Leaf Blower on a Yatch - WATCH THIS

Sorry, no cigar. Your video has no merit. It doesn't work like that.

What previous post?

Leaf Blower on Yatch

It would be easy to see the problem if smoke were introduced into the fan. The "sail" is acting like a thrust reverser on a jet engine, changing the direction of the fan's thrust.

In the classic example of the "fan on a sailboat" the action of the pressure of the air from the fan is cancelled out by the reaction of the fan. To put it in physics terms, the vectors are 180 degrees out of phase. Say the air from the fan produces a 10 newton force on the sail, pushing the boat straight forward. But the reaction of the fan is producing a 10 newton force straight backward. I used the newton since it's Newton's third law that applies here.

Not if the fan was directed aft at a 45deg. angle. You would then be sailing to windward, with the action of the fan assisting at a tangent. Right up until you ran out of gas. It does give me pause to wonder if the savonios rotor in 'Water World', used to charge batteries to run a motor would work. Probably less efficient than sail, but you wouldn't need to tack upwind. Has anybody tried that?

It's interesting (but not surprising) that when the fixed fan is turned on the sail-equipped model rapidly speeds away from the fan, but when the fan is attached to the sail-equipped model it takes noticeably longer to start moving and the acceleration is much less. That should be a good indication that all is not what it appears and that there is a big difference between a moving fan on a moving boat and a fixed fan on a moving boat.

This has absolutely nothing to do with the first dumb**s post.

There is no problem with the fan on the sailboat, blowing into the sail, propelling the boat into the direction that the fan is pointing. It is just very inefficient.

I wrote in this reply a Challenge Question:

"The fan causes the average air stream to change form 0 to +v, with a reaction component of -v (backward). The sail causes that same air stream to change from +v to k(-v), where k is some positive constant less than 1. So the fan causes an airspeed change of +v and the spinnaker an airspeed change of:

-kv - v = -v(k + 1)

The reaction component to that is v(k+1). Add the reaction components and you get:

-v + v(k+1) > 0, i.e., a reaction component in the forward direction."

In the video clip it is clear that k is not very close to 1, hence the inefficiency. This has to do with the air losses around the sail (plus of course general compression/heating losses of the air).

-J

It is difficult to identify the trick on what we have.

The sound of the internal fan remind me of :-

Long ago I had a clockwork alarm clock. It was about 4" round. The two marble sized stays to keep it upright was however missing.

After setting the time and balancing properly it was working ok but always started rolling in one direction when the alarm goes off and I had to wake up pronto to catch it before it smashes into the floor.

I never took the time to investigate (that would have shortened my drinking time) but I suppose the inertia / flywheel effect of the unwinding spring may have caused it to start rolling.

I suspect something similar.

A spirit level in the video might help.

If you go back to the original concept of a leaf blower pushing against a 300 square foot sail, Newton's laws work. The air exiting the leaf blower is pushing you back with the same force, almost, as the force exerted against the sail by the blower. Zero gain.

As someone said in the other post, if you are not on the yacht (hope the correct spelling doesn't confuse you), then a tiny amount of forward force might be generated.

I thought the question about non-water rain in another thread was much more thought provoking and intelligent.

Hi lynlynch, you seem to miss something (if I understand correctly what you said). The leaf blower/sail combination can propel the boat in the direction of the sail.

The air exiting the leaf blower has been accelerated from 0 to +v (say), while the air pushing against the sail and bouncing back has been decelerated from +v to almost -v, a speed (and hence momentum) change of almost double that caused by the leaf blower. The 'gain' is hence not zero and it is the standard Newton's laws at work.

-J

Thanks.