Waterways World December 2008 - Narrowboat Editorial

Could a new design of propeller improve your boat's handling and reduce wash?
MARK LANGLEY tries it out

When buying a boat, most people are very choosy about the equipment that is fitted -specifying in detail everything from shell thickness to the type of tap in the galley. But one item that is often taken for granted, and, after the shape of the hull, arguably affects handling the most, is the propeller. Virtually all boats are fitted with standard marine screws, usually made of a phosphor-bronze alloy, though sometimes aluminium or even plastic on outboard motors. Whatever the material, the basic shape of these props would be very familiar to shipbuilders of a century ago. Alan and David Watts are a father and son team, who, when using their converted lifeboat at sea, found that the conventional propellers would not allow it to reach hull speed in rougher weather. The marine screws ventilated (drawing air in from the surface), so they set about designing a new propeller that would give better performance. Waterways World has had exclusive access to both the development and testing of this new propeller - the Axiom. Looking very different to marine screws, the blades of the Axiom are more "spade-like". They are also designed to give equal thrust both ahead and astern, unlike marine screws. David and Alan have been testing different combinations on both yachts and narrowboats over the last few years, and it was on a very cold and windy day on the River Nene that we were invited to test several different propellers on the same 58ft narrowboat, Roebuck. We used two GPS receivers to measure speed.

TRADITIONAL PROP 
Firstly, we took out the boat for a 45-minute run, using the standard propeller, as originally fitted to the Liverpool Boats shell. The handling was about average, with the boat pulling a sizeable bow wave and some stern waves, when running upstream at 1,600rpm, making 4.5mph. 
We winded the boat in a fairly tight side-stream, which took several attempts, and then tried to handle the boat astern. In the relative shelter of the trees, we built up to 4mph and then did an emergency stop. It took 18 seconds for the boat to come to a complete stop, with severe slewing to port as we did so. Going downstream at the same engine revolutions, we made 5mph, with slightly less stern wash than our journey up stream.

AXIOM PROP 
Once back at the mooring, Alan was ready kitted-out in his drysuit to dive under the boat to change the propeller. Not something for the faint-hearted! Happily it took him only about five minutes to change the marine screw for the Axiom propeller, and we then set out on the same course as before.The first thing that was noticeable was the reduced noise at the stern and inside the boat. The tiller felt much lighter, and looking astern, the area where the 'thrust' of the propeller surfaces in the wake was much further back than with the marine screw.Underway, 1,600rpm gave us almost 5mph, and looking along the boat, we could see that the bow wave was noticeably reduced. This, in turn, meant that the stern was not squatting down in the water as much. The stern was about 2in higher than using the marine screw, so the boat was almost running level.
At the weir stream, we winded again, and I was taken by surprise by the sudden amount of reverse thrust generated - and the boat began to steer backwards!Once we had built up to 4mph forward and conducted the emergency stop again, the time of 13 seconds (compared to 18 previously) illustrated that this prop certainly can give more thrust in reverse. When doing the emergency stop, the boat did not swing to port. Going back downstream, we averaged 5.5mph at the same engine revolutions.

TIPPED PROP 
At the moorings once again, Alan dived to change the propeller. This time a tipped Axiom prop was used: the edges of the blades are T-shaped. Tipping propellers is not new science, though this was the first tipped Axiom fitted to a narrowboat.Underway, it was obvious that, although the helm was now even lighter, more engine revs were needed to achieve the same speed, while the prop seemed to pick up weed more. The thrust in reverse was not as good either. The tipped props are not really suitable, certainly at this stage, for use on narrowboats.

CANAL TESTING 
Our tests on the river seemed to prove that the Axiom design gave better handling, far greater astern thrust, and a pronounced reduction in bow wake and stern squat.
This test was restricted to the comparatively deep Nene, rather than the shallow waters of canals. However, Alan Buckle, the owner of Roebuck (who has no connection with Axiom) took a long summer cruise from the Nene, via Birmingham to Sharpness and back. The outward trip was made using his standard marine screw, the return with the Axiom.On the way back, Alan noticed that he was able to cruise faster, making less wash (again, measuring with GPS). On passing moored boats, he observed that the boats were pulled less: this would probably be due to the way the thrust from the propeller affects the alignment of the hull to the water. He also reported not having to go down the weedhatch on his trip back, unlike the frequent visits on the way to Sharpness.

IN THE LAB 
The design behind the Axiom can be seen in a complicated test rig in their workshops. The addition of smoke to the air-stream demonstrated how the Axiom gave a thrust effect without the usual 'prop-walk' effect that marine screws have: this would explain the improved handling as well as the massive increase in astern power available. The thrust is also channelled into a much tighter tunnel and does not as easily dissipate like a conventional prop. This was shown by the water surfacing much further aft with the Axiom, which keeps the stern of a boat buoyant and reduces it digging in.
Further testing is set to involve two identical boats, one fitted with an Axiom prop, the other with a standard manne screw. This should enable more information on fuel consumption to be collected: the data on our trip was inconclusive, as only total fuel flow was measured, not allowing for the return flow to the tank. 
But these first tests do show that the Axiom propeller has the potential to be a major advance in propeller design. It could well be the next must-have detail for new boats - and could even increase the handling pleasure of existing craft.

PRICES 
16in diameter £690 17in diameter £715 18in diameter £756