Sailyboat question

gravygraham

gravygraham

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There's an article in mbm with a pic of a funny racing sailyboat thingy which apparently made fifty something knots (before crashing) in twenty something knots of wind. It's true I got an E for my physics o-level but I'm buggered if I can work out how a sailboat can exceed wind speed.
Any ideas peeps? (I know this is the wrong forum for this type of question but I'm hoping for an answer which doesn't include all the techie things such as reefing furls and mizzen masts which our wind assisted associates warble on about).

Graham
 
The wind behind it just pushing it along, its at an angle to the sail, which becomes an aerofoil creating lift like an airplane wing on its end.

Thats slightly simplified but the key point is the wind is not blowing it along as such, thus it is nto limited to only going as fast as the wind. Even so its still an amazxing feat!

Ants
 
A sailboat can't exceed true wind speed downwind but can on a reach, ie when the wind is abeam.

This is because of the apparent wind being greater than the true wind.

On a still day, when you motor along, you feel a breeze equal to the speed of your boat. In the same way a sailboat can "make" its own wind.

If a sailboat has 5 knots of true wind directly abeam, and is travelling at 6 knots forwards, the apparent wind will be 7.8 knots at 40 something degrees from the bow.

Clearly here the boat is going faster than the wind, but is still slower than the apparent wind.

Now what I don't know is if the aerofoil effect already mentioned by Anthony means you can go faster than the apparent wind - someone will though (is that what you meant Anthony)......

Downwind is slow because obviously as soon as you move, you are reducing the apparent wind and you have no aerofoil just a blocking sail.
 
Salining boats like these (mainly multihulls) have fully battened sails. That makes them really stiff rigs and as such like Anthony says like an aerofoil.

Now we all know how the wing of an aeorplane works - essentially it because the air flowing across the top of the wing (aerofoil) has to travel further than the air underneath. This creates a pressure difference between the air each side of the aerofoil thus creating a force laterally to the aerofoil.

Now apply this to a big vertical (fully battened) sail and to can create a force that can easily propell the boat faster than the wind.

I come from racing cats where we tack on the downwind legs - it is the fastest way downwind. Here's the reason:-

If you sail one of these hi tech craft straight down wind (running before the wind) - say between two fixed points (one directly downwind from the other) you can only travel at the speed of the wind. Now if you travel across the wind you can travel quite a lot faster than the wind - in doing so, the "apparent" direction of the wind changes. That is the wind direction relative to the boat changes - because you are in fact travelling throught the wind as well. The faster you go the more you can point the boat further downwind whilst still having the "apparent" wind coming across the beam. Using this technique, you can arrive at the second of the two fixed points much sooner than if you simply sailed the boat directly. So the quickest distance between two points isnt necessarily the shortest.

In cat racing, you move the crews weight to the leeward side of the boat and actually tighten the rig so as to lift the windward hull. This reduces the wetted surface (by 50%) and you go even faster - then by controlling the angle you can sort of "walk" the boat downwind - in cats we call this "doing the wild thing".

This is a real test of ability.

Now for some sad news

Cat racing has been removed from the Olympics by the bafoons at the RYA/ISAF

WHAT PLONKERS.

They simply dont understand the sport that they support.
 
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Now we all know how the wing of an aeorplane works - essentially it because the air flowing across the top of the wing (aerofoil) has to travel further than the air underneath. This creates a pressure difference between the air each side of the aerofoil thus creating a force laterally to the aerofoil.

[/ QUOTE ]

Thread drifting slightly, but that's one of the strangest myths put out I think by schools at O Level, and also i believe at basic flying schools (cos, I've overheard pilots sitting in the passenger section on planes explaining it to fellow passengers). It's really strange how this misinformation perpetuates.

Even if you take as true (which it isn't) the theory that the air goes further and hence faster across the top of the wing because of the shape, doing the the fluid dynamics maths (Bernoulli) will show you that the lift so generated for a 747 is a few tonnes or something. (Air at 35000feet being pretty thin stuff). I forget the number - something like 10% of the required lift, iirc.

