L,Ofltfl DUt On
Roll stabilization must be considered only the first and prudent
step in dealing with ship motions, which problem has been
adequately solved in various ways. So far, sea experience has shown that a stabilized ship proceeds at the some speed as she
did when rolling for the same power output from the main
machinery since roll stabilization does not affect the seaworthi-ness.
The next and final step to be taken is a solution of the pitch-damping problem, though this major problem as regards sea-worthiness and speed is much more difficult and complex than rolidamping. The full-scale trials with fixed fins on USS 'Com-pass Island" presented a complete failure and the fins had to
be removed for various reasons. The bulbous bow may be held responsible for the excess vibrations during the trials since this
construction involves a wilful narrowing of the bow at the waterline. However, the excess shipping of spray and water
was not anticipated by model research, though pitch reduction
was very partial and estimated at about 20 per cent of the
vertical accelerations at the bow as compared with undamped
pitch. Since extremely large forces and severe problems of
seawotrhiness and speed are involved in pitch stabilization, not only practical seamanship and sailor's intuition but pure logic demands to fight the source of the motion, i.e., the waves. This
progressive, though for some of the scientists in this field
probably controversial conception, together with a suitable ap-plication of modern equipment, will lead to a practical solution
of the pitchdamping problem.
U.S. patent No. 3,O1S,749 (1962): "Anti-pitch stabilizers for ships" This patent of which the application has been viewed by the Examiner with expert ability, contains a full description of the working of the tins. The simple idea is comparable to that of a horizontal reversed balanced rudder of which the deflection is
automatically controlled by suitable means. The differences with
the known constructions of this type have resulted in an effi-cient, automatically controlled device.
These peculiar qualities may be summarized as
follows:-1) The combination of bow propellers and movable fins (7, line 15).
As was to be expected, neither fixed nor movable fins alone will be able of uroperly reducing pitch. The full-scale trials on USS "Compass Island" leave no doubt this statement and the
additional applicatior of bow propellers will be indispensable on
many counts, even 'hen controlled fins are provided in the
propeller race at tb .tern, which are said to be very effective. However, the censtr iction of stabilizing fins in the propeller
race at the stern '; 11 present serious difficulties for single-screw vessels bscau - of the rudder.
Bow propellers, eitir of the "screw" or the "Voith-Schneider" type will distu b tìi orbital motion of the water particles in the wave, level the iangerous top of the wave crests and check the bow wave formation thus decreasing pitch angles, heaving and vertical accelerations and, in general, ease the motions of
the ship. The suction and discharge screw currents will establish a large low-pressure region along the forebody of the ship,
thus preventing the shipping of spray and water, so that full power can be maintained even when reducing pitch. Moreover, the application of 'Voith-Schneider" instead of "screw" pro-pellers along the bow, with horizontal axes of rotation will not only facilitate construction and bring down the costs, but may provide a powerful means of opposing forces to the pitching motion, When the propeller blades are suitably controlled, the propeller thrust may be utilized for pitchdamping purposes for instance in a "heavy sea" condition. When installed parallel to
the contour at this location of the bow, the resistance of the
slipstream on the hull will be negligible, without impairing its favourable influence on the waves and bow wave formation or pitchdamping properties. For example, when an output of the "Voith-Schneidcr" propellers of 10 per cent of the total engine power will be required for said purposes, even the fair weather speed in a smooth sea will not be hindered materially.
(2) The guard frame (7, line 33), protecting the fins and the
bow propellers, permitting a safe berthing and anchoring.
36 Holland Shipbuilding
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to a so-uon oî
ju1A:
'6's
1ya,
The bearing means of the fin axis in the frame (8, line 5). This feature will fortify fin construction and prevent a flutter-condition of the fins.
Limiting the deviation of the fins in midstroke for a cavi-tation-free position in different sea conditions (8, line 12).
The idea is new for this type and includes the advantages of movable fins to fixed fins.
A margin between the lever and the abutments (8, lines
14-17). This feature is decisive for an undisturbed and efficient action of the fins. The abutments ars Only C safeguard against
severe irregularities in the pitching motion.
An examination of the patents cited (8, lines 29-37), will give an explanation of the above stated special, improving features.
A severe handicap for a realization of the project is the poor correlation between wind-generated ocean waves and the pre-sent trochoidal, swell-like tank waves.
