Re: Why does he have to pedal???


"R.C. Payne" <rcp27@xxxxxxxxxxxx> wrote in message
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Wilbur Hubbard wrote:
"R.C. Payne" <rcp27@xxxxxxxxxxxx> wrote in message
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Wilbur Hubbard wrote:
Watch this propeller driven tricycle in operation:
http://www.youtube.com/watch?v=UnntlyUiNFU


Now, seems I recall a discussion here about how a propeller driven
sailboat could use the wind turning its propeller to push it directly
upwind. If that's the case how come the poor bloke shown has to pedal
his tricycle. If your theory was right all he'd have to do is face the
thing into the wind and let the propeller start turning and off he would
go at ever-increasing speeds without having to pedal because as the
apparent wind speed increased so would the thrust increase and pretty
soon he'd be going 100kph.
Let's actually consider some fluid mechanics here. If I consider the
flow of a fluid passing through a turbine or compressor (ie wind through
the wind turbine on the top, or water through the prop in the water at
the bottom), then the force on that propeller/turbine will be given by
the mass flow rate through the propeller disc multiplied by the change in
velocity of the flow. The power consumed/delivered from/to the flow will
be given by the mass flow rate multiplied by half the square of the
speed.

For my boat to make way directly to windward, there would have to be a
situation where the force on the underwater propeller exceeds the force
on the wind turbine, and the energy liberated from the wind turbine
exceeded the energy consumed by the water propeller.

If we take a wind turbine of 2m diameter, in an apparent wind speed of
about 10 knots (about 5 meters per second), and assume that the wind
speed down stream of the turbine is reduced by 1 knot (apparent), then
the force on the propeller will be about 19 N [1]. This will liberate
170 W of power from the wind.

If we take the boat speed to be 2 knots, and a propeller diameter of
20cm, then for the force on the propeller to equal that of the wind
turbine, the water speed down stream of the propeller will be 1.7 m/s
relative to the boat [2]. This will consume about 80 W.

Clearly there will be power to spare from this process, which will be
consumed by hull drag both in the water and air, and inefficiencies in
both the turbine and propeller, but it is clear that it is possible for a
vessel to sail directly to windward in this manner. I have not attempted
any kind of optimisation for matching the size of turbine to propeller
here either, which would improve the excess power or thrust, but the
point is, it is possible.

[1] assume air density to be 1.2 kg/m^3
[2] assume water density of 1000 kg/m^3

Robin

You are basically claiming you can derive enough power from the wind
because you make the invalid assumption that the boat is somehow attached
to the water and will stay put. You are adding to the system that force
that would be there provided this windmill was attached securely to the
ground. On a vessel in the water this assumption is invalid as the wind
will move the vessel backwards.

I start from the assumption that the force of the wind on the wind turbine
acting in an astern direction will be exactly ballanced by the force on
the underwater propeller driving the vessel forwards. Newton's First Law
therefore maintains that the vessel will continue going at the same speed
in a straight line.


Boy, is that ever an ill-conceived assumption. Your assumptions says you've
somehow invented a mechanical or electrical system that is 100% efficient
and that cannot be according to recognized laws of physics. Newton's first
law states: "An object at rest tends to stay at rest and an object in motion
tends to stay in motion with the same speed and in the same direction unless
acted upon by an unbalanced force." The windmill powered boat will stay in
backwards motion being blown backwards by the wind unless and until an
unbalanced force acts upon it. It his case the force is somehow to be
produced by a spinning propeller which tends to push the craft backwards
even faster because a spinning propellar gives more resistance just like a
spinning water propeller gives more resistance. You some how assume that you
can produce enough force to arrest the backwards motion of the system and
then to propell it forward against the wind. It won't happen. Nobody has
produced such a waterborne system to date. If you claim they have then
provide proof - not some bogus model or grainy filmstrip that shows nothing.