Truth is, an aeroplane flies becuase the engines ram the wings forward at an angle of attack to the apparent wind, just like a waterski or the planing hull on a boat. And just like the propeller blades on a boat. The lift force is the Newtonian reaction to the downward push that the wing applies to the air as it is rammed forwards tilted like a waterski. That's the only way to get 400tonnes of lift to make a 747 fly. Newton explains flight, not Bernoulli who merely makes an incidental contribution. It's quite astonishing that flying schools teach such bollox. (And note, flying schools don't teach how to apply Bernoulli, that's way too hard, so they never get rumbled when teaching this bollox)

So in summary an aeroplane waterskis through the sky, and there's a small contribution from the aerofoil shape which means it can flatten its waterski a little so making for an easier ride.
 
That's a bit 180degrees though Elessar. Apparent wind is the consequence of the boat's motion, not the cause of it

The reason a boat can sail faster than the wind is that there is no reason why it shouldn't. It would be a mere coincidence if it could only match the wind speed.

The sails are aerofoils (except when being blown pure downwind) and generate power. Suppose a sailboat needs 20 hp to make it do 30knots (that's a function of its hull drag). It is entirely feasible for a big sail operating as an aerofoil (ie not sailing downwind) to generate 20hp in say 25 knots of wind, hence the boat will sail 30kts in 25kts of wind, simply because it is being pushed by a motor that is making 20hp. Sure, you have to trim the sails to allow for the now 30kts of headwind, and the apparent wind angle will move a long way forwards, but if at that apparent wind angle the sails make 20hp then the boat will as I say do 30kts and the fact the wind is blowing at 25kts isn't a limiting factor.

So the speed of a sailboat is simply a function of how much horsepower the sails make compared with the hull drag. The horsepower from a sail is a function of wind speed but also of sail shape and area, and there is absolutely no reason why the sails on a boat will always produce no more horsepower than the power needed to push the boat at (coincidentally) the speed of the wind. Keep adding sail and you will get more horsepower (till you capsize of course, but that's another matter).
 
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Thread drifting slightly, but that's one of the strangest myths put out I think by schools at O Level, and also i believe at basic flying schools (cos, I've overheard pilots sitting in the passenger section on planes explaining it to fellow passengers). It's really strange how this misinformation perpetuates.

Even if you take as true (which it isn't) the theory that the air goes further and hence faster across the top of the wing because of the shape, doing the the fluid dynamics maths (Bernoulli) will show you that the lift so generated for a 747 is a few tonnes or something. (Air at 35000feet being pretty thin stuff). I forget the number - something like 10% of the required lift, iirc.


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And a light aircraft sees much more proportional impact from the lift itself not the AoA. To be honest, your explanation is just as wrong. You can't discuss without talking about induced drag either.
 
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So the speed of a sailboat is simply a function of how much horsepower the sails make compared with the hull drag. The horsepower from a sail is a function of wind speed but also of sail shape and area, and there is absolutely no reason why the sails on a boat will always produce no more horsepower than the power needed to push the boat at (coincidentally) the speed of the wind. Keep adding sail and you will get more horsepower (till you capsize of course, but that's another matter).

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A little hole in your theory here.
Yep - I agree with your horsepower theory but how could a boat actually go faster than the wind without the aerofoil effect? - because it can. If a sail just "pushes" the boat downwind it simply cannot go faster than the wind. Its only across the wind that a sailing boat can go faster than the wind. A cat will use this effect and the apparent wind to actually go down wind faster than the wind - believe me - its true. I'm not talking about the old fasioned sailing boat - I'm talking about hi tech boats with modern asymetric rigs.

In fact the aerofoil effect is so pronounced that jibs/spinnaker are introduced which "channel" even more airflow across the leading edge of the mainsail and increases the speed further.

When racing or sailing any sailing boat its the wind behind the sail (on the sail's leading edge) thats doing the work - without an airflow across the sail you will be at the back of the fleet - believe me - I know.