Though all investigations about ocean at'es are primarily dealing with the significant wind waves, representing the regu-lar windsea, see for instance the Manual by Pierson, Neumann and James. this research concerns only the wave length and
height, but there is not the slightest indication of the profile
of the signe wind wave. In an Appendix (1961) to my book "Speed''&'Ttching" (1957), a new significant-wave theory has been developed, revealing all wave characteristics which,
to-gether with the well-known trochoidal-wave theory for the
swell, will give a perfect insight in the seemingly so complex seaway-conditions. A combination of these wave systems will yield the irregular or confused sea, Moreover, a method is
pro-posed to generate a significant-type of wind wave in the tank by
means of a set of movable pneumatic wave makers. Generating the tank waves at the working length of the tank and removing the device to let the model pass, will prevent deformity of the
wave profile. The seaworthiness and speed of a ship in a seaway
depend on this kind of wave. The ultimate goal for imitating sign. wind waves, is tu generate tank waves with breaking, or
nearly breaking crests. Prof. W. J. Pierson,
Jr.:-'If we are careful enough to provide breaking waves in the
tank and make enough measurements, then a model ship will solve our nonlinear problems for us."
Even with "flaps" there may be a method to realize this profile, by programming the (laps and concentrating the tank waves in
the midst of - the working length - of the tank. A
concen-tration of a large number of similar waves, successively only slightly differing in direction, may result in significant-type of tank waves, though much depends on the manner in which tank waves interfere and integrate.
However, a combination of the two wave generating systems will have many advantages in tank research, since any irregular sea may be generated. An irregular sea is a windsea excited by the prevailing wind but disturbed by crossing swell waves from distant storms or depressions. The windsea, represented by the significant-type of tank waves may be generated by the pneumatic wave makers and the swell waves by the flaps. Any sea condition may thus be represented.
The urgent need for an improvement of the commonly used trochoidal tank wave is widely known.
Mr. Lewis C. Host:
"All of us will subscribe to the need for more complete data
on sea conditions, It may well be, however, that the sea, in spite
of all investigations, will refuse to give up its secrets, as it has in the past, and that we will be forced to accept the fact that
sea characteristics cannot be definitely set out like the
tro-choidal-wave theory, and we must therefore lean heavily en experience for what the sea will do."
The good, old and familiar trochoidal-wave theory yields a
profile which is closely related to the swell wave as well as to
the commonly used trochoidal tank wave which, in all respects,
J
e. pcnoianpna prooem
Fig. 5 of the patent specification. Presenta horizontal sactiona through the propeller shofts of a ship fitted with two propellers 14 at the stern and two screw propellers 15 at the bow. The balanced rudder is denoted 11-13. The anti-pitch stabilizer at the bow consists of two similar parts on both sides of the bow, fixed to a common transverse shaft 3, dividing each fin in two nearly balanced areas of which the foremost poes 9 have a slightly higher moment on the shaft 3 than that cf the after parts to, when hit by an obliquely directed flow of water. lt may be noted that the center of balance on inclined planes. caused by a flow with a small angle of attack, s located at about one-fourth of the wdth of the piano from the leading edge, so that the fins in the drawing are strongly overbalanced. The shaft 3 is provided with a lever 6 and for the purpose of automatic control, the lever may be fitted with resilient devices
to check the deviation of the fins for a cavitation-free position. The partly
dotted Contours are horizontal sections at the level of the bow anchors 12, showing an adequate protection of the installation by means of the guard frame t.
is unsuitable to give reliable results in pitch or pitchdamping
research. Depending on the ratio chosen between wave height H
and wave length L, the seaworthiness features will not be dis-closed (Prof. Abkowitz, H/L is 1/30), or the motions will be strcngly exaggerated (Mr. Ochi, H/L is 1/20). However, a new sign-wave theory as stated will yield a perfect image of the
wind-generated ocean wave, the so-called "Stokian wave". Ac-cording to a careful study by Sverdrup and Munie, the steep-ness of sign. wind waves is comprised between the ratios Hi'L
of 1/10-1/20, with a realistic mean value of 1/13.3. Consequently, in seakeeping model tests and longitudinal strength calculations,
the commonly used trochoidal wave of ship's length (a heavy sea" condition) with a ratio H/L of 1/20, has to be substituted by a significant-type of wind wave with a ratio H/L of 1/13.3, to get more closely to actual sea conditions.
To what extent seakeeping model basin practice has drifted
apart from reality and from contact with sea conditions, when
the pitthing motion is at stake, may be indicated by the
fol-lowing.
