SERVICE PERFORMANCE AND SEA-KEEPING
TRIALS ON TWO CONVENTIONAL TRAWLERS
By Prof. Ir. G. AERTSSEN, Member, Ir. V. FERDINANDE, M.Sc.
and Ir. R. DE LEMBRE
9th November, 1964
SYNoPsIs.In view of a study of the service performance and the behaviour at sea of conventional trawlers and stern trawlers the Centre Belge de Recherches Navales arranged trials at sea on two conventional and on one stern trawler. To begin with, elementary measurements were made on board a conventional trawler 51 5m. in length with a trivial equipment and during one voyage only.
The second ship, again a conventional trawler 45 m. in length, was then equipped with pitot log, torsionmeter, anemometer, wind vane, gyro pitch and roll recorder, two accelerometers and a shipborne wave recorder. As a complete set of data
could be collected on this ship during two winter voyages in severe weather, it was possible, with this instrumentation, for the 45 m. ship to carry out a straightforward analysis of the effect of weather upon speed and power and upon the behaviour of the ship in waves.
Less information was obtained on the first ship and what could be collected is given merely for comparison with the more thoroughly investigated 45m. trawler. As adequate stability is required for this type of ship, the problem was given some thought for both trawlers.
Only mild weather has been encountered as yet by the stern trawler, so that
information on her behaviour in waves is rather meagre. Therefore the analysis of the results of this ship has been postponed until a complete set of data is available.
PART L
THE SHIPS AND THEIR INSTRUMENTATION
TABLE
i gives the design particulars of both trawlers with draft.
displacement, shape particulars, approximate natural periods of
motions at the time of the measurements, power and speed.
The trials on trawlers started with measurements on the trawler John during
a winter voyage in the North Atlantic off Norway. Besides a trivial equipment
with stop-watch and clinometer, the experimenter had no more at his disposal than the usual nautical instruments of the ship. Weather was not especially severe, 6to 7, Beaufort scale, the highest waves being H1110 = 6m. As
informa-tion was gained on the service-performance of this trawler as well as on ship motions, this information is given whenever it is available, as complementary
information in the diagrams concerning the Belgian Lady.
This second trawler was extensively instrumented and the two experimenters collected much data during two winter voyages in weather up to Beaufort 11.
The location of the instruments on the Belgian Lady is given in Fig. 1. The speed through the water was measured with a Sal log which is an instru-ment of the pitot tube type, giving the relative velocity, water to ship, near the
bottom of the ship.
The relative wind velocity was measured with a cupanemometer and the wind direction with a wind vane. Both instruments were
mounted on top of the bridge, the anemometer 9m. and the windvane 8m.
above the water level.
Ship's name
Builders in Ostend (Belgium Owners in Ostend (Belgium)
Length between perpendiculars, metres Breadth moulded, metres
Depth, metres .
Mean design draft, metres Design displacement vm3
Displacement with shell, metric tons.. Block coefficient
..
. Centre of buoyancy, from AP, metres 1 /2 angle of entrance of waterline, deg. Natural pitching period, sec.Natural heaving period, sec. Motor, power b.h.p.
When sailing, motor revolutions
When sailing, propeller revolutions r.p.m. Service sailing speed, knots .
Diameter of propeller, metres
Pitch at 0 7R, metres
Blade area ratio Fa ¡F Number of blades
John Belgian Lady Béliard- Béliard-Murdoch Murdoch Pêcheries Noord-à vapeur visserij
5l50
4500
940
860
525
465
425
3.75 915 689 930 7100439
0475
25'052l90
153
l50
42
4.0
4.5 4.3 1,000 1,200 325 380 130 139l30
l35
325
318
34l
326
055
070
4 5The information ori weather was completed by a Tucker shipborne wave
recorder. This instrument, built by the National Institute of Oceanography,
has been successfully used during the trials on the cargo ship Lukuga (Ref. 1). The pressure units with the accelerometers were installed in the engine room, at
14m. before A.P., the orifices port and starboard at a depth d = 3327m.
whereas the computer and the recorder were installed in a recording-room
located aft on maindeck.
This recording-room provided space also for the distance unit of the Sal log,
allowing, with the help of a stopwatch, mean values of speed to be measured
when bad weather made readings of instantaneous speed difficult.
Much attention was devoted to the recording of ship motions. A set of two
gyros was installed amidships on maindeck. These gyros, one for pitch, the
other for roll, are connected to a graphical and a statistical recorder. The heave motion was taken from the wave-recorder, using only the accelerometers. The vertical acceleration was measured fore, aft and amidships with Statham
accelero-meters connected to a Brush recording instrument installed in the recording-room. During part of the second voyage, when the weather was especially
severe, the accelerometer amidships was turned 90 degrees, providing informa-tion on lateral accelerainforma-tion.
The instrumentation was completed by a Maihak torsionrneter and an
electric impulse counter connected to the meter in order to give the r.p.m. of
the propeller. The pick-up of the Maihak torsionmeter was installed on the
shaft aft of the reduction-gear, the recorder being located in the recording-room.
The accuracy of the measurements was within the following limits of error: Speed through the water: in smooth water, 1 per cent; in waves up to
3 m, 2 per cent; in a very rough sea, 4 per cent.
Torque: in smooth water, 2 per cent; in waves, 3 per cent. Revolutions: in smooth water, 05 per cent; in waves, 1 per cent.
AND SEA-KEEPiNG TRIALS ON TWO CONVENTIONAL TRAWLERS 39
Pitch and roll angles: 4 per cent, except for small angles where error is
more important.
Wave height: about 10 per cent.
No calibrating buoy was available, but use was made of the correction factors applied by B.S.R.A. for the analysis of the data of the "Weather Reporter" (Ref. 2). The hydrodynamic factor was taken with an equivalent depth of 2 37d, where d is the depth of the pressure units measured along the girth of the hull. The vertical displacement is given by the accelerometers of the transmitters
of the wave recorder, the pressure units giving then no signal to the electronics of the computer. As the wave height transmitter is located about the axis of rotation of pitching in most conditions of loading, no correction is made for
this pitching. It is believed that the error on the vertical displacement is not
in excess of 10 per cent.
PART II
SERVICE PERFORMANCE
The main engine of both trawlers is a diesel. The trawler John has a
nominal power of 1,000 b.h.p. at 325 r.p.m., whereas the engine of trawler Belgian Lady has a nominal power of 1,200 b.h.p. at 380 r.p.m. There is a gear reduction of revolutions to propeller, when sailing, to 130 on the John,
to 139 on the Belgian Lady. Moreover the Belgian Lady has another reduction, when trawling, from 380 to 109.
At the time the measurements were taken on board trawler John, the engine
developed, when sailing at 11 5 knots, 960 b.h.p. at 314 r.p.m. As sea state was
moderate, this means a loss of speed in service of about 1 knot, together for
fouling and weather effect. Facing a sea of Beaufort 6-7, power and revolutions dropped about 10 per cent.
Trawling at a speed of 4 knots the engine of trawler John developed 780 b.h.p. at 235 r.p.m. in a calm sea. In a rough sea, power and revolutions dropped about
5 per cent, the trawling speed was about 35 knots.
The catch per voyagevaried between 53 and 72 tons. Measurements were taken during each voyage
at about one-hour intervals.
The readings were made in better conditions on board trawler Belgian Lady,
this ship being equipped with a torsionmeter and extensively instrumented in view of recording wind and waves. Moreover, instead of the rather mild
weather conditions which were encountered during only one single voyage on
trawler John, the trawler Belgian Lady had to bear, during a considerable part
of two consecutive winter voyages, the violent motions set up by a very rough sea. Therefore the analysis of the numerous data collected on the Belgian Lady
could be undertaken more thoroughly than was the case for the John. Most of the diagrams concerning service performance and ship motions concern in
the first place the Belgian Lady, but whenever possible the John data are shown
in these diagrams for comparison. Although all the information obtained at
sea on these ships is shown in diagrams, it was believed to be useful as reference
to bring together the most representative of the data of the Belgian Lady in a table (Table 2). Unless mentioned especially they relate to the condition "sailing ".
