The Wind Powered Sail-less Boat

Connecting an Air-turbine to a propeller

There's nothing wrong with this idea, but what do you do if you have any kind of failure? You have no back up. Since it is so simple to mechanically connect the turbine with the prop, why would you not want to do it
? Mechanical connection is such an elegant system. Simplicity is everything in the quest for reliability (KISS). It would be so simple to arrange such a system as a fall-back.
Of course, storage of power whilst moored is a great idea, but with a mechanically connected system it is possible to have the turbine running continuiously (head to wind) at reduced power to relieve anchor loads and prevent downwind drift onto a lee shore.

 

For a small boat with a slender hull in windspeed of 5m/s moving directly into the wind.

The turbine has a nominal pitch of 1128mm, the propeller has a pitch of 648mm. Both are operating near 87% efficiency with low slip and low velocity ratio. The connecting shaft is doing 415rpm and mechanical transfer efficiency is 94%. The boat will be doing 4m/s through the water. The boat has a drag of 80N to achieve this speed.

The drag on the air turbine is 150N so total boat drag is 230N.
The total air power applied is 1350W.
The useful power collected from the air turbine is 1183W.
The power delivered to the water prop is 1112W.
Power delivered to the boat to move it is 920W.

This highlights the power transfer requirements. The boat would only need to use 320W to do the same speed without the turbine drag.

So if you take the case where the boat sits at anchor in moderate winds with the turbine delivering at 1000W to a battery you have collected enough energy in 1 hour to operate for 3 hours. This highlights the benefit of storage.

I also add that getting the turbine and propeller to both achieve 87% efficiency is not trivial design either.

Rick W

 

A mechanical linkage is not all that simple. In fact it is highly constraining on the boat design. It also really needs to have variable ratio to get the best of the system. In low wind the effective gear ratio has to be more than 2:1.

As a pure sailing system without storage it offers little advantage over a typical sailing vessel. It has curiosity value with the ability to sail directly into the wind. However if you really think about this it does not mean much. Even a power boat will avoid going directly into waves when it can avoid it. Taking seas on the quarter softens the ride quite a lot.

Rick W

 

I wonder if you can clarify two things.
Firstly, you quote pitch settings in mm. and I am really hoping to get an idea of the angle of attack of the blades relative to the rotational axis, for this I need to know the diameters of the turbine and the propeller in your example.
When you talk about turbine drag, which drag are you referring to? the drag against rotation, or the "axial drag" along the line of the rotational axis? Sorry to be a little slow on the uptake on this!

 

To clarify, I presume you are saying the airturbine rotates twice for one rev of the prop? (Not sure which way round you mean this). Do you mean that a gear ratio of more than 2:1 is a bad thing?

 

The blades are based on thin cambered sections. The AoA under design condition ranges across the blade. Usually it ends up slightly negative in the range -1 to -.5 degrees.

The pitch angle is a simple mathematical function related to the geometrical pitch and the radial position:
pitch angle = atan[pitch/(2* pi* radius)]
The maximum radius of the turbine is something like 1.5m. The propeller is about 300mm in radius.

When I refer to drag it is the force applied by the air or water against the direction of travel.

As far as the gearing goes I stated initially for the system described that the turbine and propeller rotated at the same speed. The water velocity past the propeller is 4m/s and the air velocity past the turbine is 9m/s. They are both turning at the same rotational speed. So for roughly the same operating point on the foils the pitch on the air turbine is 1128mm and the pitch on the propeller is 648mm. The foils are not the same and operating conditions are a little different because of the different fluid properties.

Your other question regarding the 2:1 required for low wind speed. You need this sort of ratio because the efficiency of the air turbine drops at low wind conditions so if the gearing is not increased the boat stalls as the turbine cannot generate the torque required to spin the prop. When the wind speed is higher you cannot take advantage of the better overall system efficiency. My best design will operate with gearing as low as 1.4:1 under the most suitable wind conditions.

Having variable gearing means you can actually go faster for the given wind strength than if the gearing was set 2:1. So ability to change the effective gearing between the turbine and the propeller is required if you want to get the best overall performance.

