mainsail only, close-hauled

[Marshhawk]

that makes sense, that the slower air creates lift. my ds sails well to weather and tacks ok under main alone as long as i keep her footing. if i have to go to windward in waves and breeze enough that i cant carry a jib, i have screwed up. bound to happen though. that is why i want a little storm jib. does anybody else have one, or reef points in their jib?

[GreenLake]

About debunking the slot effect. I'll keep this really brief, because I think you get better mileage by reading Gentry's paper than my summary.

However, here goes. There's no disagreement that splitting the sailplan across multiple sails has an effect that's different from taking the sails in isolation. The disagreement comes on what aerodynamics result from the arrangement.

The "slot effect", which should be considered debunked as of 35+ years ago , is based on a theory that treats the area between the sails like a pinched section of a pipe (smaller diameter). Bernoulli states that fluids in such a pipe flow faster where the diameter is more narrow (also known as "venturi effect").

On a boat, the whole system contains not just the slot but the sails. If you take an airfoil, you can observe that the air sets up a circulation about it, going with the airflow on the lee side and against the airflow (slowing it) on the windward side. Split the airfoil in two, and you get two circulations. In the gap, they meet and oppose. Hence, slower air in the slot.

Which theory is correct? This is where measurements come in. When they were actually done, they showed the slower air flow, but despite this fact, the "slot effect" has taken on a life of its own.

I'll continue with a quote from one of Arvel Gentry's papers:

With accurately determined streamlines, you can see that the air passing between the two sails is quite different from the old "venturi effect" explanations many of us have grown up with. With both sails set, a large percentage of the air that was going between the headstay and the mast when the mainsail was alone now goes above and down the lee side of the jib. Less air is left to pass in the slot between the sails and this tube of air actually slows down (the streamlines spread out) as it reaches the line between the headstay and the mast.

The slot effect theory assumed that more air had to fit throught the slot, as it narrows; modern aerodynamics explains that this extra air actually goes around the outside of the jib. Bernoulli's law isn't violated, it's effect is merely to speed the air back to "normal" at the exit of the gap.

There's, of course, a bit more to the aerodynamics of sails, which Gentry all explains methodically. If you are at curious about how sails do what they do, I can only recommend that you go to the source.

[jdubes]

This weekend I did some lake sailing. One of the advantages I’ve found with lake sailing is the consistency of the wind. Constant winds for long runs. I was able to sail the boat close hauled many times, and work the Jib to help point the boat. This really impacted the tension on the tiller. It made tiller essentially float in my hand.

I have to say, I’ve been sailing a lot this summer, J24, and J80’s weeknight and weekend race series. Along with time on a 30 something foot Bristol. Learning to sail on this sailboat has been amazing. The simplicity of this boat, with its ability to be sailed alone and its un-weighted keel really makes learning to sail fun and obvious.

[dannyb9]

Is sailing close-hauled with only the mainsail much less efficient (or even possible) than with both main and jib -- or is our boat out of whack?

I should add, she sails beautifully on a reach with main only .

if you are trimming the main as you would with the jib you are probably overtrimmed and trying to sail too high (pinching) if your centerboard is all the way down extra weather helm is helping to steer the boat higher. to help balance the boat and go faster upwind with just the main i bring the board back a little and ease the main slightly. how much is a feely thing. i raced sunfish for years and it has the same tendency, easy to sail too high and stall the blades because of a lot of weather helm.

[navahoIII]

danny - Thanks, that sounds like good advice!

[navahoIII]

Greenlake,

Thanks for the explanation (I only just now read it).

I don't quite get it but I'll keep at it. I am still at the point where I (think) understand basically how an air foil works, ie - the slower moving air on the inside of the foil is denser (right?), thus creating more pressure against the sail, where the air on the outside (lee) is moving much faster, creating less pressure, so the inside (weather) air pushes the sail to the less pressure area. Pretty simplistic, I know, but is it fairly accurate?

[dannyb9]

Is that what the "slot effect" was? Is it now debunked? If not, what is the slot effect?

look at development classes where its all about speed: a class cats, (18', 166# catamaran, 'run what you brung') moth class, windsurfers, speed record holders etc... the a cat is a demon to windward. jibs are for tacking. jibs are for reducing area as a first reef. the slot effect is just spilling wind that could be more power. in my humble opinion. jibs do lower the sail plan somewhat and the forestay is already there so why not hank on some sail...but i dont see the evidence that the slot effect is more efficient than a similar sized single sail
dan in port royal

[GreenLake]

What actually happens to the airflow is quite different from simply "spilling" some air between the two sails.

If you place a mainsail behind a jib, you end up increasing the flow of air around the outside of the jib, and the air speed at the trailing edge (leech). Having more airflow on the outside (leeward) side of a sail increases the driving power of the sail. A hanked on jib has the additional advantage of not having a mast disturb the airflow.

The discussion that I've been able to find does not allow a quantitative comparison of whether or when the postitive influences on the the jib are outweighed by the effects of the jib on the main. Certainly, the combination can be pointed higher without stalling either sail.

A very tall single sail behind a specially designed mast profile might be equally effective, and, because it reaches higher into the wind, might pick up faster air in some conditions. However, the heeling forces are much greater and require wide beam (catamaran) or wings and trapezes.

Equally clearly, to windward theres a different dynamic between the two sails (blanketing of jib by main).

[phil]

the best explanation of the "slot efect" I have read is in Wallace Ross's SAIL POWER on page 24. This book has always been my bible and hasn't let me down so far.[/u][/i]