And here's a YouTube clip of a British Tornado with a spinnaker going downwind - a good example of the "wild thing" - the windward hull stays out of the water all the time. You can see that not only does the boat speed up in the gusts but you can see the apparent wind enables it to change its direction and sail deeper downwind - this boat is sailing faster than the wind.

http://www.youtube.com/watch?v=PDvChzqU5T4

And then just because I didnt want to leave out my old class - a clip of Hurricanes sailing at Stone.

http://www.youtube.com/watch?v=UeqbpaQ4OXM
 
Confused here. I agree with what you wrote 100%. I dont see that anyhting you write exposes a hole in what I said. We agree, so far as I can tell :-)
 
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And a light aircraft sees much more proportional impact from the lift itself not the AoA. To be honest, your explanation is just as wrong. You can't discuss without talking about induced drag either.

[/ QUOTE ]

Eh? Yes, sure, a LA sees relatively more bernoulli than Newton, I never said otherwise. But there are no planes flying that fly just on Bernoulli, and the vast majority of planes on the planet are sub 25% Bernoulli. Newton is the main reason for lift, Bernoulli is incidental and the aviation world would work fine if it didn't exist. Note, I said planes mostly fly on Newton with a bit of help from Bernoulli; the teaching that I'm saying is wrong is the theory that planes fly 100% Bernoulli with no mention at all in the classroom of AoA/Newton. So I dont accept therefore that I was "just as wrong".

Incidentally an acrobatic LA pilot doesn't much appreciate Bernoulli when he's flying upside down, and is very grateful that Newton keeps him up instead of plunging to the earth at 2g. The classroom teaching I'm criticising conveniently doesn't explain how a plane can fly either way up. :-)

On the induced drag thing, look, I wasn't explaining how you design a plane in full. I was just explaining the lift thing in the most basic (waterski) terms, no more. Of course induced drag etc is relevant to plane and wing design, but it's another aspect and I was just making a fast point on an internet forum about what makes the upward force, no more, fer chrissakes. Gimme a break.

Hey, why dont you explain where the lift comes from in the same word count I used, and you're not allowed to steal my waterski illustration, ok? :-)

By the way folks, although lift is wrongly taught in school physics and (remarkably) in commercial flight schools, that's as far as it goes. Rest assured it is taught and understood correctly in universities and that the engineers (not the pilots, pah!) at Airbus and Boeing know that Newton/waterskiing makes planes fly
 
heehee. Swmbo had to take another pilot along once on trip to SoF who advised that there was actually no point of flying any higher than that needed to avoid hitting hills/mountains and obeying regulations. Er so why do they fly at 30,000 feet eh? Well, it's to be above the clouds, see...
 
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When racing or sailing any sailing boat its the wind behind the sail (on the sail's leading edge) thats doing the work - without an airflow across the sail you will be at the back of the fleet - believe me - I know.


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Yes don't we all Mike, the faster the boat, the more obvious the lack of ability on new crews too, it's quite easy to get lapped when cat sailing as a beginner, even if you were previously a very experienced dinghy sailor.

Btw. You don't need an asymetrical rig just a good idea on how to trim it and a boat with minimal drag/weight. pretty much any cat, well shaped board or most fast dinghies.
 
Wrong question, I'm afraid.
It's still precisely the same jfm already explained, though the Bernoulli/Newton proportion is possibly even higher than in a light aircraft.
In fact, also a glider can't "stay up", unless it finds ascentional winds which can at least match, or possibly exceed, its planing coefficient.
Even a 747 would stay up with no engines in such conditions, but of course ascentional winds of such magnitude are pretty rare.
 
[ QUOTE ]
Hey, why dont you explain where the lift comes from in the same word count I used, and you're not allowed to steal my waterski illustration, ok? :-)

[/ QUOTE ]ROTFL, nice one!
And while he's at that, JECuk might try to explain what's keeping me from falling in my avatar.... /forums/images/graemlins/grin.gif
 
[ QUOTE ]
Thread drifting slightly, but that's one of the strangest myths put out I think by schools at O Level, and also i believe at basic flying schools (cos, I've overheard pilots sitting in the passenger section on planes explaining it to fellow passengers). It's really strange how this misinformation perpetuates.

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In physics, I seem to remember asking "So, how does an aerobatic plane fly upside down, then".

I also asked "Yes, but what causes gravitational force".

In both cases I was told to sit down, be quiet, and given extra homework: I didn't make that mistake again!

dv.
 
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