Prof. Edward V. Lewis, in his lecture "Logbook analyses"
"The greatest reduction in speed in rough seas are found to
result from voluntary reductions of power. Entries in logs sug-gest that the most frequent difficulty associated with ship motions is the shipping of heavy seas (wind waves). High ac-celerations associated with pitching and heaving do not
ordin-arily seem to be a reason for speed reduction (swell waves). The
effects associated with pitching - shipping of water and slam-ming - appear to be the prinicpal reasons for power and speed reduction in heavy weather."
This true sailor's interpretation of the meaning - the behaviour of a ship in a seaway - or the seaworthiness, is thus only in-directly associated with the - motions - in pitch, but depends on the kind of waves encountered.
Prof. van Lamrneren, lecture Royal Academy of Science (1959, in Dutch):
"A model test in regular tank waves may be used to calculate for any arbitrary sea condition -' the behaviour of a ship in a seaway."
It will be clear that this, from a sailor's viewpoint, fantastic
statement only refers to motions which take only second place in a sailor's interpretation of the (seakeeping) behaviour of a
ship under certain weather and sea conditions on which, in
pitch, the speed depends. The statement is only valid for roll
and a convincting proof that the tank waves used in this
re-search are in fact swell waves in which the seawothiness feat-ures play no part, unless abnormal swell wave ratios H/L are applied. The limiting interpretation is due to lack of seaman-ship and the use of trochoidal, swell-like, tank waves in which the motions and the seaworthiness features are a far cry from those encountered under actual windsea conditions. However, the introduction of significant-type of tank waves will change this sad picture completely, yielding results in full confirmation with the acid test of the sea.
The step from towing tank to seakeeping tank is a fast one and
the ultimato criterion for the seakeeping tanks to qualify, is presented by a solution of the pitebdamping problem. The
Western World still has a firm command of the seven seas, one of the important assets in the race for World domination and freedom. To fortify this position, research and practical applic-ation of improved techniques even in this field, have to remain )ust one step ahead.
A short recapitulation of pitchdamping conditions may for in-stance, clarify the merits of fixed and movable fins for
pitch-damping purposes. At the phase of max. accelerations when the ship is on an even keel in midstroke, oenditions are most
favour-able for damping pitch. However, the euiciency of fixed fins will decline when accelerations are reduced by damping,
con-trary to movable fins where the angle of attack of the water flow on the fins may be substantially enlarged till the cavitation
limit, see fig. B and C. Consequently, in a "light sea" condition and slightly pitching, fixed fins are of little use while movable controlled fins may still yield a max. pitchdamping force, if so desired.
The pitchdampiig research carried out by Prof. Abkowitz in
this connection is misleading and conclusions incorrect. In
tro-choidal, swell-like tank waves with a ratio H/L of 1/30 as used in this research, there is not the slightest indication of the shipping of spray or watet-, even when damping pitch! In swell waves, "the ship rides the waves like a duele" and the pitching
and heaving forces are too large by far to be effectively re-duced. However, in this cor,dition is pitchdamping not necessary since full power can be maintained without the shipping of
spray or water! At sea, swell waves do not present any
difficul-ties as regards seaworthiness and speed. Moreover, the research
was carried out by models with a length of 5 and 6tA ft. in a "heavy sea" condition and assuming a ratio H/L of 1/30 will yield wave heights of 2 and 2½ inch! The research presents a sad picture of confusion. misunderstanding and beating about the bush. For example, Prof. Abkowitz came to the conclusion that fixed fins are beneficial to the seakeeping behaviour. The error in judgment is clear since the decrease in the -motions-was considered as a corresponding improvement of the sea-worthiness which, in pitch reduction, is not true and even to the contrary, see the full-scale trials in TJSS "Compass Island". The liability of shipping spray and water will increase seriously when reducing pitch and maintaining full power. Any
imporve-ment in speed will thus be made illusory when no adequate
steps are taken, hence, the provision of bow propellers.
It will be evident that this kind of research and lack of sea-manship may be held responsible for the failure with fixed
fins on IJSS "Compass Island", which failure any sailor could have predicted. So far, research is very disappointing arid not improving but retarding a pitchdamping solution, though for obvious reasons. In spite of the disappointing results with the
application of fixed antipitching fins on LISS "Compass Island",
Mr. John Vasta was still confident that pitchdamping might
become a success:
"We will have to sharpen our theories and model-testing tech-niques to develop the necessary parameters for a successful ship installation. If we pursue the problem with renewed vigor and imagination, I have no doubt that we can solve it."
