Does a sail act as a turbine blade or a propeller blade?

Under the conditions that rulebooks used to try to define as fair sailing, a sail transfers energy from the moving air mass to the boat (but not very efficiently when it is stalled, as in running before the wind).

In roll tacking a dinghy in light airs, work done by the crew swings the mast and sail through the air, 'pumping' it backwards - in effect, you have just rotated a propeller through about 50 degrees.

In the natural roll of a larger boat in moderate seas, there is an element of using the energy of the roll to drive the sail as a propeller as the boat rolls to windward.

 

Agreed. This is why I wanted to start with a less ambiguous case where I expected we could agree on definitions. Having thought about the case of the sail, my recollection is that there were cases where the sail itself (not necessarily the craft) seemed to fall pretty neatly into the category "turbine" while other courses of sail fall into the category "prop".

An example would be an ice-boat on a 45 degree downwind course with downwind VMG greater than wind speed. In this case it seems we could picture a long row of these ice-boats running abreast, and all on the same tack. We could think of their sails as forming the actuator disk. The flow would clearly go from low-pressure to high pressure as it passes our virtual disk. In this case it seems clear to me that the sails are acting as propellers. But I haven't scrutinized it closely. Will do so at lunch.

I'd be curious though if you can think of a way or frame in which to look at this scenario in which the sails (not the entire craft) could be thought of as turbines.

 

Sort of like this drawing?

JB

 

Yes, that's exactly the configuration I had in mind. Unfortunately the left to right arrows refer to the direction the sail moves in the cross-wind sense. The fact that the whole craft is moving downwind as well may complicate matters. That's what we should scrutinize over some In & Out burgers.

 

Deadeye,

I think this illustration from Jack L. Kerrebrock's (MIT Professor) "Aircraft Engines and Gas Turbines" (Second Edition) will illustrate what I'm trying to say better than I could explain it.


Mark, Spork, et al:

I think if we use the (somewhat oversimplified) constraints that were applied in the original post, we can all agree (after an appropriate amount of thought) that my first post here quite well describes the situation in all but DDW useage of a spinnaker. Even in "Wing on Wing" DDW sailing, the sails are both harvesting energy from the wind, and exerting energy on the wind to alter its course...so both (and spinnakers were disqualified in the orig. post anywise).

 

Sorry, but consider case where a powerboat is moving backwards, and then suddenly puts all ahead. The prop would produce large force forward against motion (of boat related to free stream water outside prop stream) if you define that as a turbine , I'll have to think engine would disagree and so would most boaters.

That sounds good, if fluid energy is defined in IRF of free stream by definition, is it ?

 

It is of my opinion that Spork and TAD are one in the same. The question really is... If one person is portraying two people and both theoretical people agree on the subject, is he a split personality?

 

Very astute. I assume your logic is that we're just plain too smart for a single personality - and you're right. But I'm the better looking one.

Nonsense! I am.

 

After some more consideration I'm planning to stick with my notion for the moment that the practical definition of prop or turbine relates to whether the pressure is high on the upwind or downwind side of the actuator disk. If we use this definition I think we can say that a sail DOES act as a turbine in certain situations and as a propeller in others.

I think saying it can be thought of a turbine or a prop depending simply on the frame chosen only makes sense if we define it based on work done. And to me that's a less satisfying definition to begin with since "work done" is a far less tangible notion than forces and flows that can be measured.

 

Obviously that was my logic.

 

Now you still have to define upwind side of the actuator disk to make this definition clear. Is it defined as related to flow thrue the disk or related to freestream direction ?
Think about a case where a powerboat is moving backwards and then putting engine on forward ...
1) with very low power setting so that the flow does not reverse thrue the disk, while still going forward. The prop only tryes to work as a prop, but within this definition it acts like a turbine. However the torque on the shaft can well be same as with a prop due to inefficient prop pitch & twist for this operating condition outside of it's design envelope.

2)with high powersetting so that flow is reversed, but boat still moving backwards at this time due to inertia. Which side of the actuatordisk is now a highstreamside ? Pressure is clearly high behind the disk & boat in slipstream. Is this the turbine case or a prop case ?

With this definition both cases above are propellers, not turbines.

These 2 definitions are therefore in contradiction, unless I misinterpreted your definition. Please clarify the issue. The second definition makes more sense for me, but tells nothing about a sail.

 

I define it as the flow through the actuator disk - not the free stream. In the case where the blades are stalled, I don't consider it a propeller or turbine. In this case it's just an object I'm dragging through the water.

If it's not stalled, but the net flow through the disk is reversed, it's simply a case in which the model of abstracting the spinning prop as a disk breaks down. The flow in the region of the blade is still in the correct direction if the blade is not stalled.

 

The question and poll are meaningless as it is a frame of reference-energy boundary issue, not an absolute. All (prop, turbine, sail) are just degenerate equations from different vantage points of the total energy equation.

FWIW, all are the same, as they all do "work" on the air as work is a vector and only frame of reference places it as positive or negative, work in or work out. The same is true of "lift" and "drag" forces...i.e. it is all entropy as far as the fluid is concerned, it just depends on if it is going in the direction you want it to go. Do not be confused by sophomoric mathematical tricks.

JB, I would suggest you hit the books more; a good thermo and fluids book with historical perspective will show you just how foolish your poll was.

 

Rather than "foolish", I prefer the term "ignorant". I don't find any discussion which elicits interesting information "foolish".

Many place negative connotation to the term "ignorant", but I find that acknowledging ignorance of any topic allows learning.

JB

 

jehardiman: true, but perhaps a bit pedantic. I took the poll in the only way it makes sense - using the craft on which the sail was mounted as the frame of reference.

However, I do have a question: why rule out DDW spinnaker? Airflow is more complex, but it isn't the equivalent of a flat plate or anemometer cup normal to the wind. A mainsail is probably more completely stalled DDW than a spinnaker.