Artemis Technology--Sustainable Future

Discussion in 'Boat Design' started by Doug Lord, Jan 29, 2019.

  1. OzFred
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    OzFred Senior Member

    Perhaps you meant that sarcastically. The (entirely theoretical) Artemis ASV uses stored energy and propeller.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    No

    Whatever "that" was........
     
  3. OzFred
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    OzFred Senior Member

    Using a turbine in one medium driven by kinetic energy in another medium is very different to using a sail. There are zero videos or even theories of a sailing boat with a sail being able to sail directly upwind, or sail faster than the wind directly downwind. Even for a turbine driven vessel to achieve that feat will require very special conditions.
     
  4. OzFred
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    OzFred Senior Member

    The wonders of modern technology provide links to the quoted post. I'm sure you're very well aware of what the quoted post is.
     
  5. OzFred
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    OzFred Senior Member

    Which shows that beyond a critical angle downwind, speed is rapidly lost. Exactly as Ian Perry predicted (which wasn't that difficult to predict). :)

    Given the accuracy of GPS, particularly for speed, and instrumentation measuring apparent wind, it should be a trifle to accurately determine the true wind for the boat at any instant. Especially if they are correlated using the GPS time signal. Even free programs like GPS Action Replay do a pretty good job of estimating apparent wind purely from boat velocity. Anyone who can write software to resolve GPS data to speed and position can do that in their sleep.
     
  6. tlouth7
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    tlouth7 Junior Member

    I think kerosene's point was that a boat with a sail moving at some angle of attack is fairly equivalent to one of the blades of a propeller on a vessel moving directly downwind, because that blade by its rotation achieves that same angle of attack. His video illustrates this point very well, and demonstrates how a real yacht which can sail with VMG downwind > windspeed supports the concept of a vessel that achieves DDWFTTW (which Artemis are not claiming to do anyway).
    This only works in areas of water with 0 current. It is more correct to use a log on the boat in question (and some calculation of leeway) to derive TWS & TWA.
     
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  7. OzFred
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    OzFred Senior Member

    I understand that comparison, however a sail can't achieve any angle of attack going directly down wind faster than the wind, at that point the apparent wind is from directly ahead.

    And while it may be theoretically possible for a boat to use the same principle as the propeller driven carts to go faster than the wind directly down wind, there is zero evidence that anyone has done it. I think the added drag of the boat in the water and inefficiency of paddles in water vs wheels on the ground preclude it in practice. But I'm happy to be proven wrong about that. :)

    Yes, data from a log (i.e. speed through the water) would help, as would heading vs course made good, so not just GPS + apparent wind. :)
     
  8. tlouth7
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    tlouth7 Junior Member

    Unless I have missed something I am pretty sure no-one is trying to claim that.

    This is my understanding as well. I imagine the maths to show whether a particular configuration would work is not particularly hard, you just need to find various drags and thrusts.
     
  9. sharpii2
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    sharpii2 Senior Member

    <I appreciate your post. But I believe you have an imperfect understanding of how these things work (as I may have). My understanding is that one must have a vector difference if one is to extract energy from a moving fluid. There is but one exception to this rule. That is if one is simply allowing a portion of one's machine to be carried along by this moving fluid (like a square sail before the wind). In this one case, the portion of the machine will always be moving slower than the speed of the fluid.

    When I was a boy, I once made a windmill powered cart out of Tinker Toys (r). Its design was unusual in that the windmill portion, which was not allowed to pivot in relation to the cart, was oriented sideways so its axis was at a 90-degree angle to the forward travel of this cart. I wanted to see if it could travel upwind despite this arrangement. I had the plastic vanes of the windmill oriented at a 45-degree pitch.

    I took this contraption outside where there was a substantial wind blowing. The pressure-fitted parts of the windmill quickly flew apart. But not before the cart had advanced several feet--dead upwind.>
     
  10. tlouth7
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    tlouth7 Junior Member

    If I understand what you are saying correctly then yes, I agree that there must be some relative movement between the mechanism that is extracting energy (boat, turbine) and the fluid it is extracting energy from. Consider that Work = Force * Distance, so no distance, no work. This relative movement is achieved by the yacht moving at an angle to DDW such that there is always apparent wind, and by the moving blades of the air-propeller in a DDWFTTW vehicle.

    If you are being carried along by a fluid (e.g. a cork in a river) then no work is being done so this is not an exception to the rule. You need two fluids, moving at different speeds, to extract any work (analogous to how you need two reservoirs of heat to extract work in a heat engine).

    Not sure what your point is here, this will work if there is some difference in flow speed between the top and bottom portions of the turbine, or some non-zero angle of attack.

