Larry wrote:
wrote in news:1153667730.918826.254080
@p79g2000cwp.googlegroups.com:

Would this work?


No.

Airboats with just the fan go 70 mph, even over wet grass. I haven't seen
any sails on any of them to increase speed.

Let's think....If we put an airboat engine/fan on the bow blowing through
all that wind resistance of rigging and sails and tackle and measure the
speed.....then, clean the deck of all sailing paraphenalia and let the
airboat fan power the boat with no sailing equipment, its drag, its weight,
its moment pulling the boat over...

Which do YOU think would be faster?

I'm picking Door Number Two!

Thanks for the response. I didn't know airboats could go that fast.
The key fact that makes me think it could go faster is that with a
motor driven boat there is created apparent wind from its motion even
in still air. The wind speed of this apparent wind is the speed of the
boat of course (in still air.) But it is known that sailboats can
*exceed* the wind speed:

The physics of sailing.
"How can boats sail faster than the wind? Lots of boats
can---especially the eighteen footer skiffs on Sydney Harbour. Ask a
sailor how, and he'll say "These boats are so fast that they make their
own wind", which is actually true. Ask a physicist, and she'll say that
it's just a question of vectors and relative velocities."
....
"The faster that the boat goes, the greater the relative wind, the more
force there is on the sails, so the greater the force dragging the boat
forwards. So the boat accelerates until the drag from the water
balances the forward component of the force from the sails."
....
"Why are eighteen footers always sailing upwind?
In a fast boat, there's no point going straight downwind: you can never
go faster than the wind. So you travel at an angle. But if your boat is
fast enough, then the relative wind always seems to be coming mainly
from ahead of you, as these arrows show. So the eighteen footers never
set ordinary spinnakers: they have asymmetrical sails that they can set
even when they are travelling at small angles to the apparent wind."
http://www.phys.unsw.edu.au/~jw/sailing.html

Furthermore it has been proven that propeller driven boats that have
sails can achieve higher speeds with the sails than from the propeller
alone:

Motorsailing - Some Thoughts.
"Based on my experience the motorsailer is the most practical and
comfortable vessel for serious ocean passagemaking. However these days
power-only long range cruising yachts seem to be the rage. We sell
about 8 power boats for each motorsailer we build. I personally can not
understand the long range powerboat skipper generating a beautiful 7 to
9 knot wind, at some cost, and than just throwing it away - while
worrying about fuel?"
....
"Plus the sailing rig increases the speed of the vessel so less fuel
capacity is needed. This weight savings coupled with the additional
propulsion power available results in faster passages and excellent
fuel economy.
"How can this be? Well most power-only trawler yachts cruise at around
6 to 8 knots per hour depending on power, weight, hull shape and water
line length. All these vessels are creating, at some expense, a wind
equal to their speed. This wind, a vital and reliable source of energy,
combines with and adds to the true wind to create an apparent wind
across the boat which is just thrown away. The motorsailer, on the
other hand, uses its rig to regain this energy resulting in increased
boat speed, up to 25% above a stabilized powerboat, without increasing
fuel use."
http://www.seahorseyachts.com/seahor...emarine_3.html

So a fan at the stern blowing *rearward* with the sails forward, so
the fan does not blow over the sail, could likewise increase speed,
just as with the motorsailers.
What I'm asking is if in fact you did put that fan in front of the
sails so the air did blow over them, would this increase speeds even
better than the fan at the stern case.


Bob Clark