Thrust greater than drag

The string stops it sailing away directly upwind. The wind is coming towards the camera.

 

You're right, I shouldn't have bothered commenting on your post.

 

There have been a few sail systems tested in ships, and many proposed. Of the systems trialed so far, most have been either some variation on a wing sail, or Flettner Rotor devices. Relative to the ship size, these devices have been quite small and intended only as fuel saving devices, not as the main source of power. Despite the small size of these devices, fuel savings of five to ten percent have been claimed. As far as I am aware, no-one has tried using a wind turbine for this purpose.

As I understand it, the argument against wind turbines as the primary power source at ship scale is that because residuary resistance scales with the displacement (length cubed), you cannot generate enough power with a wind turbine to propel the boat. There is an issue with this argument: For geometrically similar hulls, the length cubed scaling factor for residuary resistance only applies if the same Froude number is maintained for both the small and large vessels.

I have done a bit of a study to see whether wind turbine power can be scaled up to ship size. Initially I made some powering estimates for Jim Bates’ ‘Tango’ (see video earlier in this thread). From the video, I estimated that the catamaran was somewhere between 12 and 14 metres long, and I had a guess that the displacement might be around 6 tonnes. I made use of a method for making ball park estimates of hull resistance, which I derived from the results of some tank testing I was involved with many years ago, together with results from various research papers. I used this method to get an estimate of the residuary resistance fraction, as well as estimating the required power.

In the video, it is claimed that Tango could do 8 knots directly into the wind. Based on a 12 metre waterline length and 6 tonnes displacement, I estimated that to do 8 knots, Tango would need around 7 kW brake power, plus the power required to overcome the drag of the turbine. I did a search to for a commercial 10 kW wind turbine, and found that the swept diameter is 10 metres. This agrees with the video, where it is stated that Tango had a 10 metre turbine. At 8 knots, the Froud number for Tango is 0.38, and the residuary resistance makes up close to 50% of the total.

For the ship comparison, I chose a Large Capesize bulk carrier, because I was able to find some powering information for one. I used a ship length of 270 metres and a displacement 200,000 tonnes. For the same Froude number as Tango at 8 knots, the ship speed is 38 knots. At this speed I estimated that the residuary resistance makes up 97% of the total resistance, and the brake power to reach this speed is close to 4,000 MW.

Bulk carriers of this size operate a maximum speed of around 14.5 knots, a Froude number of 0.14. The powering information I have for this size ship gives a brake power requirement of around 17 to 18 MW (depending on propeller choice). My own estimate of 20 MW for the ship at 14,5 knots is reasonably close to that figure, and it shows that the residuary resistance makes up about 60% of the total, which is in the same order of magnitude as for Tango.

So, in order to power a ship of this size we need a wind turbine that can generate close to 20 MW. While there are wind turbines of that size in the design stage, existing offshore wind turbines produce a maximum of 10 MW of power. These turbines are 150 metres in swept diameter. On a 270 metre long ship, it would be possible to install two such turbines along the length of the ship.

Wind turbines generate peak power in 15 knots of wind. From this we can conclude that, with the wind abeam at 15 knots, the ship would be able to achieve 14.5 knots. Directly into the wind, the drag of the turbines will reduce the maximum speed achievable, but the ship will still travel at a respectable 12 knots at half power. Directly into the wind the ship is generating apparent wind of its own. If the ship is able to travel at 12 knots, then theoretically it could travel directly into the wind in three knots of true wind speed.

Of course, offshore wind turbines generate electricity, so is this a good choice for ship propulsion? Marine electric motors are available up to 24 MW, and the efficiency of electric generators and motors converting mechanical to electrical energy, and vice versa, is over 99%, so it’s entirely do-able.

I would conclude that not only is it possible to use wind turbine propulsion at ship scale, but it could be done with existing technology. There is still a question as to why you would do it, given the development costs and commercial risk. The answer would be that rising fuel costs and the move towards sustainable energy solutions may make wind power viable again, so solutions such as this will have to be considered.

Anyway, don’t take my word for it, I’m allegedly ignorant of such things. Feel free to do the calculations yourselves.

 

I sent earlier posts in this thread with comparable calculations to those you present above. Since we are considering a ship that will not actually be built any time soon I thought I might as well think big - so I picked 400m LOA and maybe 350,000 gross tons. And I was thinking in terms of a sailing ship that would rely 100% or close to 100% on wind energy with alternative propulsion only for emergency use or possibly for harbour use. Clearly there is no requirement for such a vessel at the present time, but in the future who knows how that might change. Although most current windship projects are targeting fuel savings of around 10% there is at least one project for a full sailing ship. The Wallenius shipping company Oceanbird project aims at 90% power from wind. Mind you, the wingsails on the current drawings of Oceanbird seem to have shrunk relative to what was originally proposed - are they giving up on the principle?

Since I am considering a very large heavy ship travelling at less then 10knots, hull drag would be increasingly dominated by skin friction as linear dimensions are scaled up. Skin friction and windmill power both scale approximately with the square of linear dimensions so such scaling considerations should not limit the maximum size of a windmill powered ship. There may of course be other limits!