In the same way as had been done for the cargo ship Lukuga (Ref. 1) an attempt was undertaken to separate the effects of wind and waves. No wind
tunnel tests were carried out for the trawler but use was made of the resistance coefficients of the Lukuga to establish the wind effect (Fig. 4 of Ref. 1). From
the ahead resistance coefficient X/05 p AT VR2, the longitudinal component X
of wind resistance can be deduced for a given relative wind velocity VR at a
given angle of attack. The transverse projected area AT is 53 5m2. From the longitudinal component of wind force the power due to wind resistance is
calculated. Finally, subtracting the air resistance power in calm from this wind resistance power gives the wind effect.
On the other hand it is possible to establish on a basis of power data in a calm sea for various values of speed a d.h.p. V curve in calm. The power at
the propeller d.h.p. is deduced from torsionmeter power by subtracting 2 per cent for the shaft losses. Moreover this power d.h.p. which is measured in
various load conditions is corrected to the standard displacement of 650 tons.
The total increase of power due to weather is then determined for each
observation by subtracting the power in calmtaken from the d.h.p. V curve
in calmfrom the power obtained in any weather condition.
By simpledifference the increase of power due to waves only is finally obtained.
This increase of power due to waves is plotted against wave height in Fig. 2. As cumulative energy density and mean height of tenth highest are believed to be most representative for a wave height, two scales v'E and H1110 are chosen
for the basis of this diagram. From a large number of observations made
during the two voyages, curves of increase of power at a given speed are deduced
for different wave directionshead sea, beam sea and following sea.
Thediagram shows a scale of wave heights related to wind force for large fetch and
duration as observed in the North Atlantic during these two winter voyages. These wave heights are somewhat higher for a wind strength Beaufort 5 and somewhat lower for a wind strength Beaufort 9 than the heights given in the
scale published by the Hydrographic Office U.S.A. Publication 603 (Ref. 3).
The power absorbed by waves is spectacular for a wave height about o = lOm. and a speed of 95 knots.
The figures H1110 = 96m. and V= 95
knots plotted on the diagram relate to an observation (Obs. 5) which took place in specially severe weather conditions. The experimenters were fortunate in having the company of a daring captain who on this occasion put to test the seaworthiness of the trawler to the utmost of her potential. The speed of 95 knots was maintained in this sea just long enough to make the readings of power, motions and weather. Normal speed in these waves is somewhat
lower (Obs. 15).
It is remarkable however that, again in a very high sea of about 20m., the trawler maintained her speed of 9 knots. This occurred on the second west-bound crossing of the North Atlantic to Iceland when during more than two
hours the trawler sailed with a speed of about 9 knots in a gale of 44 to 58 knots blowing from 10 to 15 degrees on the port bow, the waves 20 to 25 degrees off
the bow being as high as H1110 = 17m, to 21m, with a length of more than
200 m. Again this appeared to be a rather exceptional performance as many
trawlers in the area sailed with reduced speed (Obs. 20 to 23).
The capabilities of the Belgian Lady in these very high waves will be discussed
in Part III but the severity of the reactions of these head seas upon the trawler
is sensed in a certain way by the apparent slip Sa. Therefore Fig. 2 shows the
variations of Sa with different wave heights for a sailing speed of 95 knots.
It is now possible, knowing the amount of power absorbed by waves of given
height at various values of speed, and the increase of power caused by wind
resistance, to construct a service-performance diagram (Fig. 3). This diagram
gives the power d.h.p. required in various weather conditions for a standard displacement of 650 tons, trim by stern about 1 7 m. In addition propeller revolutions r.p.m. are given in a calm sea and for various head seas as well.
The left side of the diagram refers to the ship en route, whereas theright side
refers to trawling.
The d.h.p. curve for a calm sea covers the results obtained during the two consecutive voyages of the Belgian Lady at moments when the sea state was
Beaufort i or 2. The tank prediction from model results at design displacement
689m.3 is given for comparison. The diagram shows also the results of the trawler John for a displacement of 880 tons and a trim by the stern of about
25m. Power is given for two sea conditions, a calm sea and a head sea in
*North Sea. **Trawling. P = Port. S = Starboard. Mean of tenth highest acceleration (double amplitude) is 36 '
Trim by stern varied from i 7m. to I 9m. except n° 15 and n°24 where trim was 22m. and n° 16 where trim was 24m.
Plate I
"Service Performance and Sea-keepitig Trials on Two Conventional Trawlers" by PROF. ir.G. AERTSSEN, Member, Ir.V. FERDINANDE, M.SC. and ir. R. DE LEMBRE
TABLE 2Salient Data of Trawler Belgian Ladi'
Obser- Speed
Wind Waves Pitch Heave Roll
/of
Bow
\/Eof
sk'rnd.h.p.
corr. Displace-meli!
Mean Mean Vatio,? n° knots Beau- Relative fort speed Scale knots Relative Direction degrees Height H111, metres .4ppar-eilt length metres Heading degrees of how Meaiz period encounter sec. Angle
.,
degrees mean Period sec. Total2,,
metre Period sec. Angle 1/1O degrees Period sec. Accel.times g ti,nes gAccel. to 650
toils tons
i
1I9
5 20 20S45
50 066
lOO-
-
-
-
-
0243 0175 806 6482 110 6 28 55P
79
70 SOP66
-
-
-
61
-
0.344 O249 854 6473 103 7 35 45P 8'I 70 SOP
66
-.
-
-
-
-
-
0365 0261 833 6464** 3.9 7-8 35 0 105 80 lOP
96
17758
88
95
273
75
O304022i
553 6485 9.5 7-8 42 0
96
-
073
20349
-
-
222
-
0529 0383 863 648 668
7-8 39 0 104-
O77
19651
-
-
222
-
0494 0344 565 648 736
8 39 0 115-
O91
200
54
93
85
284
79
0354 0279 190 648 8 116 7 32 70P96
-
90P75
14544
7.5 7O 37578
0347026l
848 648 9 11 8 35 0 100-
083
20962
88
8l
29577
0355 0290 170 647 10 0 7 32 7OP90
80 90P 10784
61
86
lO7256
78
0151 0132 0 648 11 102 8 40 30P 104 80 7OP 1O8 12164
98
123 36984
-
0216 470 658 12 123 7 18 18096
-
180 125 122 10458
12l
3O781
0082 0079 757 647 1328
7 31 SOS96
-
45S87
15756
76
82
32781
0301 0234 136 647 14* 125 7 33 7Ø55
50 85P63
-
-
58
-
25576
0218 O165 887 650 1572
7-8 38 lOP90
-
20P 81229
55
-
-
248-
0382 542 664 16* 125 10-11 55 IOOS68
50 lOSS77
11078
59
75
456
82
-
0187 854 67917 0 10-11 55 hOP ISO
-
lOOP 121 155-
-
-
34683
-
-
0 65418
Il5
lO 55 45P 169 80P 100 19657
135 104408
78
-
-
804 654 19 100 10 57 25P 190 45P 110 22754
18M 110 36177
-
-
817 651 2088
10-11 66 15P 213 25P 118258
56
2O6 113 41176
-
-
707 651 2190
10 62 iSP 20M more than 200 25P 110249
57
139 106346
78
-
-
664 651 2292
9 52 lOP 171 20P 107 26055
18095
309
78
-
-
766 650 2367
7-8 39 lOP 183 200m. 20P 115262
60
i49
104339
8O-
-
390 650 24 8M 8-9 32 170P 130-
170P 153 181 121 112 149282
81
-
0099 129 662 25** 4.4 4 12 130P 116-
120P 125 156 10198
I26
26176
-
-
485 650The John results are merely given as a comparison. All the other information of the diagram concerns the Belgian Lady. Examining the sailing section it is
apparent that power and revolutions go down as weather worsens. Not only is there, at a given setting of the main engine, a drop of revolutions when the trawler is sailing ahead in waves, with the inference of a loss of power; but, owing to the violent motions, the captain has to reduce speed deliberately in a
very rough head sea. This is shown in the diagram where all lines in full refer to head waves. One run against waves of about H1110 = 10m, at a sailing speed of 95 knots lasted just long enough to make the readings. The data of this run are given on the diagram and in Table 2 (Obs. 5) but heavy slamming
prevented the captain from maintaining this speed in these waves and therefore
this observation is to be considered as beyond normal service. A much easier condition is given by the data of Obs. 15.
