Windmill or Wind Turbine- powered boats: how many are out there, and are they viable?

<<<Irrespective of all of the above I have determined that with good mechanical system you could go maybe 20% faster than the wind in either direction with a well designed system on a slender hull.

You can do performance determination solely on the efficiency of the system and the drag on the hull at a given velocity. A good air turbine/prop will run up around 88% efficiency and a water prop/turbine around 86%. In both cases these are asymmetric foils set up to lift correctly with respect to flow.

These numbers are for a scale suited to a single person size boat. Such a boat has the hull drag is a function of speed^2 and is 40N at 3m/s.>>>>

Great to see some numbers. So V/Wr =1.2 is about optimized , huh?
I was hoping it might be a bit better, maybe one should analyze the hydrofoil option for a pretty substantial resistance reduction...
The reversible airfoil problem is solved-at least in the case of the airturbine-by using a compromise airfoil with zero twist and trailing edge/leading edge symmetry. It disturbs me to think of an untwisted rotor foil but according to windmaster it doesn't seem to hurt too bad, although i wonder has windmaster ever used a correctly twisted foil? Also i notice that his foils do not go all the way to the hub lessening the negative impact of the absent twist, although this produces another tip vortex reducing the efficiency somewhat. At least the inner tip vortex does not have the same amount of moment as the outer (unavoidable) tip vortex. I would also like to point out here that one blade is the most efficient but is very difficult to engineer because of the assymetrical torque loads and needless to say can only be considered for pretty low RPM's . Two blades is the next step but they too have some of the problems of the monobladed rotors, namely instabilty due to highly axial mass distribution. Therefore the most practical option seems to be the three bladed design which behaves very nicely and is still fairly efficient. Any more blades just generates more induced drag. The only real reason i can see to have more blades is if there is some kind of constraint on overall diameter.
Another thing, Rick, hull resistance proportional to v^2 is only correct for v/rootlwl of less than 1 or so. After that it becomes increasingly innacurate due to wave making resistance augmenting faster than the square of V...

 

In my above message there is a quote from Rick W.

Also i want to congratulate Rick on having obtained such close results and taking such an interest in human powered propulsion, a topic dear to my heart, and one which is sadly overlooked by most people,even sometimes ridiculed, which is tragic considering the times we live in.
I also want to add that the wave making term which would create an deviation from resistance=kV^2 is pretty small IF the hullis slender enough, as in VERY slender.

 

CORRECTION
i earlier said that a reversible prop (from driven to driving) must be twistless. This is wrong in fact the twist works both ways- A big boost to the efficiency of the dual purpose rotor.

 

The dreaded axial forces...

First, welcome to Tcubed into the discussion, we need as many stimulating minds as possible in this effort.

I don't mean to flog a dead horse (which I believe is very much alive) but the issue of the axial component of lift that creates a force opposite to the desired thrust is the limiting factor for unducted turbine blades used as propulsion sources. As soon as this force exceeds the tangential component it is pointless to try and make the turbine produce more power, unless of course you are stationary, at anchor, and just want to produce electricity to store. Once you are freefloating, you are at the mercy of every force exerted on the turbine, including drag on the pylon, drag on the blades, and any uncompensated component of force acting on the blades in a direction aft of the rotational plane. As a thought experiment, imagine a generator disconected from the driving propellor, but on full output. The negative axial forces (thrust) developed and applied to the pylon would tend to drive a free floating vessel backwards. Before any forward motion is possible you would need to overcome this first, leaving only a small fraction for further forward propulsion. It is clear that this will soon exhaust itself, resulting in diminishing returns with added forward speed, and pretty quickly a balance ensues, and presto, top speed!

I have repeated my diagram of these axial forces for reference.

It may actually be necessary to have an enclosed turbine with the blades at a NEGATIVE angle of attack in their orientation to the shaft, and with a better inlet design cause the air to strike the blades at the correct relative wind. Will try and make a simple diagram that illustrates this a little later on.

 

The ideal angle of attack.

This is a schematic representation of why the angle of attack should be negative for a propulsion turbine. Notice that the resultant vector aligns with the tangential direction, and a component of the lift is used to cancel the drag on the blade. This tubine theoretically could spin without a thrust bearing.

 

Not only theoretically- It's the same principle helicopters can use when landing with free (without engine power) rotating blades having negative angle of attack. However this technique means having brakes on...

 

The things that get in the way of a fast practical solution are the power handling capability of the system. The real cost is the power transfer that is going on. The water prop has to be bigger than otherwise because it has to make up for the drag on the air turbine. The weight of things go up to handle the bigger forces.

One thing to remember though is that the VMG is the boat speed if you can go straight into the wind. This makes a big difference.

I often see wind surfers flying across the local lake. They can do the 500m width in a matter of seconds on a broad reach. Some of them simply cannot make way to windward. They get further down wind and cannot make their way back. Have to go ashore and walk back. Even 16 foot cats can fly across the lake but on a triangle course I have not yet been beaten on my best pedal boat. They are getting to twice my speed when reaching but I go direct to the next buoy. It also takes a strong breeze for them to move faster than I can down wind.

