"David CL Francis" wrote

The boat can move
because there is a relative velocity between the two fluids. It is
obvious you can sail if there is no wind at all provided you are in a
tidal flow.
********************

Not really. Relative velocity between the wind and water does not enter in,
much. In the case of a tidal flow you cite, it would have to be one hell of
a tidal flow; enough to make apparent wind, and a LOT, at that.

With very slow wind speeds, there is not enough wind to keep the sail in in
an airfoil shape, then you go nowhere, other than the speed of the water.
Most flows do not reach a minimum speed.


Every one knows a sailing boat can tack up wind but can it tack down
wind? The answer is not much, mainly I believe because of the much
higher drag of the water. Sand yachts and ice yachts can, and they can
sail on a reach, downwind faster than the wind is blowing.
--
David CL Francis

More problems. Sailboats *can* tack down the wind. They often do, if they
do not have a spinnaker. (Large parachute shaped sail, added while going
down wind) Problem is, you have to go a lot faster to justify the extra
distance. Usually, you just go down wind slower, and don't worry about it.

The drag of the water *is* an issue, but not as you state. The boat, until
you ride on top of the water, can go only so fast, almost irrespective of
how much extra power you add. It is called hull speed, and generally
speaking, it is higher, the longer the boat is.

When you are going into the wind, the hull speed is not tough to get to, if
the wind is strong enough. The maximum angle needs to be about 15 degrees
or more away from straight into the wind. (again, generalities) The sail is
acting like a wing, and airflow is accelerated around it to pull the boat
into the wind, just like an airplane wing provides lift. The boat slips
sideways, due to the wind pushing against the sail, which pushes sideways on
the water, through the keel and rudder. That is what gives the drift.

What kills downwind speed, is the fact that the sail is only acting as a
barn door, or flat plat, using the wind pressure, and does not have lift in
addition. Those are the main factors limiting down wind speed.

In the case of ice boats and land boats, the lower friction removes the hull
speed limitation. The craft continues to accelerate, until a wind that
*was* coming from the side or partway from behind, now with increased hull
speeds, will appear to come from the front of the craft, now called apparent
wind. The sail can then act like a wing again, and use the lift to continue
to go faster and faster. Problem is now, you can only go around 45 degrees
into the wind, without slowing way down again.

I hope I have made this all clear enough.
--
Jim in NC


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