Classic Power

Steve Turner reviews successful engine installations

Article taken from Seapeople number 11 (1989)

Reading the article by John Chitty in Sea People No. 10, which described his engine installation, made me realise that there is a whole new generation of P.C.A. members who may not be familiar with the gradual evolution which has taken place with Polycat engine installations. There have been many attempts to provide reliable, effective auxiliary power, particularly on the larger designs - some very successful, some less so.

Some builders fitted engines in one or both hulls, with conventional prop shafts and propellers in apertures in the skegs. This set-up gives the best manoeuvrability under power, and is a straightforward installation similar to most other boats. However, unless one spends quite a lot of money on sophisticated folding props, there is a big penalty to pay in increased drag which can really take the edge off the boat's performance. The engine is also sharing your accommodation with the associated problems of noise, heat and smell with diesel, or fire with petrol (gasoline). Others mounted their engines centrally, building some sort of pod to house it. This left the problem of getting the drive to the water, fairly easily solved by the application of money - purpose built outdrive legs from firms such as Sillette providing a neat if rather expensive answer.

However, the (generally fairly impecunious) average Polycat builder was looking for a cheaper answer and preferably something he could make himself. The breakthrough came in the early seventies when Ernald Pearson fitted a Yanmar diesel into his Narai 'Cheetah'. The engine was mounted in the centre of the boat, driving a retractable propeller through a long swinging shaft.

This system was widely copied by many Tangaroas, Narais and Oros using variations on Ernald's basic idea. The next step was taken by Jeff Fallon with his Narai IV 'Nick Of Time'. Jeff used a 25 h.p. petrol engine to drive a similar installation but his stroke of genius was to simplify the whole arrangement by doing away with the cumbersome ladder arrangement thought necessary until then, replacing it with two wire stays and a simple line to lift the prop clear of the water. To prevent the prop from rising to the surface, two vanes were positioned in front of it, angled to fly down through the water. The previous shaft running inside a tube with its associated bearings was dispensed with by making the tube itself act as the shaft! Altogether a very worthwhile saving in weight and complication.

Having sailed with Jeff on 'Nick', I was soon borrowing his ideas for the installation on our Oro 'Imagine'. Our engine is a twin cylinder Lister diesel, which is air cooled, slow revving and extremely reliable. My contribution to the development of the drive was prompted by visions of the prop carving its way through the side of the hull should one of the supporting stays fail. I decided to put a steel ring around the propeller as a guard, this also protects the prop in shallow water.

Having the ring around the prop also meant that I could move the paravane from in front of the prop to on top, where it forms an effective anti-cavitation plate, as well as keeping the shaft down.

The engine is extremely reliable, hand starts, is miserly with fuel (just over one litre per hour) and is rather noisy, a plus factor I feel, as it encourages me to shut it off and sail! Running at only 1800 r.p.m. maximum, driving through a two to one reduction gear, it turns a large three bladed propellor at 900 revs. 'Imagine' will never motor fast enough to ski behind, (the engine is only 8 h.p.) but she will plod on steadily at about 41/2 knots at a little over half revs, even when pushing against a head wind or towing other boats. (See report of 1987 cruise in company, Sea People No. 9). Jeff's 22 h.p. petrol engine gave 'Nick of Time' a top speed of 81/2 knots with an economical cruising speed of 41/2 knots. The 12 h.p. Yanmar used by Ernald gave a maximum of 6 knots, cruising at about 5.