Assessment and improvement of ship handling characteristics

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AS8ESSMEN'1 AND IMPROVEMENT OF SHIP HANDLING CHARACTERISTICS

KJELL MARTINUSSEN MARINTEK A.S.

P.O.Box 4125 Valentinlyst, N-7002 Trondheim, Norway

ABSTRACT

The paper describes briefly the work leading to the proposal for a standard for ship manoeuvrability by International Maritime Organization (1MO). The present version of the

proposal is presented and the choice of criteria is discussed. Methods for improving ship steering and xnanoeuvring

characteristics are then discussed. This discussion is based mainly on experience gained from model tests. Results from some of these tests are presented.

INTRODUCTION

Manoeuvrability has probably never been considered the most important aspect of ship design. The reason being mainly the lack of a good basis for assessment of ship manoeuvrability. As a consequence of this, the manoeuvring characteristics of a

new ship have seldom been the subject of contractual requirements.

This situation will probably change when 1MO completes its development of a standard for the manoeuvrability of ships. Even if international acceptance of a standard is a prolonged process, it is expected that it will find its way relatively quickly into contracts for new ships. This

prediction is based, of course, on the assumption that the criteria of the standard are well chosen and are considered relevant by the shipping community.

Such a development will give new emphasis to the design of ships for good manoeuvrability. Knowing that some modern hull forms may be the cause of xnanoeuvring problems, it is expected that the means of obtaining a manoeuvrable design will be considered more important in the future.

BASIS FOR ASSESSMENT OF SHIP MANOEUVRABILITY

The definition of characteristics and development of criteria for ship manoeuvrability have a long history. Several

individuals as well as national and international institutions have made efforts in this area. However, so far no

internationally accepted standard has been developed.

Manoeuvrability of ships has been on the agenda of the Sub-Committee on Ship Design and Equipment (SD&E) of 1MO since 1968 as far as the author knows. During much of the time a working group for this subject has been active. Some progress has been made in the last few years. The characteristics

describing ship manoeuvrability has been defined, and the test manoeuvres necessary to quantify the characteristics have been explained. The information about a ship's marioeuvrability required to be found onboard a ship has also been defined. Lately the working group has made an effort to solve the remaining and most difficult problem, the development of a standard for the manoeuvrability of ships. A draft version of the standard has been developed and has already been modified a couple of times. The target date for completing the standard

is the meeting of SD&E in 1993.

Including the modifications suggested at the 34th session of SD&E in 1991 the important aspects of the standard are as given below.

Characteristics Defining Manoeuvrability

(Steady) turning ability, quantified by results from the turning circle manoeuvre.

Yaw checking ability, quantified by results from the zig-zag manoeuvre.

Initial turning ability, quantified by results from the zig-zag, turning circle or course change manoeuvre.

Course keeping ability, quantified by results from the pull-out manoeuvre and, if applicable, from the spiral manoeuvre.

Criteria for Manoauvrability

(Steady) turning ability: Advance should not exceed 4.5 ship lengths. Tactical diameter should not exceed 5 ship lengths.

Yaw checking ability: First overshoot angle in 10/10 zig-zag manoeuvre should not exceed 15 degrees. Second to fourth overshoot angle in 10/10 zig-zag manoeuvre should not exceed 20 degrees. First overshoot angle in 20/20 zig-zag manoeuvre should not exceed 25 degrees.

Initial turning ability: With application of 10 degrees rudder angle the ship should have travelled no more than 2.5 ship lengths by the time the heading has changed by 10

degrees.

Course keeping ability: If the pull-out manoeuvre shows that the ship is dynamically stable there is no further requirement. If this cannot be determined from the pull-out manoeuvre the following applies:

Length/speed ratio less than 12 s: The width of a hysteresis loop in the spiral manoeuvre should not exceed

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ship will be classed as inferior due to small initial turning ability and small steady turning ability. A consideration of the 1MO proposal will therefore to a great extent consist of seeing how the criteria apply to ships with different degrees of instability.

In figure 1 are seen results from a 20/20 zig-zag test and a spiral test for a very unstable ship. This ship is considered very difficult to handle in normal operation. Comparing these results with the criteria given above we see that at least a ship with these unsatisfactory characteristics will not comply with the proposed 1MO standard.

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Figure 2. 1st overshoot angle in 10/10 zig-zag tests.

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Figure 3. Ship handling cha-racteristics. ist overshoot

in iO/iO zig-zag tests.

Figure 2 shows results for the first overshoot angle in 10/10 zig-zag tests for some relatively modern ships. We see a very large scatter of results. Ships found at the upper end of the diagram are probably very unsatisfactory with regard to

handling characteristics. Somewhere in the lower part of the diagram there is probably possible to draw a rough line

between ships with satisfactory and ships with unsatisfactory characteristics.

The problem here as with any collection of ship trials results is where to draw this line. Taking for granted that there exists a relation between ship handling characteristics and accidents at sea, a reasonable approach may be to draw the

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well founded criteria for ship maneouvrability. The project will be continued under a grant from the Norwegian Maritime Directorate.

IMPROVEMENT OF SHIP HANDLING CHARACTERISTICS

As mentioned, we generally mean a lack of dynamic stability when we use the terms inferior or unacceptable handling

characteristics. Requirements of loading capacity and

propulsive efficiency will often lead to hull forms lacking dynamic stability. Only as a last resort will a hull form be changed because of unsatisfactory oanoeuvring characteristics, if it is optimized with respect to the above mentioned

parameters. Consideration of the vessel's manoeuvrability should therefore be included in the optimization _procedure early in the design process. If this is done it is very likely that designs that are found deficient with respect to steering and manoeuvring, can be sufficiently improved without

resorting to hull form changes of any consequence. The means available in the first place are rudder design and addition of lateral area in the afterbody. Other means are installation of special stabilizing fins and various devices for improving the flow to the propeller and rudder. At the design stage it

should normally be sufficient to consider the rudder _design. If the space is too restricted to fit a conventional rudder of sufficient size, a special high-lift rudder might be

considered. Such a rudder will in most cases provide the control forces necessary to make possible the improvement required.

If inadequate manoeuvrability is discovered after the ship has been put into service it is still possible to resort to rudder improvement or a new rudder design. However, freedom of action may now be more limited in _{some cases due to}

inadequate steering engine power, due to insufficient strength in the afterbody structure to support a rudder providing

larger control forces, or simply due to cost. In such cases some of the other devices mentioned might be considered.

Some solutions for improvement of course keeping ability and manoeuvrability has been tested in the Ocean Laboratory of Marintek. Most of the tests have been _{performed with}

relatively large (length 4-6 m) free-running, _remotely controlled models. In these tests the _{excess frictional}

resistance of the model compared to the ship _{is compensated by} the force from an air propeller onboard the model. This

propeller is automatically controlled _{as function of the model} speed. This force is equivalent to the towrope force in

ordinary propulsion tests and ensures that the main propeller loading and rudder force is at the correct level in the tests. Rudder or steering thruster angular speed is also modelled in these tests.

Figure 8 shows the effect of different rudder designs on three different ships. The effect of the rudders on

manoeuvrability is exemplified by the _{influence on the first} overshoot angle in the 10/10 zig-zag test. _{Considering ship 1}

other had a pronounced IfS section shape. The figure shows full scale values in MNm of total moment about a vertical axis through the vessel's centre of gravity. This is the sum of the moment induced by the flow around the hull and the moment induced by the rudder. Positive moment turns the bow to

starboard, negative moment to port. Results are presented

for

(port rudder), and for a range of negative (bow directed to port of direction of motion) and positive (bow directed to starboard of direction of motion) drift angles. It is seen that the rudder with IfS section shape provides a marked improvement in moment induced by the rudder. From another series of tests with a high-lift flap rudder we have estimated the effect on the ship in figure 11. It is seen that the flap rudder provides a further marked improvement of rudder induced moment.

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Figure 12. Effect of vertical course stabilizing fins in the afterbody.

Figure 12 shows examples of the effect of fitting vertical course stabilizing fins in the afterbody of the ship. The effect is again demonstrated by the influence on overshoot angle in 10/10 zig-zag tests. In the case of ship 1 in the figure an increase of relative fin area from 1.5 to 3.1 per cent of ship length times draught resulted in a decrease of first overshoot angle from 16.9 to 10.8 degrees. Ship 2 experienced an overshoot angle of 20.6 degrees with no fins fitted. Installing fins of relative area 2.95 per cent reduced

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the overshoot angle to 13.3 degrees.

Considering ship 3 an overshoot angle of 32 degrees was found with no fins fitted. Installing a kind of stern bulb above the rudder and fitting vertical fins of relative area

1.67 per cent reduced the overshoot angle to 28.0. Here it should be mentioned that the stern bulb alone increased the overshoot angle to 35 degrees (not shown in the figure) Removing the stern bulb and increasing the fin relative area to 2.79 per cent reduced the overshoot angle to 18.5 degrees. The fins fitted to ship 3 so far were of flat plate section shape. Fitting fins of relative area 1.25 per cent with NACA aerofoil section shape, and fitting a horizontal bottom plate to the tip of the rudder resulted in an overshoot angle of 25.5 degrees.

Ship 3 was also tested with horizontal finlike devices fitted to the afterbody hull to improve the flow to the rudder and propeller by removing what was suspected to be a certain recirculation in the vicinity of the upper part of the rudder. This modification reduced the overshoot angle from 32 to 28 degrees.

Figures 13 and 14 show results from model tests for a fast waterjet-propelled vessel. This vessel is equipped with a horizontal hydrofoil on vertical supports in the afterbody. The lateral area of the supports provide an appreciable course stabilizing effect. Results from spiral tests at 34 knots are

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Figure 13. Effect of vertical Figure 14. Effect of verti-area. Turning circle test. cal area. Spiral test.

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