Thrust greater than drag

Why does it sound impossible? A conventional sailing boat can sail directly into the wind, it just does so by tacking. Essentially there is no difference between a conventional sailing boat and a wind turbine powered boat. Both generate motion by extracting energy from two moving fluids, they just each use different methods to do so. One uses a linear wing and propeller arrangement, the other uses a rotating wing and propeller arrangement. No magic here.

 

Well, Solario, in the supplement in your post #38 you can easily check the validity of your statements. The writer of that document knows what it's all about; use the equations. Just substitute his transmission efficience of 95% with the more reasonable 60 à 65 % for a propeller with its transmission, and a realistic drag estimate for a boat hull and the paraphernalia necessary to hold the turbine in position (and the hull upright). Then see what surplus there is left to propel the boat, simple!

When done with that excercise and still inclined to trust your gut feelings instead of physical evidence, there is nothing that stops you from producing solid evidence in the form of a prototype to prove your point; just go ahead and do it instead of endless rantings! But don't wait for the applause the community of NA's and ME's until you have proof. Besides, when I go sailing, I do it for the pleasure of silence, absence of machinery and for the sake of harnessing the force of Nature by the simplest of means; a textile sheet, some rope and a wooden (ok, metal...) stick. If you sail to pay for your salt, then the propulsion has to be reliable, controllable and safe in all conditions; your suggestions don't tick any of those requirements....

 

It's quite possible that you can get good efficiencies from a wind turbine setup. Just looking up some numbers, aircraft propellers can get up to 90% efficiency, and the larger diameter and the slower it turns, the better. So a wind turbine may be able to get better than 90% efficiency, added to which the turbine blades are often of higher aspect ratio that aircraft propellers, so higher efficiency again. For the propeller, on a wind turbine boat you should use a large diameter slow turning prop, again for grater efficiency. You should be able to get up towards the same efficiency as for an aircraft propeller. Even standard water propellers can get over 70%.

For the transmission, all you need is three shafts and two sets of bevel gears. If the shafts run on ball bearings, efficiencies of up to 99% are possible, and bevel gears are about the same. So the transmission losses can be quite small.

Regarding hull drag, I mentioned earlier that a sail powered boat has to travel about 1.4 times the speed and assuming similar hulls, the hull drag will be about twice that of the propeller powered boat. The sail efficiency will be less than that of a wind turbine blade, due to the lower aspect ratio of the sails. The weight of the wind turbine is likely to be more than that of a sail rig, however if built of lightweight composites, and if the size is optimised to be the minimum possible, there might not be a lot in it. For stability you will need a catamaran hull for the wind turbine boat, because heeling will degrade the efficiency of the wind turbine, when on courses other than dead upwind.

Given the above, I don't doubt that a well designed wind turbine boat will have no trouble sailing dead upwind faster than a sail powered boat. It's not as if wind turbine boats haven't been built and shown to work. I imagine that a well designed and efficient one would not be cheap though. It will need additional features, like the ability to feather the turbine blades.

 

A lot of speculations; put relevant numbers into the equation and check the balance. It is not a question whether it can be done, but for what purpose and to what expense. Now, if you have no doubt about the performance, why don't you go ahead and prove its success in real life then? Come along, stop talking and get it done, wise man!

 

I don't have any real interest in building a wind turbine boat. I was just countering your more pessimistic estimates. It's a discussion. Anyway, the concept has already been proven by others, so I don't need to do so myself.

 

I don't understand why increasing size would, in principle at least, result in drag and windage that would inhibit forward progress directly against the wind. Other things being equal and assuming wind velocity constant with increasing height the power produced by a wind turbine is limited by Betz's law and is proportional to the square of linear dimensions. This power drives a water propeller that produces thrust to oppose hull drag and aerodynamic drag, the latter comprising the windage of hull and superstructure plus drag on the wind turbine and its blades (the 'tower force' in wind turbine parlance). So for the minimum windward boat speed at which a wind turbine boat could be considered to be functional, i.e. a speed to windward just above zero, the hull drag is effectively zero and the aerodynamic drags are proportional to the square of linear dimensions. Other things being equal, the thrust force that a water propeller can generate at zero boat speed (i.e. 'bollard pull') is proportional to power input so as the linear dimensions of the craft are scaled up the thrust from the water propeller increases in proportion to the aerodynamic forces that it needs to overcome so for a simple analysis there is no size at which the propeller becomes unable to balance the aerodynamic forces. If one takes into account the increase in wind speed that occurs with height due to the atmospheric boundary layer then a windmill boat scaled up to a large size can potentially perform better than a small one, this is one reason that wind turbines for power generation are being built in ever larger sizes and the big ones are now built on a scale comparable to large commercial ships. As for the stability of a wind turbine boat, that increases with the forth power of linear dimensions while the aerodynamic heeling moment increases with the third power so a scaled up boat has a stability advantage, this also applies to a conventionally rigged boat. I suppose that one ultimate limit to the size of wind turbines is the blade tips moving out of the earth's atmosphere at the height of their trajectory.

