15 cc 4-stroke

by Peter Chinn
("Model Boats" July 1968, p278)

To the best of our knowledge, the 15 c.c. four-stroke Gannet is the only model engine of its type currently manufactured anywhere as a finished ready-to-run power unit. No previous model four-stroke engine that we can call to mind has, in fact, managed to survive as a commercial proposition for anything like as long as the full decade that the Gannet has remained in production. One recalls, for example, that superb 10 c.c. four-stroke, the Jensen Channel Islands Special (which, incidentally, the Gannet resembles in overall design) made by J&G Jensen Ltd. of St. Helier, some twenty years ago but which, despite a remarkably modest selling price, failed to achieve a reasonable volume of sales and had, in consequence to be withdrawn. Of course, there have also been other, less praiseworthy, examples of model four-strokes, whose demise is neither surprising nor lamented.

It has to be admitted that, in general, commercial model four-strokes do not compete with the better two-strokes on price or power (particularly power/weight ratio) bases. Their very much greater complication must inevitably lead to considerably higher production costs, and the higher retail price which must result obviously restricts overall sales and thus production volume which, in turn, may push the price up still more.

A similar vicious circle exists in regard to power, weight and overall dimensions. Quite apart from the weight of its many extra parts, a four-stroke needs to be made more robustly than a two-stroke of similar swept volume if it is to develop equal power. This is because, in order to deliver the same torque (and thus equal power at a given crankshaft speed) the four-stroke much achieve a mean effective pressure twice as high as that of the two-stroke.

Stated simply, this means that since the four-stroke has a power stroke only once in every two revolutions, the physical stresses encountered will be twice as severe and the stressed parts will, in consequence, have to be made that much stronger. In addition, the fact that a power impulse occurs only every other revolution means that torque fluctuations are much greater than in the two-stroke and a heavier flywheel is therefore necessary, particularly if low idling speeds are required.

So far as model engines are concerned, the present day two-stroke also has the not inconsiderable advantage of over thirty years' intensive development. This development, spurred on by keen commercial competition, has been responsible for raising specific power outputs in the most powerful racing type two-strokes to something like eight times the levels of the mid-thirties. Understandably, the lack of a similar impetus in the development of model four-strokes has meant that these engines have not progressed at anything approaching this rate.

Because of these factors, model four-stroke engines have never achieved any recognition as practical power plants for model aircraft, but for boat work the situation is rather different. Except for racing purposes, a high power/weight ratio is of little importance and the average user is far more interested in ease of starting, reliable performance and trouble-free operation. For big boats and particularly where the engine size is to be 15 c.c. or more, a good four-stroke, such as the Gannet, has much to recommend it.

The Gannet 15 c.c. Marine engine has now been with us since 1958. During this time it has been held in high regard by those who specialize in the larger type of hull and whose attitude to model boating perhaps tends more towards a 'marine engineering' approach. That the Gannet's acceptance for such work shows no sign of diminishing seems to be borne out by the fact that, last season, at least two dozen major regatta successes went to craft powered by these engines.

The Gannet is, of course, a spark ignition motor and is intended for operation on straight petrol. Lubrication is by crankcase depression from a separate oil reservoir mounted on the crankcase nose and feeding through the crankshaft to the big end. The engine is of the overhead valve type, the valves being placed vertically in the head and operated by pushrods and rocker arms from a skew gear-driven cross camshaft.

Earlier Gannet engines were intended for use with a high-tension coil and battery ignition system and standard engines of this type are, in fact, still available. The motor submitted for review, however, was equipped with the Gannet rotary magnet type magneto. Complete with magneto and 14 oz. fly-wheel, the engine weight 3 lb. 10 oz. The standard engine (less coil condenser and battery) is about 13 oz. lighter.

The great thing about any four-stroke engine, of course, is that, especially to the engine enthusiast, it is so much more interesting than the average two-stroke. The fact that the Gannet not only has a spark-ignition system (which all but disappeared from the model two-stroke world twenty years ago) but also has exposed push-rods and rockers that can be seen working, is something that will fascinate many modellers who have only known model glowplug and c.i. two-strokes. The parts that go together to make the Gannet, in fact, more often have their counterparts in full-size engines than in the familiar model two-stroke as we know it today.

The Gannet crankcase is a substantial sand-casting in D.T.D. 424 aluminium alloy, complete with integral main bearing housing and camshaft housing. The counterbalanced crankshaft is machined from a steel forging with a separate, hardened, tin. dia. crankpin. The shaft has a tin. dia. rear journal, where it is carried in a It in. o.d. ball bearing, while at the front end, journal diameter is reduced to 7/16 in. and runs in a phosphor bronze bush. Ahead of this, the shaft is reduced to 5/16 in. dia. on which a steel split taper collet is mounted, carrying the flywheel. The flywheel is of nickel-plated mild steel. 27/8 in. nominal diameter, and is retained by a 5/16 in. BSF nut.

Between the two crankshaft journals is mounted the camshaft drive gear which is of steel. This meshes with a phosphor-bronze gear mounted centrally on the camshaft between the two hardened cams, to provide the necessary 2:1 reduction for the valve and ignition timing. The camshaft has a diameter of 7/32 in. On the right hand side it runs in a phosphor-bronze bearing. At the other end it runs in a flanged alloy bush which also forms the mounting for the contact-breaker assembly, the hardened mild steel c.b. cam being secured to the end of the camshaft with a grubscrew. The hardened tappets are 7/32 in. dia. and operate in phosphor-bronze guides. The inlet valve is timed to open at 20 deg. before T.D.C. and to close at 60 deg. after RD.C. The exhaust valve opens at 60 deg. B.B.D.C. and closes at 20 deg. A.T.D.C.