    This is what happens any time we go sailing, but is quite hard to describe without diagrams. Consider the case of sailing on a beam reach in still water. You have no component of velocity in the direction of the wind, and yet clearly you are extracting work from it. As you go faster the apparent wind shifts forward until eventually the angle of attack on the sail is too small to provide sufficient thrust (forward component of lift) to overcome drag.

    Now consider the case where your heading is a broad reach (45 degrees below beam reach relative to true wind). As you accelerate the apparent wind shifts forward. When boatspeed (STW) = SQRT2 * true windspeed (TWS) the apparent wind will be 45 deg off the bow, and VMG(DDW) = TWS. The vector diagram here is a right angle triangle with TWS downwards, STW as the hypotenuse (direction downwards), and apparent wind horizontal. VMG is the downwards component of the STW vector, which can be seen to be equal to the TWS. Clearly boats are capable of sailing with AWA 45 deg off the bow, so all that is necessary is low enough drag to sail 1.4 * AWS in this configuration, which the fast foilers (and some skiffs) can easily manage. Again the only limit to STW (and therefore VMG) is how close to the bow the AWA can get before drag beats thrust.

    The reason this doesn't work with no true wind is because the AWA would always be dead on the nose. Effectively what the true wind does is shift the apparent wind aft (for all sailing angles other than dead down- or upwind) which provides an angle of attack for the sail to operate in.
     
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  11. tlouth7
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    tlouth7 Junior Member

    Now to address the mechanism for extracting work in a DDWFTTW vessel:

    First remember that Work = Force * Distance, or more usefully Power = Force * Velocity

    Consider a craft that has very high air drag (e.g a very large spinnaker) but very low water drag. If you point it DDW it will accelerate to almost TWS. Its velocity relative to the air (va) is very low, while its velocity relative to the water (vw) ~= TWS. Imagine our vessel has a water turbine connected to an air propeller.

    All power generated by the turbine is passed to the air prop (ignoring various frictional losses). You can see that we have a turbine working at low force and high velocity connected to an air-prop acting at high force and low velocity. Thus we can extract a net thrust from the turbine/prop mechanism. This wouldn't work at 0 TWS (even if we towed it up to some starting speed) because there wouldn't be a difference between va and vw, and so we couldn't have the difference in force.

    An interesting idea to achieve this would be to have a forward facing scoop underwater joined to a backward facing nozzle above. This could develop a net thrust from the water jet pushing on the air. Edit: I have convinced myself that this wouldn't work after all.
     
    Last edited: Feb 13, 2019
  12. sharpii2
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    sharpii2 Senior Member

    Tlouth:

    I believe there is a considerable amount of confusion caused by comparing the treadmill cart and the downwind windmill powered cart to a sailboat.

    As an example, suppose Team Artemis made a vessel which was extremely light and had a propeller which was as efficient at extracing energy from the river's flow as the wheels on the treadmill cart were at extracting energy from the treadmill. Now suppose this prop was connected to a much larger propeller on a mast and geared to get the maximum efficiency out of both props. Let us further suppose the drag imposed by the hull was similar to that of the rolling resistance imposed by the cart's wheels.

    If all these suppositions proved to be true, this craft WOULD actually make progress up stream, even though there is no vector difference between the river's flow and the boat's ustream travel.

    This is because there is definitely a vector difference between the blades of the water propellor and the river's flow. Also, as the boat heads upstream, the water prop experiences greater flow velocity which one would expect would pump more energy into the system. One could not be blamed for assumming this could go on forever with the boat going ever faster and faster upstream. But this would clearly not happen because the vector difference between the river's apparent flow and the water prop's blades travel is getting smaller and smaller. The smaller this difference gets, the less energy the system can extract.

    This is like when one is sailing and wants to get upwind quickly. So one points the boat further and further upwind hoping to cut the sailing distance. What one quickly discovers is that, after a point, the closer the boat sails to the wind; the slower it goes. This is because the vector difference between the wind and the sail is getting smaller and smaller. A more experienced sailor knows that sailing less close to the wind often gets him to windward faster than sailing the boat as close to the wind as it will sail. Do you agree?

    Suppose this windmill boat I have just described manages to make about 1 kt progress upstream and Team Artemis is celibrating the huge amount they won from a bet with a hedge fund manager. The hedge fund manager then bets Team Artimis (TA) double or nothing that they cannot accomplish the same feat with a sailboat. Should TA accept the bet?

    I say nooooo! Please don't.

    This is because a sailboat is nothing like a windmill boat other than the fact they are both powered by the wind.

    I will try to explain why by supposing TA takes up the bet.

    They construct the most efficient sailboat the world has ever seen (I know. Been there; done that).

