The application of non-cylindrical nozzles for large tankers and bulk carriers

__ _______

Tchmsche Hoçjeschcd

595

Delfi

1

The application of non-cylindrical nozzles

for large tankers and bulk carriers

ARCH

Shipbuilding and Shipping Record, November 8, 1968

WHEN SELECTING PROPULSION devices for ships, propeller induced vibration and cavitation must be considered in addition to the ever-present desire for high propulsive

efficiency. In an attempt to provide large tankers and

bulk carriers with superior propulsion devices, the

appli-cation of ducted propellers has been the subject of

exten-sive investigations performed at the Netherlands Ship

Shape of ducted propeller systems

Insight into the shape of ducted pro-peller systems can be gained by Fig. 1.

Here, the flow through different types of ducted propellers is compared with

the flow through an open propeller.

Both open propeller and ducted

pro-pellers are designed for the same mass

flow rate and velocity in the ultimate wake.

The momentum law leads to the con-clusion that the ideal efficiency and the thrust of these systems are equal. In the ducted propeller, the

axial force on

the impeller differs from the net thrust of the system. A positive or negative force

will act on the duct depending on the operating condition. Due to the effect

of the nozzle, the inflow velocity of the

impeller can be either less or greater

than the inflow velocity of an open

propeller under equal conditions. The shapes of accelerating and flow-decelerating nozzles are schematically

shown in Fig. I. This figure shows also

that the ratio exit area of the nozzle

over the disc area of the impeller f0r the flow-accelerating nozzle is larger than that for the decelerating nozzle.

Investigations into the optimum nozzle shape from an efficiency point

of view have led to the conclusion that a maximum acceleration of the flow by

the nozzle should be aimed at. The

accelerating nozzle

itself produces a

positive thrust. However, the

accelera-tion of the flow is limited by the risk of flow separation on the interior sur-face at the aft part of the nozzle.

Tests with series of ducted propellers

Investigation performed at the NSMB

concerning nozzles of the accelerating type have led to the development of a standard nozzle (No. l9A) applied by the NSMB in the case of heavy screw loads. This nozzle meets various prac-tical requirements: it has an axial cylindrical part at the inner side of the

D

nozzle at the location of the impeller,

the outside of the nozzle

profile is

straight and the trailing edge of the

nozzle is made relatively thick. For this

nozzle the Ka-screw series were

specially designed. These screws have

wide blade tips, uniform pitch and flat

face sections.

Experimental investigations showed that with regard to efficiency and

cavi-Fig. I. General form of steamlines enforced by an open propeller and by different nozzle types

Flow deceteratin9 Open propeller Flow accetertirg

type of nozzle type of nozzle

ve +Ua

Model Basin. The results of these investigations vere

given by Van Manen in several publications. Recently,

non-cylindrical nozzle shapes adapted to the

circum-ferential non-uniform flow behind the ship have been

investigated at the NSMB. A short description of the

purpose and the results of the investigations is given

here.

_-1EE

Propel1er disc

M. W. C. Oosterveld

Netherlands Ship Model Basin

tation this impeller type is just as good as those calculated according to Vortex

theory. Besides, they have reasonable

stopping abilities. Optimum curves for

open-water efficiency ti0, diameter coefficient 6 and impeller thrust-total

thrust ratio - of the Ka 4-70 screw series

in nozzle No. 19A and the B 4-70 screw

series are compared on a base of the

power coefficient B in Fig. 2. Screws of

the B 4-70 screw series

are usually

applied behind single screw ships. Fast

modern tankers and bulk carriers have B, values in the range of 35-60. lt can

be seen from this diagram that from an

efficiency standpoint application of a ducted propeller for these ships is attractive.

Non-cylindrical ducted propeller

Based on the

results presented in Fig. 2, propulsion tests have been per-formed with tanker models equipped with ducted propellers. The results of these tests confirm the conclusion that

an increase in propulsion efficiency can

be obtained by application of a ducted

propeller for this ship type if compared with a conventional screw. The propul-sive efficiency can be further increased by application of a ducted propeller, in combination with a cigar-shaped stern. The cigar-shaped stern tends to

homo-genise the flow and to bend it

in a

horizontal direction. In this respect. the app1 ication of a non-cylindrical nozzle

behind a conventional stern may also

be attractive.