In other words ,you fail to account for the fact that there's nothing
pushing back on the wind propeller other than some slight friction from
the water.

No, what is pushing back is the thrust from the underwater propeller. In
the same way that the keel under a conventional sailing boat pushes back
against the sideways action of the wind on a conventional sail, and the
boat only makes a little bit of leeway (because the keel needs positive
angle of attack to generate lift).

Wrong! You can't produce more force in the opposite direction than is
available in the original force unless you add the force from a stationary
object such as the Earth that pushes back in the first place with equal
force. On the water this pushing back simply doesn't exist.



If you add to your figures the amount of energy you are assuming is there
in forward mode (because you assume a fixed platform) but is not there
then your figures come up lacking. It seems to be a basic human fallacy
to assume the ground provides no forward push to a wind mill.

I have made no such assumption. The only two forces I have considered in
my evaluation are the force of the wind on the wind turbine and the force
of the water on the propeller.

In other words you ignored the natural propensity of the craft to be pushed
backwards by the wind which not only reduces the force available in the wind
but gives a net negative forward energy to your system.


But consider how such a windmill suspended in the air by a helium balloon
would simply blow backwards. A windmill suspended in the water on a hull
would go backwards albeit a bit slower due to the increased density of
the water.

Consider how a conventional sailing boad can sail on a beam reach. If it
were suspended by a helium balloon, the only force on it would be the
wind, and it would proceed dead downwind. Instead, the force of the water
on the keel counters this force, and between the force of the wind on the
sail and the water on the keel, there is a net force forwards to counter
drag (due to both water and air).

But the keel works in conjunction to the sails to give a net vector forward
as long as one is not proceeding closer than 45 or so degrees to the wind.
Directly upwind all the vector arrows face towards the stern of the boat and
there is NO NET force forward being produced. You presume to produce a net
forward force by using a water propeller but you can't because of the
inefficiencies of the mechanical or electrical systems. Again, the best you
could do with a 100% efficient system would be to stay in one place and not
get blown backwards. To expect to go forward directly into the wind would
require an additional input of energy that can't be gotten from the wind
propeller.



So, for any equation to be valid, one needs to take the force that the
wind is pushing the system backwards and add it to that which needs to be
produced just to hold it in place.

The force to hold it in place is provided by the propeller. DO

Only if you assume the system is not dynamic. In other words only if you
assume it is sitting on and attached to a solid surface with pawls or some
other device in the wheels that keep it in one place. In other words you
ignore the force pushing the system backwards because your mindset is one of
a lubber who is familiar with concepts that are tied to solid surfaces.


A perfect system would just manage to hold it in place. But there is no
perfect 100 percent mechanical or electrical system that exists in real
physics. Therefore it would go backwards.

If it were 95% efficient it would go backwards at 5% of the wind speed.
If it were 50% efficient (which is probably realistic) it would go
backwards at 50% of the wind speed.

You can't get something for nothing.

Go back and look at the numbers. The force of the wind on the turbine is
19 N, calculated from the steady flow momentum equation. This means, for
the vessel to hold station against the wind, the propeller would need to
generate exactly 19 N (in the other direction). By applying the steady
flow energy equation, the power required to drive the underwater propeller
so that it can produce this 19 N force is 80 W. Now for this vessel to
hold station against the wind, it will have to derive 80 W from the wind
turbine. If the wind turbine generates less than 80 W, it will be blown
down wind. If the turbine generates more than 80 W, there will be spare
power to turn the propeller faster, and not only will it generate the 19 N
it needs to hold its station against the wind, it will have spare to make
way to windward. The turbine in this case can generate 170 W of power
from the wind, so there is plenty to spare, to allow for inefficiency and
various forms of drag.


Sorry but your numbers are flawed unless and until you account for the force
you are ignoring - the force that pushes the system backwards because it
isn't attached to anything to hold it still. That's the basic flaw that you
and others refuse to include in your calculations.

Wilbur Hubbard


.

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