NABO Editorial February 2009
Mr Stuart Sampson - Readers of Waterways World may have seen an article making claims for a new design of propeller manufactured by a firm called axiom. They may also have shared my initial skepticism and disbelief that anything further could be done with the Marine Screw, since it has been honed and refined ever s ince Brunel built the SS Great Britain, and probably before. It has such functionality and beauty that it is  ashamed to hide it under a boat. Surely a propeller with blades shaped like spades can't out perform something that looks at one with its fluid environment.
The familiar marine screw blade is made with a sinuous twist so that the pitch is the same over the full length of the blade. Nearer the middle, where its is moving the slowest as it revolves, it is at a shallower angle to the flow than it is at the tip. This means that you would expect on each revolution of the few a cylindrical, flat ended slug of water is passed through it. The length of the slug is the pitch of the propeller.

Wouldn't it be wonderful if that happened in practice? Unfortunately it doesn't and it isn't too difficult to understand why if you look at the screw fro end on.

When a blade of the screw rotates it pushes water away at right angles to its inclined surface, and this is not just backwards, it is also outwards at a tangent. So, as well as as causing what the mathematicians call "Grad", the intended pressure gradient backwards, it also causes what hey call "Div", Divergence, and Curl, a twisting motion.

Instead of the ideal cylinder of water, what actually comes out is a rotating cone of water. The divergence is not only wasteful of energy but it also annoys the environmentalists, stirring up the roots of their beloved aquatic plants and muddying the waters for the fish. It also draws more water than necessary from under the stern of the boat causing the dreaded "squat".

Apart from the squarish outline of the blades of The Axiom Screw, one is also struck by the lack of twist of the blades. This means that the pitch of the blades is less towards the middle of the theoretical slug of water leaving the screw is concave ended because there is less flow through the middle. I can see this counteracting the divergence effect and effectively focusing the stream, improving the general efficiency. I would also assume the screw need ti draw less water through nearer  the middle and so the streamlining of the boat's swim would be less critical.

How the claims of the Axiom's zero, prop-walk come about is not so obvious, mainly because the explanation of how the effect is caused in the first place is far from obvious, although one would expect the curl to be involved. The notion that water is denser for the lower blades is fine as aide memoir, but doesn';t hold water (sorry), as water doesn't compress to any measurable extent except at extreme depth differences. In all other respects everything is symmetrical, so the blame must go to the prop's surroundings, notably the counter above the prop. This funnels the water into the upper blades so it is already traveling through them quicker and the blades have less bite. This is quite a reasonable idea because the effect invariably fails when the canal bottom is closer, acting like the counter, but below - just when you are relying on prop-walk to tuck your stern into a mooring!

I can't see why the Axiom should be less prone to this effect by Design. Axiom claim a rather special blade cross section which improves efficiency in reveres and certainly this will help, but, short of a contra-rotating design, the blades must be trying to put some curl into the water and be less loaded when they are uppermost. Of course the behavior of water round a  screw when suddenly reversed is bound to be complex and turbulent, and if the Axiom can establish an orderly flow quickly under these circumstances then there is much less time for the prop walk effect to become established and noticeable.

I am keen to see the Axiom in action. If anybody has tried one, please get in touch.