Jib Sheet inhaul

[Bandit]

What is the measurement racers are using for the jib sheet distance off centerline while sailing upwind? The North tuning guide says 16" off centerline for "normal" wind speed upwind and 18" if sailing in breezier conditions with chop. The Dieball tuning guide says the inhauler should be set at 10" off centerline. That is quite a difference. Hopefully I'm not misinterpreting the Dieball guide.

thanks in advance

[GreenLake]

There's the theoretical possibility that different jibs are cut differently, thus requiring different settings.

However, when I installed mine, I placed marks according to the instructions from my sailmaker at the time (Jotz) and I just verified that they appear to be at 16" off the center line. From my sailing experience, 10" would seem rather aggressive.

Also, as the winds get lighter there comes a point where you might want to let out the inhauls progressively, because you'll find that you're punching instead of pointing.

When mounting inhauls they should cross each other, that is, the cleat should be on the opposite side of the jib sheet being controlled (so you can operate them from the windward side). That means that these measurements should not affect placement of the cleat. And the distance that you place sheet by hauling on the inhaul would thus be freely variable (not limited by cleat position).

Thinking of this, what is 10" off centerline (on the opposite side) on my boat are the cleat positions. I have a starboard cleat about 10" off the center which controls the inhaul for the port jibsheet. If I pull that tight, there are about 26" between cleat and jib sheet.

[GreenLake]

Actually, come to think of it, these are "inhauls" not "outhauls". The latter exist as well on some boats, used for reaching to allow the opposite: a deeper angle on the jib. Usually not found on DS. So that others can find this discussion later, I've gone ahead and changed the thread title.

[tomodda]

Hi Bandit!

I'm going to chime in, but with two observations: I'm 1/2 asleep tonight and I don't race anymore. So take my opinion for what it's worth!

IMHO, there is no "fixed" inhaul offset (sheeting angle), it depends highly on conditions and your sails. Let's unpack the whole question for a minute - what are we trying to DO with our inhaulers? Obviously, modify the "slot" between foresail and main. But our jib doesn't overlap, so what we are really doing is "fitting" the air flowing off the leach of our foresail onto the leeward side of our main. Why? A few reasons - create more airspeed across the back of the main, thereby creating more power (Venturi Effect in action); melding both airflows together creates one giant airfoil; and we can now balance the power between foresail and main, affecting our helm balance. Of course, it's just one more tool and we need to work it TOGETHER with all the other controls.. sheets, halyards, jib car, vang, traveller, outhaul ,even your leach tensioners (assuming you have them). Not to mention your body weight, helm, and centerboard. All works together. The jib can never be trimmed in isolation, as it has a critical effect on the mainsail.

So, what are we actually trying to achieve with all these controls? I say "one airfoil", balance, etc... but to what end? Well, it's the old tradeoff in sailing - speed vs pointing ability (height). Visualize it... moderate 12-14kt winds, sailing maybe 30 degrees off the wind, jib and main with draft about 1/3 of the way back on each sail, very little twist. Close the slot up with your inhauler and now the wind coming off the jib leach blows straight over the deepest part of the main's draft, nice laminar flow.. That's your SPEED MODE. Now think of the start, the fleet is still on starboard and you have boats above you... Slack the inhauler a bit (an inch at most!), slack the jib a bit, maybe even move the draft back (halyard tension), get your boom up to centerline or maybe even to windward, open the draft on you main, this is "Flaps Down" (like an airplane landing) or POINTING MODE. Big difference for only an inch or so of inhaul change! Paradoxically, at least on my boat, I open the slot up (slack the inhauler) quite a bit (5-6 inches) for lighter as well as heavier air, it really all depends. What you DON'T want is for the backwash of the jib to wash out the main. What you DO want is for the two curves - jib and main - to match each other (so now you have to play your jib car, etc). Just to complicate things further, if you are making more leeway then you'll have to ]sheet the jib more narrowly, because the jib’s angle of attack is influenced by how much or how little the boat is slipping sideways through the water. Sail the boat FLAT - hike, hike! It literally a real balancing act.

Speaking of matching curves, the reason that Dieball gives you a different "default" inhaler offset than North is becuase their sails are cut with a flatter "exit" (rear 1/3rd or so). More curved will need a larger offset. Likewise as sails get older and bag out.. I sure notice the difference between my new suit of sails (year old) and my "reserve" set of rags. And then there's sail twist, let's not go there except to say that once again, you want to match curves between both sails.

Anyway, bottom line is that you need to get out and trial your boat. Just like any other tool in racing, you need to put together a plan of what you want to do (Height? Speed? Power?) and then get out and sail, test, measure. Do take notes, make marks on your control lines, put tape markers on your spreaders, etc... you should know the drill by now. What works on my boat probably doesn't work at all on yours and there's only one way to find out. Good news is that GPS tracking is not forbidden for trialing, so that should make life a bit easier and you can do a lot of it without a partner boat. But you're in Fleet 38, right? And they are fairly active, maybe you can get a trialing partner, especially if you find someone who wants to geek out on inhaulers (who wouldn't?!?).