The new parameters, unknown in rolldamping, are represented by the seaworthiness
properties:-The shipping of spray and water,
slamming, and
racing of the propellers.
In pitchdamping, these parameters and not the -motions- are decisive for the behaviour of a ship in a seaway, on which the speed and even the course may depend (racing). Compared to
roildamping, pitchdamping presents a formidable and very
com-plicated problem, thoug a solution will come forth as soon as
model-testing techniques improve and corne up to expectations. The evolution in ship propulsion, behaviour and speed will then
have made a radical step forward, presenting a valuable con-tribution to progress. In any ship it will be important as regards ship design, maintenance of speed, regular engine drive, the wetness of decks, the comfort of passengers and crew and, last but not least, the economy of fuel consumption.
Structural problems and difficulties associated with berthing and
anchoring:-The breadth of the installation should not exceed the beam of the ship.
Movable fins will provide a formidable device for pitch
reduction, though the final form, dimensions and loads required
can be settled only by model research in conjunction with the effect of bow propellers. The combined damping-effect should
be large enough to prevent cavitation in a "heavy sea"
con-dition, i,e, when ship and wave length are about the same. The tank waves should be of the significant-type. In a "light sea" condition, movable controlled fins may still exert max. pitch-damping forces, if so desired.
(Contind ori page 62)
ortlinirr'! front pupe 37)
ONTRIBUTION TO A SOLUTION OF THE ITCHDAMPING PROET EM
Scheantie atraia er an autceatically controlled
peeurwtle anti tcheng device st tine bay.
The lever in neutral psuuttion at S atmoepireric prceeure l-6,
A
Zata, l-5
connecting rad pieton
21 Hea'ry eea candttion. lever at
ut i abutnent /atm. 5. 62 HolLand Shipbuflding at. ate, l-6 regulato r le,er >rin a,rts
deviation 5 deg, In neutral 6 atm.
in Ya g. ti
/
eLatn.
6-eliding onu turning lever in the Idet of tre rid L. deviatIon i deg.
O
(3) Light eva condition, lever at L, deviation 20 deg. in neutral 1 ate.
ig,C
Fixed fins, however enormous in area, are unsuitable and inadeguate in pitchdamping. Even in a "light sea" condition
fixed fins are of little use, since the efficiency will decline during the process of damping when accelerations are reduced,
The criterion for protecting movable fins and bow
pro-pellers should ho, permitting the successful performance of a flying or running moor. This manoeuvre is practiced in res-tricted waters and in a tideway. The headway of the vessel is used to run out double the desired length of chain on the first anchor, dropping the second anchor and heaving in half the
amount of cable on the first anchor. The guard, as proposed, will provide adequate protection, though streamlined bow-shaped struts to the hull may strengthen the guard and improve protection when applying "Voith-Schneider" propellers.
Bow propellers will be effective at a moderate or even
slow speed. Moreover, the suction screw current will provide
a continual flow on the fins, enlarging the flow velocity due to the speod and improve damping. Only a small fraction of the total engine power, depending on speed, will suffice for an adequate improvement of the seaworthiness, preventing the
shipping of spray and water.
Effective means for preventing the racing of the bow
propellers should be provided, by limiting the max. number of revolutions.
Since large forces are involved in pitch reduction, the change in load on the fins between successive strokes should take place according to a methodical transition. In the proposed design the max. damping force on the fins will take place in
the midst.of a swing when acceleLations are a maximum but will gradually dimiñish iiIFThe transition between the swings when accelerations arc zero.
A combination of movable fins and bow propellers seems
to be the only practical solution. Because of the continual change
in direction of the water flow at the bow duringpitch reduction, being dependent on accelerations, cavitations presents a major problem. Therefore, automatically controlled fins will be more
effective, with simpler means, than activated fins,
A survey of many crossings of the North Atlantic by two
17-knot motor freighters of about 7,000-10,000 tons d.w. (De Zee,
no. 4, April 1962, by Mr. A. Wepster, in Dutch), showed the following mean difference between the best summer and the worst winter crossings; 190 and 238 hours sailing time
respect-ively. Loss of time by fog or ongine trouble, etc., has been
taken into account, so that the difference amounted to an
average of 48 hours or 25 per cent, i.e., a lOSS of two days by opposing weather and sea conditions. Even the summercrossings
will improve by the installation of an efefctive antipitching
devico.
