Thrust greater than drag

I will go with MalSmith's comments rather than Ad-Hoc's since I am assuming that the speed of a large wind powered ship relative to wind speed will not change significantly with the size of the ship.

It sounds as though Rumars knows a lot more about wind turbines than I do so thank you Rumars for your information about the weight of wind turbine components, that allows me to take my 'back of envelope' sums a stage further.

I did a quick internet search and found a paper 'Extreme load estimation of the wind turbine tower during power production' by Atsushi Yamaguchi et al, September 2019. This give predictions and measurements for the total tower bending moment for a certain wind turbine at the Choshi demonstration site in Denmark. For this 2.4MW wind turbine they expected maximum tower bending moment near the base in a 25m/s wind to be in the region of 35MNm. I roughly scaled this figure for the greater tower height and greater rotor diameter of the 15MW turbine I considered in my post above and obtained a figure of 470MNm.

I then considered that a 400m long ship might have a beam of 60m and a draft of 15m giving a gross tonnage of say 350,000 tons. The centre of bouyancy could be around 8m above the keel or 7m below the waterline. The centre of gravity will be affected, at least to some extent, by the weight of the wind turbine high above the deck. Let's assume that the nacel and blades are 1000 tons at 150m above the deck and lets take the tower to be another 1000 tons at say 60m above the deck. Those are heavier weights than Rummars provides but I am thinking that a ship mounted wind turbine would need to be more robust than a seabed mounted one. That shifts the centre of gravity of the 350,000 ton ship to about 0.6m above where it would be if the wind turbine were removed, that's not a big shift given the scale of this thing. So lets assume for an initial calculation that the centre of gravity is at the waterline although this clearly depends on how the ship is loaded, we may not want to stack too many containers on the deck of a big wind ship and we may need plenty of water ballast when running light. And we may not need the weight and windage of a conventional bridge structure if the ship is piloted by a computer and any crew on board are there mainly for maintenance work. I think the metacentric height works out to be about 20m above the centre of bouyancy so 13m above the assumed centre of gravity. From that I make the ship's heel angle resulting from the aerodynamic loading on the wind turbine at 25m/s wind speed to be about 0.6 degrees which does not seem much to wory about. Let me know if I have slipped a power of ten somewhere!

Please note that I am not proposing that wind powered ships should necessarily be wind turbine powered, a lot more research would be needed to make a decision on that and I would expect that clever people will be looking into these matters. A wing sail powered ship does on the face of it seem a simpler option than a wind turbine powered ship. I think the Wisamo wing sails would look rather splendid on a 400m sailing ship. On the other hand large wind turbines are becoming a mature technology whereas large wing sails are not at that stage. A wind turbine powered ship would need the complication of a propeller but I imagine that any large future sailing ship will in any case need that for emergency propulsion, together with a power plant running on green fuel (Hydrogen/Ammonia/Bio-diesel, whatever). Or maybe we just go nuclear?

(Note: I have edited this post since the windspeed for which Atsushi Yamaguchi et al predicted approximately 35MNm bending moment at turbine tower base is 25m/s, not 50m/s as I originally stated. Not sure how I came to make that error! I would have thought that for designing an ocean going wind turbine ship one should consider design maximum wind speed to be well above 25m/s and so one could expect extreme heel angles due to turbine wind forces to be perhaps several times higher than the 0.6 degrees that I calculated. Even so, such heel angles would seem to be quite modest and would not invalidate the concept of a large wind turbine ship. Have I made some other slip up here - let me know if I have.)

 

I don't claim to be an expert on the subject, and it has been a while since I was involved in managing any tank testing programs, but I did spend many years analysing and interpreting tank test data as part of my job as hull designer for a fast ferry company. So forgive me for thinking I can speak with some authority on the subject.

I don't have an issue with your analysis per se, only with it's application to this particular problem. In my opinion, we are not comparing apples with apples, inasmuch as using my previous example, you wouldn't apply the resistance curve of a 420 to the Maltese Falcon.

If you wish to criticise my logic, that is perfectly fine, but please don't attack my character, it adds nothing to the discussion.

 

And yet, you make endless claims...

Managing, is NOT an a prerequisite to understanding of the science involved....

No problem, but, once you make this statement, do not profess to make any statements based upon said subject...in which you are clearly ignorant.

Nothing of the sort, that is your own misguided interpretation of the words and inferences noted.
Engineering is based upon facts, not emotions.

I cannot control what emotions you have/feel...