Again it is shown in the diagram that, although a speed of 9 knots cannot be maintained reasonably in head waves of H1110 = 10m., the trawler was able to sail at this speed in waves of H1110 = 20m. at 20 to 25 degrees off the bow.
The same relation wave height/wind is adopted as in Fig. 2. It is worthwhile mentioning the parallelism of the iso-weather lines, as was suggested by Telfer (Ref. 4).
The right side of the diagram makes clear that only 60 per cent of the sailing power is developed by the Belgian Lady when trawling. This shows the trawling particulars in a calm sea. The trawl pull is obtained by calculation.
During these two voyages the Belgian Lady developed, when sailing in fair
weather conditions, from 800 to 900 d.h.p., whereas the required power at the propeller when trawling was from 500 to 550 d.h.p. The sailing power 900 d.h.p. is developed at about 130 r.p.m. in good weather conditions, whereas
revolutions when trawling go down to 88 r.p.m. at 500 d.h.p. This means that,
unlike the trawler John, where only one r.p.m. reduction is available, both for sailing and for trawling, the Belgian Lady, equipped with another reduction
from 380 to 109 r.p.m. uses this second reduction when trawling.
It is now possible to deduce from the performance diagrams Fig. 2 and 3 the increase of power in percentage due, firstly, to waves only (Fig. 4) and secondly to wind and waves together (Fig. 5). Fig. 5 is based on the assumption that, when
sailing in bad weather, the trawler develops the power which she is capable of
maintaining in the weather considered. The increase of power is spectacular
in Beaufort 7 for the ship when sailing: 150 per cent in a head sea as against 32 per cent for the ship when trawling.
It is certainly of interest to compare the wind and wave effect upon this
increase of power. This is shown in Fig. 6 where the ratio of power increase due to waves only to the total increase due to wind and waves is related to weather in the Beaufort scale. The diagram concerns the ship when sailing.
It is remarkable that, in mild weather conditions, the wind effect is an important factor, whereas it is no more than 25 per cent of the total in very rough weather.
Both effects are equal in a sea state Beaufort 5.
Increase of power due to weather is perhaps a rather intricate consideration From the point of view of the economy of the ship, it is easier to speak of a loss of speed. This again is given in Fig. 7 for the trawler en route. In Fig. 5
it was assumed that the waves had a height in agreement with the scale of wind and waves given in Fig. 3. This is not always the case. In limited areas, the North Sea for example, the wave height is less than the corresponding height for the given wind strength (Obs. 14 and 16 of Table 2). On the other hand
it happens, after a storm, that the wave height is much more than would
correspond to the wind strength.
For the conventional curves of loss of speed related to weather (Beaufort) the authors refer to the scale of wind and waves given in Fig. 3.
42 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
(Obs. 24). The
PART ifi
SHIP MOTIONS
Pitch and roll have been recorded graphically as well as statistically. The vertical and lateral accelerations were recorded on the Brush instrument. Pitch and roll and accelerations were recorded simultaneously, as readings were made on the navigating bridge of wind strength and direction and a wave record was taken.
Unfortunately, heave, which was recorded by means of the wave
recorder, could not better be recorded than immediately after the wave. Only for this motion was the record not simultaneous with that of the wave.
The analysis has been carried out on a statistical basis. In all the diagrams \/E, (square root of the cumulative energy density) has been given for wave
height, heave, pitch and roll angle, vertical accelerations at the bow, amidships
and at the stern and lateral accelerations amidships. On the other hand, how-ever, the mean of the tenth highest of wave height and double amplitude of
motions and accelerations appears to be more representative for all these oscilla-tions. Therefore the diagrams are doubly scaled and the ratio of the tenth
highest and
/E
is in agreement with a Rayleigh distribution.Numerous histograms have been prepared. Only some of them, namely for
the periods of some motions and waves, where they may be a help for a better understanding of the behaviour of the ship, were reproduced. Instead of reproducing the histograms of the periods, it appeared to be more feasible to range them in sextiles, as was initiated in the analysis of the Dutch destroyers
(Ref. 5).
The oscillations of a given motionpitch for instanceare divided
into six groups ranging from the small to the large periods. Typical are then three values of the period: the upper limit of the first sextile, indicated by the
subscript i (low sextile T,), the overall mean period indicated by the subscript m (mean T1tm) and the lower limit of the last sextile, indicated by the subscript h (high sextile Tfrh). For a given observation, the period of pitch is described by three values ranging from T1r1 to Tfrm andT?Jrh.
A new feature of the instrumentation of the Belgian Lady was the shipborne wave recorder. This instrument has its weaknesses and it cannot be expected
to give the wave height with an accuracy better than 10 per cent, even when using
the correction factors found by B.S.R.A. during the trials on the Weather
Reporter (Ref. 2). As the Weather Reporter is not so different from the Belgian
Lady the correction factors of B.S.R.A. can be used with confidence. The agreement during the two winter voyages between the wave heights recorded by
the instrument and these heights, as estimated visually by the captain of the
trawler, was very good. Therefore a better knowledge than before, when wave height was only estimated by eye, can now be gained on the relationship between waves and motions.
Fig. 8 shows the relation between pitching and heaving amplitudes and wave
height for three different wave directionshead sea, beam sea and following
sea. Again the diagrams are doubly scaled with the statistical values ./Eand the mean of tenth highest double amplitudes. As the pitch angles are obviously less when the Belgian Lady is trawling than when she is en route, the statistical values are given for both conditions. It has_been emphasized that the trawler has to heave to when she meets ahead waves H1110
=
11m, but that occasionallyshe faced waves H1110 = 17 to 21m. at a speed of 9 knots. The latter
circum-stance is shown in dotted lines on the left part of Fig. 8.
The part of this
diagram showing pitch and heave amplitudes in a beam sea makes clear the capability of the trawler to maintain a sailing speed of 11 knots in a broadside
sea as high as H1110 = 17m. (Obs. 18). Trawling is possible in a sea Beaufort 7to 8 at all headings: in head waves H1110
=
l0'5m. (Obs. 4), in a beam seaH1110 = 11 6m. (Obs. 25) andin a following sea H1110 = 87m. Theright part
of the diagram shows pitch and heave amplitudes in a following sea where the trawler has a sailing speed of 8 knots in waves of H1110 = 13m.
pitch angles of the trawler en route in a following sea are two-thirds of what they are in a head sea, and in broadside waves they have values intermediate between the values attained for these two extreme headings. Heave amplitudes are not very different for the three headings. The highest pitch angle attained by the
trawler en route when facing the waves H1110 = 21 3m. was
= 258 deg.
with_a maximum of 36 deg. (Obs. 20), whereas the total heave in this condition was Z1110 = 206m. with a maximum of 24m.
Pitching and heaving produce vertical accelerations and occasionally slamming
The accelerations were recorded at the bow, amidships and at the stern. The accelerometers fore and aft were in the ML, but the accelerometer amidships was
located in the chartroom on the starboard side and there may be an effect of pitching and rolling on the acceleration recorded by this instrument. Special
manoeuvres were carried out with the Belgian Lady in a sea Beaufort 7-8 to study the behaviour of the trawler for various headings into waves. Not only was the ship sailing on various courses, but facing the waves the speed was
reduced by degrees to i knot and the trawler went even on drift in these waves.
