Confessional: admit to boat-related things you've never quite understood...

[maxcampbell]

Uffa said

Uffa fox said "My belief....is that sails are deflectors: that they turn the wind backward towards the stern and that the reaction pushes the vessel forward. If, as some theorists maintain, they have lift on their lee side, corresponding to the upper-surface of an aeroplane's wing, then this force is taking the boat to leeward; so the only time it acts ahead is when the sails are squared right across the centreline of your vessel."

 

[JumbleDuck]

Uffa fox said "My belief....is that sails are deflectors: that they turn the wind backward towards the stern and that the reaction pushes the vessel forward. If, as some theorists maintain, they have lift on their lee side, corresponding to the upper-surface of an aeroplane's wing, then this force is taking the boat to leeward; so the only time it acts ahead is when the sails are squared right across the centreline of your vessel."

He was right about the first bit but not about the second. One component of the lift is pushing the vessel to leeward but another component is pushing it forwards.

Basically the "wind deflection" theory is fine, as long as you realise that the front surface of the sail deflects the wind by pulling on it rather more than the back surface deflects the wind by pushing it.

 

[onesea]

Basically the "wind deflection" theory is fine, as long as you realise that the front surface of the sail deflects the wind by pulling on it rather more than the back surface deflects the wind by pushing it.

I figure boats are "sucked" along by the slight vacume caused in front of the sail, where the wind bends around it.

The pressure pushing must help a bit but keep that vacuum working for you does allot more. Hence "tacking down wind being quicker you maintain the follow across the sail, instead of relying on the windage of the sails....

 

[Hydrozoan]

You see a lot of nonsense around about "Bernouill doesn't explain lift". These claims invariably come from people who either don't understand Bernouilli's theorem or who misapply it, usually by neglecting momentum changes.

So can you point us to a good up-to-date article, for the non-physicist or non-mathematician, on the generation of lift by a sail? A modern equivalent of "A review of modern sail theory" by Arvel Gentry http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf), perhaps?

 

[JumbleDuck]

So can you point us to a good up-to-date article, for the non-physicist or non-mathematician, on the generation of lift by a sail? A modern equivalent of "A review of modern sail theory" by Arvel Gentry http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail%20Theory.pdf), perhaps?

I keep meaning to write it ...

 

[Hydrozoan]

I keep meaning to write it ...

Too busy paving the road to Hades, eh! But seriously, perhaps someone will come along and offer us something.

The Gentry article (of 1981) is readable by the lay person, and he was a research supervisor in the aerodynamics research department at Boeing. This one (http://twodresslers.com/Fun/FunDocuments/APSSailing.pdf) in Physics Today (2008) is by an experimental nuclear physicist (and sailor), but he seems to me to use the shorter/longer path argument which I thought was no longer accepted.

Is there a professional aerodynamicist in the house?

 

[AntarcticPilot]

Is there a professional aerodynamicist in the house?

Surely the problem is that the flow regime sails work in is very different from those an aircraft wing operates in? After all, flow velocities round a sail will be tens of MPH at most. If it gets to 100 MPH, must of us would be under bare poles! Even low performance aircraft are operating at higher flow-rates than that, and that's what your average aerodynamicist will be used to.

FWIW, my understanding was that a rig with two sails will get closer to the wind because the slot effect between the sails accelerates the air-flow across the back of the after sail., thus increasing the lift experienced by the after sail. So, I tend to follow the "lift" way of thinking of how sails operate. I also suspect that flow separation at the leading edge is important in defining the different path-length - I suspect the flow on the windward side of the sail is detached from the sail, and that on the leeward side is attached. But I'm NOT an aerodynamicist!

 

[Hydrozoan]

Surely the problem is that the flow regime sails work in is very different from those an aircraft wing operates in? ... I tend to follow the "lift" way of thinking of how sails operate.

Well, yes to both of those - but what I was getting at (and thought the OP was, too) is exactly how the lift is generated by a sail (which is different as you say from a wing in section). It seemed to me that Gentry provided a physical explanation, different from the simple explanations I had read (which he rejects, with explanations) but I wondered if what he says is still accepted - albeit put in layman's terms. He also rejects a simple explanation of the slot effect.

Worth a read (or in my case several, ongoing ) when you have some time. And there's even an explanatory demonstration one can try with a bath and some talcum powder!

PS Although Gentry worked at Boeing, he was a successful sailing boat racer who assisted with research on America's Cup boats and was engaged in various other sailing projects - see biography at bottom of article linked.

 

[alant]

Surely the problem is that the flow regime sails work in is very different from those an aircraft wing operates in? After all, flow velocities round a sail will be tens of MPH at most. If it gets to 100 MPH, must of us would be under bare poles! Even low performance aircraft are operating at higher flow-rates than that, and that's what your average aerodynamicist will be used to.