A number of wind turbine powered boats have been built over the years and the best of them have performed well enough to satisfy their owners. An example of a smaller one was built by Amateur Yacht Research Society member Peter Worsley- see Rotary Sailing Homepage http://www.sailwings.net/rotaryhome.html. Larger examples that were briefly described in the December 2022 edition of newsletter of the Amateur Yacht Reseach Society are Jim Wilkinson's catamarans 'Revelation 1' and 'Revelation 2'. Revelation 1 was built in 1982 and was based on a Prout 'Siroco' catamaran, 7.9m LOA fitted with a 6.1m diameter three bladed wind turbine. The six bladed water propeller had a diameter of 0.9m and pitch 1.0m, the gear ratio between wind turbine and propeller was 1.652, the propeller turning the faster. Speed on all courses was around 4knots in true wind of 6 to 8 m/s, increasing to 5.3knots reaching with 8.8m/s true wind speed. Revelation 2 was a larger version at about 11.0 m LOA.

In the above I have used the term 'wind turbine boat' since most people describe the modern generation of wind energy devices as wind turbines rather than windmills but ideally I would prefer the term 'windmill' for a device that produces power from the natural wind. My professor of Mechanical Engineering made a clear destinction between 'windmill' and 'turbine' explaining that a windmill operates in free air so its power is limited by Betz's Law whereas a turbine operates in a duct and is not subject to Betz's law. To me this is a clear and fundamental distinction between these two types of energy conversion devices so I would prefer the modern electricity producing wind power devices to be called windmills - they are an evolution of earlier windmill designs used for processing cereal crops and also pumping water, sawing timber and other purposes. I am pleased that Wikipedia includes a page titled 'Windmill ship' but I guess it is too late to stop the general public describing windmills as wind turbines.

 

The drag is primarily a function of the volume of the hull, and speed; ie length^3 for volume and length^n for speed. The propulsive power, on the other hand, is a function of swept area or similar, ie length^2. This means that there is a crossing point in physical size, where the "budget" becomes negative. You'll find similar limitations in many systems, where dynamic forces interact with static forces; you can trust Jehardiman on this......

For instance, this is the reason an adult elephant can't jump; the "upscaled" size has resulted in a body mass proportional to length^3, while the square area of its muscle fiber bundles (delivering the necessary acceleration to overcome gravity) has increased with length^2.

 

Hull drag generally scales with the second power of linear dimensions, not the third. But no matter, if we are considering wether or not there is a 'cross over point' at which the boat fails to move through the water we can ignore hull drag since at such a point the hull drag would be zero. If the propeller thrust can balance the aft acting aero forces for a small size boat then it will do so for a larger boat since all these forces increase with the square of linear dimensions.

Agree about the jumping elephant but that's another matter.

 

In my world, the resistance of a surface vessel is mainly a combination of friction and wave-making; the former a function of surface, ie L^2. But the wave-making is scaling differently, since it is a function of displacement, ie L^3. What is argued here is not station-holding or even the possibility of propelling a wind-driven vessel directly upwind, but whether the arrangement is indefinitely scalable.

The non-jumping(!) elephant is quite relevant here, because its "dynamic structure" is not scaling with the same factor as its "static structure". The same goes for a surface vessel.

 

Agree that wave making drag complicates drag calculations to some extent although as you scale up at constant boat speed it becomes an ever smaller part of total hull drag so I think it is the skin friction drag that matters here. I dont want to argue this for ever but surely if a wind turbine boat can, in principle at least, be indefinitely scaled up and still hold station against the wind (or of course do a lot better than that) then there is no 'cross over point' at which the propeller thrust becomes less than the contrary aero forces.

There are of course some structural and logistical problems that would apply to building a very large wind turbine powered vessel but the size of modern wind turbines shows that wind turbines built at the scale of large ships are feasible - from the internet the current largest wind turbine is 280m tall which is a height approaching the length of the largest ships (not as heavy though).