The data of these manoeuvres give a picture of accelerations and slamming
(Fig. 9). Again, the statistical value of is/E of double amplitudes of accelera-tions is used together with a slam number dec/N, where dec is the deceleration
for each slam and N the number of low cycle oscillations of the record. The
left side of the diagram concerns only head waves and gives for a speed of 95
knots and a height of waves H1110 = lOm. the accelerations fore, aft and amid-ships, and the slam number, on a basis of the mean period of encounter of waves of 7'3 sec. As wave height decreases at constant speed the period of encounter of waves decreases and this is shown at the left of the vertical of 95 knots and 10m. On the other hand, as speed decreases at constant wave height the period
of encounter of the waves increases, and accelerations and slam number at
various speeds are given to the right of the vertical. The right side of the diagram
gives accelerations and slam number for various headings of the ship in the same sea H1110 = 10m, and at a speed of95 knots.
Vertical acceleration decreases with speed, so the accelerations are lest important when trawling than sailing. Fig. 9 shows that the acceleration as the bow is 60 per cent greater at 9 knots (sailing) than at 4 knots (trawling).
The acceleration in a head sea is higher at the bow than at the stern, which means that the axis of rotation of pitching is further aft than amidships. In a quartering
or a following sea this axis moves forward and in a following sea there is no indication that the acceleration is higher or lower at the stern than at the bow There is, when sailing, an important scatter in the values of the accelerations in a beam sea, due to the course keeping and the relative importance of the
directional spread of wave energy on pitching in this sea.
The situationis better defined when the trawler is on driftand this is frequent in the open ocean even in waves as high as H1110 = 1 5m.because the ML of the trawler is then parallel to the wave crests and keeps a position corresponding to the smallest pitch angle.
In some circumstances it happens (Obs. 8) that the
acceleration at the bow, sailing in a beam sea, is the same as when the ship is
"hove to" in this sea (Obs. 4) and about twice the acceleration in drifting
condition (Obs. 10). The pitch angles are in the same proportion. This high
bow acceleration when sailing in a beam sea is, however, exceptional and the
diagram gives mean values which for the sailing condition are not much higher than for the drifting condition. Due to the central position of the axis of rota-tion of pitching when sailing in a beam or in a following sea, vertical accelerarota-tion
is the same at bow and at stern; but amidships this acceleration is 40 per cent
less. The acceleration in a head sea is about 38 per cent higher at the bow than at the stern. It is clear from a comparison of the accelerations at bow, at stern
and amidships that the axis of rotation of pitching in a head sea is located at
O38 Lpp before the after perpendicular.
When drifting in a beam sea Beaufort 8 the maximum acceleration O 55g
44 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
was 029g. These figures are about three times the /E values given by the curves of Fig. 9, which corresponds to the highest acceleration on 10,000 oscillations in a Rayleigh distribution. Applying this same coefficient to the
accelerations of Obs. 5 gives a maximum value at the bow of 1 '58g, and at the
stern of i 15g (Obs. 5).
The maximum observed accelerations were i -04gat the bow and i - l0 at the stern, It must be said that Obs. 5 refers to an extra
manoeuvre and it is most unlikely that the trawler in normal service will face waves H1110 = 10m, at a speed of 9-5 knots. Moreover the accommodation
is located amidships and aft on these trawlers and the maximum 'v'E value
amidships is Q-16g in a bow sea H1110 =9'6m., which corresponds to a maxi-mum acceleration of O -45g. The maximum acceleration recorded amidships
was 045g when sailing in waves H1110 = 13-3m. at a heading 50 degrees off bow.
The Belgian Lady was slamming most of the time during these two winter voyages. An extensive report on the slamming of this ship together with the
slamming on a cargo ship has been given in Ref. 6. The violent motions,
pitch-ing and heavpitch-ing, produced frequent impacts of water on the forebody. No
cracks or other effects of fatigue were observed on the Belgian Lady when she docked immediately after the second voyage. The flat parts of the fore bottom
of cargo ships are more liable to damage by slamming than the forebody of a slender trawler. The decelerations, however, produced on a trawler by the
impacts are important and they are best sensed by the accelerometer at the bow.
At every shock the record of the accelerometer shows a deceleration and it is
hazardous to assign a level to the deceleration produced by what should be called a slam. Therefore all the decelerations (dec) revealed by the accelerometer record in a given number of low cycle oscillations (N) are summed up and divided by
this number of oscillations.
The "slam number"
dec/N thus obtained is given in Fig. 9, on the left side of the diagram for various wave heights andspeeds and on the right side of the diagram for various headings. The maximum deceleration recorded during the special trial (Obs. 5) was 2-75g. The form of the forebody of the Belgian Lady, which has a certain importance with regard to slamming, is given over a length about a quarter of the length of the ship
(Fig. IO).
The rolling amplitudes and the lateral accelerations associated with this
motion are given in Fig. 11 in relation to wave height for a head sea, a beam sea
and a following sea. There is not a wide scatter of the observations on the
curves showing roll angles related to wave height and therefore it was possible
to draw these curves with a tolerable certainty. There is a steady increase of the roll angles with growing waves: for a wave height H1110 = 10m, the roll angles1110 are 25 degrees in a head sea, 28 degrees in a following sea and 36
degrees in a beam sea. In a beam sea growing to 17m. wave height and a bow
sea growing about 20m. the roll angles 1/1O increase to about 40 degrees.
This concerns the behaviour of the trawler in the open ocean where for these
very high waves there is not much difference in roll amplitude between bow and beam seas. It is remarkable that the maximum roll angle (double amplitude)
has the sanie value, 58-6 degrees in a beam sea H1110 = 16'9m. (Obs. 18) and a bow sea H1110 = 21 -3m. (Obs. 20). The behaviour of theBelgian Lady is different
in the steep waves of the North Sea where synchronism occurs frequently
between waves and rolling. The higher curve of the left part of Fig. il concerns
rolling in short steep waves and the mean period of encounter of the waves as
well as the mean period of rolling is given in tuned circumstances. The diagram makes clear also that in beam seas the lateral accelerations which are recorded amidships are important when the roll angles are large. Again a higher curve is drawn for these lateral accelerations in the tuned circumstances of the North Sea (Obs. 16). The roll angles in these rather small but very steep waves are
spectacular, = 45'6 degrees with a maximum of 63 degrees, the highest roll angle recorded during the two voyages; and the lateral accelerations were A1110 = l-18g with a recorded maximum (single amplitude) of 0-8g. This
AND SEA-KEEPING TRIALS ON TWO CONVENTIONAL TRAWLERS
lateral acceleration v'E = 0327g is about twice the vertical acceleration at bow or stern in the same circumstance. Notwithstanding the heavy rolling the ship kept dry.
Since it is so difficult to relate the roll angles to all the wave dimensions the
authors adopted, for the analysis of rolling, the presentation introduced by St Denis and Pierson on wave spectra and ship's response spectra (Ref. 7). The method was worked out for numerous records for waves, rolling and pitching.
With the shipborne wave recorder this straight-forward analysis was made
possible. A Fourier spectral analysis of 12 minutes records of waves and
motions was carried out over 180 harmonics. Only the periods in the range of
4 to 30 sec. were used for the plotting of the spectra.
Computing a roll response is affected by the errors of wind effect and
direc-tional spread of wave energy.
Korvin-Kroukovsky commented on thesedifficulties in Ref. 8. Fig. 12 shows the wave and roll spectra and the " response
operator" for Obs. 14 in the North Sea. The highest response factor relates to a component which is synchronised with the natural roll period and this seems
to confirm that the chosen beam sea was correct in its direction. Calculated
values of the roll amplitudes at various frequencies, using a damping factor k = 009, are plotted for comparison (bar keel 020m; bilge keel 035m.)
The relation between the periods of the motions and the mean period of
encounter of the waves is shown in Fig. 13, 14 and 15. Typical for the periods
are the low sextile, the high sextile and the mean periods and these values are shown in the diagrams.
The variation in the roll period is small, from 75
to 82 sec. for the mean period and this period is the same, sailing and trawling.
This corresponds roughly with the variation of the natural period during both
voyages. In a following sea, but only sailing, the mean periods are somewhat
higher, 82 to 90 sec., which means that the rolling period is sensitive to the
higher values of the period of encounter. The same statement has been made
during the Lukuga trials (Ref. 1). The difference between the mean period and
each of the sextiles is about I 5 sec. As roll is coupled with sway the period
of the lateral acceleration is given in the diagrams, but only for beam seas and following seas. This period is about I sec. lower than the rolling period.
