Why Not a Tractor Screw?
CONVENTION appears to have as large a hold in the outboard motor field as in every other branch of motoring, and it is therefore not surprising that we find the propellor of an outboard situated behind the gear housing in nearly every case.
The origin of this presumably lies in the fact that ordinary motor boats have their ” props ” at the rear of the boat. While not wishing to suggest they should be anywhere else, there is no reason why, on outboard drives, the propellor should always be aft of the housing.
From a mechanical point of view there is nothing to choose between the two positions, as it is perfectly simple to arrange for the thrust, whichever direction it is applied. Prom the point of view of hydraulic efficiency there are many things to recommend it. The normal arrangement necessitates a streamlined gear housing, but at the speeds now being attained it is impossible to avoid eddies and the propellor efficiency is consequently interfered with. If the screw is in front of the housing the water has absolutely unobstructed access to it, and therefore the full area of the blades is effectively used in undisturbed water, and the slip is thereby reduced.
When cornering with a normal design considerable trouble is caused by cavitation. This is set up by the violent sideways movement of the whole mechanism which makes eddies behind the gear-housing, which fill with air. These passing back to the screw cause the familiar cavitation, which requires throttling right down, before the screw will grip again.
With the tractor arrangement this will not occur, as any eddies that there may be will start behind the screw. Another great advantage is that the steering can be arranged to be self centering. As the thrust at present is behind the turning point of the motor, there is a constant tendency to pull out of the straight. If the screw is forward of this point it will tend to pull straight, and therefore it will be far simpler to hold a powerfurmotor
on a straight course. These are only a few points in favour of the tractor screw, but they are important enough to show that this arrangement merits greater consideration.—W.S.
B .0 .R.0 .
OWING to the decision of the committee, Mr. E. P. Garrard, the popular secretary of the British Outboard Racing Club and the British Motor Boat Club, has temporarily given up his appointment. It was thought that a full-time secretary was unnecessary during the winter months, and Mr. F. T. Bersey, the hon. treasurer is now also acting as hon. secretary.
Last month the committee of the B.O.R.C. went into the matter of the future policy of the Club in relation to its parent organization, the B.M.B.C., and it was agreed that the former has now been in existence long enough to justify its functioning as a separate body. The headquarters of both clubs will remain as before, however—at 2, Whitehall Court, S.W.1.
“MISS ENGLAND I” AT SOUTH KENSINGTON.
THE many historical exhibits to be found at the Science Museum, South Kensington, have been further added to recently by the inclusion of Lord Wakefield’s motor boat ” :1liss England I.” It will be remembered that the late Sir Henry Segrave raced this craft successfully in 1929 at both Venice and Miami, and Lord Wakefield has now presented her to the nation.
GAR WOOD SHOWROOMS.
TAR Wood Motor Boats, Ltd., ask us to point out that they have only temporarily vacated their London showrooms, their display having been transferred to their depot at Cowes. New models of both Gar Wood and Dee With craft are now in stock there. The offices remain at Trafalgar House, II, Waterloo Place, SAVA .