So 20% faster upwind than the wind and likewise downwind may not be flying but look at what speed a typical yacht has to achieve to make the same progress. (Getting this sort of performance is not trivial either)

One of the features about sailing directly into the wind is that the healing loads are nil. I have not studied the reaching case but there are gains here because the power transfer requirements are dropping off. I expect the healing forces would be quite low as the centre of pressure does not have to be as high as a sail.

All this means that you can get away with a more slender hull that it easily driven. You do not need beam to carry sail.

With a good CVT connection, performance could be impressive compared with most vertical foil sailing boats. Just that it needs a lot of development.

Rick

 

There may not be thrust on the rotor but there is on the stator and this has to be pushed through the air so will add drag and has to be accounted for.

How is the trailer thruster coming along?

Rick W.

 

The attached provides a calculator for a wind turbine boat sailing directly into the wind.

It contains a little macro that has to be operated with the command button to balance the turbine load when any values in the white cells are changed.

This is somewhat simplistic because it does not consider near field conditions for the fluid flow. Basically it assumes lightly loaded propeller and turbine.

The turbine power goes red if it exceeds 10kW. This is the sort of peak power level consistent with a boat having the drag I have set.

The point of this is that it shows how quickly the system power climbs once the boat starts to exceed wind speed. With turbines and props of this efficiency you will see that the potential is to go much faster than windspeed but practically there is no way of making lightweight transmissions to handle the power transfer.

It also shows how quickly performance deteriorates if the component efficiency drops off. You need to do a good design to get an air turbine to 88% efficiency and a water prop to 86% efficiency. This is the problem with cobbled together systems that use the classic windmill turbine and a heavy duty water propeller. Efficiencies for both are typically around 70% or worse.


Rick W

 

You're right Rick 1.2 is of course a VMG that is uncomparably superior than any other boat. I just thought maybe it can be optimized even more..

The duct that bends the airflow such that a zero thrust turbine can be made creates more drag than what it reduces in turbine axial force so it's not really worth it. However, get this people: By using Counter Rotating turbines they operate in each other's wash. The leading turbine operates in the upwash of the trailing turbine which means it must be set at a shallower pitch, which increases its axial load. The trailing turbine operates in the downwash of the leading turbine, which means it can be adjusted for a steeper pitch, which reduces its axial force much like the ducted example. Now for the good part: Upwash is approximately half the angle of downwash so the NET effect is a reduction in the OVERALL axial force. On the downside, there are now twice as many tips which increases tangential drag on the turbines lowering efficiency, as well as more mechanical losses, but i think there would still be a net overall gain. Also, for practical purposes the turbines must not be set too close to one another for the interference gets much too drastic when the blades pass each other, apart from creating a dreadful thumping noise. (which is why helicopters make so much noise, you're actually hearing the thump as the blades pass over the tailboom) So these turbines would have to be on either side of the pylon to separate them by at least 25% of turbine diameter.

Now for heeling forces. There is a general consensus that the axial force is a big hamper to upwind achievable speed, probably the biggest drag force in fact, more than hull resistance. Yet Rick does not think there will be much in the way of heeling force. I do agree that heeling moment from a turbine is considerably less than a regular rig but i think it is more than what Rick is estimating for. If you've made any mathematical analysis, Rick, please post it so we can get an idea as to the sort of numbers you think the axial force would translate into heeling moment with a beam apparent wind. This would be very interesting.

I want to bring back the theme of the gyroscopic forces, because i think that it is the number one obstacle to creating a practical seagoing rotoboat. There are two factors that i will define before carrying on.
First Precession. This is what is called the effect of a spinning disk to react at 90 degrees to the torque applied.
Second gyroscopic effect which is the spinning disks reluctance to change plane of rotation.
The first is the worst, as it creates serious handling anomalies. Consider the case of a turbine that spins counterclockwise as seen from the eye of the wind mounted on a boat that is sailing on a reach on port tack. When a wave comes and attempts to roll the boat to leeward, the p effect makes the turbine respond by torquing it to starboard yaw. This is annoying at best and dangerous at worst.
There are several solutions to lessen the problem. The first and most obvious is decreasing the mass of the turbine to as little as possible. Secondly to keep it at low RPM (high torque, high pitch-less axial force anyways so better all ways).
Third, CR turbines eliminate all p effects. However there is still the gyroscopic effect which is quite considerable. Let me illustrate with another example: The navy experimented with gyroscopically stabilized ships. A metal flywheel was mounted inside a very strong metal housing which was solidly bolted to the ship's structure. It was found that with a flywheel as small as just one per cent of the ship's displacement a substantial effect was achieved. Unfortunately i cant' remember what RPM's they used, much higher than our turbine's of course. I'll try to find out, unless someone beats me to it.
Fourth, allow the turbine to move differently from the hull. The pylon can be counterweighted with the boat's ballast. Because impedance in the roll axis is not really detrimental and can even be beneficial the movement of the pylon can be constrained to independence for pitching only where impedance is VERY DETRIMENTAL. Basically the weight gets slung in a kind of sealed centerboard case with the pylon extending up from it. This is still not perfect but is a big improvement.
Fifth, by way of universal joints at the pylon head to allow the turbine completely free movement. This is a bit complex to engineer but i think the most elegant solution as long as the threat of the blades hitting the pylon in extreme movements be somehow designed out.
Sixth by using a VAWT system the mass distribution and RPM's are more favorable, but the efficiency is compromised... Material for a different post.

