Difference between revisions of "PropellerEfficiency"

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(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
  
for the mass of water pushed out the back at some velocity. The energy put into that water by the propeller is:
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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 is very efficient at 15 MPH is very inefficient at 2 MPH.  At slow speeds it goes through less water and gives it a much higher deltaV.   
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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.