Heaving occurs roughly in the period of encounter, in head as well as in beam and in following waves. When the trawler is sailing ahead in very small waves,
the mean heaving period is exactly the natural heaving period of the trawler. The
natural pitching period is also the pitching period in small head waves. But,
as the weather worsens, the trawler proceeding in head waves augments its
pitching period slowly as the period of encounter increasesonly 2 sec., whereas this period of encounter increases from 4 to 12 sec. This means that proceeding against a sea Beaufort 9 (Obs. 20 to 23) the pitching period of the trawler is only half the period of encounter. Sailing in a high beam or following sea however,
the trawler has a pitching period not so very different from the period of
encounter.
This prompted the authors to compare the statistical values, the mean periods
and the sextiles of ship's motions with the histograms of the waves for various
headings and in various sea-states. Fig. 16 shows the histograms of waves and
pitching in the most severe head sea, H1110 = 9 6m., against which the ship proceeded with difficulty at service speed (Obs. 5). The wave oscillations
between 4 and 6 sec. have a frequency of occurrence of 31 per cent whereas the frequency of occurrence for the same range of pitch oscillations is 55 per cent.
Fig. 17 shows the histograms of waves and motions in the highest sea, H1110 = 21 3m., the ship encountered at service speed on the bow (Obs. 20). The
wave oscillations between 4 and 6 sec. have a frequency of occurrence of no more than 7 per cent, whereas the frequency of occurrence for the same range
of pitch oscillations is 54 per cent. Considering that the frequency of occurrence of the roll oscillations is 51 per cent in the range 6 to 8 sec., it is stated that the pitch spectrum of the trawler in this sea was as sharply tuned as the roll spectrum.
46 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
might explain, both with the slight slope of these waves of a height more than 20m., why the trawler was able to proceed, although with difficulty, in these
waves at her normal speed. For both observations 5 and 20, the apparent slip,
which is a measure of the severity, is the same 0'27.
It may be of interest to show the wave spectra for these two observations 5 and 20 (Fig. 18). _The first spectrum H1110 = 9 '6m. is broad whereas the
second spectrum H1110 = 21 3m, is more sharply tuned.
This spectrum,which relates to the highest waves ever recorded by the Centre Belge de Recher-ches Navales, is shown on the right side of the diagram, after being reduced to zero speed.
The special manoeuvres in a sea Beaufort 7 were undertaken to establish the variations of the periods of motions with heading and speed. Fig. 19 gives the
relation between the periods of pitching, heaving and rolling and the heading of the ship in this sea Beaufort 7. Dependent upon whether the ship is sailing
or trawling, there is some difference in the periods of pitching and heaving.
Only the rolling period is not really sensitive to heading. The figure gives also the rolling period of the trawler John. Fig. 20 finally shows in the same sea, the trawler sailing against the waves, the variations of pitching and heaving periods when ship's speed decreases. A measure of the speed is in this case the period
of encounter of the wave and that is why the periods of motion are plotted against the mean period of encounter. The diagram shows also the variation of apparent slip Sa, which increases from 025 to 050 as speed decreases from 9'S to 3'S knots in this head sea Beaufort 7.
PART IV
STABILITY
The Belgian Lady proceeded at a speed of 9 knots against waves H1110 = 10m.,
had to hove to in higher waves, but again in some circumstances proceeded at
9 knots against waves = 20m. She sailed at the same speed in a beam sea H1RO = 17m. and was drifting ¡na sea = 15m. She had a sailing
speed of 13 knots in a following sea H1110 = 13m. She was trawling in a sea H1110 = 12m. (Beaufort 8).
When readings were made on board trawler John these high seas were not met,
but there is some evidence that her performance in heavy seas is of the same
order. The stability requirements must be severe in these circumstances, so accelerations will be rather high.
The relation between rolling period and metacentric height Tçbm
-holds good for this kind of ships and this gives the means at any moment of the
voyage to compare the GM obtained by simple calculation of moments of weights with the GM estimated from a recorded roll period. This was done for the single voyage of the John and both voyages of the Belgian Lady. The results are given in Fig. 21 which shows the change of GM during each of these voyages. This GM, calculated on a basis either of weight distribution or rolling period, is high enough to prevent the ship from heeling dangerously against the wave slope. This relative heeling angle must be held within reasonable limits to elude shipping of water.
There is however another criterion of stability. In Ref. 9 one of the authors
suggested for coasters a righting lever at 30 and 45 deg. of at least 25cm. Rahola,
arguing along the same lines, suggests 80mm. as minimum dynamic stability
at 40 deg. for fishing boats. This prompted the authors to establish the curves of righting moments and heeling moments for both trawlers. A certain number of assumptions form the basis of the calculation.
The ship with the ML
AND SEA-KEEPING TRIALS ON TWO CONVENTIONAL TRAWLERS
and 3m. high. The buoyancy calculation is purely static, without Smith effect.
The righting moments are calculated for two conditions, the wave trough
amid-ships and the wave crest amidamid-ships. The forecastle is taken in account, but not the other superstructures. The heeling moments are produced by water on deck, free surfaces of partially filled tanks, icing and wind.
Dangerous heeling conditions arise for a trawler when sailing in following or
quartering large waves which have more speed than the trawler herself. In a
sea state Beaufort 6 the period of encounter is then about 15 sec., which means that every 15 sec. about 10 tons of water may come on deck if the trawler has the size of the Belgian Lady or the John. Calculating the allowance of water flowing
through the freeing ports of the bulwark it was estimated that after a time 20
tons remained on deck of the Belgian Lady, whereas 30 tons remained on deck
of the larger trawler John. The lateral wind component in this following or quartering sea fortunately will be small and was estimated in the present case
to be 25kg. per m2. There may be some overrating of the heeling moments in assuming icing effects in a sea state Beaufort 6, nevertheless this icing effect is considered because a swell may persist even in a high pressure area.
Wendel in Ref. 10 comments on the various capsizing influences on ships of
small size and mentions 20 tons as the amount of water coming on deck of a coaster of 80m. from each wave in a following sea of a period of encounter of
15 sec. This is a support for the 10 tons water assumed in the case of our trawlers. Establishing figures for the icing effect is more difficult. Lackenby in Ref. Il describes the B.S.R.A. tests on the effect on stability of the formation of ice and his comments on trawler design in this respect are certainly of interest
to the naval architect. The authors adopted for their calculation the figures
given by Traung (Ref. 12) and which derive from Russian stability rules. These figures allow for 30 kg per m2 of ice on deck and 164. kg per m2 of ice on the
rigging and spares up to a height of 10m.
A basis for the calculation of the righting moment is the initial stability. 1f
this initial stability GM is low much water will be shipped in following or quartering seas. When, on the contrary, GM is reasonably highand this was the case of the Belgian Lady and the Johnlittle water is shipped. Assuming an amount of 10 tons shipped by these trawlers in a following sea is again
over-rating the heeling moments and this must be considered when drawing
con-clusions from the final stability results. The righting moments of the Belgian
Lady and the John have been calculated for the actual conditions of trim and
GM during the trial voyages. Fig. 22 shows the stability curves of the Belgian
Lady, Fig. 23 shows these curves for the John. It is seen at once that, these
trawlers being situated on top of the wave crest, the righting moment disappears
at a heeling angle of somewhat more than 30 degrees on the Belgian Lady, at
somewhat more than 40 degrees on the John. Due to the large heeling moments the position of these ships appears to be critical on the wave crest in this following
sea and this position might become dangerous if it lasted. Therefore skippers
of trawlers reduce speed when ship's speed is approximately the speed of the
waves, as happens sometimes in a following sea. The trawler is then overtaken
by the waves and the ship is not long enough on the crest of a wave to be
endangered. This prompted the authors to calculate, for each heeling angle, the mean of the righting moment crest and trough and the mean of the heeling moment crest and trough These means give what might be called a balance stability diagram for "crest and trough" which might as well give the picture
of what happens in reality when crests and troughs follow one another reason-ably quickly. Again the John, where righting moment and heeling moment
are equal at 55 degrees heel, appears to be somewhat better in this respect than the Belgian Lady, where this occurs at a lower heeling angle, 45 degrees These amplitudes being never attained, even in tuned rolling, it may be concluded that there is little risk that these ships might be endangered, even in the most severe storm.
