ATLANTIC 21 AND D CLASS inshore lifeboats, out on trials, are a familiar sight in the Solent; testing and proving the vast amount of detailed development work which, over the past 15 years or so, has been quietly going ahead at the RNLI Cowes base, under the inspired guidance of Lieutenant David Stogdon, on every aspect of inshore lifeboat design. One of the most important aspects, under continuous review, is of course engine installation.

If character is given to a power boat by the design of her hull, vitality comes from the engine installation; and there is great, pent up vitality for the RNLI's inshore fleet of semi-rigid Atlantic 21s and inflatable 16' D class boats in their outboard engines: high speed (30 knots for the Atlantic 21, more than 20 for the D class) and controlled reserve power to give the impetus and manoeuvrability on which both efficiency and safe navigation depend.

When choosing an outboard engine for an ILB, the desired characteristics must be sought among those models available at any one time. Outboard engines are produced in very large numbers for international distribution and lifeboats are only a tiny part of the market. But if there is no possibility of outboards custom built for lifesaving at sea, the development work of the past twenty years or so in the marine engine field has resulted in a range of good, powerful and reliable machines from which selection can be made.

The RNLI needs, above all, a good work-horse engine which will not give up when faced with difficulties beyond the normal bounds of duty. It must be capable of providing high speed, but, even more important, it must have enough power to give that vital kickwhich will get an ILB out of trouble in bad weather: launching, beaching or picking her way among hostile seas.

The engine must also be so designed that it can always be started; if the starting cord should break, there must be the possibility of rigging a jury line.

As Evinrude/Johnson (OMC) engines meet all these requirements, it is from this range that most of the present RNLI engines come.

Choice of engine is the first step. The next is choice of propeller. Power, transmitted through the drive shaft, must be converted by the propeller into thrust. Only if the right propeller is selected will the engine be run without undue stress and its potential power used economically and to good purpose.

Engine and propeller must match each other in the conditions of the work which, between them, they have to do.

What is the propeller's share of the work? With a screw-like action, its blades draw water from in front and push it out to the rear, creating what is called a thrust cone (Fig. 1). It is this thrust cone, pushing against the undisturbed surrounding water, whichpropels the boat forward, and the nature of the thrust cone is, of course, determined by the design of the propeller.

The diameter of the blades (Fig. 2) and their pitch (determined by the angle at which they are set to the hub) are the two basic variables. Pitch (Fig. 1) governs the distance travelled and that is how it is expressed: a 10" pitch means that, theoretically, the propeller will move forward 10" in one revolution working in a solid. Working in fluids, not solids, marine (and aircraft) propellers have to contend with slip and turbulence which in practice reduce the theoretical pitch to the 'effective' pitch. The higher the effective pitch, the greater the forward travel, and the greater the load.

Thus, if a propeller is specified as 12" X 10" it means that it has a 12" diameter and 10" pitch.

The basic guide which shows whether the right propeller has been fitted to the right outboard engine for a specific job of work is the tachometer (Fig. 3) which shows on its dial the revolutions per minute (rpm) of the engine. The faster the engine runs (the more revolutions per minute), the more horsepower it produces, up to a point.

At peak performance the revolutions of the engine give 100% of available horsepower. After that, even if the engine is run faster, no more horsepower will be produced. This is because the exhaust port sizes of an outboard engine are so designed that they can clear all waste gases from the cylinders at the peak rpm recommended by the manufacturers; if, however, the engine is run at a higher speed, the gases cannot be cleared quickly enough and are left in the cylinders; thus there is a loss of efficiency.

If, at full throttle, the rpm reading on the tachometer, say 5,000 is within the range recommended by the makers, all is well. If the reading is too low, say4,000 rpm, the engine is being overloaded and will oil up; the pitch should be reduced (distance travelled will be shortened). If, at full throttle, the reading is too high, say 6,000 rpm, the engine, underloaded, running too fast and uneconomically, could be damaged and pitch should be increased (distance travelled will be lengthened).

So, power and loading must be balanced. Known data and experience will lead to the right area of propeller dimension, and in commercial ships this is sufficient for accurate design of propellers from standard series charts.

For small boats and outboards, on the other hand, fine tuning can only be done at sea, using, as a first guide, tachometer readings. So, for inshore lifeboats, it's out on the Solent with Michael Brinton, Engineer Overseer (Cowes) and his team; running trials over a measured mile, looking for rough water, trying another set of combinations and gradually building up a bank of data which will reveal the right solution.

