THROTTLE INTERLOCK

In the course of preliminary trial running it became clear that steps should be taken to protect the reverse/reduction gears from the possible effects resulting from mishandling the bridge control levers.

It can be understood that if, for instance, while running at high speed in “ahead” one of the levers should be suddenly put into the astern position, very considerable damage could be expected to be the result.

It was decided, therefore, that an interference pin should be arranged so that the lever could be prevented from moving from ahead into the neutral position until the ship’s speed had fallen to a reasonable figure, which was agreed to be 15 knots.

A spring-loaded pin was arranged in the “Bloetube” control run in such a way that while the shaft revolutions exceeded the figure corresponding to 15 knots ahead the magnetic relay kept it in a position to interfere with the control lever with¬drawal to neutral, nor for that matter could the lever be put from neutral into ahead.

When the revolutions had sufficiently dropped then the spring would overcome the magnetic relay and withdraw the pin from its interference position. An Iso Speedic centrifugal switch was fitted to the shafts in the Allen’s gearbox where it is driven from a gear train connected to the output shaft.

This protecting arrangement worked very well in practice and is very necessary in eases such as this where the whole of
the function of the engine control in manoeuvre and speed is vested in the bridge personnel.

EXHAUST ARRANGEMENTS

The exhaust arrangement seems very satisfactory, certainly from the point of view of noise and water exclusion. It has not been found very satisfactory to run for a long period with the exhaust flaps at the 350 setting owing to the deflection of the hot gases in a forward and downward direction, which has caused much distortion and cracking in the neighbouring aluminium structure.

During the course of trials it was frequently necessary to proceed out of Portsmouth harbour from the berth and well out into the Solent before sufficient increase of speed could be maintained to open the flaps to the full extent. It is, of course, quite unnecessary for the flaps to be at the 35° setting for all of this time, so it is suggested that in the future the automatic functioning of the flaps should be eliminated and that the exhaust flaps should be manually operated with safe¬guards against opening up towards full power with the flaps partially closed. Also it must be certain that those in charge close the flaps in case of an engine being shut down for any reason.

This should not be too difficult. One method which suggests itself would consist in arranging for an interference pin to prevent the opening up of the throttles till the pin is removed by the opening of the exhaust flaps. It could also be arranged that a horn blows at the control panel in the event the engines are reduced below say 3,000 power turbine revs, with flaps wide open, or if the turbines exceed say 5,000 r.p.m. with the flaps partially shut.

TRANSOM FLAP OPERATION

As originally installed the incidence of transom flap was arranged to be adjusted by means of an ON-OFF type of switch on the bridge console.

However, it was found that this was not satisfactory owing to a tendency for the gear to creep, probably due to an element of leakage in the hydraulic operating ram.

A closed loop feed back system was therefore arranged so that the ordered angle could be maintained by means of a Muirhead hydraulic relay unit in conjunction with the Keelavite ram and pump. This arrangement is shown in Fig. 10. This system, after modification as described above, functioned very well and maintained the required angle with precision in all conditions. The only slight drawback com¬pared to the ON-OFF system is the necessity to keep the hydraulic pump and its associated motor running continuously.

Exhaustive trials over the measured mile were carried out to ascertain the optimum transom flap setting. It was eventually agreed that 1½° down appeared best, but from recent evidence we are not quite sure that 0° incidence may not be best, at least for resistance and therefore speed.

Undoubtedly if it is required to drive fast to windward in a head sea the use of up to 3° of down flap is a help in avoidance of “slamming.”

There are times when a rather tiresome “slam” or “bang” takes place in relatively small steep waves, but this can usually be overcome by putting on some more speed, which can be shown to reduce the effective angle of attack on a given section of planking for constant vertical velocity downward as in pitching. (See Fig. 22.)

Seakeeping

Quite effective trials of short duration were carried out at sea outside the Nab Tower in wind ESE. Force 6 on Monday, 5th October, 1959. The sea was confused and relatively short but steep, wave height varying between 5 ft. and 3 ft.

In this sea we were able to drive the ship quite reasonably comfortably at her maximum continuous speed of 45 knots at all headings.

We consulted the literature for statistical data on the incidence of this type of weather in the Channel area and also for the Baltic (Ref. 7). (See Fig. 23.)

So far we have only been able to find this data for the Irish Sea area from the data presented by Mollie Darbyshire, B.SC., of the National Institute of Oceanography at Wormley, Surrey (Ref. 8):

(1) Percentage time height exceeds 5 ft.

Feb. Mar. April May June July
15 5 10 10 10 i6
Sept. Oct. Nov. Dec. All year
40 30 24 32 23

(2) Percentage time period exceeds 5 secs.
Jan. Feb. Mar. April May June July
78 45 52 14 23 i6 19
Aug.Sept. Oct. Nov. Dec. All year
44 37 25 37 69 41
This shows that over the whole year the probability that such weather will be equalled or exceeded amounts to 20% or thereabouts. It is less in the summer and more in the winter as one might expect. For 1957, January and September showed a 40% probability, while March, April, May and June averaged less than 10% probability.

This indicates that over the year this craft would be operational at 45 knots for four out of five days.
One rather interesting and so far inexplicable phenomenon which has been observed in the case of all seagoing fast craft built to this form is that in a high wind the ship invariably heels inwards towards the wind even at high speed. The reason is not at all clear, but may possibly be related to the use of helm required to prevent the head paying off to leeward.

This paper has been read with the permission of the Admiralty, but the responsibility for statement of fact and opinions expressed rests solely with the author.

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