How many people have wondered if you can use a windmill to drive a boat? Many I suspect, but very few have ever tried it. You might think that history would be littered with unsuccessful "Windmill Boats" but that isn't the case. There are very few. This is surprising, because it is so simple – it’s not rocket science. You just have to connect the windmill to a water prop. Where is the difficulty in that?
It has been suggested that by the time the screw propeller was invented - steam power was well underway and nobody thought of going back to the wind in any way. Perhaps?
But the idea is not a new one. The earliest instance I have found dates from 1335 when Guido Vigevano made a drawing of a wheeled craft using the principle.
But the contraption was never built as far as is known.
In the age of wooden sailing ships, windmills on
land for grinding corn were very common.
Back in the 1980's I was curious. I made working models using a few ball bearings and wooden shafts making a crude gearing system that would connect the "windmill" to an underwater prop. These models were floated in a small water trough and placed between two houses where the wind blew something like a natural windtunnel. After some adjustment of blade angles, I succeeded in getting the windmill boat (floating on pop bottles) to progress directly against the wind - and quite well too!
Further models followed - small radio controlled boats which were filmed, and another design for a version which only had one single shaft. Details of which I published for any person who wished to prove to themselves that the idea was viable. (see below)
Progressing further, I purchased a 12ft "Seaskater" catamaran and mounted on it a 6ft diameter wind-rotor with a variable pitch which connected to an underwater skeg with a modified aircraft propeller to push it through the water.
It was a man-carrying boat. I could ride on it. It worked. Some video was taken of this craft in action which can be seen linked to the website www.windthrusters.net and its further links.
Direct into wind sailing was achieved and the rotor head could swivel to provide drive when the wind came from either side of the boat. Drive downwind was by the drag of the rig, in its feathered state.
A process of analogy
I was successful in finding the best settings, gearing, and blade angles by a process of analogy.
In fact the setup was exactly the same as a conventional craft such as a sailing dinghy sailing on a close-hauled course. How?
I considered one of the blades of the wind-turbine to be the equivalent to a sail operating in the normal way providing lift at a certain velocity through the air. I considered one of the blades of the underwater propeller as the keel or centreboard moving through the water at the same speed as the wind-blade is moving through the air. This was arranged by suitable gearing between the two. Because the water prop is much smaller diameter, then the rotational speed had to be stepped up to achieve these same velocities. Keeping to this idea, you don't end up with a wind-turbine whirling round at high speed, but a "sail" moving on a rotating course through the air at the same speed as the wind, and a propeller rotating at the same velocity underwater. The equivalent of a normal sailing boat.
Because of this analogy I called the system "Rotary Sailing" (This description has been usurped more recently by makers of Flettner rotors who call their products "Rotor sails" - so I will have to call my process "Rotary Sailing (Horizontal axis")).
Most of my work has been collected on my website at www.sailwings.net/rotaryhome.html
At the time I made this research, success in making a boat that could travel direct upwind driven by the wind was an amusing aside and a novelty which most thought was of no consequence.
The chances of such a system being adopted by the yachting industry were nearly impossible, given the traditional attitudes of conventional sailors.
But now, because of climate change and the drive to de-carbonise the shipping industry, this form of assistance from the wind starts to become important.
There is a movement to add wind assist systems to commercial ships, this is made imperative by rules starting to be imposed by the IMO section of the United Nations.
The ships are already steaming using conventional power systems, and wind-assist is an idea to reduce fuel consumption.
Since the ships are already making progress into the wind, then the apparent wind moves more ahead and a system that can produce thrust directly into the wind becomes more relevant.
Many companies have started producing wind-assist systems for large ships.
One of the most successfull so far is Norsepower, whose Flettner rotors have been fitted to several ships including a cruise liner.
Other companies have been fitting suction systems, - tubular towers with a slot which sucks in air which makes the tower act as a high-lift aerofoil. (These are based on the "Turbo Sail" pioneered by Jacques Cousteau in the 1980s). Others are proposing wingsails aligned to the best angle, often by computer control.
If you want to check the various types, the website of the International Windship Association is a good place. https://www.wind-ship.org/en/grid-homepage/
All of the systems so far make use of sidewinds to assist the ship in its progress, as indeed did all the sailing ships of old.
But Rotary Sail system I propose here can additionally add the ability to use headwinds.
If the wind comes directly against the direction a ship wishes to sail, both Flettner rotors, and suction towers, both of which are tall circular cylinders cause a lot of drag which the ship has to overcome. They only work producing lift at 90 degrees to the wind direction, so when going directly into the wind they cannot provide any assistance to the ship, only a vast amount of drag. Wingsails are better because they can be aligned to provide a minimum of drag, although they cannot produce any forward thrust when the wind comes from directly ahead..
A rotary sail system such as I have researched is unique in that it can provide thrust in exactly the opposite direction of the windflow.
Here below is a typical polar of a sailing boat which also applies to the Flettner systems and suction towers, proposed and fitted to some cargo ships so far.
You will see there is a large gap of 90 degrees where no power can be provided when the wind is ahead. A rotary sail system can fill this gap with positive power to push the ship. So before, when there was the "No Go Zone" which was described as an "unfavourable wind" exists no longer and every wind direction can be used to push the ship along. 360 degree windpower!
Such a craft need not fear anymore a "lee shore" because it can sail directly away from it.
The Rotary Sail system is the only system that will provide direct into-wind thrust, but it is not so suitable for using sidewinds, because of the high rotor speeds that would be required. This is what I found during my testing. Wingsails are still the most effective when using winds coming from the side of the ship.
This is why a "hybrid" type of ship has been proposed, which uses both self-trimming wingsails and also a horizontal axis rotary sail (both my fields of research) to provide the 360 degree wind thrust to the ship (General layout shown below)
More information about the Rotary Sail
Amateur Yacht Research Society