Re: Sailing, sailing....


"Dave Typinski" <nospam@xxxxxxxxxx> wrote in message
news:ldj7h495u8lf0j6ikpjfabkfqs8vbirbss@xxxxxxxxxx
Posit: still water, boat motionless wrt water. Reference frame is the
still water.

Te boat's air turbine creates a change in momentum within a column of
air impacting the turbine. In so doing, it absorbs energy from the
air in relation to the change in the air's momentum.

If the boat is static, then to conserve momentum the screw has to
impart the same momentum, oppositely directed, to a column of water.
Assume the column of water has the same mass flow rate as the air
column's mass flow rate. It doesn't have to, but why make things
complicated.

Since water is 830 times denser than air, the velocity of the mass
flow of water required to produce the same amount of momentum is lower
than the air's velocity, all else being equal.

Since energy goes as v^2 while momentum goes as v, that lower velocity
means the water mass flow contains less energy than the energy
absorbed form the column of air. Actually, when dealing with columns
of mass flow and densities, energy is proportional to v^3 and momentum
is proportional to v^2, but the point remains. Namely that there's
energy available in excess of that required to keep the boat
stationary wrt the main body of water.

That energy can be used to drive the hull forward by pushing even more
water aft.

This works for a boat in water driven by air. You cannot drive an
airplane this way because the densities of the two mass flows are
identical.
--
Dave

Apparently you can make a windmill boat work without a water prop.

Energy available is proportional to the lift produced by the turbine blades.
Energy consumed is proportional to the drag produced by the turbine blades

Lift >> drag

If the boat/turbine was highly efficient drag approaches zero so no problem.








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