A propeller behind a ship operates in a non-uniform flow. The intake velocity

is lower in the upper part than in the lower part of the screw disc. Conse-quently, the propeller is more heavy

loaded in the upper çart of the

pro-peller disc. The inflow velocity of the propeller can be made more constant

over the screw disc by surrounding the

propeller by a non-cylindrical nozzle which is adapted to the wake distribu-tion and the flow direcdistribu-tion behind the

ship. This nozzle accelerates the flow in

the upper part of the screw disc (by

increasing the exit area of the nozzle) and decelerates the flow in the lower part (by decreasing the exit area of the nozzle).

Fig. 3. View of stern of tanker with

non-cylindrical ducted propeller

10 15 20 30 40 50 60 70

ap

Fig. 2. Comparison of the K, 4-70 strew series in nozzle no. l9A and the B 4-70 strewseries

A view of a tanker with a conven-tional stern and equipped with a

non-cylindrical ducted propeller is given in

Fig. _{3. The non-cylindrical nozzle is}

cylindrical inside from the leading edge

of the nozzle to the impeller; only the

aft part is non-cylindrical. The outside of the nozzle profile is again straight.

Reductions in DHP due to application

of ducted propellers

In the table below the

results are compared of a large number of model self-propulsion tests, performed with tankers with conventional and cigar-shaped stern arrangements and fitted with conventional screws, ducted

pellers and non-cylindrical ducted

pro-pellers.

Comparative DHP reductions

Contigu ration

Ship with conventional

stern and cylindrical ducted propeller

Ship with cigar-shaped

stern and cylindrical ducted propeller Ship with conventional stern and non-cylindri-cal ducted propeller

Loaded Ballast condition conditon red u tions about 2-4% greater

From this table it can be seen that

the conventional stern with non-cylin-drical ducted propeller gives a reduc-tion in DHP which is still larger than

can be obtained by application of a

cigar-shaped stern with a cylindrical nozzle. 5 1.3 i:i L g .7 05

The ship with conventional stern and

non-cylindrical ducted propeller as com-pared with the ship with conventional

stern and screw gives a reduction in

DHP which is for the loaded condition

6-9% and for

the ballast condition 8-13%.

In addition, it may be concluded from

the homogenising effect

of the

non-cylindrical nozzle on the inflow velocity to the impeller that the non-cylindrical ducted propeller offers a definite means of minimising propeller inducted vibra-tion and cavitavibra-tion problems.

Economy

Economical considerations will give the final answer on the question in how of minimising propeller induced

vibra-far application of the non-cylindrical ducted propeller is attractive for large

tankers and bulk carriers.

The increase in building costs has to be compared with the decrease in DHP and the smaller risk for propeller induced vibration and cavitation dam-age. Besides, a comparison has to be made between the building costs of a conventional stern arrangement with

non-cylindrical ducted propeller and

those of a cigar-shaped stern with cylin-drical ducted propeller.

Finally it may be noted that

appli-cation of the non-cylindrical ducted

pro-peller also offers a means to improve

the propulsion characteristics of existing

ships without expensive alterations of

the hull shape.

With reductions in DHP of 6-13%

it _{is evident that the range for}

econo-mical application of non-cylindrical

ducted propellers for tankers and bulk-carriers has been reached.

VU 0,90 0.80 070 0.50 .040 0,30 0,20 010 8p_0_! VA _0__ P power .__ per _OIL' V D screw iarneter D-maximum dameter nozzle outsde T tott thrust Tpropetter undisturbed stream mn _" vetocity at the thrust VA

-- -

Tp T ..----, nozzle noi9A

L

-. B 4-70 ozte nol9A '4 noi9A

596 _{Shipbuilding and Shipping Record, November 8. 1968}

400 360 320 280 240 200 150 120 80 40 0 2-5% 5-8% 6-9%

Shipbuilding uzd S/zippingRecord, November 8, 1968 597

Linearisation of ship's

steering characteristics

STEERJtG INSTABILITY has been

experi-enced particularly with large bulk car-riers and mammoth tankers having blunt

hull forms.

The tendency has been

apparent in tankers having a length to breadth ratio of less than 7 : I and in

ships with a smaller L1B than 6 :

(see S.R. 98 Report, Japan Shipbuilding

Research Association).

The head of the Decca Arkas opera-tional research team. Mr. M. Bech. has

devised and patented

a method of

modifying the existing Arkas autopilot

and manual steering systems whereby

iT IS ACCEPTED that a ship's steering

characteristic is linear in only a small area around low values of rate of turn.

Some ships are dynamically unstable.

This dynamic instability shows up around low values of rate of turn, but

it decreases and totally disappears when the rate of turn exceeds a certain value. These facts can render the automatic and particularly the manual steering of certain ships very difficult.

The following development is

intended to compensate for any

non-linearity in the steering characteristic,

through a modification to

the ships

steering equipment in such a way as to make any ship behave like a normal and

well-steered ship both from the point

of view of helmsman and the autopilot.

An investigation has proved that a ship's manoeuvreing can be described

with good approximation by the follow-ing equation:

aY,N8 2m N Y

q'.

3-6wF2 6-iE( - 1812 2m q' t

which is valid when the ship is sailing at a constant speed. The symbols are

as

follows:-6 Rudder angle.

'Y Heading angle.

in Ship's mass.

12 Ship's moment of intertia

around a vertical axis through

centre of gravity.

a

Constant for a given ship.

'Vr

'VV

Nr N6

Jl(q') Ship's steering characteristic. Hydrodynamic derivatives.

vessels with unstable or marginally

un-stable steering characteristics, can be electronically cured of the defect and

made to

behave like normal,

well-steered ships.

Decca offer a service whereby Mr. Bechs reversed spiral test

is used to

assess a vessel steering stability or other-wise

during trialsUniverse

Ireland

was so tested and found to be stable

steering-wse---and modify the autopilot

to iron out any instability discovered. The actual patent application is generally set out below.

where

constant.

the rudder angle commanded wheel or autopilot (i.e. wheel angle).

li applied to i renders:

av .N 2u . N Y, 1.

36ui 12 aq'

-or

It can be proved that A. B, and C

with good approximation are constants.

As the

rudder velocities normally

applied in ships are normally rather

low, the following is valid with good

approximation: From this is obtained:

which is a linear differential equation with constant coefficients.

This equation describes a ship with

ideal linear steering characteristics over the whole range of manoeuvres.

The constant a denotes

the ratio

between applied wheelangle and finally

obtained rate of turn"j'.

Fig. i describes a possible mechanisa-tion of the invenmechanisa-tion.

a

Fig. I

The rate gyro (2) measures the rate of turn and applies this signal to the function generator (3) and to the

signal converter (4) which multiplies by

the constant a.

The signals H(4) and a"4f thus

obtained are also applied to the sum-ming device (6) which accordingly

derives the wanted output signal b.

The signal & is now used in normal

ways to control the rudder position.

Instead of obtaining the signal b from the wheel (1) it may as indicated

by obtained from the autopilot (7), (8),

(9), (10)

As the functions H('4-)

for any

given ship will depend on the ships

peed and loading condition the

possi-bility may exist to apply to the function

generator (3) the ship's speed (eventually

automatically from ship log), as well as

probably manually adjusted

informa-tion about loading condiinforma-tion.

It is to be observed, however, that even if H('4.i-) is not exactly compen-sated for, the invention will still provide

a marked improvement of the control

of a difficult ship.

The invention is thus not depending upon these extra signals but may well

make use of them.

This is valid also for conditions external to the ship such as e.g. water

depth. channel effect or influence from nearby vessels.

The constant a, which denotes the

ships " new steering characteristic", may by simple means be adjusted at will.

The development applies a

mechan-ism which controls the rudder according The wheel (I) transmits the signal &

to the equation: _{which via the switch (5) is applied to}