Some authors, Rahola (Ref. 13) and others, commenting on the stability of ships of small size, emphasize the importance of dynamic stability. On the
48 AERThSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
balance stability diagram the righting lever is deduced straight-forward from the righting moment and it is seen at once that both trawlers fall in with the Rahola criterion of a minimum dynamic stability of 80mm. at 40 degrees. Moreover the righting lever at 30 degrees heel is 21cm. for the Belgian Lady and 33cm. for the John whereas this righting lever at 40 degrees heel is 17cm. for the Belgian
Lady and 32cm. for the John. On the other hand, from information gained on board these two trawlers, one is justified in believing (and the same state-ment was made by Möckel (Ref. 14)) that a GM varying from 07 to 09m is a commendable estimate of the initial stability with regard to deck dryness.
and possibility of deck work. In conclusion it may be said that the stability
and the behaviour in waves of these ships was satisfactory.
ACKNOWLEDGMENTS
This investigation was carried out under the auspices of the Centre Belge de
Recherches Navales (CeBeReNa) with the financial assistance of the Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et
l'Agri-culture (I.R.S.I.A.).
The authors wish to acknowledge with thanks the co-operation of Professor
C. Grosjean, Director of the Computing Laboratory of the University of Ghent,
of Mr. J. van Maanen, Director of CeBeReNa, of Mr. A. Maes, civil engineer
in shipbuilding, who was responsible of the measurements on board the trawler
John and of the shipbuilders Béliard-Murdoch SA. who were of great help in
REFERENCES
AERTSSEN, G., "Service-performance and seakeeping trials on m.v.
Lukuga", R.I.N.A., 1963, p. 293 to 335.
CANHAM, CARTWRIGHT, GOODRICH, HOGBEN, "Seakeeping trials on O.W.S. Weather Reporter ", R.I.N.A., 1962, p. 447-492.
Pirrso, NEUMANN and JAMES, "Practical methods for observing and
forecasting ocean waves ", Hydrographic Office, Secretary of the Navy, Washington, Public. 603, 1955.
TELFER, E. V., "The power-loss factor in ship service performance ",
Centre Belge de Recherches Navales, Brussels 1953.
BLEDSOE, BUSSEMAKER, CUMMINS, "Seakeeping trials on three Dutch destroyers ", S.N.A.M.E., 1960, p. 39.
AERTSSEN, G., "Hull vibrations excited by waves ", 6° Convegno Triestino di Tecnica Navale, 1963.
ST DENIS and PIERSON, "On the motions of ships in confused seas ",
S.N.A.M.E., 1953, p. 280-357.
KORVIN-KROUKOVSKY, "Theory of seakeeping ", S.N.A.M.E., 1961.
AERTSSEN, G., "La stabilité des navires de cabotage ", Annales
trime-strielles de l'Association des Ingénieurs sortis des Ecoles Spéciales de Gand, no 2, 1934.
WENDEL, K., "Safety from capsizing ", Fishing Boats of the World, Second World Fishing Boat Congress, FAO, Rome, 1959.
LACKENBY, H., "Formation of ice on trawlers ", Fishing Boats of the
World, Second World Fishing Boat Congress, FA. O., Rome, 1959. TRALJNG, J. O., "On the Stability of Fishing Vessels ", Shipbuilding and
Shipping Record, 22.8.57 and 19.9.57.
RAHOLA, J., "The judging of the stability of ships and the determination of the minimum amount of stability ", Helsinki, 1939.
MÖCKEL, W., "Behaviour of Trawlers at SeaIL ", Fishing Boats of the
50 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE NOMENCLATURE AND SYMBOLS
p density of water
VR relative wind velocity
AT = transverse projected area (air)
= displacement (metric tons salt water)
g = gravitational acceleration, metre per sec2 b.h.p. = brake horsepower of motor, metric
d.h.p. = at propeller delivered horsepower, metric r.p.m. revolutions per minute
Sa apparent slip
Teh, Tern, Tei= high sextile, mean, low sextile period of encounter of waves, sec
Ttfrh, Tfrm, Ttl'1 = high sextile, mean, low sextile period of pitch, sec.
Tç5, Tç51 = high sextile, mean, low sextile period of roll, sec.
T, Tzm,
T,
= high sextile, mean, low sextile period of heave, sec.Tam, Tal = high sextile, mean, low sextile period of lateral acceleration,,
sec.
Subscript ¡ = upper limit, first sextile Subscript h = lower limit, last sextile
GM = transverse metacentric height, metre, in.
= direction of advance of waves, degrees
P, SB = heading of ship to waves port, starboard, degrees off bow (10 SB'
of tables means = 170 degrees)
1/O, H113 = mean height of 1/10 highest, of 1/3 highest waves, metre I/10 = mean double amplitude of 1/10 highest pitch angles, degrees c5 i/lo mean double amplitude of 1/10 highest roll angles, degrees
mean double amplitude of 1/10 highest heave, metre
= mean double amplitude of 1/10 highest lateral acceleration, x g VE = square root of cumulative energy density of waves, motions and
accelerations, meter, degrees, x g
N = number of low cycle oscillations in one observation (vert, accelera-tions)
dec. = deceleration due to a slam, x g
- I iP 'r? 300 o-(n LU p, g w 200 z w o o-
u-o
)!
UI u, w z o'i4
I#4
//
/
WAVE HEIGHT METER
5 10 15 i
il
1 2 3 4/
I I 5 loFig. 1Instrumentation on Trawler Belgian Lady
An=Anometer V = Wind Vane
W = Wave Height Transmitter G = Gyros Pitch and Roll
Ac = Accelerometer P =Pitot Tube System log (Sal Log)
T = Torsionme ter 1.0 - .- - - EXcEPI1ONAL soW\A u) HEAD SEA z BEAM SEA z FOLLOWING SEA < Q-o- -S1 9 KNOTS 20
V OF WAVE HEIGHT, METER
Fig. 2Belgian Lady: Relation Between Power absorbed by Waves and Wave Height
5 18 3,0 23
6 23 5 35
7 30 0
8 37 12,0 9,4
9 44 18,5
V9,5KNOTS SEA WIND WAVE WAVE
SEC STATE VELOCITY HEIGHT H1110 EIGHT
....
