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

Discussion in 'Projects & Proposals' started by Duma Tau, Oct 9, 2006.

  1. MPraamsma
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    MPraamsma Junior Member

    They said it could't be done....

    Are you guys ready for this one...?

    Attached Files:

  2. Tcubed
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    Tcubed Boat Designer

    lets clear this whole mess up

    Wow, you are all extremely confused! (especially Rayk)No offense..
    First let me preface this by saying i was born on a sailboat (at sea) and raised on that sailboat. My father was an engineer and naval architect and I've been studying this field my whole life. My bachelors of science is in Mathematics and Fluid Dynamics, and i have also extensively studied physics, mechanics and structural engineering.
    First of all, The turbine car stops at the beginning and the fella says "oups i had the brakes on". He then relaunches it with the brakes released and at the end he brakes again and says so. Listen carefully this time. Believe me if not as i have my computer connected to big speakers.
    Now,.. of course a turbine boat will sail directly upwind exactly as a sailboat gets upwind through the manipulation of vector forces. The difference is that a sailboat must physically tack back and forth, whereas on the turbine boat the turbine alone tacks back and forth along with the underwater part the prop. Notice that none of the propeller blades go directly into the wind. They all follow angled paths relative to the wind, meanwhile the boat takes the overall (average path) In this case directly into the wind. It's possible because the driving surfaces are now no longer rigidly attached to the boat.
    Just think of this thought experiment: Imagine a sailboat tacking towards a destination straight upwind towing a mastless hobiecat, say, and that the tow line is a kilomet[/U][/I]er long. Imagine the sailboat does short tacks, no more than 200 meters per board. The towline then doesn't deviate more than six degrees or so either side of dead upwind. A helmsman easily keeps the towed boat heading straight upwind. Now think of the sailboat's sails and keel as the turbine and prop respectively and the towed boat as the turbine boat and you have a very close analogy of the turbine boat's force dynamics.
    How much faster than the wind can it go??
    Energy is conserved. Can we at least agree on that?
    So, the energy extracted by the turbine goes towards propelling the boat and the whole thing reaches a steady speed (0 acceleration) once all the energy extracted is being converted into all losses involved with maintaining that steady speed- Aerodynamic drag, hydrodynamic resistance, mechanical loss... We already know that sailboats can sail faster than the wind when close hauled. Even a good monohull can sail faster than the wind in light wind. Very efficient sailing systems, such as iceboats which have very little resistance, can and do, consistently achieve higher than windspeed VMG. I'll repeat that: Very efficient sailing systems, such as iceboats which have very little resistance, can and do, consistently achieve higher than windspeed VMG.
    The limiting factor is the sum of the aerodynamic and interface lift to drag ratios. However, in the case of a turbine boat, the powering surfaces are moving along different paths to the entire boat so this is no longer an issue. In fact if there would be zero resistance, zero drag, zero losses of any kind, the turbine system would accelerate indefinitely, straight into the wind, with an acceleration a function of the initial real windspeed. So in the real world the ratio (Boat speed straight into the wind wrt substrate)/(real windspeed over the substrate) is entirely a function of efficiency. I don't see why a highly efficient system couldn't go twice or maybe even more the windspeed into the wind.
    Now for downwind. If you go exactly downwind the apparent wind drops until you reach windspeed by which point apparent wind is zero, and you automatically slow down to let the wind "catch up with you again" so you can carry on extracting useful energy from it. Basically this is the turbine boat's slowest point of sail and no it cannot outrun the wind, unless it deviates from dead downwind which is material fopr another post.......
    In the video it is clear that conditions are quite gusty and that the vehicle smooths out these irregularities in the wind due to its inertia and rotational mass moment of inertia. The video would be useful if they sailed alongside a row of little flags so we could observe what the real wind is doing at that moment right where the vehicle is sailing.
    P.S. remember the only wind that is relevant to a foil is the apparent wind.
  3. Guest625101138

    Guest625101138 Previous Member

    You have missed the key point with regard to sailing downwind faster than the wind. The turbine is in the water and the propeller is in the air so roles reverse to the upwind condition.

    On another thread, I said it was difficult for most people to understand how boats can sail directly into the wind and even fewer can understand how it is possible to sail faster than the wind downwind.

    If you are competent in all matters of physics and hydrodynamics it is simply a matter of properly analysing the situation based on my first paragraph. Forget about apparent wind relative to the boat just consider the operating conditions at the AIR PROPELLER and the WATER TURBINE. (Just the reverse of a boat moving in a current extracting energy from the air and that energy being used to make the boat go faster so the air turbine spins faster so it can extract more energy. It is not perpetual motion. Performance is limited by the system efficiency and hull drag. It is easier to demonstrate on land because there is little wheel slip with the propelling wheels and the rolling friction is quite low compared with hull drag.)

