As will be understood from the list of alternative machinery arrangements we were ordered to investigate there was a choice between high speed diesels, compounded diesels, and propeller gas turbines or a combination of diesels and turbines.
After very careful study all concerned came to the con¬clusion that the optimum arrangement was represented by an installation consisting of three Proteus marine gas turbines in conjunction with Allen’s reverse, reduction vee drive gears.
Basically the case for the gas turbine as prime mover rests upon the very high power to weight ratio available combined with the torque output characteristics inherent in the use of the free power turbine.
The latter feature is of outstanding importance for the case of the high speed planing type of craft where it is necessary that the machinery output in the form of available torque must always exceed the requirements of the hull at any par¬ticular load, trim or speed.
All piston engines possess a characteristic torque output curve which is rather sensitive to overloading. This can best be explained by study of Fig. 8 where a hull requirements curve is shown matched to the available power curve for both a diesel and a turbine.
The propeller must be designed to absorb the full power of the engine at full speed.
In both cases we design the propeller for a point o at which full power can be developed. If now the displacement is increased by 20%, the hull, instead of requiring power as indicated by line D, will require power as indicated by line C. Considering the engine output characteristics which must remain the same for the same engine and propeller (fixed pitch) we can see that whereas the reduction in power available from the diesel is nearly 30% in the case of the turbine it is negligible.
For this reason one can design the propeller for the turbine to absorb full power at the lightest condition, while for the diesel we must design the propeller for the heaviest and most resistful condition likely to arise due to fouling, etc.
This feature in itself can account for a difference in speed available from a given horse-power of anything up to 15%-20%.
There is, of course, another side to the question in that the s.f.c. for the turbine is nearly twice that of the diesel, so that if very extended range is a requirement the diesel may be best, but then the speed will also have to come down fairly substantially.
In this particular case speed was officially stated to represent the paramount consideration, while the normal range require¬ment at 400 nautical miles was able to be fulfilled.
A gas turbine is unidirectional so that for good handling qualities, which calls for outward turning propellers on the wing shafts, the rotation must be reversed by means of a reversing gear train able to take full power in the astern direction. This was eventually decided upon as representing the most practical solution following a design study by Messrs. W.H. Allen, who put forward a gearbox designed on the Allen-Stoeckicht principles (Fig. 9), which includes an epicyclic reduction and a vee drive. In this case two operated in the direction for a right-hand propeller and one for a left-hand propeller. The two wing shafts therefore are outward turning for ahead running, which is known to give good manoeuvrability when handling alongside another craft or, for instance, a quay.
As regards turning at speed, which is really a separate problem from that of manoeuvring in harbour or alongside, the three rudders are operated by an electro-hydraulic gear developed by Vosper making use of Keelavite hydraulic rams. The rudders are designed to incorporate a section with fine entry forward and blunt trailing edge (Ref. 3). In this way appendage drag and cavitation is reduced to a minimum, certainly while the rudders are at or near midships.
Each rudder is placed in line with the axis of a propeller.
It was a little unfortunate that the gearboxes were originally designed for an output shaft speed of 850 r.p.m. at full speed. Subsequently this output speed was changed to 1,720 r.p.m., which involved the gearbox designers in a reduction to 850 and then a further step up to 1,720. This is a somewhat wasteful process, but was unavoidable.
Probably in the event of a design envisaging at the start the high speed of output shafting, certain economies in weight and space occupied could be affected.
While on the subject of the selection of the turbine machinery, it should be mentioned that it was appreciated that when operating in heavy seas a liability for salt water spray to reach the turbines via the air intakes existed.
Certain special tests were undertaken which will be described by Mr. Markham dealing with the engine developments.
By virtue of the machinery lay-out which locates the turbines aft, the exhaust has to be led out through the transom. This is a good arrangement from the point of view of the deck lay-out because a funnel can be avoided. However, there was a possibility that in the event of the transom becoming immersed sea water might find its way to the compressor and/or power turbine.
This was not considered to be a serious likelihood while any turbine was running, but in the event of one or more being stopped it became essential to avoid water ingress. To this end a flap system was devised which would enable a complete closing of the exhaust orifice for any particular turbine, which for any reason remained stopped at sea. Additionally it was arranged that while starting and slow running the flaps should only be open about 35° and that on increase of power output the flap should thereafter be opened to the fullest extent. (See Fig. 10.)
This was affected automatically by making use of an air cylinder operated from the compressor pressure.
Tests in a seaway were conducted in the fast launch Swordfish belonging to Vosper, which is built to the same form, to observe the amount of spray or water likely to cover the exhaust outlets in the transom. Reproductions of the exhaust piping to scale were fitted to the transom of the Swordfish and she was even run astern into waves 2 ft. 6 in. to 3 ft. high at slow speed.
The results were encouraging but the incorporation of a shelf below the exhaust penings was demonstrated to be well worth while. (See Fig. 11.)
In passing it should perhaps be mentioned that an alterna¬tive arrangement of turbine combined with small high speed diesel was carefully considered and can in fact meet certain requirements rather well. This consists in arranging for the necessary combination of vee drive and reduction gear which will include an “idler” for “handing” the rotation. A marine diesel complete with reverse gear is then coupled to the main gearbox through suitable clutching arrangements so that for harbour manoeuvring and slow running or long range cruising the diesels alone are used. In this manner a reasonable power (150-250 b.h.p.) is available for manoeuvring and slow cruising, while very extended range is available at this speed of 10-12 knots. By making use of voltage regulators or some form of constant speed drive the auxiliary generation can be affected to a very large extent.
Figs. 12 (a) and (b) will show the machinery lay-out eventually adopted and the alternative diesel cruising arrange¬ment discussed above. The latter arrangement is fractionally heavier than the lay-out adopted incorporating Allen’s gears, but there is not a great deal of difference.
As the extended low speed cruising range was not a require¬ment of the British Admiralty and the Allen gear arrangement was lighter, this arrangement was selected.
The control of this machinery lay-out has been arranged to take place as far as reversing and manoeuvring are concerned, from the bridge by a single lever. (See Fig. 13.)
However, there is a control instrument panel located in the Action Information Office under the bridge. From this point an engineer can supervise the functioning of the machinery as a whole as well as having the exhaust flap and various cooling inlet scoop controls. He is in a position to run the engines up to low powers but he cannot override the bridge control as far as concerns the actual handling of the ship.
The control levers from bridge and A.I.O. operate by means of “Bloctube” controls, what is in effect a three-way cock on the Allen’s box which is, of course, oil operated. Interlinked with these controls on the gearbox is a fuel metering device by which the compressor revs, are ordered, which in turn controls the output of the power turbine. (See Fig. 14.)