Muirhead-Brown controlled tank stabilizer

Muirhead-Brown Controlled Tank Stabilizer

FIG i

Diagrammatic

passive tank

stabilizer

Early tank stabilizerstheir merits

and demerits

One of the earliest types of ship stabilizer to be

tried took the form of tanks containing water which was free to move as the vessel rolled due to the action of the sea. A scientific paper given

by Sir Philip Watts' described the early work and

from time to time further contributions to the art have been made by technicians in Britain,

Germany, U.S.A. and Japan.

Most of the work has been devoted to the use of 'passive' tanks and, in general terms, their use is based on tuning the tank and fluid system so that

it has a natural period of oscillation matched to

the rolling period of the vessel. The vessel having

its own righting moment is a tuned system and, by mounting in it a similarly tuned system it is

possible to achieve a good measure of roll

stabilization under what would be conditions of maximum rolling, namely when the vessel is

subjected to a steady swell of matched frequency.

Fig. i gives an impression of one type of tank used. The tuning depends on the proportions of

the connecting channel and the port and starboard tanks. It is also influenced by the depth of fluid in

the system. Water can be used or, alternatively,

reserve fuel oil or other fluid.

While the passive system has the advantage of simplicity and economy since no control or

power is used, it suffers from some disdavantages,

and patent literature has many examples of

attempts both to apply controls and power to

improve performance.

Around the year 1910 Dr Frahm carried out

experimental work and, with the aid of Bloom and Voss, installed a number of anti-rolling tanks in

ships, some with manually set controls, some having tanks with the connecting pipes resemb-ling 'U' tubes, others with openings to the sea, and some damped by throttling the air flow in pipes connecting the tops of the port and

star-tEngên.ering Consultant, Muirhead & Co., Ltd.

I.ab. y. Scheepsouwkunde

Technische Flogeschool

board tanks. These were not sufficiently success-ful to warrant general adoption, the main disad-vantage being that while under favourable condi-tions a reasonable degree of damping was obtained,

under other conditions, when the ship's rolling period and the waves to which it was subjected were not in tune, the vessel tended to roll more

than if the tanks had not been fitted and it would

appear that an expert had to be on board to get

the best results. There was also a certain amount of noise due to throttling of air in the control pipes.

Assessment of a recent tank stabilizer

More recently, following experimental work done

in the model ship tanks in America, a passive tank system under the name of McMullen has been installed in many vessels. The design of these passive tanks is a compromise in which

the tuning of the tank is made less critical by the introduction of resistance to the flow of water. In

consequence, the efficiency at synchronism is reduced but so also is unwanted rolling in the

non-synchronous condition. A suitably designed

stabilizer of this type can be expected

sub-stantially to reduce the roll due to a synchronous swell but have much less effect on the roll due to a non-synchronous or confused sea. A limitation

in the application of such tanks, which may be serious in certain cases, is

that the natural

rolling period of the vessel may change appre-ciably due to variations of loading, e.g. with or

without cargo, and unless the tuning of the tanks is changed to correspond, the stabilizing efficiency

will fall.

An improved activated tank stabilizer

with control equipment for the fin type ship

stabilizer for over twenty years but it was

ciated that this type of stabilizer did not fulfil all requirements. For example, fins will not stabilize

a ship laying-to in an open anchorage, or a factory-type ship which steams along at perhaps six knots; or again, fins are difficult to apply to an icebreaker where it is undesirable to have hull penetrations for

retraction. The field of possible systems to fulfil these requirements was surveyed and, while the system of moving a large solid mass across the

vessel by controlled power is possible, its general

adoption by shipowners seemed improbable. Work was, therefore, initiated in 1958 on tanks

holding water and various methods of controlling

the amount and .movement of water in the tanks were examined.In an early form the water was pumped into 'vrtical tanks having a small deck

area but the amount of power required was found

to be prohibitiveanks open to the sea through the l)ottom of the vessel in which the flow of water in and out of the tanks was controlled by

air tinder pressure was found to be quite efficient

but the presence of holes of considerable size in

the hull was not likely to find general favour with

shipowners. Work was, therefore, initiated on 3tanks resembling the original tanks which were tried out in the 1880's but with modifications directed to pumping the water and controlling its flow in the most efficient manner instead of

leaving it free as in the passive system.

Figs. 2 and 3 show, respectively, the plan and

elevation of such a system. It will be seen that the

installation consists of port and starboard tanks with appropriate connecting channels. A central tank is also provided in which is located a

pro-peller driven by a motor or other means of power.

In Fig. 3a is shown the condition in which the vessel is on an even keel and no control signal is being applied. The motor runs continuously arid

water circulates between the central and port and

starboard tanks without hindrance except for a

certain degree of throttling due to the

inter-mediate position of the control valves. The central

tank, therefore, has a small head of water above the level of the water in the port and starboard

tanks.

In Fig. 3b it is assumed that the vessel has been subjected to a wave on the port side and a small roll to starboard has been initiated. The control valves in the channels are operated so that the flow of water due to the pumping action is now

CONTROL VALVES

(a)

Even keel no signal.

Water circulates to both tanks

FIG3

(b)

Wave on port side and small roll to starboard.

Water flows from starboard to

port tank

page 11 altered, the water being transferred rapidly from

the starboard tank to the port tank.