A speaker at the recent Innov-Sail conference commented that the wind ship industry has just taken a 100 year break and will soon be back in business. It does seem that within, say, the next couple of decades we will see real commercial interest in wind assisted ships, or even fully wind powered ships, rather than just the small scale and mainly 'greenwashing' activities that we have seen to date. But my guess is that the new sailing ships will not be wind turbine powered. Wing sails or even conventional fabric sails would seem to be a simpler solution for big vessels just as they are for smaller vessels.

 

Just for fun, lets do a quick 'back of envelope' calculation for a big wind turbine powered ship. Take that 280m tall wind turbine that I mentioned in a previous post and stick it on top of a 400m long ship of 200,000 dead weight tonnage, which means a considerably larger loaded gross tonnage. At some stage it might be an idea to check stability but lets not worry for the moment, stability of ships against wind loading tends to improve with size. That particular wind turbine is projected to produce 80GWh of electricity in a year. If I have done the sum right that's a mean output of about 9130kW. If mounted on a ship it might actually do a bit better since it could take advantage of weather routing but we will ignore that. Assume that power all goes to an electric propulsion motor with losses of 10%, that probably also allows a bit for 'hotel loads'. So on average we have about 8200kW of shaft power to drive the ship. A fossil fuel powered ship of that size and weight might have 30MW maximum shaft power and a service speed of 14knots. I am not a ship designer so I am not sure what percentage of maximum shaft power is actually used at normal service speed so lets assume it is 100%. So our hypothetical wind turbine ship has 8.2MW vs. 30MW for the fossil fuel version. The wind turbine will add to the ship's wind resistance but if we start by considering the apparent wind to be on the beam we can ignore that. Since power requirement for a ship is roughly proportional to cube of speed the wind turbine ship can be expected to make about 9knots rather than the 14knots for the fossil fuel ship. We dont have to worry too much about going to windward since wind turbine vessels have proved to be at least as fast on a dead to windward course as they are reaching. The weak point of such a ship could well be heading down wind when the apparent wind would be reduced by the ship speed, weather routing could help with this. 9 knots rather than 14 is a considerable reduction in speed but it is recognised that if the talked about reductions in shipping CO2 emissions are to actually be achieved there will have to be a significant reduction in the speed of most ships (hydrofoil sailing ships perhaps excepted) and 9knots or less may well be the new normal. With a ship of that size its still a lot of cargo being shifted. This is of course a simplified analysis.

 

Let's stick to the basics. Hull size is determined by the needed righting moment. Haliade X 12MW has a nacelle weight of 675t sitting 150m high. To this we add the blades, another 165t, plus they are 107m long and moving. Then there is the tower the nacelle sits on, you need to add that to the overall weight.
Or we can take the new Siemens Gamesa 14MW, it's nacelle is around 100t lighter at only 500t, the blades 1m longer at 108m.
Nacelle height is site specific, but it can't sit lower then the blades, so we are talking at a minimum about 650t sitting around 120m above deck. You can make the tower in carbon fiber if you like, it's still going to add a few hundred tons to the total.

The hull needs to safely support the turbine in storm conditions taking into account following limitations: under no circumstances the hull can be allowed to heel enough for the blades to touch water, and your draft is limited to a maximum of around of 24m.

 

Sorry incorrect, it is not a squared function.
Baeckmo gave you the correct guide..but you seem unwilling to accept basic facts.
If you want more of a detailed breakdown as to how this is derived....you can read HERE.

 

Your calculations assume that the speed increases along with the scaling of the resistance. However I think in this discussion we need to consider that the speed does not increase significantly with scaling up of the hull, because wind speed doesn't increase when to boat gets longer. Furthermore you calculations assume equal parts wave making resistance and frictional resistance. Generally this is not the case. For narrow hulls e.g. multihulls, the frictional resistance can be as much as 80% of the total. So for equal speeds the resistance will generally scale at a bit more than scale-factor ^2.

Another consideration is that in the real world we don't scale boats proportionally. Longer sailing boats tend to be relatively lighter than shorter ones. Consider the proportions of a 420 vs the Maltese Falcon. People don't get bigger as the length of the boat increases and we only need to increase stability by the square of the length, not the cube, because the sail area squares with the length. This makes a significant difference to the resistance increase with length. We know for a fact that longer sailing boats go faster than shorter ones, so we can conclude that in reality, resistance is increased by less than length squared. Given that the performance of a windmill powered boat will scale the same way as for a sail powered boat, there is no reason to suggest that the windmill boats performance will be degraded with length any more than a conventional sail boats performance.

 

You clearly have no idea what you're talking about.
May I suggest you read several, not just one, books on Resistance and Hydrodynamics...to understand your ignorance on the subject.