The Growing Demand
an
the Economìc
Case for Flume
Stabilization
A Review of Existing Tank-stabihzation
Systems and the Gains in Speed,
Fuel
Economy and Maintenance
Costs
Offered by Advanced
Methods
4ANY classes of ship, including
iVI tankers, are today operating with
some form of anti-rolling equipment. hut
it is the passenger-carrying vessel which
demands the best forms of stabilization. Many ncw ferries entering service are
tilted ith various types of tank stabiliza-tion systems, rather than the usual lin-type
stabilizers and to sonic extent this may
be attributed to the relatively high initial
cost and redction in speed of the ship when such fins are in use. This. speed
loss has, on occasion, been measured as more than I knot, although a more repre-sentative speed loss is between O'SO and 075 knots. Considering the extremely
tight schedules with which ferries operate,
the elimination of any speed loss
trace-able to stabilization has been an extremely
important factor in the consideration of
tank stabilization systems by ship owners. Tank stabilization systems result in a speed gain, rather than a speed loss. This speed
gain is obtained by the removal of bilge
keels which, when coupled with the reduc-tion of rolling, increases the sustained sea
speed of the ship by 05 knots.
The fact that tank systems operate at any ship speed, even when the vessel is hove-to, has added to their increasing
favour with owners and operators of
ferries. lt must again be realized that
many ferry services require long periods ot low-speed manuvring when entering and leaving port, during which time a lin
system is relatively ineffective, while a
tank system would operate with full
efficiency.
The possibility of damage to a lin
system in heavy weather is another factor
hich must he considered, along with
maintenance and operating costs, and this
makes the investment required for lins about four times that required fora tank system: there is the additional factor that
a tank system, depending on the type
selected, requires little or no maintenance
over the life of the ship.
Most tank systems are dependent on
the proper phase relationship between the
motion of the ship and the moment
generated by the motion of the liquid
within the tank. Very simply speaking. the moment generated by the liquid is
made up of two force components, a vertical force component which reflects
the difference in weight of liquid on each
side of the tank, and a horizontal
com-ponent caused by the motion of the
liquid transversely across the tank. The
horizontal component is generally small
in relation to the vertical component, although its magnitude is dependent upon
the distance of the tank from thecentre
of gravity, and the location of the tank with respect to the centre of gravity. ihe
horizontal component increases the
stabilizer moment if the tank is above the
centre of gravity, and reduces the
stabi-lizer moment if the tank is below the
centre of gravity.
While the basic idea of tank
stabiliza-tion is not new, the systems offered today differ widely in concept and, insome cases in principle. The types of tank systems
offered today may he described as
undamped free surface tanks, damped free surface tanks. U-tithe tanks, active
tanks, and controlled tanks. These systems are briefly described below
together with a description of their
prin-cipal characteristics:
AUndaniped Free Surface l'anks.
Although undamped free surface tanks represented the first effort toward tank type stabilization, and such work was reported by Watts in 1883 (I) and
1885 (2), this type of tank was not thought
to be sufficiently worthwhile for further
investigation. The publications of the
staff of the University of Delft (3), (4),
and recent installations in two ferries (5).
represent attempts to justifY the work of
Watts in view of the present
popu-larity of tank systems. The principal danger of undamped tank systems lies in the fact that large angles of stabilized
roll may be encountered at wave frequencies away from the natural
frequency of the ship, and severe and
possibly dangerous destabilization can
occur.
According to the theory of Chadwick and Klotter (6), a completely undamped
By J. J. McMULLEN,
John J. McMuUen, Associates inc.,
New York, London, Hamburg.
tank will destabilize to an infinite point
at frequencies removed from resonance.
and will stabilize to a zero point at the resonant frequency of the tank, if this is
matched to the resonant frequency of the
ship. As every tank must contain some
damping, frictional or otherwise, the usc
of the terni " undamped tanks " is relative.
hut the possibility of destabilization is real, lt
is difficult to control the fre'
quency of undamped tanks, since Ire-quency changes appreciably with the angle
of oscillation, and precise tuning of an undamped tank is much more important
than in the case of a damped tank, which is described below.
8Passive Free Surface Tanks with
Internal Damping. This type of tank system. of which the Flume Stahilizatioiì System is the best known (7), (8). (9). is primarily responsible for the resurgence
of interest in tank stabilization systems.
lt was not until after the Flume system
was introduced in 1960 that passive tank systems were even considered. More than
300 such installations have been
con-tracted, with very satisfactory results,and
a large number of ferries have used the Flume system with good effect. In this
type of tank system, the adverse effects of the system described in paragraph A have
been eliminated by the introduction of flow orifices within the tank to provide
damping of the liquid motion. This damp-ing has the effect of alterdamp-ing the response
curve of the stabilized ship until it is
nearly flat in character, this eliminating the possibility of destabilization. The introduction of damping by nozzles does not affect the natural frequency of the tank or the moment developed by the
tank. hut alters the phase curve of the tank to allow a response which is closer to 90. at frequencies away from resonance, than would he the case in an undamped tank.
