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Propeller walk
"Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? 1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi. http://www.youtube.com/watch?v=u5KKcBQen-Y |
#2
posted to alt.sailing.asa
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Propeller walk
"Charles Momsen" wrote in message ... "Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? Don't think so. Drag increases to a greater extent than lift. If this weren't the case your average airplane or boat propeller would never cavitate. 1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi. http://www.youtube.com/watch?v=u5KKcBQen-Y Excellent! That is one static psi difference in a foot. Take a three-bladed prop with a surface area of about 600 square inches and you're dragging the blades of the damned thing through 600 psi on the bottom half? Correct? What kind of pressure do you have on the top half? 500PSI? Makes your mere ..98 psi seem suddenly more significant, doesn't it? Wilbur Hubbard So you drag the top of the prop through |
#3
posted to alt.sailing.asa
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Propeller walk
"Charles Momsen" wrote in
: "Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin...ity.html&edu=h igh Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? Yes, you are absolutely correct. blondie |
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