So I did an internet search for the largest wind turbine in the world, this came up with one in Denmark rated at 15MW and standing 280m high, but looking again I see that there may be even larger ones under construction or even operational. For example a Chinese one expected to start electricity production next year. The way the wind turbine industry is developing, by the time any large commercial windmill powered ship is launched windmills of this size may be considered rather small! I then considered that a single 15MW windmill would fit nicely on a 400m long 350,000 ton cargo ship. The average power that turbine would produce over a year would be about 9MW and I estimated that this power would give something like 9knots allowing 10% for auxiliary machinery, hotel loads and losses in power transmission to the shaft (I think looses would be more than MalSmith sugests) Mind you, the speed at time averaged power is not exactly the average passage speed since the gains due to above average wind speeds would be less than the losses due to below average wind speeds. More detailed analysis would need to be based on actual weather data for likely routes. Perhaps the big ship naval architects here can say if 9knots is at least in the right ball park. I then looked at the heel angle of such a ship with strong wind on on the beam and that seemed rather minimal, which is perhaps not surprising since other things being equal righting moment for a given heel scales with the forth power of linear dimensions. A catamaran configuration appears to be unnecessary and would have greater skin friction.

Downwind might be the least favourable course. Here is a question - assuming that variable pitch is available for both the wind and water rotors would it be an advantage, theoreticaly at least, on a downwind course to use the water rotor to extract energy from the water flow and power the windmill to augment thrust? I havent got around to doing the sums on that one yet!

So, rather to my surprise, this all seems basically feasible, at least to this point. I realise that such a slow ship would not be commercially viable today, but the commercial and legislative environment could be very different in a few decades time, i.e. within the expected lifetime of ships being launched today.

MalSmith sugests having two wind turbines on a large ship but I am not sure about that. I think that when the apparent wind is away from the beam there would be interference between two rotors so close together. That is the reason for the carefully optimised spacing for wind turbines on a wind farm. Going directly to windward I suspect that total power from two rotors one ahead of the other would not be much more than a single rotor of the same size (presumably the two rotors would be contra-rotational) There might also be vibration effects from the effect of one rotor on the other? Given that heel stability does not appear to be a serious problem I think I would prefer one large rotor to two smaller ones and this would also give some benefit from the wind gradient with height, although at hundreds of meters height that may be a diminishing return. All pies in the sky!

It occurs to me that operation in confined waters could be a significant advantage of a windmill powered ship over a sail/wingsail powered ship, or a Flettner rotor ship. I have watched big container ships going in and out of Felistowe port in the UK - I cant imagine a wingsail powered ship tacking and gybing then coming neatly head to wind alongside the quay, presumably with all the cranes rolled clear - if that could be done they could certainly charge spectators to watch! A wind turbine ship on the other hand could have an array of electric thrusters allowing it to move in any direction under computer control, so docking would be relatively simple as long as there is some wind - if no wind it would just have to wait outside port as ships always used to do in the good old days. And when the ship is alongside in port or when waiting for cargo it could sell electricity to the grid. Mind you, it might be safer to have the rotor stopped if loading by crane - and with a three blade rotor and the lower blade vertical there would be little obstuction to cargo handling.

Another advantage of a wind turbine ship is that large wind turbine technology is well established whereas large wingsail technology is still at an early stage with many differing concepts being trialled and rather complicated arrangements being considered for reefing. I expect that some further development would be needed to make a wind turbine suited to ship mounting rather than seabed mounting but at least the basic rotor and power generation seems to be well developed and capable of withstanding extreme wind speeds.

A proper comparison of a windmill powered ship with a sail/wingsail powered ship and also perhaps with a Flettner rotor ship would be interesting. If anyone here can offer funding I am in contact with a University department that would certainly be interested!

 

The 99% eficiencies I mentioned were for the generator and motor only. I used a QPC of 0.65 for the powering calcs.

I did consider that contra-rotating would be the best option. Generally, there are net gains in efficiency with contra-rotating propellers, although the wide spacing may affect that. I have seen a proposal to put two contrarotating propellers on the same mast, one on either end of the generator nacelle, but I assume that design has not been progressed much. Seemed like a good idea though for keeping the swept radius smaller. One large rotor may be better, but 20 MW units are not available yet. They will be 200 metres in diameter when they are.

I meant to mention in my earlier post that a two bladed turbine may be a better option. When docked, the turbine could be parked vertically, which would keep it within the confines of the beam of the ship.

I checked up on the definition of 'windmill' and 'turbine'. The term windmill specifically refers to a wind powered grain mill, although the usage has been extended to other devices. A turbine is any device that converts the kinetic energy of a moving fluid into another useful form of energy, e.g. mechanical. So 'turbine' is the more general term and is the more correct term to use in this case.

 

I'm very surprised to learn that Hillary went to the South Pole in 1958. If Bates suggested his windturbine boat to him it must have been before he went. So we come out with the fact that Tango must have been built some time in the 1960s - so long ago! Since the boat was apparently so successful there is no reason why another similar could be built nowadays, there is nothing rocket science about it! Someone should make one!