the process--it will change if you fall off). Racing crews with new boat designs (or jibs) typically use twings (little lines to adjust jib clew up and down) and barberhauls (little lines to adjust jib clew side to side) until the "correct" spot is found. Most production boats use a pretty standard number of degrees off centerline--but with monohulls, the boat's beam has something to do with where they get put. A boat is a dynamic machine-everything affects everything else-and every boat is FULL of compromise decisions.

If you want to continue to use a self tacking jib and you have the ability to do so, you might consider putting the jib on a boom that swivels at the bow. I've never had one like that, but cruising sailors seem to swear by them. If you do so, you can optimize the jib design and actually employ horizontal battens which will help the jib keep the optimum shape AND you can have some roach on the jib leach...T

I still think, tho, that a cat-rigged boat can't get much benefit from a jib since the more power you put at the bow, the further you affect the boat's balance and therebye also end up using more rudder to keep you on course. The reason most boats have the balance intentionally set so it gently rounds up when you release the rudder is because that is SAFE...the boat will go head to wind and you stop. If you have lee helm by adding a jib, if you release the rudder (say you are unconscious because of a health issue), the boat will fall off and keep sailing-probably into trouble.

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R/Thom
SeaRail 19
Triak
BMW C600
Formerly Getaway with Custom Spinnakers
Formerly raced F24 Mk II

I understand cat-rig benefit and the lee helm concerns. I posted awhile back to see if anyone had issues with lee helm before adding a jib to an island. No response so I just built it and took the risk. For whatever reason I got lucky and the boat remained balanced. I never fight the rudder. In fact, when I let go of the rudder it continues straight on course. Now, a little weather helm would be welcolmed but definitely no lee helm. My jib is moderate in size and the main's mast sits further back in the AI vs TI, maybe these make the difference. I don't recall fusioneng had any lee helm when he put a jib on his TI. Maybe Hobie left us a little room to play with foresails.

On the issue of the cat rig benefit. I certianly point higher and there is a modest speed difference of about 1-1.5 mph when measured with the GPS. But your right, it's not the same turbo boost that you feel when you open up the jib on a monohull.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Since this is an interesting topic (to you and me anyway). Here's a short link to the debunking of a couple of the most common misconceptions of lift physics (e.g., equal transit time). I've found that you get much more science by looking at NACA shapes and airfoil information from airplane hobbyists and not sailors. Can't use the info directly usually, though, because the slowest airplane is going much faster than my boats will ever sail.

http://amasci.com/wing/airgif2.html

I do have a cat-rigged boat (triak). Even though it doesn't do much, I added a jib. It does help a little in light breeze when I'm not adequately powered up. When reefed because of high winds, it helps point a little higher. I located the tack point only about a meter from the mast so as to minimize lee helm. It is probably more trouble than its worth (extra halyard, extra snuffer, extra snuffer strings), but I like to play with strings when sailing-otherwise I get bored since the boat is pretty slow compared to the other boats I sail.

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R/Thom
SeaRail 19
Triak
BMW C600
Formerly Getaway with Custom Spinnakers
Formerly raced F24 Mk II

Wow. I would have to say that was a great read. I'll never look at air foils in the same way again. So the question begs where is lift in a sail really taking place? When you watch a sail there appears little doubt there is lift occuring in the forward third of the sail. Yet these air foil studies would indicate that we are also getting lift just aft of the leach as air is redirected windward. This could certianly explain the mild collasping of the main's luff.

So you would think that the further the jibs leach is from the main, the less drag and dirty air on the main. Yet set the jib too far out and we lose the ideal angle of attach thats maximizing lift. Deep breath. I guess in the end everything is a compromise and about getting out and experimenting on the water.

Thanks for the link.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

The mild collapsing you see can be mostly gotten rid of if you rig a cunningham or pretension the mainsail luff before you go out with a 2 or 3:1 (you won't need it if it isn't windy--and you will go slower (a little) going downwind). That will help keep the max camber where it is designed to be. Race crews call these "speed wrinkles" and usually you are happy to see them since you are having fun.

What happens with sails (since they are flexible and not rigid) is that as you speed up, the designed maximum camber moves aft-since the sail tapers toward the top there is less cloth to deform near the top (plus the wind direction at the top of the mast is a tad different than the wind direction the bottom of the mast sees -- even these short masted boats. That's why there is an amount of twist designed into the sail.

As far as your other question (where lift occurs)...some people think it occurs before the wind reaches the boat (and they have equations to prove it). The theory goes that since air has dynamic viscosity, the disturbance of the air in front of the sail is transmitted further and further forward and the air "lines up" for the sail even before the sail gets there. I dunno how much of that I believe, but it is a pretty cool theory.

_________________
R/Thom
SeaRail 19
Triak
BMW C600
Formerly Getaway with Custom Spinnakers
Formerly raced F24 Mk II

Blasius, I had posted a response to your post a couple days ago, but I don't see if for some reason. Keep us posted on your progress and the things you learn along the way. Check the archives, I seem to remember one poster who said they built a pvc mast top but it did not hold up to the jib forces and cracked.

One option is to get the reacher when it comes out. The mast topper can double for the jib.

With my jib setup you can quickly take it down or put it up independent of the main. At any time it is up you can also furl it with the main. Lots of designs out there. Good luck and feel free to ask questions or request pictures along the way.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Perhaps this is demonstrated on the long Mulsanne straight at LeMans, where drivers of racing cars travelling at over 250mph can feel disturbance to their aerodynamic stability caused by a car travelling more than 50 yards behind them....

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

Same here. I've explored every waterway within 400 miles of me. Now I look for ways to explore the boat and it's possibilities a bit further. I don't mind more strings to play with. It's going to be a sad day when I can't find anything else to change on this boat.

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

Tony, teach me how you quote someone in a text. Why doesn't the quote in my last post list who wrote it?

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Greg

2016 AI - Spinn & Jib

“Out of sight of land the sailor feels safe. It is the beach that worries him.”
– Charles G. Davis

The mast top I made is just a prototype, I will do many tests before I will mount a jib on it.
The way I would connect the lines and pull the jib will be completely different than the other solutions I have seen on the web.
Unfortunately I cannot do tests in my garage because of the heigth of the mast, so I will do some tests during next exit before launching.
If you want I can try to do a drawing to show my Idea.



It will be nice if someone will make an autopsy of it and share the pictures ....


This is something still unclear to me: how you can raise, remove or use it independently of the main while sailing?
Can you please clarify it?

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Biagio

Geez Vetgam, I don't really know. I hit the quote button on your post, and here is what it showed.
left square bracket quote equal-sign double quotes vetgam double quotes right square bracket - your words left square bracket right slash quote right square bracket.

Hope this helps

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker