Stopping of ships using propellers

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STOPPING OF SHIPS USING PROPEL:fa-3

E. P. Lover, MRINA, RCNC,

Deputy Superintendent, Admiralty Experiment Works, Haslar, Gosport, England.

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PROPMLER

STOPPING OF SHIPS USING PROPELLERS

By far the most effective apparatus for bringing a Ship to rest is the. device used to accelerate it to speed in the first place.

This tote exqmines the various factors governing the effectiveness

of

the propeller when used as a brake.

2. The behaviour of a ship when braking depends upon two equations

of motion,

that governing the rotation of the main machinery-shaft-propeller system, and that determining the translational speed of the ship. The two equations are interdependent, the propeller being a common factor providing a torque reaction to the shaft system on the one hand, and a braking force to the ship on the other. Even with the grossest simmlifications and the use of an

analogue computer, the solution of these equations presents a formidable pro-blem. Fortunately, however, the naval architect is not often presented with a braking requirement stated in precise terms, The owner is normally only con-cerned that the braking performance should be qualitatively of a given stan-dard and it is for this reason that this study confines itself largely to matters of cause and effect. The independent variables, or causes, are taken to be displacement, horsepower, and propeller characteristics, and the dependent variable the distance advanced whilst stonping (or head reach).

3.

An ex mination of the records of stopping manoeuvres shows, that the brak-ing sequence occurs in three overlappbrak-ing stages, viz:

A delay occurring after the executive order for astern rpm (or pitch) has been given,

A rapid loss, of speed due largely to the hull resistance and the removal of propulsive thrust and the drag of the stopped (or zero pitch) propeller.

A final phase in which the hull resistance has reduced to negligible proportions and the ship is brought to rest by the astern thrust developed by the propellers.

These are illustrated

in Figure 1.,

It should be noted that the first two stages depend upon the rapidity with which the propulsive thrust is removed and that only the final stagy is dependent upon the astern horsepower available, and the effectiveness of the propeller in

converting this power into a braking force.

4.

FUrthermore it can be demonstrated from records taken at sea that the complex and unstable transient flow conditions that apTly as the propeller moves from the locked (or zero pitch) condition with the ship moving ahead, to the quasi steady state, lastern bollard pullt condition at zero speed, results in a more or less constant astern thrust during this phase of the manoeuvre (data given in Reference4 are typical in this respect). It is therefore possible to take the propeller braking characteristics as being. reasonably

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well represented by those exhibited at the single condition at the end of the manoeuvre, ie with astern rpm (or negative pitch) and zero speed of advance!.

5.

With some fairly plausible assumptions it can then be shown (Reference I) that the distance required to stop (s) should be dependent upon the displacement,. the ratio

of

the power driving the ship ahead as the manoeuvre commences Po and

that available for braking during the final phase (Pb), and a, propeller factor trEt

ie

C

where U . (volume of displacement)

MECT OF ASTERN PO;JER

The results from a number of ship trials are shown in Figure 2 as

plotted against o. An interesting and unexpected feature of the data for

Pb

British warships is that although the vessels concerned ranged from 440 to 44,000 tons displacement, their results follow a common alignment. This is considered

to be due to the fact that their hull forms are reasonably fine and not grossly dissimilar, and that the requirements for efficient propulsion tend to give similar propeller-hull proportions.

The most significant feature of the results is, however, that for indi-vidual ships the relationship between astern power and stopping distance is given by

where n

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between 0.40 and 0.67.

When stopping from a slow speed it can be shewn that the power index Int should tend towards the value*. The lower value of 0.4 corresponds to brak-ing manoeuvres from higher speeds and reflects the point made in paragraph 3

above in that the astern power is relatively ineffective until the final slow speed phase.

THE EFFECT OF THE PROPELLER

The propeller effects a significant braking force during the second of the three braking phases as well as during the third and final phase. The drag of a locked propeller, or a OP propeller passing through the zero pitch condition

is

of the order of one half of that of a transverse disc of the same diameter or about the same as the propulsive thrust it -would exert to maintain self-pro-pulsion at that speed. Its effect therefore is to double the total resistance

to motion, and to double the rate of reduction of speed.

As the speed falls and the astern rpm builds up (or astern pitch is achieved for a CP propeller) the corresponding increase of astern thrust or drag offsets the natural square law decrease with speed leading to the approx-imately constant braking force during the final phase, referred to in paragraph

4.

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If the propeller-hull interaction effects are either neglected*, or are assumed to be constant, this constant braking force from the propeller during the final phase can be expressed in terms of the astern power and the astern bollard pull condition,

''':[= KT and K being the astern bollard pull

(J = 0) thrust and torque coefficients and D is the propeller diameter.

The criterion measuring the effectiveness of the propeller in converting shaft horsepower into astern thrust is therefore

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The immediate implication is that a large propeller of a given type is a more effective stopping device than a smaller one, not only because of its greater drag in the second of the three braking phases, but also due to its superior ability to convert power into astern thrust at the lower speeds of the final phase.

The effect of propeller shape, as distinct from size, is illustrated in Figure 3 where the parameter

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-is plotted against blade pitch for constant pitch and CP propellers, using model data taken from a variety of sources. Trends due to blade area ratio and pitch ratio are obvious, but it is also apparent that normal designs aimed at optimum propulsive efficiency are so near to the optima for braking effectiveness that it would be unwise to prejudice the former in an attempt to improve the latter. Only by adopting the maximum possible propeller diameter is it possible to obtain the best of both worlds.

The effect of the number of propellers is an interesting one. If a multi-propeller installation is considered, it can be shown that the total' braking force available from a total braking power is independent of the number of propellers if the size of each of the individual units has been determined by a common thrust loading (CT). This is normally the case and the overall effectiveness in braking is then only affected by the differences in hull propeller interactions in the different installations and not by the number of propellers per se.

CONCLUSIONS

If the distance to stop from full speed is taken as the criterion for brak-ing performance, then apart from the size and speed of the ship herself, the

Data obtained from trials of British warships suggests that these inter-action effects can be neglected for fine twin screw vessels.

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factors dominating this performance are:

i. The size of the propeller or propellers.

The speed with which the ahead propeller thrust is removed. The astern power available.

If slow speed manoeuvring is the criterion, the astern power becomes of greater importance, but the propeller size is still the dominating factor.

The fact that these two criteria are different can be illustrated by the example of two small British warships. One of these was fitted with propellers capable of producing a braking force several times more effective than her sister ship. On trial they were found to have the same head reach when stopping from

full speed. The only clue to their difference was the much longer time taken by one of them in stopping within this same distance, and her very inferior performance when stopping from a low speed.

8th July,

1969.

SES

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References

Reference 1. AEW Technical Memorandum 50/67 (November, 1967).

Reference 2. "Screw Propeller Characteristics." H. F. Nordstrom 1948. Reference 3. "Open water tests on propellers with fixed and controllable

pitch blades, running ahead and astern." F. Gutsche and G. Schroeder.

Reference 4. "The Braking of Large Vessels tII)." H. E. Jaeger and M. Jourdain. Netherlands Ship Research Centre.

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'BRAKING SEQUENCE.,

INITIAL DELAY IN EXECUTING ORDER.

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CI LOSS OF SPEED PS: CIPALLY ma' 'TO DRAG.

0 SHIP OROUGHT TO REST sy PROPELLER

BRAKING FORCE.,

OPOLE FULL AHEAD HALF ASTERN

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FIG. I.

NOTE

_{THE APPARENT RELUCTANCE OF THE SHIP TO DECELERATE}

'DURING PHASE CI IS ALSO DUE TO THE RATE OF CHANGE OF

VIRTUAL MASS THAT OCCURS AS THE VESSEL SLOWS DOWN

WITHIN ITS OWN BOUNDARY LAYER AND WAVE SYSTEM AND

THE RESULTING NEGATIVE DRAG EFFECTS THESE CAUSE

AN UNDULATION OF THE VELOCITY TIME CURVE DURING

THE WHOLE BRAKING MANOEUVRE, BUT ARE PARTICULARLY

SIGNIFICANT iN THE INITIAL PHASE.

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FIG. 2. HEAD REACH CURVE.

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