    To say it cannot be done indicates a lack of understanding of what is going on.

    Once you have done the numbers then it would be a helpful contribution if you correct the misunderstanding in your earlier post.

    Rick W
  4. Tcubed
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    Tcubed Boat Designer

    Ok so i joined today and am not yet used to the layout. My apologies for this post that actually refers to the discussion on page 1 of this topic. i got a bit frustrated by the ignorance level and couldn't help myself. Afterwards i noticed how far back i was in the discussion.
  5. Tcubed
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    Tcubed Boat Designer

    Ok Rick W. I am all about learning and have no problem with admitting to being wrong, so please explain to me how it works. Specifically this; you're sailing the turbine boat exactly downwind and as you approach windspeed the apparent wind drops to zero. Now let's assume inertia is enough to "punch through to the other side" and now you're sailing exactly downwind but with an apparent headwind. It is a pivoting windmill so it doesn't matter, it just spins 180. However the apparent wind is Wr-V=Wa, so you are implying that the boat here must be extremely efficient (example Wr=10knt , V=11knt for 1knt app. wind plus it presumably must keep accelerating to get to a more usable amount of headwind say V=15knt for app. wind of 5knt and a V/Wa of 3 still somewhat improbable efficiency)
    Do you see my thinking ?
  6. Guest625101138

    Guest625101138 Previous Member

    I went back 6 pages to find rayk you referred to so was at a bit of a loss to work out what you were referring to.

    It takes a while to get to grips with any forum and it pays to step lightly until you have explored the history. Most things end up getting worked through many times over because there are always new comers who do not take the time to review the past. This is not a bad thing because there can always be new ideas and it pays to keep an open mind. We are all constrained by our learning. Doubt if anyone can put aside all their prejudices and what they believe to be true. We all work within some framework.

    Still it is worth the exercise to understand the SDWFTTW principle. I would like to see someone do it on the water. I was thinking about a demonstration but the air prop needs to be quite large to be effective even on a single person boat. I have too many other things to toy with. The amateur sailing research group in the UK who have demonstrated it on land do not seem to have taken to the water yet.

  7. Tcubed
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    Tcubed Boat Designer

    Good, point taken.
    I am definitely going to have to explore this concept of the turbine and the prop reversing roles. The first i'm thinking of right now is that turbines do not make efficient propellers and vice versa. This is because they're cambered a certain way which upon reversing roles is now -ive camber. So upon reversing roles the efficiency automatically drops...
  8. Guest625101138

    Guest625101138 Previous Member

    Just to be clear on definition. THIS IS IMPORTANT.
    A turbine extracts power.
    A propeller delivers power.

    Going downwind the air propeller is delivering power to the air. The water turbine is extracting power from the water.

    The turbine and propeller are geared in such a way that the rotating torque, under and condition, from the turbine (in the water) will always overcome the torque from the propeller (in the air).
    You have to think about the gearing between the turbine in the water and the propeller in the air.

    Think about the boat with the air prop sitting still and the wind pushing on all the super structure to drive the boat though the water at slow pace. The water turbine starts spinning. The gearing will cause the air prop to spin to drive the boat forward in the direction of the moving air stream. This adds speed. The water turbine generates more power which is applied to the air prop. And so it goes on accelerating until the energy that you can collect from the water matches the system losses including those from hull drag.

    Once the air propeller is spinning, the far field air conditions are different to the local conditions because it will be increasing the velocity of the air relative to the boat as it passes through the blades. It would not be adding energy if there was no change in velocity.

    The air turbine does not need to spin 180 degrees to become the air propeller. It is just a change in gear ratio so the torque from the now water turbine can overcome the torque from the air propeller. A practical system requires the ratio to change from 1:2 upwind to 2:1 down wind. With a CVT you could trim to get better performance. The small model that has been demonstrated would have worked better at larger scale and variable gearing. It needed a push to get going so the air prop was in a reasonable operating regime.

    One of the problems with a practical system is that the most efficient turbines and props have asymmetric props. So to get the best upwind and down wind you would ideally flip both the air and water blades 180 degrees.

    If you look at the model you will see it has a flag that trails once it gets going indicating it is exceeding wind speed. No need for course flags. This tells the story:

    Rick W
  9. Guest625101138

    Guest625101138 Previous Member

    You will see I made this point in the above post on this aspect.

    My view on all of this is that you really do not want a turbine/prop boat without some form of energy storage. There are lots of issues compared with sails. However wind turbines are good for continuous extraction and storage for a battery electric system.