C-U-tube Tank System. The U-tube
tank system, or Frahm System, is a fur-ther example of renewed interest in pas-sive tank systems previously thought to
he unworthy of use. The history and
background of such tanks may he found
in papers by Frahm (IO) and Biles (I I).
Among the many modifications available
FLBRUARV, 1967 THE MOTOR SHIP
541
542
today are systems fitted with valves in the submerged portion of the U-tube or
in the air passage, systems fitted with blocks for varying the cross-sectionalarca
of the U-tube and air passage, and systems
which are precisely the same as the
systems used by Frahm. U-tube tank frequency is extremely difficult to adjust
by variation of liquid level, since the prin-cipal factor In the determination of
frequency is the area and length of the
submerged cross connection.
DControiled Tank System.
Super-imposing a gyroscope control for the air
or water valve, upon the systems described in paragraph C. results in a controlled tank system, and such systems are being
offered today (12). (1 3). The stated advantage of a controlled-type system is
that it renders the tank less sensitive to variations of stability, offering in effect the same advantages obtained through variation of the liquid level in a free
sur-face passive tank system, without the necessity for such variation. Since a
U-tube type system is. by nature, less effective than a free surface tank, the
introduction of flow variation and control
a'?
THE MOTOR SIHP
is a further reduction of efficiency. Thus
a controlled tank must he larger than a free surface passive tank and, since the horizontal component is of more import-ance in a U-tube tank than in a free
surface tank, should be located above the centre of gravity.
EAclivated Tanks. Activated tank
systems use a propeller or pump to
over-come the drawbacks of the controlled
system described in paragraph D.
Although the system may be theoretically
Sound, the cost of installation, power
requirements, and the impracticality of pumping large masses of water make it
relatively unsuitable for all except smaller
vessels.
From the above, and the large degree of acceptance obtained by damped
pas-sive tank systems, it is easily understood
why these systems have made great
inroads on th'e popularity of fin stabil-ization systems in ferries. lt will he interesting to note further developments with regard to the stabilization of ferries. and the role which the various types of
fin and tank systems play in future
developments.
FEBRUARY, 1967
REFERENCES
1.Watts, P., On a Method of Reducing The Rolling
of Ships at Sea. TINA. 1883.
2.Watts, P.. The Us of Water Chambers for
Reducing The Rolhng of Ships at Sea. TINA, 1885.
3.Stigrer. C.. The Performance of U-Tanks ¡a a
Passive Anti-rolling Device, Nech. Ship Research Center, TNO. Report 81S.
4.Van den Botch, J. J.. and Vugts. J. H., Roil Damping
By free Surface Tanks Neth. Ship Research
Center, TNO. Report 83S.
5." Norwane ' Shipbuildìng and Shipping Record.
December 30, 1965.
6.Chadwick. J. H.. and K. Kiotrer, On the Dynamics
of Anti-Roiling Tanks, Technical Report 2. Dept.
of Electrical Engineering. Stanford University.
19S3. Sch,tfstechnik 3, 1955-56
7._Bridges. T. F.. Hiiliard. B. A . and McMiller,. J. J., The Influence of Bilge Keels and Rolling In
Waves ort Sea Speed and 1-lorsepower. Tr.
SNAME. 1964.
8.McMullen, J. i., and Field. S. B., Passive Tank
Roll Stabilization and the Fiume Stabrirzat,on
System. Canadian Shipbuilding and Ship Repwrivg Assoc,ation. 1965.
9.Webster. A. R.. The Design of the Canadian
Weatherships, TINA. 1964.
10.Frahm, I-4.. Results of Trials of che Anti-Rolling
Tanks at Sea. TINA. 1911.
11.Biles, I-I. J. R., Model Euperismients with
Anti-Rolling Tanks, TINA, 1925
12.Bell, J., " Muirhead-Brown Controlled Tank
Stabilizer" Part 1 and 2, Technique-House
Journal of Muirhead and Co. Ltd.. January and
April 1965.
13,Bell, J,, and Walker. W. P., Activated and Passive Controlled Fluid Tank System for Ship Stabiliza-tion. SNAME. 1966.