The cylinder comprises a machined D.T.D. 424 casting forming the water jacket and crankcase mounting flange into which is fitted a Meehanite wet liner. Water inlet and outlet stubs are fixed with what appears to be an epoxy resin sealant. The piston is also machined from an aluminium casting, has a flat crown and is equipped with two 1/16 in. compression rings. It has a hardened, fully floating gudgeon-pin with brass end-pads. The connecting rod, again of sandcast aluminium, has a phosphor-bronze bushed big-end and a plain little end and both ends have oil holes.

The cylinder-head is another D.T.D. 424 sand casting and has pressed-in phosphor-bronze valve seats. The valves are of 100-ton steel with 3/8 in. diameter heads and 1/8 in. dia. stems. Valve lift is approximately .090 in. The valve guides, pressed into the head, are of phosphor-bronze. Valve spring retention is by means of a horseshoe cotter which engages a groove in the valve stem and is retained by a counterbore in the spring collar. The hardened rocker arms are mounted on eccentric bearings on a cross shaft to provide for tappet adjustment. Pushrods are 7/64 in. dia. hardened steel. The suction feed carburettor has a cast aluminium body, a brass throttle barrel and a 1/4 in. dia. choke. Mixture strength adjustment is by means of the usual needle-valve and the needle is arranged to drop as the throttle is closed and so avoid an excessively rich idling mixture. The carburettor is provided with a choke flap for starting.

The magneto employs a high flux rotary magnet of 1-7/16 in. dia. mounted on a tin. dia. silver steel shaft which conveys the drive from the crankpin and runs in two Oilite bearings in the crankcase cover. This latter is of cast aluminium and includes a rectangular flange to which the stator limbs and condenser are attached. The magneto supplies h.t. current to a 'full-size' 10 mm. short-reach sparking-plug, timed by the contact breaker assembly which has tungsten points and is adjustable for advance and retard.

A test to determine the Gannet's actual b.h.p. was not possible as the engine's width of nearly 3-1/4 in. across its mounting lugs, over 3/4 in. wider than the current largest (13.2 c.c.) commercial two-stroke, was too great to fit our test rig, but we were able to check the engine starting, handling and running qualities and to get some idea of its capabilities.

Starting was very good indeed. Magnetos provide only a very weak spark at starting speeds (hence the use on some cars, in the days of magneto ignition, of a supplementary coil-ignition starting system) but, with the Gannet, a brisk pull of the starting cord invariably gave a first time start provided that the engine had been adequately (but not excessively) choked. Excessive choking, will, of course, wet the plug and kill the spark, although this may not be so troublesome as with the petroil mixture used for spark-ignition two-strokes. A quick solution here is to remove the plug and if one does not have a spare one handy, to adopt the old mechanic's trick of 'burning the plugs' - in this case, the application of a gas lighter flame to the plug electrodes for a few moments. We are not for one moment suggesting that anyone should really need to resort to such measures with the Gannet but it is worth bearing in mind that a hot dry plug often works wonders when starting any spark ignition engine - especially full size marine engines on a cold damp morning!

The throttle worked well. From a maximum speed under load of around 5,000 r.p.m. (and with the needle valve and ignition set for best performance) the Gannet could be throttled down to fifteen to sixteen hundred revs. Actually, of course, it sounds, to a 'two-stroke ear' as though the idling speed is only about 800 (due to 'the lower four-stroke exhaust frequency) and, at all times, the Gannet, .even without any form of silencing, is noticeably quieter than would be an equivalent capacity two-stroke.

A torque of approximately 120 oz. in. is claimed for the Gannet. This is equal to a B.M.E.P. of nearly 104 Ib/sqin. for a four-stroke engine which is about fifty per cent more than for a good throttle-equipped 10 c.c. two-stroke running on alcohol fuel. Certainly the hefty torque impulses of the Gannet become noticeable as the throttle is opened and the manufacturer's insistence that the engine must be rigidly mounted in a substantial bed is obviously sound advice and should be strictly followed if excessive vibration is to be avoided.

The magneto equipped Gannet is a completely self-contained power unit. All it needs is a fuel supply and to have its oil cup filled with motor oil of S.A.E.30 viscosity rating. Recommended fuel is premium grade petrol and we used 95 octane Shell spirit but it is doubtful whether as high a grade as this is really necessary in an engine of only just over 1 in. bore and with a compression ratio of 8.5 to 1. Therefore, if some overseas users are obliged to use fuels of lower octane value, it seems improbable that the Gannet would show any objection to such a diet. This is an engine which, treated with reasonable care, should last a lifetime. Full service facilities, ranging from a routine decoke to major overhauls, are offered by Gannet Engineering Ltd. who will also supply various accessories, including silencers, float-chambers, heavy-duty fabric couplings, stainless steel prop shaft, sterntubes and propellers.


TypeSingle cylinder spark ignition o.h.v. four-stroke petrol engine with magneto. Displacement type lubrication system. Crankshaft supported in one ball-bearing and one bronze-bearing. Aluminium piston with two compression rings.
Bore1.0625 in.
Stroke1.030 in.
Stroke/Bore Ratio0.967
Swept Volume0.913 cu. in. = 14.96 c.c.
Weight3 lb 10 oz including flywheel.
FlywheelMild steel nickel-plated, 2i in. dia., 14 oz.
Cooling systemWater cooled circumferential cylinder jacket (wet liner).
Throttle systemBarrel throttle type carburettor with automatic mixture compensation.
Dimensions:Overall height: 6-1/2 in.
Height above bearers: 5-1/2 in
Overall length : 6-1/2 in.
Overall width: 4-3/4 in.
Crankcase width below mounting lugs: 2-5/32 in.
ManufacturerGannet Engineering Ltd., 7 Reeves Yard, Whitstable, Kent.



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