    There is another thread on this topic and MP is in the process of building a reasonable scale upwind machine:
    He has some good ideas on ducting which I have not really thought much about.

    The more people that play with this idea with a proper understanding of the physics and blade design then the faster it is likely to develop. I am a keen observer but I am working on electric system with both solar and wind collection.

  10. Tcubed
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    Tcubed Boat Designer

    Rick, thanks for reposting the video and the thorough explanation. I had just finished reading all 200+ posts and was just about to go back to that video when i saw you reply. I have now looked at it with greater scrutiny and it is indeed as you say because of the angle of the blades, they are pitched for thrust and not to be turned by the wind. In fact were they to be pitched to be wind driven, by the time the vehicle reached windspeed the airprop would be acting as an air brake!
    I never have anyone to talk seriously in depth about aero/hydro (until signing up here) and i see it is good to be exposed to fresh thoughts!
    Now what i am curious about is the transition between the two modes and how this would graph on a polar diagram.
    Your point about making sure the camber is obeyed upon changing from DRIVEN prop to DRIVING prop is definitely an improvement on -ive camber but it would mean that the trailing edge becomes the leading edge and vice versa, so it is still somewhat of a reduction in performance, but now having understood the downwind concept i guess there's energy to spare and a bit of loss doesn't matter too too much.
    Your plan on using electric transmission and energy storage is very good at expanding the boat's flexibility and is no doubt worth the efficiency penalty for your particular requirements. Myself, i still like the "elegance" and efficiency of mechanical transmission. It seems no one has yet commented in depth on the various options here, so let me just quickly point out the obvious: Bevel gears, apart from being inefficient, will not work with windvane selfadjusting rotorsails due to it then wanting to "walk" around due to torque... The efficient alternative is the twisted chain or belt but it must be remembered that the rotor must be"untwisted" every 360degrees, something that you also avoid with the electrical transmission.

    Also, i must add to a previous post where someone commented on your rudder being very small especially considering that the main contributor to lateral forces will be due to the turbine, which you have conveniently placed at the extreme rear. I must profess that the whole wetted surface thing has been blown way out of proportion ever since the craze of cut away forefoots began.. You will get more resistance from an overloaded small foil than from a lightly loaded larger one despite the little extra wetted surface. Remember that optimal lift/drag for a zero camber foil is typically less than 0.1 coefficient of lift. You want your foils to be operating at more or less this value under typical operating conditions so as not to incur a hefty induced drag penalty. Besides in rough conditions the yawing torque of the waves can be tremendous and i'de much rather dominate the boat with a generous rudder than be struggling for control with an undersized one. Also i must add that surface piercing rudders do NOT ventilate IF they are BIG enough to do the job. If you think that by tucking the rudder under the counter you've made the rudder much more effective and ventilation proof, you are only partially right. If the rudder is so small that were it to be surface piercing, it would ventilate, then it is too small for under the counter too, basically overloaded..Also being removed from the surface is no help against cavitation. Remember too, that if you move the rudder forward one foot that is one less foot of moment arm you have and the rudder should be correspondingly enlarged to preserve the desired yawing moment. Nuf said....
    Adding the solar panels definitely rounds out your design. (although that is off topic.)
    Another factor that i think has been under mentioned is the ratio of power to drag ratio of the turbine. Specifically, that the smaller the pitch the worse this number gets. Shallow pitch creates higher RPM's with less torque, Steeper pitch makes for slower turning turbine but with more torque. somewhere in there one can surely optimize the power, but keeping in mind that the smaller the pitch (high RPM turbine) , the more force it will create along the axis of rotation-it's not really correct to call this drag at this point, since it is actually due to the lift (normal to the foil's chord surface) of the turbine. This is why-as has been pointed out earlier-sailing with a freely rotating propeller is worse than locking it, a point lost to many present day sailors....
    Therefore i would tend to favor high pitch (of the order of, at most, two times windspeed=tipspeed) turbines with a corresponding gear ratio to compensate, in order to minimize backwards thrust from the turbine on most points of sail. Lower RPM's also has the great advantage of drastically reducing vibration and noise.

    Thanks again for revealing to me an entirely new concept. Now I want to know (let's for the sake of argument assume that in this case the boat has no energy storage capability) what you think the polar would look like of the threshold of mode transition......
    Last edited: Sep 11, 2008
  11. Windmaster
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    Windmaster Senior Member

    This all seems unnecessarily complicated


    Nice to see new contributors.
    Tcubed seems to have everything right in the main part of his first post.
    I was not sure about his position on the ddwfttw video - whether he said it is possible or whether he did not believe it.

