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#11
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I'm not convinced that birds don't stall. They're experts at stalls, spins and recovery. (Ospreys, kestrels...) Last night I was reading a commercial rating textbook and one section addressed the question of whether, like humans, birds ever forget to lower their landing gear. Below the paragraph is a series of three photos showing a seagull landing feet up, sliding his ass across the ground. It would appear he forgot to do his GUMPS check. -c |
#12
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On Mon, 8 Nov 2004 at 19:42:55 in message
, Morgans wrote: "David CL Francis" wrote The boat can move because there is a relative velocity between the two fluids. It is obvious you can sail if there is no wind at all provided you are in a tidal flow. ******************** Not really. Relative velocity between the wind and water does not enter in, much. In the case of a tidal flow you cite, it would have to be one hell of a tidal flow; enough to make apparent wind, and a LOT, at that. I've done it, although I have sailed where the tide can reach 9 knots and 5 knots is quite common. The whole driving force for a sail boat _is_ the difference in velocity between the two fluids. It must be. In an 8 knot tide and an 8 knot wind in the same direction you have no option but to start the engine (or in my case I used to get out the paddle). With very slow wind speeds, there is not enough wind to keep the sail in in an airfoil shape, then you go nowhere, other than the speed of the water. Most flows do not reach a minimum speed. Again I have made headway when you could scarcely detect any wind and you lean the boat way over to make the sails nearer the right shape to catch the tiny winds. In my boat it also tended to reduce the wetted area and lengthen the waterline (I used to think!). Every one knows a sailing boat can tack up wind but can it tack down wind? The answer is not much, mainly I believe because of the much higher drag of the water. Sand yachts and ice yachts can, and they can sail on a reach, downwind faster than the wind is blowing. More problems. Sailboats *can* tack down the wind. They often do, if they do not have a spinnaker. (Large parachute shaped sail, added while going down wind) Problem is, you have to go a lot faster to justify the extra distance. Usually, you just go down wind slower, and don't worry about it. I thought I said that - my words were 'not much'? My boat used to have a spinnaker thank you. It came out of a chute on the foredeck. The drag of the water *is* an issue, but not as you state. The boat, until you ride on top of the water, can go only so fast, almost irrespective of how much extra power you add. It is called hull speed, and generally speaking, it is higher, the longer the boat is. Can we call it drag please? Drag rise at hull shape speeds is not totally unrelated to approaching the speed of sound. I know the speed of sound in water is very high but the natural speed of waves is not do so high. That's why bores occur - they are a sort of shock wave travelling down the surface. When you are going into the wind, the hull speed is not tough to get to, if the wind is strong enough. The maximum angle needs to be about 15 degrees or more away from straight into the wind. (again, generalities) The sail is acting like a wing, and airflow is accelerated around it to pull the boat into the wind, just like an airplane wing provides lift. The boat slips sideways, due to the wind pushing against the sail, which pushes sideways on the water, through the keel and rudder. That is what gives the drift. What is the difference between your visualisation and mine? Not many boats that I have sailed can get within 15 degrees of the true wind and there is no chance of them getting to their maximum speed while doing it. Maximum speed is on a reach when I could get up to 12-15 knots while planning. In stronger winds I could have gone faster but I was not usually brave enough. Incidentally the keel and the hull are also acting like a wing but in the water. Hence the sideways 'slip' but worded differently. What kills downwind speed, is the fact that the sail is only acting as a barn door, or flat plat, using the wind pressure, and does not have lift in addition. Those are the main factors limiting down wind speed. Because you cannot effectively tack downwind you are forced to do that. In the case of ice boats and land boats, the lower friction removes the hull speed limitation. i.e. lowers the drag The craft continues to accelerate, until a wind that *was* coming from the side or partway from behind, now with increased hull speeds, will appear to come from the front of the craft, now called apparent wind. The sail can then act like a wing again, and use the lift to continue to go faster and faster. Problem is now, you can only go around 45 degrees into the wind, without slowing way down again. I agree at last - more or less. I hope I have made this all clear enough. It made no difference. Just because you find somebody expresses things in slightly different terms from yourself please don't automatically assume you have to disagree with them. The drag law for boats is different from aircraft but it is still drag as far as I am concerned. -- David CL Francis |
#13
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"John Bell" wrote in message m...
2. We know it's possible for an aircraft to fly at angle to the direction in which it's pointing. Does anyone know why does such a thing not happen in boats or ships? After all, both ships and aircrafts use media to float on... any sailors here could answer that, I guess In addition to the issue of boats traveling through water with a current, there is actually the issue of sailboats traveling through the water at a different angle from which they are pointed due to the push on the sail. This slip angle creates an angle of attack on the centerboard, daggerboard, or keel to provide an opposing force. As a sail boat buys I take issue with the comment "push on the sail". "Pushing" on the sail is your last choice and used only when sailing directly away from the wind. However, this is the way the old 1400's sailer did it. Today we have sails that are airfoils so the air "sucks" us around. In fact you will move MUCH faster when sailing up wind than when sailing down wind. -Robert |
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