On 28 Sep, 16:45, Ronald Raygun wrote:
harry wrote:
On 28 Sep, 10:19, Ronald Raygun wrote:
harry wrote:
On 27 Sep, 21:31, Gib Bogle wrote:
Yes, I know about sailing. *Did you read the post? *Here, I'll cut out
the appropriate part for you, to make it easier:
"A wind powered boat or vehicle that could sail or drive directly into
the wind using a turbine facing the wind to drive a water prop or
wheels" Do you see the reference to "using a turbine"? *OK, now the
question is this: "Can a WIND TURBINE-POWERED vessel or vehicle go
faster than the wind DIRECTLY DOWNWIND?" *Think about it.
By the way, a sail-driven craft cannot go faster than the wind
DIRECTLY DOWNWIND either. *Get it? *Your slight knowledge of physics
should be adequate to grasp this.- Hide quoted text -
Obviously is soon as it went "as fast as the wind downwind" there
would be no wind (relative to the machine/boat etc). Therefore
impossible.
No, that's a faulty conclusion. *It correctly answers the wrong question.
The question was whether it is possible for a boat to sail directly
downwind faster than the wind, powered only by a wind turbine. *That
means whether it is possible for a boat *to sustain* such a speed.
The question you've answered, however, is whether it's possible for the
boat *to reach* that speed, subject to the constraint that it must start
from rest and only ever head directly downwind whilst accelerating.
That's a completely different question, doomed to a negative answer by
the fatal constraints imposed.
Anyway, your answer correctly observes that the relative wind speed
approaches zero as the boat's speed approaches that of the wind, and
so the turbine is faced with the impossible task of extracting power
from a dying wind. *Therefore crossing the barrier of actual wind speed
is going to be impossible in those circumstances, and in practice there
will be a top speed which it is possible to reach but not exceed.
Suppose this practical top speed is about 90% of wind speed, so that a
10% difference or so is enough to provide the power needed to keep up
with water resistance etc.
But if a 10% difference is enough, then travelling at 110% of wind speed
should also be sustainable.
The only problem remaining is how to reach that state to begin with.
Clearly it can't be done subject to the constraint of your scenario,
but it could be done by other means, including by "cheating" and using
stored energy.
I dare say another way would be to use the wind turbine to accelerate
to beyond wind speed by going downwind *but not directly downwind*, and
then quickly turning directly downwind, relying on inertia to lose not
too much speed in the turn.- Hide quoted text -
- Show quoted text -
The speed the boat reaches is determined by the drag through the
water. When the thrust from the sails *= the drag, equilibriibum is
reached and it ceases to accelerate.
Indeed. *Drag increases with speed, but available power (and hence thrust)
reaches a minimum (of zero) at 100% of wind speed. *I postulated that if
equilibrium might be achievable at 90%, that, on the basis that the
relative wind speed is then 10%, equilibrium ought *also* to exist at about
110% when the relative wind speed is also 10% (I appreciate that water drag
at 110% will be a bit more than at 90%, so the difference might need to be
a bit more than 10%).
*The only way to increase the speed in the same conditions is to
increase thrust with a bigger sail.
Or a bigger turbine in this case. *But if doing this on a dead downwind
course, you still can't break the 100% barrier, you will merely push
your equilibrium a bit nearer 100%.
My point, which you may have missed, was that you could exceed 100%
wind speed by making your initial acceleration on a course which is *not*
directly downwind, and where therefore there is no barrier effect at
which relative wind goes to zero. *And then suddenly turn dead downwind
where you should then be able to sustain the 110% you had already
reached earlier.- Hide quoted text -
- Show quoted text -
Nah. The turbine would slow it down. It would be driven by the wind
if going faster.
Anyway drag and speed relationship is not as simple as that.
Drag is divided into two components.
One is caused by the shape of the object in question and increased
with speed through the fluid. (Gas or liquid). Not linearly either,
it's a square law.
The other is caused by friction with the surface of the object and
decreases with speed.
This means there is a low pont on the combined curve known as mininimu
drag.
So there is and optimum speed that consumes the least power known as
the "best" lift/drag ratio. ie the cruising speed for ships and
aircraft.
There is a picture of the graph here and an explanation.
http://en.wikipedia.org/wiki/Lift-to-drag_ratio#Drag