    Rick, as always, brings in his electrical solutions, which are not without merit, but serve to confuse and complicate the issue.

    Most agree that upwind for a turbine boat, directly against the wind is no problem.

    Downwind faster than the wind. Here is the SIMPLE way of visualizing why it is possible and in fact, no problem.

    Given that upwind directly against the wind works ok. The interface vehicle in the water receives energy from the relative movement of the air which is converted to move the vehicle through the water.

    Downwind faster than the wind works the same but the other way round.
    The interface vehicle receives energy from the moving ground (it has not been done on water yet) which is converted to move the vehicle through the air.

    Note: in the last case people will say "but the ground is not moving!"
    Well, the ground is not moving relative to them. But the ground is moving relative to the vehicle. Unlike an outside observer, the vehicle does not know whether the ground is moving or the air is moving, it only sees relative movement. Which allows it to move through one or other of the mediums.

    Tcubed mentioned that there is a problem of the turbine head wanting to precess around a vertical drive shaft on a mechanical system. If you have seen a picture of my wind-turbine boat somewhere in this forum you will see that I used a belt drive which eliminates a vertical shaft, and therefore did not get this problem.
  12. Tcubed
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    Tcubed Boat Designer

    Yeah, windmaster, at first i did not believe it because i was still thinking (assuming, dangerous!) that the airprop was driven instead of driving. Upon closer inspection of the angle of pitch i understood exactly.
    But it now opens a whole other issue as to how exactly do you operate the beast? I speculate that at some point of sail round about a broad to very broad reach one must do the switch in order to stay on the better of the two curves.
    Basically there's a polar curve corresponding to driven turbine mode and another curve for driving airprop mode for each windspeed. The locus of the points of intersection of these lines would determine when to switch modes. But what would it actually look like? I will over the next few days/weeks attempt a mathematical solution or at least a good estimate of what we could expect...
  13. Guest625101138

    Guest625101138 Previous Member

    Without going into a lot of detail on how things have developed since these original posts I can provide background as well as make a couple of possibly useful observations.

    Boats have been a hobby for a long time but it was only the last 5 years that I decided to educate myself on the possibilities as part of a retirement project. To that end I progressed from model boats to pedal powered boats.

    I have now got these boats to a nice state of development. My latest design set a 24 hour record this week:

    To get to this stage I have learnt a lot about hull shapes and lightly loaded, high efficiency propellers.

    I have only built one wind turbine to play with and gave myself a bit of a scare with its power in high wind. I damaged the blades and have not yet rebuilt.

    The main development in going from my lightly loaded boat props to a turbine is the higher velocity ratios to extract reasonable power for size and the Betz limit. Now MP on the other thread has suggested ducting and this overcomes the Betz limit.

    Issue is I have not really developed correct far-field conditions into my turbine modelling. In any event ducting overcomes some of the limitations. Anyhow I have the dilemma that my modelling is not as accurate as I would like for the turbine.

    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.

    This is all you need to determine performance in any particular wind speed but suggest it be at least above 3m/s. In practice you cannot achieve the efficiency figures given for all points of sailing and all wind conditions. In particular the air turbine/prop will operate at Re# that is not conducive to the nominated efficiency. With ducting I believe it is possible.

    So I have a large gap in my knowledge regarding turbine ducting that demands some worthy analysis. Hopefully MP will do some testing to provide data that I can compare with a model.

    If you are not familiar with JavaProp, JavaFoil and Michlet/Godzilla then you should take time to educate yourself on these tools unless you already have similar tools. They are all very useful in developing VPP for turbine/prop wind boat. Sadly I have not found anyone doing a good turbine model so I need to do my own. (I do have my own prop optimisation model and VPP for pedal powered boats that is accurate to about 2%)

    Rick W
  14. Guest625101138

    Guest625101138 Previous Member

    Should add that my models are mostly based on basics physics using an analytical approach rather than empirical data. None of them are purely analytical because things like viscous resistance on hulls are based on measured data. The results for this compare well with my results and I have not seen CFD output that betters it.

    Rick W

  15. TeddyDiver
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    TeddyDiver Gollywobbler

    But please tell me where the energy is coming from in this case.. "Not moving ground" water will gain more acceleration due the "screw" working in it (so far I'm not ready to call it either propeller nor turbine). Air screw ;) on the other hand by it backwardsfromship propulsion brakes wind down.. The only sensible outcome of this is a negative acceleration just with the same principle as Tcubed explains the positive acceleration in the upwind case. "In fact if there would be zero resistance, zero drag, zero losses of any kind, the turbine system would accelerate indefinitely, straight into the wind, with an acceleration a function of the initial real windspeed."
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