 

Fair enough. As I'm not qualified to speak on this subject, can you please explain the observed fact that longer sailing boats are faster than shorter ones, given that sail area only increases with the square of the length increase, and resistance increases at greater than the cube of the length increase. If it is true that longer boats perform better than shorter ones, can you please also state whether or not this applies to windmill powered boats, and why.

 

You started off ok...with the short v long, but..then you begin to conflate the whole (complex issue) by bringing in variables which are nothing to do with the statement.

Sails, per se, are "in air"....
The hull, is " in water"...

You are introducing two totally separate issues into the short v long debate....which is perhaps the source of you confusion and ignorance.
A sail is not a hull..and a hull is not a sail..and they are not governed by the same mechanisms and certainly do not scale and the same manner.

 

I wouldn't have thought it was that complicated. If the available thrust is increasing with the square of the length and the resistance is increasing at greater than the cube of the length, then longer sail powered boats should be slower than shorter ones. But they are not. My point is that longer sailing boats are faster not because the scaling factors you mention are wrong, but because in general longer boats are not geometrically similar to shorter boats. Therefore we can say that for broad estimates of performance in a real world situation, we can use a resistance scaling factor of less that the square of the length when relating it to the sail area or windmill swept area. It would be an empirical scaling factor taking into account the general geometric dis-similarity between boats of different lengths, much the same as we use for rating formulas.

The other issue is: can we apply the same logic to windmill powered boats as we can for sail powered boats? I believe we can, but that is in the absence of hard data.

 

And that's why you won't be able to understand...as you are biased and ignorant from the outset.
It cannot be boiled down to a simple one liner - which 99% of posters appear to want. Almost like a gotcha moment....

May I suggest you read and study resistance and hydrodynamics before answer any questions on a subject you know nothing about.
Then you may be able to frame a question with an open and unbiased mind...and build on the information you have learnt from reading said books.

 

I should be glad that I'm not the only person subject to your contempt. Anyway, this discourse has wandered significantly from the topic of discussion, so I'll accept my ignorance and apologise now for any stupid things I may say in the future. Hopefully we can get back to discussing windmills.

 

The direction the ship can go in is surely and important consideration. The windmill ship would have the advantage that it could advance nearly or directly against the wind despite the extra complication. This is, I would suggest a great advantage. On the other hand the wingsail ship would have a quadrant of approximately 90 degrees where it cant progress - I would say this is a disadvantage. Everybody assumes that it is either a windmill ship OR a sail or wingsail powered craft - how about combining the two in a hybrid (never before done) such as proposed here: https://windthrusters.net/images/hybridship.jpg"target="_blank"

 

I think this is a good idea. The arrangement proposed in the sketch you have linked seems to show that the windmill does not need to rotate into the wind, which would on first glance seem like an advantage. But thinking about it further, on courses not directly upwind I think the windmill is going the be less efficient. Also on down wind courses you would need to do something about the fact that the windmill would spin in the opposite direction, like a gearbox to reverse the direction, or lock and disengage the windmill. I think it would be better to be able to rotate the windmill into the wind so that it is always providing power.

I thought of an arrangement whereby the windmill is mounted to the leading edge of the wing. The windmill nacelle could be made to rotate a bit so that it was always pointing directly into the airflow when the wing was set at an angle of incidence to the airflow, for best efficiency. If using multiple wings, each wing could carry a windmill.

 

Ok guys...Solario, MalSmith...prove to me that a windmill boat can complete for the Jules Verne Trophy....I'll wait...

 

You might be missing the point a bit there. A windmill system is not going to be very useful on a lightweight high performance boat. It would be much more useful on a heavier displacement boat which can't go much above 'hull speed'. On such a boat it would give you the ability to travel at much the same speed on all points of sail, including directly into the wind. It's just an interesting idea that may be worth exploring.

 

No, I'm not...you say it is better than what exists...prove it, good bad, evil.... I know that all the vessels you have listed have never been beyond their local bounds.....What you quote are toys..., nothing else. As a professional, I would never recommend a windmill propulsion system...all oceans, all situations... You may accept the death other persons to prove your point, but I cannot espouse that. Windmill vessel exist in a very narrow system of conditions...outside of that they are deadly...so therefore a responsible naval architect cannot recommend them. SHOW ME THE MATHS....and I know you can't....so let us just end it here.

 

Have you seen that video



Jim Bates built his windmill cat towards the end of the 20th century. He says he sailed 4000 miles on it. Hardly a toy. He also says it was the most stable boat he ever owned. Looks convincing - are you saying he was not telling the truth?

 

2:28 "In 20 knots, heading directly into the wind, it gets dangerous at 8 knots speed, with the wave action "

True, its not a toy.