OK, so I basically just told you to throw the sailmaker's tuning guides off your transom! Hey, they are a good starting point, right off the bat you can call your sailmaker and ask them about your jib profile and sheeting angles, and maybe get some intelligent answers. This is what makes our sport fascinating - continual learning!

Fair winds,

Tom

[GreenLake]

Anybody who's made it this far in the discussion: here is an article that describes the theory of the interaction between jib and main (traditionally, it not very correctly, called the slot effect). The website it's posted on calls it "Another look of the Slot Effect" even though the actual title is a more forbidding "The Application of Computational Fluid Dynamics to Sails". Well, we are not interested in learning how to do the calculations, but despite the title, the paper gives easy to understand breakdown of how air actually flows around the two sails when they are used in conjunction and the conclusions we could draw from that. They are part of the work of Arvel Gentry who, in his time, did a lot of work to dispel popular myths on the subject and replaced them by sound aerodynamics.

The section called "The Generation of Lift" is very clear and easy to follow and avoids any formulas. The main departure from uncritically regurgitated popular understanding is that the "slot" between the sails is not a Venturi nozzle: instead of increased airflow as you might get from naively applying Bernoulli's principle, you have a reduced flow in that area. The surprising reason is that the flow around each sail is combined of two components: a somewhat linear flow past the sail and a counter-circulation flow that goes upwind on the upwind side of each sail.

For two sails with a slot, the two circulations meet head-on in the slot, therefore reducing the effective air speed, instead of increasing it. With two sails in close proximity, some of the circulation around the main will go forward and all around the jib instead of through the slot; the same for the jib. This is the way the presence of the jib will influence the main and vice versa.

His discussion starts out by establishing that even an angled flat plate creates lift despite its apparent symmetry: due to viscous effects the flow remains attached along leeward side. This asymmetry id what mathematically corresponds to that circular counter-flow, and produces lift (which a fully symmetric flow would not). While Newton guarantees us that a lift force must correspond to an acceleration of air masses downward (think prop wash from a helicopter), the simple conservation of momentum argument doesn't tell us how this is done in detail and thus doesn't tell us how to influence it by trimming our sails, let alone how to maximize the lift.

Those details you find in Arvel Gentry's paper (and the other ones on the site). If you're not an engineer or don't remember any math or science, you may still get a lot of useful information out of them by skipping to the part where he describes things in general terms.

[tomodda]

GL:

Thank you for bringing up Mr Gentry Yes indeed, Bernoulli Principle might not apply to the slot, it's all about overall flow. Nevertheless, I think my point still stands - jib angle sheeting is very dynamic, not at all a static "set it and forget it", especially in one-design racing where every little advantage matters. And the only way to understand it (for racing purposes) is to go out and trial.

For what it's worth, I brought up Avrel Gentry over a couple of beers with an Aeronautical Engineering PhD candidate here at our local Uni. He certainly knew of his body of work but essentially told me that it's two different ways of looking at the same phenomena. "OF COURSE there is circular flow around the whole aerofoil, where do you think the high-pressure air goes?"..... words to that effect. All I know is that the "traditional" Bernoulli-effect way of thinking about sail aerodynamics is probably incorrect, but is just fine as a mental "working model." Where Avrel Gentry really gets me confused is when he writes about the airflow being already affected (disturbed) AHEAD of the airfoil, in the space it is about to enter.

Well, getting way off track here! Mr. Bandit, I hope you get the time/inclination to try out working your inhauler controls. On my own boat, I got so barmy about it that I rigged up a single, central inhaul control line that splits out into "Y", one side to each jib sheet. That way both sides are equal (why would I ever need one side at a different sheeting angle from the other?), less lines in the cockpit, and simplification. My crew has to tend the jib sheets, the one inhauler control, and the jib car, that's it. Besides fetching beer and HIKING, of course .

Best,

Tom

[GreenLake]

Tom, having entertained serious thread drift, we might as well continue

Yes, your central point is totally on point: the wind is variable and sail trim needs to be responsive to conditions; and that seemingly minor adjustments make a huge difference. Adjust the main, the Barber hauler or what have you by an inch and suddenly the boat "comes alive". We've all seen that.

I'd also like to throw in here that some of the settings can affect how much of a "groove" you have, in other words how narrowly you must steer to the wind to keep the effective angle of attack within the limits. There seems to be no single answer to that, even among top sailors some sail with a much narrower groove than others. (I remember reading a discussion of that somewhere). But if you set your groove so narrow that you can't reliable stay within it then any advantage of that narrower setting would be lost to you.

