Thrust greater than drag

It would be interesting to know what he meant by dangerous. Did he mean that the motion of the boat heading straight upwind was throwing the passengers/crew were being thrown around too much? Did he mean that the windmill was moving around too much? I had assumed the former, but who knows. Either way, we could say that this boat was a prototype, so lessons learn't could be used as a guide to improvement. Anyway, 8 knots dead upwind is impressive. The same boat with a conventional rig tacking upwind would need to do 11 to 12 knots to equal it. I'm not sure if that is achievable, I would guess not, and it would be just as uncomfortable if it could.

 

It's pretty clear what he means by dangerous, he said it clearly, "the boat will sail of the face of the wave". It's the same as any other boat bearing to much sail for the given conditions, it's overpowered, wich in a multihull means it's to fast, launching off the waves and in danger of stuffing the bow into the next wave. The boat becomes airborne, falling off the face of the wave and crashing on the water between waves, wich if you ever experienced it is like landing on concrete.
If you go even faster instead of crashing in between waves your bow(s) hit the wave in front of you, piercing it, wich runs the risk of pitchpoling. That's why race boats have very fine inverted bows, to be able to pierce the top of the wave without pitchpoling.

A windmill can't be reefd, you can only feather it out of the wind killing drive to make it turn slower. This makes the boat slower (speed is determined by propeller rpm) and thus safer, keeping it on the wave face. Now for the real problem, you can only feather so much before the blades stop turning and you are dead in the water. This means there is a windspeed where even feathered to the max the boat is to fast and you have to resort to other things to slow it down.
You can read all about the different heavy weather multihull strategies, but in the end having a windmill complicates things.
Monohulls are even more complicated because of heeling, under no circumstances can you allow the blades to touch water, they won't survive it.

 

The boat will still have parasitic drag (windage). It would be possible to feather the windmill blades to the point where there is just enough thrust to keep you stationary against the windage. Steerage might be a problem, but maybe that could be overcome by mounting the propeller on the rudder to gain directional thrust. Variable pitch windmill blades offer continuously variable speed control.

Monohulls are probably not the best platform. Multihulls make more sense.

Certainly safety needs to be considered. It's part of the engineering problem. Every year millions of people climb into aluminium tubes and fly 40,000 ft above the ground at 600 knots. By comparison, a windmill powered boat should be fairly trivial (famous last words).

 

It's not rocket science!

The explanation is that the boat acts as a lever between two media (the water and the air) The windmill disk blades rotate through the air a certain distance and like any lever we can trade a small force over a large distance for a larger force over a smaller distance - that is what happens, the windmill and the boat are connected in such a way that the boat moves less distance than the windblade but with greater force. The force from the windblade is amplified by the gear ratio and a suitable propeller fitted to apply that force. That is how there is more thrust than drag

 

I see. You mean there is more thrust than wind drag, not total drag. You should clarify that in the thread title.

 

No, actually it IS...rocket science.

You still do not understand the difference between FORCE, ENERGY, and POWER. Please go to school and learn these. I'll wait....

 

In order for a windmill boat to work, all that is required is that the windmill blades need to be at a higher pitch than the propeller blades. For the windmill, higher pitch gives less drag for the same torque. For the propeller, higher pitch gives less thrust for the same torque. The torque of the windmill will be equal to that of the propellor (Newton), and the thrust of the propeller must be greater than the drag of the windmill (plus parasitic drag) in order for the boat to move forward. Therefore the pitch of the propeller must be lower than the pitch of the windmill.

Looking at it another way, you wouldn't expect to make much progress to windward if you oversheet or backwind your sails.

Edit: to be clear, when using the term 'pitch' above, I mean pitch ratio (advance/diameter).

 

What exactly do you take issue with in this explanation. Do you understand the principles of leverage?

Yes, the boat progresses through the water because the windmill provides power to overcome the wind and the water drag.

 

This explanation is incorrect for a few of reasons:

1) Air and water have different densities. You can't really compare the two directly and therefore it has nothing to do with the speed of the boat vs the speed of the airflow. Imagine you have two streams of water separated by an interface, moving at different speeds, with a turbine in one and a propeller in the other, both the same diameter and blade area and 1:1 gear ratio. The device will move in the direction of the propeller (against the turbine) relative to the mean stream speed, as long as the pitch ratio of the propeller is sufficiently less than that of the turbine. For a propeller in water and a turbine in air, the system will actually go backwards relative to the mean stream speed due to the different densities, but with a boat, what we are concerned about is that the boat moves forward relative to the water (our frame of reference).

2) The gear ratio has no effect on whether the system works or not. Gearing up the propeller, increasing the RPM, allows a smaller propeller to be used. You will loose some efficiency if you do that, but there are practical benefits such as shallower draft and maybe cost.

3) The leverage argument doesn't make any sense. With a lever, the moments on either side are equal and opposite. Similarly the torques of the turbine and propeller are equal and opposite.

 

Thinking about it a bit more, it may have little to do with the density difference. The viscous properties may have some effect, but the main difference is that the hull resistance is significantly greater than the windage.

Edit: On third third thoughts, I'm an idiot. It's absolutely to do with the density, as well as the other factors. Due to the higher density of water the propeller must be a lot smaller than the turbine for the same RPM, or else geared down a lot. Therefore the advance of the propeller will be a lot less than for the turbine.

 

Are you saying this is not true? - " in a lever we can trade a small force over a large distance for a larger force over a smaller distance" Also, what do you mean by "moments"?

 

For example, 1N at 1m and 5N at 0.2m are equivalent. They generate the same torque or turning force at the fulcrum.

 

Yes but they provide a different force at the 1m load and 0.2m effort positions (or visa versa) that's called leverage - an example of the 1st class lever. We don't take power at the fulcrum we take it at the load and effort positions. So the force may be increased at one end and the distance increased at the other end as necessary by moving the fulcrum position. https://www.teachpe.com/wp-content/uploads/2019/08/lever620.jpg - so in this case the windblade moves a greater distance than the boat providing a mechanical advantage.

 

You are confusing power with force. You would only have a case of a lever, if there was a surface generating a force forward on the boat and the water being the fulcrum. The fulcrum may be at the end. Foils are not levers though. A small foil turning at high speed can power a larger foil moving at lower speeds and/or with a lower pitch. The analogy does not hold true. Further, foils have slip, without which there would be no lift.

 

If the fulcrum was at the end it is a 2nd class lever, this is a 1st class lever - I could not make any sense of the other things you said. Who said foils are levers? So what is your verdict on windmill powered ships sailing direct upwind? Do you believe what you have seen or do you continue to deny what is obvious to anyone?.