FWIW, my understanding was that a rig with two sails will get closer to the wind because the slot effect between the sails accelerates the air-flow across the back of the after sail., thus increasing the lift experienced by the after sail. So, I tend to follow the "lift" way of thinking of how sails operate. I also suspect that flow separation at the leading edge is important in defining the different path-length - I suspect the flow on the windward side of the sail is detached from the sail, and that on the leeward side is attached. But I'm NOT an aerodynamicist!

I'm sure, I've seen a paper which states that the slot effect is a myth.

PS
Uno/windsurfer rigs, seem to manage quite well, without any slot, myth or otherwise.

 

[AntarcticPilot]

I'm sure, I've seen a paper which states that the slot effect is a myth.

PS
Uno/windsurfer rigs, seem to manage quite well, without any slot, myth or otherwise.

The slot effect is sufficiently real for commercial aircraft to rely on it when taking off and landing. Next time you're sitting in a seat next to the wing of an aircraft, watch all that goes on when it takes off and lands! Basically, the slot effect allows the wing to generate higher lift at low speeds than it would otherwise.

 

[TonyBuckley]

New topic: My boat has a spinnaker pole and sail (no idea if asymmetric) and lots of lines that I have never touched.

What is the best way of learning how to use such sails. I realise that setups and rigs differ so not sure reading the web will help.

My confessional is that I have never sailed with such a sail.

 

[awol]

New topic: My boat has a spinnaker pole and sail (no idea if asymmetric) and lots of lines that I have never touched.

What is the best way of learning how to use such sails. I realise that setups and rigs differ so not sure reading the web will help.

My confessional is that I have never sailed with such a sail.

Join a yacht club!

 

[AntarcticPilot]

Join a yacht club!

Or get your brother who is fanatical about such things to come and show you how.

 

[dancrane]

Join a yacht club!

Now, gentlemen! Surely it's tacit that the forum isn't anyone's sole source of information. The nice thing about this confessional is that no-one need face their absolvers/instructors.

I've got a nice blue spinnaker, and a nice anodised spinnaker pole, and lots of nice blocks and excellent spliced 10mm-into-6mm sheets...all the gear, in fact...

...but like Mr Buckley above, I've no idea how to use it. In fact rather than try it out, I began looking at whether an asymmetric would be easier to handle...

...and I'm not yet persuaded that an asymmetric isn't a good idea, but I'd like a pointer towards what in hell all the string and sticks on a conventional spinnaker actually do.

 

[JumbleDuck]

Too busy paving the road to Hades, eh! But seriously, perhaps someone will come along and offer us something. ...
Is there a professional aerodynamicist in the house?

I taught fluid dynamics at university for 20+ years. Not sure if that counts, but I really would like to write something on it one day.

Surely the problem is that the flow regime sails work in is very different from those an aircraft wing operates in? After all, flow velocities round a sail will be tens of MPH at most.

Fluid dynamics makes a lot of use of dimensionless coefficients to compare flow patterns. The most useful and best known one of these is the Reynolds number, Re, which is velocity x characteristic length / viscosity. Similar Reynolds numbers mean similar flow patterns, so 10m/s flow past a yacht sail with a chord of 5m will be comparable with 25m/s flow past a glider wing with a chord of 2m. Slow speed aerodynamics is definitely of interest, and well studied.

 

[JumbleDuck]

New topic: My boat has a spinnaker pole and sail (no idea if asymmetric) and lots of lines that I have never touched.

What is the best way of learning how to use such sails. I realise that setups and rigs differ so not sure reading the web will help.

My confessional is that I have never sailed with such a sail.

I went to a YM demo at a boat show on "easy spinnakers for the beginner" or some such. By the time they had added the twenty seventh control rope (or so it seemed to my jaded eye) my determination never, ever to have one of the accursed things on board was well reinforced.

 

[alant]

The slot effect is sufficiently real for commercial aircraft to rely on it when taking off and landing. Next time you're sitting in a seat next to the wing of an aircraft, watch all that goes on when it takes off and lands! Basically, the slot effect allows the wing to generate higher lift at low speeds than it would otherwise.

You still flying Tiger Moths?

 

[TonyBuckley]

Join a yacht club!

Thank you for this. I have never found a yacht club in the Solent either a) worth joining or b) open to me joining. As for my brother, he does not exist unless you know different Dad.

I suspect that effectively using a spinnaker or an asymmetric is the elephant in the forward cabin for many sailors. I respect those I see totally screwing it up (many and often) as at least they are trying.

I do have a yachtmaster ocean jedi friend who has offered to help but I would like to get ahead of that curve.

 

[prv]

You still flying Tiger Moths?

You've not noticed the dirty great holes that open up in the wing of an airliner near the ground?

Pete

 

[AntarcticPilot]

You still flying Tiger Moths?

No, Boeing's and Airbusses. They all use flaps and leading edge slots.