How about those models?

Bernoulli is often discussed in the context of a pipe with a narrow section, where the flow speeds up. The key point for the discussion of sail trim is that the gap between jib and main does notwork that way. Doesn't mean that Bernoulli's principles are suddenly no longer applicable: quite the contrary. The driving force of a wing (or sails) is due to the pressure difference between the lower pressure on its upper (leeward) side and higher pressure on its lower (windward) side, and the relation between speed and pressure follows the standard principles.

It's these pressure differences that can affect the airflow in front of a sail, which seems like magic because we can't see or feel it directly. But imagine the shape of a T. If the crossbar was a plate moving downward in the direction of the stem, then, in the absence of any pressure differences, the air would flow along each side of the stem, and then abruptly turn sideways to get around the edges of the plate. However, in real life, there would most definitely be some pressure building up: think of the air experiencing a "traffic jam" in front of the obstacle. That pressure will cause some air molecules to start deviating early. Just like in traffic, if space is available on other lanes, cars would decide early to get out of the way of the obstacle.

In case of the sails, you don't have a 'square on' case but you have a similar line along which the airflow separates. With combination of pressure and counterflow, that line, which would hit the windward side of the sail a bit aft of the luff it it was straight, bends forward a bit as most of the air aft of the luff streams down the windward side of the sail, but some manages to be pushed around the luff to the leeward side. It's a bit as if the apparent wind got lifted right before it hits the sail.

If you can change the pressure distribution (or the counter flow) by adding a second sail, this deviation can become stronger, and the apparent lift becomes more pronounced. As a result you can point higher.

You can often construct a number of equivalent descriptions of some complex phenomenon (pressure distribution, counter flow, etc.). The question is not whether any is different (they shouldn't if they are truly equivalent) but whether your particular model makes it easy to understand some aspect of the phenomenon you are most interested in. In our case (sailing) we are most interested in finding out how to maximize lift (while minimizing draft), with our two main controls: camber and angle of attack. Plus the relative position of jib and main for a sloop.

The circular counterflow model arises out of laminar flow attachment on the leeward side of the sail. It is this attachment that we influence by our sail controls (including relative position) and which we can make visible with tell tales. It is thus a useful model for our purpose. And therefore worth reading and understanding in some detail (without trying any calculations ourselves ).

That said, you could probably learn how to sail effectively by just practicing standard responses to various conditions without deeper understanding. However, most people do better if they understand (at least qualitatively) why some change has the desired effect.

How you respond to gusts and wind shifts, and how you recover speed after a tack might probably have a larger impact than some of the minutiae of sail trim, but that's a discussion for another day.

[tomodda]

GL:

The thread drift is Real!

Thank you for a clear explanation of Avrel Gentry's model.. and how the wind can shift in advance of the sail. Makes sense. I had never thought of my foresail as a method for increasing apparent lift, as you described. Maybe intuitively, but never as clearly as you and Mr. Gentry are laying out - Brilliant! Will have to give a think and experiment some more out on the water.

In the meantime, I fully agree with you - for one-design racing, acceleration out of a tack, and general tactical "feel" with headers and lifts is definitely more important than just straight-line speed. Like in Formula 1 racing, which they say is won in the corners.

Tom

[GreenLake]

Tom, if you read Bethwaite, he claims that gust response that doesn't reflexively use the tiller alone is (reliably/noticeably) faster. His suggestion appears to be that angle of attack (apparent wind direction) and control of heeling are, respectively the task of tiller and sheet and that the fastest way to sail to windward in (stronger gusts) requires you to use both.

This works well if you have your sheet on a ratchet block, so you can let it out reflexively in a gust to arrest the roll to windward and then sheet it in continuously to bring the mast vertical again. One key consideration is that at some point you may be at the hiking limit (fully extended), so hiking to control the roll isn't going to make a change. My translation is that "ease-hike-trim" below the design wind speed turns into "ease-trim" with a bigger ease.

Instead of luffing up to help roll control (reduce heeling), he suggests limiting tiller movement to that required to keep the tell-tales flying. That is, just account for the shift in wind direction (usually towards the aft) as the stronger true wind contributes more to the apparent wind.

The reason he gives for the slowness of luffing up for heel control is increased drag from the sail plan; I haven't looked at the lift/drag curves enough to be able to wrap my head around that aspect. Interestingly, he claims that as long as you are below design speed (and can still hike out further) keeping the sheet fixed isn't that detrimental to speed.

Now for broad reach, I know that sailing "down" in the gust, and luffing up in the lull is advantageous, during the lulls, sailing a bit higher increases the apparent wind speed, while during the gust, you may have enough extra that some reduction in apparent wind can be tolerated. That seems to work well in lighter airs. I don't have that much occasion to sail the DS downwind in strong enough winds that I need to worry about rolling the boat, so light wind technique it is.