    But they fail. ): ):

    This is because, with a 10 kt river flow and no wind, their sailboat operates in reverse. It is the keel which extracts the energy and not the sail. The sail then imparts this energy onto the still air and this causes lift and moves the sailboat across the river. The boat moving across the river imparts some apparent wind, which enables it to go a little faster. This extra speed is used to enable the boat to steer a bit up stream.

    What happens next? does the boat speed up further? Or does it slow down? As with the hedge fund manager, my bet is that it will slow down. This is for two reasons. 1.) The vector difference between the keel and the river flow is getting to be too small. And 2.) it is imparting less energy to the sail (which is moving slower relative to the still air).

    Now one might ask: " Isn't the same thing true with the windmill boat?"

    To some extent, yes. The same thing is happening with the blades of the windmill and those of the water prop.

    The difference is that, with the windmill boat, the boat is moving through the water much slower than than the blades of either the windmill or the water prop. The poor sailboat, on the other hand, has to go as fast through the water as both the keel, through the water, and the sail, through the air. This, by the way, is why a windmill boat can sail directly upwind and a sailboat can't.

    Does this make any sense?
     
  13. tlouth7
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    tlouth7 Junior Member

    Perhaps, certainly there is a distinction between a normal boat which has its sails stationary on any given point of sail and a DDWFTTW craft with moving parts.
    Absolutely. What you call vector difference I have been calling angle of attack.
    I disagree with this point. Because the propellers are spinning there is no reason that the angle of attack (vector difference) should change with boatspeed. In fact for a turbine of fixed pitch and fixed rpm the AOA would increase as the boat moves faster. The reason that you reach a steady state speed is that drag goes up until it balances the thrust.
    Again I do not agree. It is possible to luff arbitrarily far while maintaining constant AOA between the wind and sail, simply keep sheeting in (a traveller may be required). The reason that the boat slows down is that the lift (generally considered perpendicular to the chord of the sail) rotates as you sheet in which reduces the forward component (thrust). As above this is not an issue for DDWFTTW vessels.
    Indeed, for the simple reason that a boat with fixed sails cannot sail directly upwind (w.r.t. apparent wind).
    I find this to be an odd way of thinking about it, for the simple reason that it suggests that the mechanics of sailboat operation in current+still air are different to normal. In fact there is no difference from sailing in wind+no current, such that someone on board who cannot see the shore could not possibly tell which situation they were in. Therefore I tend to work on the assumption of stationary water (the more familiar situation) and then trivially convert to still air.
    My bet is that it will speed up. Starting from a beam reach (heading across the river) as the boat points more upstream it is bearing away. This allows the crew to ease the sheet, resulting in a larger forward component of lift. Note how for most boats the fastest TWA is greater than 90 degrees e.g.:
    life on the layline: 49er Polars from TacticalSailing.com http://lifeonthelayline.blogspot.com/2012/02/49er-polars-from-tacticalsailingcom.html
     
  14. sharpii2
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    sharpii2 Senior Member

    Not really. This is because the boat is speeding up because the turbine is spinning faster. Yes, the boat speeding up does widen the vector difference, but by an ever dimminishing amount. Eventually it gets down to fractions of a degree, to the point of not being suffient to overcome the turbon blade's own additional frictional drag through the water. This, by the way, is why even a windmill type craft may not be able to do this. The water prop is too damned inefficient (about 0.40 at best) to extract enough energy to spin the air prop at sufficient speed to do the job. Theoretically, this is possible, but practially, it probably isn't.

    Although its true that, with a better vang, a sailboat can sail closer to the wind. It is also true that a sailboat often makes better upwind progress when sailed at a less oblique angle to the wind than most oblique one it can manage. This is because the greater distance it has to sail is more than made up for by the additional speed.

    All this relativity is getting me relatively confused. But my reasoning goes like this: We are asking this sailboat to move, relative to the land, in the direction opposite that of the water, which is also moving relative to the land, by using the still air, which is not moving relative to the land. Therefore, the only energy there is to extract is that of the water which is moving relative to the land. Using this reasoning, I have come to realize that it is theoretically possible for a sailboat to do this. But it must have extremely low drag and a very efficient lateral foil, as well as a very efficient wing sail. If the vertical lift foils have drag similar to that of the rolling resistance of the wheels of the windmill cart, the boat can tack back and forth across the river at a very high speed. In doing this, its upstream gains may be greater than its down stream losses. This would certainly not look very glamorous, as it would appear that the boat is merely going back and forth across the river. Only after several tacks (maybe many) would it become apparent that the boat is further upstream than when it started.

     

  15. kerosene
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    kerosene Senior Member

    Ok hear this:

    Narrowish canal from east to west. Still water. 10mph wind from east. How fast do you think a super hyper high tech foiler can travel east in said canal (naturally tacking). Distance made good - or whatever the term is - eastward component of the speed.

    Above is exactly analogous to the still air 10mph current state.
     
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