Norwegian !icensee appointed
for the cons±rucion of Brons
marine diesel engines
The Brans marine Diesel engine will be constructed in Norway
by Messrs. AS. Nortrade Ltd. of Kristiansund Nord who have been appointed licensees recently by the NV. Appingedammer Bronsrnotorcnfabriek of Appingedam, fer the construction of
the Brons Vee-type Diesel engine. The engines will be marketed
under the trade name Nor-Brons and the production will be initially concentrated on the eight and twelve cylinders Vee-type version, developing 630 and 950 h.p. respectively. However, it is intended at a later stage to also construct the heaviest Brons marine Diesel engine in Norway, the 16-cylinder Vee engine developing 1,300 b.h.p.
Through the design and special lay out in Vee form of the
cylinders, the Brans Vee-type engine fitted with supercharging.
gives an important saving in weight/length ratio. Mechenicativ driven scavenging pumps nor auxiliary scavinging pumps are required resulting in a high mechanical efficiency. The Brons marine Diesel engine is also known for its extremely low fuel consumption,
Orders have already been received for a number of Nor-Brons
marine Diesel engines including orders for er.pines to be in-staled in trtiwlers.
In England, the Drypool Engineering & Dry Dock Co. Ltd., Hull is constructing the Brens Vee-typE Diesel engines under license.
/ Labutcent
Z tcI. le L ata.
1-n, indicatea tire position
of tine pistons when tte
lever in in neutral.
°ig. A-C. Position of the lever In neutral, and irr a 'heavy and "tight" sea tondition relaitive to the ribritments.
he fins are assumed to be cavitation-free at 25 dep. angle of attack of the eater flow. The direction of the water flow depends on the speed of the ship und the vert, accelerations at the bow, left ritter damping. For example, when
n a ''heavy sea'' a 20-knot sHp has to slow down to 12 knots, the angle of attack in midstroke may be as high as 45 dep. in undamped pitch. However, when damping is effective and the fair weather speed of 20 knots can be maintained without the shipping of spray and weter or slamming, the angle of attack may be reduced to 20 dep., which reduction indicates that the lever and the fins may be given a deviation Out of the neutral oosition of 5 dep.,
tompare Fig. A and B, under (i) and (2). In a ''light seo'' condition, pitch
'eduction by movable fins will be far more effective as compared to fixed ins and the deviation may be on hìgh as 20 dep., compare Fg. A and C, inder (1) and (3).
The maximum unbalanced force on the fina in midstroke is proportional to the speed and thus about equivalent in different sea conditions by adjusting the ins to a cavitation-free position and maIntaining speed.The mejor problem is, o pet the same torque with dfferent angles of turning of the lever, which oroblem may be solved by adjusting the atm. pressure in the cylinders to the state of the sea. During this switch operation by means of the regulator, the lever hes to be kept in its neutral position when the Ship S at the end of a swing. This may be done by o brake, working on the fin axis and releasing the brake after adjustment when the ship is once more at the end of a swing and the leve' in its neutral 005ition. Any readjustment of the max. deviation of the lever involves a siniiar readjustment of the abutmnta, limiting the
stroke of the lever to s small movement beyond the normal cavitation-free
action of the fins, see Fig. B and C
Atmospheric pressures, etc., in tine Fig. A-C are arbitrary only.
"Voith-Schraeider" propellers directed for driving power only, together with movable fins, will be adequate in a "light" and "moderate" sea condition to effectively dampen pitch and to
maintain full power of the engines, without the shipping of
spray and water or slamming. Only in a "heavy sea" condition the additional damping forces of these propellers, by a suitable control of the blades, will be necessary to maintain full power. In this case, the disturbance of the orbital motion of the water particles in the waves will be even more thorough than when
the propellers are applied for driving power only, improving the seaworthiness accordingly.
It may be estimated that in a "heavy headsea" the accelerations at the bow will have to be reduced by at least 50 per cent, to avoid cavitation and permitting a deviation of the fins out of the neutral position in midstroke. Pitch angles in this damped condition are in a range of about 3 deg. and the uptvard and
downward motion of the bow by the reduced amplitude, is
about half the wave height. The pitching period is proportional with speed and wave length and for a 29-knot ship of 500 ft.
in a "heavy headsea", while maintaining speed, amounts to 3.0
sec. for a single swing. A 500 ft. wind wave is caused by a near
gale, 7 Bft., and assuming a main ratio E/L of 1/13.3 will yield a wave height of about 38 ft.