As a result of the trials of the past few years it had been established that the engine/propeller combination which gave the best all-round performance for the Atlantic 21 was a 50 hp Evinrude engine with an llf" X 17" OMC propeller, and for the D class ILB a 40 hp Evinrude engine with a 10J" X 13" OMC propeller. However, it was learnt early in 1976 that both these engines were to be withdrawn from the Evinrude range by 1978, to be replaced by a 55 hp and a 35 hp engine respectively.

While the manufacturers had designed them to do the same jobs as the engines they were replacing, their performance would have to be proved for lifeboat work, and propeller matching, of course, plays a vital part because, as has already been seen, the amount of thrust that ultimately comes from any given horsepower depends on the design of the propeller.

It was back to the measured mile . . .

Two fully equipped boats of the same class, with full fuel tanks and full complement of crew, go out to the measured mile in Southampton Water.

First, fore and aft trim of the boats and angle of tilt of the engines, both of which are critical to performance, will have been checked (they will vary from boat to boat). Boat No. 1 will be fitted with the established engine/propeller combination, the performance of which is known. Boat No. 2 will be fitted with the new engine and will have on board several different, though closely related, propellers. A series of runs will be made over the mile, the two boats side by side, first at full throttle, then at reducing revolutions per minute, say 5,500 rpm, 5,000 rpm, 4,500 rpm, down to 2,000 rpm, a record being made of the speed achieved by each boat on each run.

Then the propeller will be changed on Boat No. 2 and the series of runs repeated. Each propeller taken out will be tried in turn. Other points will be noted as well as speed: How much punch is there? How long does it take to get from standstill to full throttle? Next there will be comparative trials in rough water to test speed and power in these conditions; to see how much cavitation (air infiltrating between blades and water, causing 'pitting' damage to the blades) there is on tight turns or when re-entering the water after a leap from a wave; to see what performance can be expected from one of a pair of twin engines on its own—an Atlantic 21 and all dual motor ILBs should be capable of completing a service safely even though one engine has failed.

When as much as possible has been learnt about one range of propellers, the whole programme will be repeated with another range. Gradually, a clear picture will emerge and the best solution established. Ultimate selection is almost bound to be a compromise, the four factors carrying the most weight being: performance at full throttle; 'pick-up' time from standstill to full throttle; cavitation under all]conditions; and the best possible performance of a single motor on dual motor installations.

During the 1976 trials it was found that for the Atlantic 21, the 55 hp engine with the same Hi" X 17" propeller gave the best performance; the punch was as good, the speed if anything slightly better than it had been with the 50 hp engine.

For the D class ILB, as the horsepower of the engine would be slightly reduced (35 instead of 40 hp) even greater care was needed to make quite sure that the right propeller was found so that the eventual power developed, as well as the speed, was up to the standard demanded. The 35 hp engine does start off with some definite advantages over the 40 hp. It is 231bs lighter, so that the load of the boat is reduced, and it has a through-propeller exhaust system, which is a good feature, making the engine much quieter.

After trials with propellers by different manufacturers, the Michigan PJ96, 10J" X 11", was selected. While research still continued at Cowes, preliminary coast trials were begun, 35 hp Evinrude engines with PJ96 propellers being fitted to the D class ILBs at three busy stations: Eastney, Bude and Bembridge.

All three being near at hand, it was easy for results to be followed up, and crew members at all three stations were reasonably satisfied; in fact Bude was very impressed with the new engine, which appeared to solve problems which had been experienced with the 40 hp engine.

During 1977 the scope for getting further data will be still further broadened.

While at Cowes investigations into any promising alternatives which may appear will go ahead, fifteen 35 hp engines will be in operation on the coast, the crews feeding back information and comment to base. In this way as much knowledge over as wide a field of experience as possible will be built up, so that before the 40 hp engine has been withdrawn, new engine specifications can be accepted with confidence.

Another investigation will also be in progress in 1977. In estuaries such as the Thames and the Mersey considerable damage has been caused to alloy propellers by floating debris. The Atlantic 21 at Southend-on-Sea is being fitted with stainless steel propellers which are claimed to be five times as strong as the alloy type. They will, of course, be much more expensive—• about three times—but should prove to last much longer and be more economical in the long run.

And so the work goes on—looking for solutions to present problems, while remaining alert to any developments which can usefully be applied to the work of saving life at sea..