Difference between revisions of "PropellerEfficiency"
(New page: ==Why we need big propellers at slow speeds== The push you get from a propeller is proportional to the change in momentum the propeller gives some water which is: mass * deltaV for the ...) |
(→Why we need big propellers at slow speeds) |
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==Why we need big propellers at slow speeds== | ==Why we need big propellers at slow speeds== | ||
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mass * deltaV | mass * deltaV | ||
| − | + | where deltaV is the change in velocity of the water. The energy put into that water by the propeller is: | |
1/2 mass * deltaV^2 | 1/2 mass * deltaV^2 | ||
If a boat is going fast a propeller passes through lots of water and so can push a little bit on many different cubic feet of water and so only change the velocity of any given bit of water by a small amount. | If a boat is going fast a propeller passes through lots of water and so can push a little bit on many different cubic feet of water and so only change the velocity of any given bit of water by a small amount. | ||
| − | A propeller that | + | A propeller that makes efficient use of 50 Hp at 15 MPH makes very inefficient use of the same power at 2 MPH. At slow speeds it goes through less water and gives it a much higher deltaV. |
An efficient propeller for a slow seastead that will be large so that it can give a small deltaV to a lot of water. Using a small fast propeller on a big slow seastead would take far more energy for the same push. | An efficient propeller for a slow seastead that will be large so that it can give a small deltaV to a lot of water. Using a small fast propeller on a big slow seastead would take far more energy for the same push. | ||
Revision as of 01:59, 29 November 2008
Why we need big propellers at slow speeds
The push you get from a propeller is proportional to the change in momentum the propeller gives some water which is: mass * deltaV
where deltaV is the change in velocity of the water. The energy put into that water by the propeller is: 1/2 mass * deltaV^2
If a boat is going fast a propeller passes through lots of water and so can push a little bit on many different cubic feet of water and so only change the velocity of any given bit of water by a small amount.
A propeller that makes efficient use of 50 Hp at 15 MPH makes very inefficient use of the same power at 2 MPH. At slow speeds it goes through less water and gives it a much higher deltaV.
An efficient propeller for a slow seastead that will be large so that it can give a small deltaV to a lot of water. Using a small fast propeller on a big slow seastead would take far more energy for the same push.