-_4fl,
----,/
-'9' /
..,,'
-. 44:''
fl):)q/
-:
-9,6m Ter 7,3sec SAILING 4)4,
Ad
ir
150 = 130z
W -110 z o 90 Ui 70 z o I-' -J,.1 7 -TRAWLING p wil
8 9 10 11 12 133 4 5 SPEED, KNOTS SPEED. KNOTSFig. 3Belgian Lady: Power-Speed Diagram at 650 To,,s Displacement
9V
1000
SEA
WINO
WAVE
STATE VELOCIÌY HEIGHT
900
BEALFIRT KNOTS METER
5 18 3,0 6 23 4,5 7 30 8,0 8 37 12,0 9 44 18,5 700
EPTX)NAL BOW SEA FOLLOWING SEA - BEAM SEA
-600 HEAD SEA -TANK PR DICTION Ui 500-..r
»
4Q0 Iii o s 300 o Ui 4, Ui o 100 WAVE HEIGHT ME T ER 2,3 3.5 6.3 9,4 14,5 600 500 400 (-J I- w Q- z w oQ- W U) o z o W (-Li -j W oI
RO 2 4 WEATHER. BE/,IJFORT WEATHER. BEAUFORTFig. 5Belgian Lady:
Increase of Power due to Weather
2
3
200
1'
i
lEAD SEA FOLLOWING SEA REMI SEA EXCEPTIONAL BOW SEA
whi
J
/
/////
.1
.---,7
-.--__.JZJQLQi--
E-i-I WAVE METER OF WElS IIG4FT. METERFig. 4Belgian Lady:
Increase of Power due to Waves Versus Wave Height
200
SAILING
FEAS SEA BEAM SEA
60
SEA
_HEAD
TRAWLING
SEA 4-4.8 IXNOTS
---F.LOWING .---BEAM SEA
z
IOO
4O
-4
z
ni
54 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
50
O
Fig. 6Belgian Lady: Ratio of Power Increase due to Waves to Total Increase
due to weather, Sailing
SAILING HEAD SEA FOLLOWING S BEAM SEA
/1
I/
o 2 4 IO WEATHER. BEAUFORTFig. 7Belgian Lady
Loss of Speed due to Weather, Sailing
INCREASE OF POWER DUE TO WAVES TOTAL INCREASE DUE TO WIND AND WAVES
o e
:
b S333 '313NV dJi3M HO1I 3ÂYi-i 40 I 40 j J e --w Ui 2 p--TI
e
(3OrurldP« 3j3 0L/1Z 3A3II 1fl0G S33030 W 310NV 91 '1 H3EkI e wJ 33003i31NY I-131Jd'AYBH 40403j j
o 9
Ui
(n w W o p--WN,.
e'
4, 44,(alirldfr«3191100) S33030 - OLAOL/Z. AVH 31')NY
21 M' I41Id S33030 '31ONY H1Id 40 3i3I' 3Y3H 40 e t 'S 'S 'S - S!-Ui W t'- +
,'I
-T Q H3Id o (3onifldIlY 3J3p 319flQ) S33030 91 'OI1 BAY3H 'OII 31ONY 91Fig.
9-Belgian Lady:
Vertical .4cceleraiio,is and Slanining
AHED
SAILING
I 2.0 o BOW:AT 0.02L FROM FP (AFT) 0.25 6 0.50 0.25 + STERN: AT 0.03L FROM AP (AFT) pp 0.50 V9.5 KNOTS kf10M AMID3HIPS AT 0.60 L FROM FP pp Z :5 15 5 x SLAM NUMBER 1.5 0.20 0.40 20 VERT. ACCELERATiON 0.40 15 w 4-15 4-Ui SLAM NUMBER Zn\o7
(I) wz
8 al 'n Ui 0.15 ° 0.30 OiS al\
Q3O Q¡w
+9 < Luz 2
Z'-\\
+ xi.ODl!
\wN
« rn ./iZo
N LL.\
°
Q2Q0.1O m\
+11 Q.I
W 02' 1z
Zz
I
w 1 Z -i\
o NJ L) 16 0DRIFTING(BOW) mo
DR IFT ING ( STERN
1JìDsHIp5\
w 0.O5 Iz
Oj0 0.05 DRIFTING(AHIPS O10 Z g AMIDSHIPS 12 N\7
2 ' ( >...
-METER V. KNOTS W o 51 109.5 4 0 0 Q. O 5 6 7 8 9 10 D 30 60 90 120 150 180MEAN PERIOD OF ENCOUNTER, SEC
BELGIAN
Fig. 10Belgian Lady: Lines of Forebody
-a*
i 2 3 5 600 BEAM SEA Tem7.7
ÁT
+1 SAILING FOLLOWING SYNCIR.QUARTEHNG SEAHEAD SEA o SYNCHR.OW
w i_i C, z -TUNING NORTH S SEC 8,2 SEC To8SEC 8SEC DRIFTING w 9 < -J O 50 ,-SEA
ii
40 SEAji
g w uf-j C,z 4 o u- o o Ui Li ern:7.8SEC+L-
p 30 Tern Z? SEC 1Pm Z5SEC W 30 Tem735ECî7,9SEc\
T 8,7 SH -ISECT"00-'
IDi
Iu,
w, ZL
</ 1
/
r ROLL ! 00 - j w e I9' W- <
..J 4 ")-,
f
/
//0
<6
g lO;
-J Tm7,1SEC T,mo7,5SEC»J
WAVE HEIGHTTUNIN. LAI ACCEL
Di
'-c0/ /<Terno765Ed Tjpr7.55EC -j/
WAVE HEIGHT 5 10 METERii
4,'-,J4L IeT79
WAVE SEC SEC HEIGHT 5 i bMETER f o 10, METER 0 1 2 3 4 5 0 1 2 3 4 1 2 3 4 5 YOF WAVE HEIGHT, METER
OF WAVE HEIGH1 METER
OF WAVE HEIGHT, METER
302015 ¶210 8 0,3 46 0,9 1,2 1,5 u 5EC' 6 5 4T SEC 6 WAVE ENCOUNTER SPECTRUM
H
/)
0 43 46 (0.9 1,2 1,5 w 5EC1 i i i 1, i 16 Ifl
/ \ ROLL I RESPONSE +CALCULATED VALUESIi'
i
I \ \ o o-.
¡0
/
o °\
O\NTHISRANGE:POOR ACCURACY
I
O
10000
3020151210 8 6 5 4TSEC
0 2 4 6 e 10 12
)JL WAVE - LENGTH RATIO Fig. 12Belgian Lady:
15 o Ui li)
':
loz
o o o o5W
û-Titi PITCH o Ti9ROLL .
- -I-Tz HEAVESAILING
o * o 0° 0 -TZI ----.--°-° T'1i -Vm / Tzh/
/ /
/
/ /
+/
+ Tzm 15 10 5 TRAWLING Izh +/ o---9*(
/
o_..-49_
Tipn- -- T
z 4Jm + 5 lo 15 5 10 15MEAN PERIOD OF ENCOUNTER, SEC.
MEAN PERIOD 0F ENCOUNTER,
SEC.
Fig. 13-Belgian Lady: Periods
of
z z I, o z C r) o z -e z o z zI, 20 10 q 014 -Iz Ta PITCH SAILING /1..
i-/
4-,/
-..--.--/
-,/
jip, 20 15 UI
W -e Q TRAWLING + ..j jTzmROLL..-
HEìWE---
LAtACCELER.uJ (-I (J
/
4 4-//.
-,/
C 4/'
/
/
0//
0/A._1h
/
-iIE
+ 11'm/
+C0f*:
I +/
/
/
i
:-i
o E 10 15 5 10 15MEAN PERIOD OF ENCOUNTER, SEC.
t.EAN PERIOD OF ENCOUNTER, SEC.
Fig. 14Belgian Lady:
Fig. I 5Belgian Lady: Periods of Motions and Lateral Accelerations
versus Period of Encounter of Waves, Following Sea
20
/
.1 + f SAILING 15 10 lpl
.4 T o w(I) PITCHF TRAWLING
-(J UiIl) 151ir
/ *
x +/
/
,.'
K --f -, x ,,,,.-",,
K .tT,. - in 0.ROLLHEAVE LAI.
ACCELER...--..--+ + 1zh
/
j
::
K/
/
/
' f Ui.J .+ // /
0 o°°°,
:Tam
z +/1>Am
lo 15 205 lO 15MEAN PERK)D OF ENCOUNTER, SEC.
WAVE OF ENCOUNTER
8 12 16 20
PERIOD, SEC
o
Fig. 16Belgian Lady: Histograms of Periods of Motions, Head Sea, Obs. 5 = 180 deg.
= 96m. V = 95 knots
4
6 8 10PERIOD, SEC
PITCH
Fig. 17Belgian Lady: Histograms of Periods of Motions, Bow Sea.