Variable gearing can be achieved with variable pitch prop but these are sub-optimal for all but the design condition. Having variable transmission is a better solution I believe. As the speed builds and the apparent wind increases you can reduce the ratio and go even faster. The limit is the power transfer capability.

Rick W

 

well reading all the replies, I take it you have the info you might need, or that i may have been able to help you with...

BUT, going to the UCT mechanical engineering expo of thesis' I found a project with sounds exactly like the one you have.

the wind turbine, was horzontaly mounted (it looked somewhat like a windmill-well the ones you find in South Africa-they pump water from a borehole, this shows this type of 'prop' has a lot of power) and his huls were, well, trimaran. He had a normal propeller to drive the craft forward.

Best of luck with the project

 

A sailing craft sailing faster than the wind achieves this by tacking, since it must have an apparent wind at an angle off the bow.

Can a fan-driven craft achieve FTTW speed by simply heading directly downwind from a standing start, or must it first achieve FTTW speeds by tacking or using an engine?

 

Faster than the wind downwind with a "fan-driven" watercraft is possible in theory, but as far as I know has never been achieved in practice. However, on land, a model has been able to run downwind faster than the wind and this can be seen on youtube. The land model can do it from a standing start and does not need to be assisted.

 

Yes. It is complete reverse of the upwind case. You have a water turbine and an air propeller. For 1:1 mechanical connection you need the prop to have roughly half the pitch of the turbine. The downwind system is really incompatible with the upwind system because of the compromises. For example you need a wide gear ratio and you would be restricted to symmetric foils.

As soon as the wind starts pushing the boat the gearing enables the water turbine to overpower the air propeller and the propeller assists in moving the boat forward through the air that is already moving. Ideally you start with a large gear ratio and as the boat picks up speed and the prop gains efficiency you can back off the gearing.

Rick W

 

According to theory, but in fact it has never happened. If you can do it, you will achieve world fame!
Jack Goodman's demonstration of land-based dwfttw received worldwide admiration, respect from AYRS and and also some disbelief.

 

I doubt that world fame will ensue. You have not read through the youtube comments. The world is full of skeptics and most of the comment is negative. A bit like Windmaster's skepticism of anything with a little maths.

It is actually easier to do it on water because the initial friction is zero. Doing it on land is harder because there is static friction. The boat will start moving as soon as there is a slight breeze. As long as the gearing is right it will start accelerating.

The air propeller has to be quite large to be effective because it is operating at quite low velocity. It is not compatible with sailing upwind.

So other than curiosity it has no merit. Boats already sail downwind faster than the wind only they cannot do it directly down wind.

It is interesting to have a look at the speeds that the Aeolus Racing cars get to and compare with land yachts:
http://www.ato.nl/?category=wind&id=30
You can see all that mechanical complexity and performance way below the conventional sail.

By the way, take note of the design of the turbine blades and have a look at the Youtube explanation of the turbine design of the winning machine.

On land the ability to sail at any point has merit but on water you have much more room so does not make a lot of sense.

I believe any useful system for boats will have energy storage. That is where the system outperforms existing sailing vessels - the ease of energy storage.

Rick W

 

Here is a video of a FTTW turbine on a downwind run; it's a model land yacht not a boat but you can see the wind tell tale reverse diection as it speeds up: http://www.youtube.com/watch?v=aJpdWHFqHm0

For further reading there are several threads on turbines:-
Another idea thread http://www.boatdesign.net/forums/projects-proposals/another-idea-1289.html
http://www.boatdesign.net/forums/pr...ats-how-many-out-there-they-viable-14182.html

 

This is the Aeolus Race winner:
http://www.youtube.com/watch?v=IWNEXBBAGNw
It gives you an idea of the best turbine blades. The comment made by the student about 1/3rd in is spot on.

The best turbine for a boat would have the same proportions as would the best propeller. When you get foils like this the performance really starts to improve. They can deliver close to 90% of the input power as useful output.

Rick W

 

Well, I have read through the youtube comments. Do you base success or failure on the commenters on Youtube? It's rather sad if you do, I doubt that any of them are informed enough to make serious judgements.