Fig. 3c shows the condition of maximum stabi-lizing torque. It is assumed that the flow of water to the port side has not been sufficient to neutral-ize the initial roll to starboard and, consequently, the control remains in the same position as in Fig. 3b until the maximum stabilizing effort has been exerted, namely, with the starboard tank empty and the port tank full. The small excess head of water in the central tank expedites the initial flow to the port tank but has no influence on the total time required to fill the port tank and empty the starboard one. It will be seen that the essential difference between this system and the passive tank system is that water will flow from one tank to the other giving a stabilizing torque when the vessel is displaced from its vertical position only by the amount required to initiate

the control action. To transfer a comparable bulk

of water by passive means would involve a

greater displacement. To get the best

stabiliza-tion in a passive system a train of waves is required

in order to build up the movement of the water from one tank to the other. This leads to a dis-tinct advantage for the activated system in that it can deal immediately with individual waves and, also, waves of various periodicities, while

the passive system can only deal effectively with a train of waves whose period is synchronous with the tuned tank system.

Tests on a rolling table

It was obviously necessary to make tests on the proposed system in order to establish practically

that it would function. The earlier forms of water tank systems which had been tested, in particular those with openings to the sea had been built into

models and tried out in the model ship tank in Messrs Wm. Denny & Brothers' shipyard but

this new development was capable of being tested

completely on the analogue table, described in

Technique Vol. 12 No. 3 and used for testing the

control equipment for the fin.type stabilizers. A model-sized stabilizing tank, complete with pro-peller and motor drive for circulating the water, was mounted on the table; also a sensing unit

consisting of a velocity gyroscope together with a

differentiating network thereby providing an acceleration signal. A small hydraulic relay unit

(c)

Wave on port side and small roll to starboard.

Water fills port tank and flow ceases. Propeller continues to rotate

with proportional response

controlled by the sensing

unit operated the valves in

the water tank system. To

provide the disturbing

element corresponding to the sea waves, a weight is moved across the 'beam'

of the table by a servo

which follows a cam the

contour of which simulates a sequence of waves of any desired character. The righting moment and

inertia of the table are

adjustable by loading with

static weights so that the analogue table represents to scale an actual vessel;

for example, a trawler of 700

tons could be represented to approximately 1/10th scale, and a 50,000 tons vessel to 1/39th scale.

Tests were made through a range of wave

fre-quencies both below and above the natural

rolling period of the model and response curves

were taken of the resultantrolling: with no stabilizing action.

with the stabilizing tanks operating passively

(without the pump in action).

with the activated tank system in full operation. Aset of typical results obtained is shown in Fig. 4. In the arrangement tested, the passive operation

(Test 2) corresponds to a passive tank system having considerable resistance to the flow of water, but not accurately tuned to the model period. The resultant curves, therefore, show a

significant reduction in rolling at the synchronous

period with only a small increment of rolling at

roiling periods longer than the natural frequency.

A passive tuned system without resistance was

also tested and the resultant curves are shown in Fig. 5. Considerably increased rolling is apparent

both above and below the natural rolling period

while excellent damping (over 90%) is obtained at the tuned frequency of tanks and vessel.

Referring again to Fig. 4, it will be seen that the roll under conditions of stabilization by means of

the activated tanks is substantially reduced over

the whole range of periods of the disturbing sea.

The above tests were made with a disturbing force almost equal to the maximum stabilizing

power available. Should the disturbing force be larger than the stabilizing force, additional free

rolling will occur due to the amount of the disturb-ing force in excess of the stabilizdisturb-ing force. Addition-al tests were subsequently carried out on a model vessel in the Ship Division of the National Physical

Laboratory and, further, on a 60 tons sea going

vessel fitted out for the express purpose of

demon-strating the stabilizer. An account of these tests

will appear in a subsequent issue of Technique.

Design limitations

Reference has been made to the maximum

Q O FIG 4 FIG 5 DISTUUINC t041 W*& sioPe 2 3 4 7e

ROI.I. PERIOD (secoNDs)

stabilizing power of a tank stabilizer system and

this merits some further examination. With a water tank system having a free surface, as is well known to naval architects, the free surface

effect is equivalent to loss of righting moment of the vessel and, therefore, the stabilizer cannot be

made larger than a certain size without serious loss of righting moment and the consequent

endangering of the safety of the vessel. In practical

terms this amounts to limiting the weight of fluid used in the stabilizer and thus the power of the stabilizer is limited to a value capable of countering the equivalent of say a 2-degree sea slope applied to the vessel. At first sight this

would appear to be a very small angle but it must be remembered that a 2-degree synchronous sea is capable of rolling a vessel without bilge keels to well over 30 degrees out-to-out and with bilge

keels, over 20 degrees if the sea disturbance

corresponds with the natural period of the vessel.

Reference to Fig. 4 will show that the analogue

table represents the former correctly.

Additional sea forces may be encountered in which the worst conditions might correspond to a 5-degree or 6-degree wave slope in severe Atlantic storms. In general, stormy seas do not have a regular frequency characteristic, and the

very large rolling which would result if such a sea was synchronous, does not occur. To counter all

conditions, an ideal stabilizer might very well

consist of a fin-type stabilizer capable of exerting a torque corresponding to a 3-degree wave slope plus an activated tank.type stabilizer of 2-degree power.

Such an arrangement would be capable of

dealing with extreme storm conditions and would

also cater for the vessel at anchor in an open anchorage or cruising at reduced speed, under

which conditions the power of the fin-type

stabilizer is very much reduced.

In the subsequent article, examules of the

acti-vated tank system will be given including weights and the power required to operate the system.

Refsr.nce 1. Trans.R.I.N.A. Vol.26. 1185. The use of water chambees for reducing the rolling of ships at sea.' The copyright of all articles in this journal is strictly reserved, but the material may b. reprinted without permission

provided definite acknowledgement is made to Muirhead & Co.. Limited, the publishers

BM 63517 Printed in England 30 2$ 26 24 22 2O I. Ib 4 12 o 30 2$ 26 24 12 WE ₂₂ s Q O 20

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