Obs.20c = 155 deg.Íi110 =213m. V=88 Knots
810
SEC 40I
z
w
o
w
C->-o
20o
LU 10 Teh Tern TeL= 9,3SEC
7,3 SEC 5,0 SEC 40F-z
w
300
=Tm
1li)I61SEC
4J9SEC 3,8SECw
Q->_ L)20z
LUo
LU lou-LU z w a lOW-
-
-LU z Lu o 10w 20! z W-
-
u. u-O 4 8 12 1620 0246 8
10 0 4 8 12 6PER OD, SEC PER IO D SEC PER OD, SEC
WAVE OF ENCOUNTER PITCH HEAVE ROLL 1th = SEC
160 SEC 7.1 SEC Tzh 15.2SEC 7.6 SEC Tem 1t8SEC2P
- Tm
56SEC 30 Tzm 11,3SEC3Q = 6/.SECZ2SEC Ty1 41SEC !zI USEC
J
u. L) z4
20 0246
tir
PER OD,Fig. 18Belgian Lady:
Wave Spectra Obs. 5 and Obs. 20
350 300 350 300 350 300 250 WAVE OBS. A ENCOUNTER 5, V r 9,5 KNOTS, BEAUFORT SPECTRUM 180 DEG., 250 OBS. WAVE B ENCOUNTER 20, V r8ft KNOTS. BEAUFORT 10 SPECTRUM 155 DEG., 250 I I REDUCED C WAVE SPECTRUM TO ZERO SPEED B 200 7-8 200 I 200 U)N
I5Q'.
150UI uil
U)'
N 150 o w U) N I I I 100 100 I, 3 IL) I lOO Iii 50 50 ' 50 O' J O' i I ,\ O 0,3 I I Q6 Q9 1,2 1,5 I I I I I SE1 0 0,3 06 0,9 1,2 1,5 I w SEC ' I 0,1 0,3 0,5 0,7 0,9 w 5E1 I I I I I 30201512 10 8 5 4 3020151210 8 6 5 4 30 20 15 12 10 8 6 Te SEC Te SEC T SECo
j
E SAILING Tzmz
o I-O o D O w 10T.
TzmTm_
z
w Tm JOHN 5 0 45 90 135 180 HEADING. DEGREESFig. 19Belgian Lady:
66 AERTSSEN, FERDINANDE & DE LEMBRE: SERVICE PERFORMANCE
5 10
MEAN PERIOD OF ENCOUNTER, SEC
Fig. 20Belgian Lady: Relation Between Periods of Pitching and Heaving and Period of Encounter of Waves (for a Speed Decrease from 95 to 35 Knots)
in Sea Beaufort 7 I I 7,0 3,5 SPEED, KNOTS
///// Tzh
1,0Fig. 21Belgian Lady and John: Stability Changes During
the Voyages FRG*.I PERIOD 40, Sp,, DISTRIBUTION
!:;.--__
---FROM WEIGHT-
\:_
i_'l"
y
/
/ ,/ -I BELGIAN LADY -S I ST VOYAGE \ 2 VOYAGE 28 V ---'S,' -I BELEAN LAOY o 5 10 20 NUMBER OP DAYS o uJ loz
o o Li oFig. 22Belgian Lady:
Righting and Heel/jig Moments in Waves
I
I
I
I
I
WAVE TROUGH AMIDSHIP
WAVE CREST AMIOSHIP
MEAN CREST AND TROUGH
300 100 200 20) g
----t----.
w w CtNG I- w 'ç,, Ç, o---/0
%
';
200 '% 100 4 wiQ-CD
z
w w w wo
z
II
w ciz
/
f
O z o i-'4
Ll.I Ui x MOMENT 100 -ioo 1' o z ___wD____ -(Dr
ICi3 S-. -w -wr
ciz
.5-(D -.200 -.100 -.10 10 20 30 40 50 6010 20 30 40 50 60 10 20 30 40 50 HEEL, DEGREES HEEL, DEGREES HEEL, DEGREESFig. 23-John: Righting and Heeling Mo,ne,,ts in Wat'es 4
z
g o 400 ' WAVE I TROUGH AMIOSHIPI
w WAVE CREST I AMIDSHIP w 200MEAN CREST AND TROUGH
V
z
tu oz
w w X2,
///
z
w o---ICINGIEE
ACES g 4 EEUNG MOME g---10 W4 OIOOA
II-
X 12 o ICING ._i--- _i---
-w -(D
z
i X -200 CI( ,4t4
o w w X (Dz z
12 0t.
to 30 40 50 60 10 20 30 40 50 60 10 20 30 40 50 60 HEELS DEGREES HEEL, DEGREES HEEL, DEGREESDISCUSSION ON "SERVICE PERFORMANCE
AND SEA-KEEPING TRIALS ON TWO
CONVENTIONAL TRAWLERS "e
Dr. F. LEATHARD, Member:
This paper is of particular interest to nie
since the company for which I work is engaged in the design and building of
trawlers.
In connection with Part I, I should like Professor Aertssen to comment upon two points in Table I. Firstly, the block
co-efficients for both trawlers appear low
compared with normal British practice, particularly in the case
of the
John. Secondly, the service speed of 134- knots quoted for the Belgian Lady appears rather high in relation to the power/speed curve given in Fig. 3. In calm water, 900/1,000 d.h.p. are required for 134- knots, and as only 1,200 b.h.p. are available, there is little power margin for a service speed of 134-knots.Turning to Part 11, I find a trawling speed
of 4-44- knots is shown in Fig. 3 for
Belgian Lady with associated d.h.p. of
500/550. Published references to trawling
speeds are few and far
between, andalthough four knots is often assumed,
confirmation of this speed by measurement in service is particularly welcome. Further, power measurements represent very useful data, since, within certain limits, the size of
trawl is the same for a range of trawler
length, and consequently the trawling
power will not vary greatly with trawler length.
This latter point leads to consideration of the use of a 2-speed gearbox. Propellers designed for maximum free-running speed will always produce a drop in r.p.m. when trawling, with the consequent risk of over-load torque being developed by the engine, with resulting high temperatures. The 2-speed gearbox allows the continuous power
to be developed, if required, at normal
r.p.m. both free-running and trawling and its use on the smaller type of trawler, where the maximum installed power may be of the order of 500/600 b.h.p. would obviously allow adequate trawling power to be developed without giving cause for
com-plaint by the Engineer Superintendent.
Since adequate power is available on the Belgian Lady, the 2-speed gearbox seems to be superfluous and indeed, a single speed gearbox was fitted on the John.
* Paper by Prof. Ir. G. Aertssen, Member. Jr. V.
Ferdinande, M.Sc., and Ir. R. De Lembre. See
p. 37 ante.
The great interest of Part IV lies in the discussion of various stability criteria which have been suggested in the past. I consider it to be a deplorable state of affairs that no official standards of stability for trawlers
exist in the U.K. Based upon certain simple standards we use for tug design, I
would suggest a minimum GM in any
condition of 30ins. in association with a
range of stability of 60 degrees with
maximum GZ of l2ins. Such a standard must be related to a clear definition of these parts of the hull to be used in the
calcula-tions, together with the method to be
adopted of taking account of free surface. Although a standard of the type proposed refers to still-water conditions, there must be some consistent relationship with condi-tions in waves and in this connection, it would be interesting to see the stability curves for Belgian Lady and John in still
water. Further, why was 3m., chosen as
the wave height for the stability calculation for both trawlers? From the diagrams
shewn, it is clear that the standard of
stability for Belgian Lady in waves is rather less than for John but this could be due to the fact that 3m., represents a relatively greater wave height for the shorter length of the trawler Belgian Lady.
Mr. A. EMERSON, Member of Council: During the last 10 years there have been considerable developments in the analysis
of irregular seas and ship responses by
statistical methods. There has also grown
the feeling that the problem is so
com-plicated that observation of ship perform-ance can only be carried out satisfactorily
by large organizations and large staffs.
Professor Aertssen has shown that this is flot the case. By careful instrumentation and technique he has obtained invaluable information on cargo passenger ships, cargo liners, tankers and now trawlers.
In presenting the results the Author has brought home to us the conditions under which trawlers work in the North Atlantic
the vertical accelerations of one "g"
seem more appropriate to fighter aircraft
than to ships. In contrast to the more usual ocean going merchant ship the trawler is short with a small pitching period, and thus is well clear of synchronism with the long high waves generated by North Atlantic gales. Professor Aertssen has shown