There is an article in a wooden boat magazine from about twenty years ago that describes a motorlaunch that got converted to windturbine power. They used the centerboard case counterweight system, but still reported p effects to be rather awkward to deal with. They also used a manually orientable turbine reporting (if i remember correctly) that the p effect prevented the practical use of a vane.

 

The thrust on the stator is not exerted along the axial direction, because it is actually part of the venturi inlet, and the pressure is uniform on all wall surfaces that are at the same section. What does develop is a counter-torque that tries to rotate the entire unit in the opposite direction, but since the entire boat (or vehicle) is connected to this it has little to no effect. If it were a problem on larger units, it would be easy to make two turbines of opposite direction and mount them side by side to cancel any forces.

The bicycle trailer is all done, and we are preparing to take her out this weekend to the desert. Pray for wind.

Michael

 

I like some of these ideas, but they may prove to be unnecessary if you simply put two counter-rotating units side by side, each cancelling the gyroscopic forces of the companion unit. I do like the idea of a pendulous support of the pylon with a universal jointed gimbal at deck level or so. This would lessen the stresses on the pylon from waves and the natural rocking and so forth from the vessel at sea. This also puts the ballast to work instead of just being dead weight.

 

Since everyone seems to be posting over here...

....I thought I would repost this from the other thread...

This website describes a turbine with some of the ideas I have raised earlier for the more efficient production of mechanical output.


http://peswiki.com/index.php/Directory:FloDesign_Wind_Turbine#Patents

Notice the use of a fixed stator at the inlet to create a swirled inflow that strikes the blades at a much higher angle of attack.

 

Michael, you can get rid of the p effect by using two turbines rotating in the same plane but in opposite directions, but you cannot eliminate the gyroscopic effect. And why would you want to put them side by side when, as i've just explained, if they're placed in line you get a net gain?
All those flow augmenters no matter how brilliantly designed have their drag cost. This makes no difference for land or at anchor energy extraction but it does under operation. There is no such thing as a free lunch. Besides all those duct and cowl ideas represent more weight up high and worse yet unreduceable aerodynamic surfaces. not so good for surviving storms on the ocean.
A simple bladed turbine actually has to endure very minimal stress if correctly designed, so can be made extremely light which is so very important to reduce the gyroscopic effects as much as possible.
If the blades are correctly designed they are under tension alone. If you look at a helicopter's blades you'll notice that they are pivoted to be entirely free to rise up. They stabilize at a certain dihedral angle when the vector forces due to centripetal acceleration balance out the vector forces due to aerodynamic lift.

 

The much maligned venturi.

Tcubed

(These statements refer to idealized venturis, not extreme cases like pinholes or lengthy pipleline etc.)
There is much misconception about what a true venturi is, and how they perform. Many people think that a venturi squishes the air through a tiny hole, and that it accelerates the air, this however is not really true. Air is sucked through a venturi, because the pressure at the constriction is very low. There are no differential pressures at each section along the duct, because each section shares a common wall, there is only differential pressure along the longditudinal axis. In an ideal venturi, the pressures are in balance from the inlet to the outlet. Air is not accelerated in the duct, it always has the same velocity, which is the speed of sound. The difference between stagnant air and air at the throat is that one has completely random motion of the molecules, and the other has polarized or collimated motion. This polarization is caused by the physical geometry of the venturi wall, and the air passing through it sees it simply as equivalent to other air molecules. This collimated motion appears to us as flow. There is no energy given to or surrendered by the air in a passive venturi, the process is known as adiabatic. Unless an object is placed in the flow and normal to it, no energy can be extracted. Venturis are limited by the speed of sound of the gas used, the inlet pressure, outlet pressure and geometry. This allows all kinds of variations, from rocket nozzles operating at hundreds of atmospheres pressure and thousands of degrees, to giant cooling towers which operate at a few feet per second.

As long as a stator has the basic venturi form, it will not create drag, it may create a torque, but this is for entirely different reasons. Cowling and jet ducting used on high performance aircraft do not loose energy at the duct, but at the turbines that feed high pressure gases to them. Augmentors are not venturis, although they look a bit alike, they are unbalanced, meaning the inlet and outlets are not even nearly the same size.

Michael