Stability, propulsion system and rudder evaluation of a riverine support vessel to optimize its operational performance

Stability, propulsion system and rudder evaluation of a riverine support vessel to
optimize its operational performance

Javier Serrano Tamayo, Naval Mechanical Engineer
Naval Academy “Almirante Padilla”
Colombian Navy, Office of Education
Tel: (57) (1) 2742410
lj.serrano230@uniandes.edu.co

Abstract—The present article summarizes the study of the
stability and the hull integration with the propulsion system of I. INTRODUCTION
a riverine support vessel, in order to optimize the efficiency of
the propulsion plant and improve its maneuverability in its
operations area. The relevance of this study originated from the
fact that the vessel originally was a Tug Boat converted into a
T he consolidation of the Democratic Security Policy in
Colombia requires the highest possible performance of
naval vessels, for which the Navy exercises sovereignty
mother vessel to transport troops. Other vessels of the same over the navigable rivers of the motherland. Within the
type were used in operations to control public order, due to the organization, the Riverine Brigade has twelve riverine tugs
lack of methodology in the conversion process the end results built in the 80’s to serve as a tugboat and cattle transport
is not optimal. vessel , work done until at the end of 90’s when they were
stranded and converted in riverine supported vessels. But
To initiate the work, it was necessary to forego a Field here was the extent of repairs, without considering that the
review that permitted to measure the vessel in 3D; change in mission required a different conception of the
modeling and executing the estimation of weight and other components of the vessel, particularly the integration
components of the vessel using SWBS (Vessel Work of the hull with the propulsion system.
Breakdown Structure), as well as determining the location
of its center of gravity (CG), work performed using GHS The increase of weights, was construed by the shielding
software (General Hydrostatics) and Rhinoceros. made out of steel plates and sand interspersed, suggests that
there have been significant variations in displacement and
An important evaluation for the criteria of stability was in the vertical position of the CG, so it was necessary to
applied using standards such as DDS-079 USN and study the loading conditions and to evaluate its transverse
046CFR170 USCG. Once a study was undergone, the stability with acceptable criteria for such boats, like DDS
resistance was predicted using systematic series with the 079-1 NAVSEA, U.S. Navy for intact stability. The main
NAVCAD software, as well as the optimal propeller engine, designed to develop an average of Rated Power 180
selection and evaluation in terms of fuel consumption and BHP @ 1800 RPM did not exceed 1500 RPM in its best
operating autonomy. In regards to the controllability case, indicating it was overloaded or the propeller was not
system, a study was performed utilizing the current rudder properly projected. In the same term, it was a desire to
in which recommendations were formulated in achievement overcome the presented cavitations. Regarding the rudder,
of the appropriate one, better located and which absorbed manoeuvrability was being affected by a tactical diameter
the propeller turbulence, as well as state of the art that could be reduced.
recommendations to improve maneuverability.
The purpose of this document is to summarize the
The results turned to a stable vessel and the studies undergone to evaluate the stability as well as to
improvement in the propulsion system efficiency, which show the procedures to obtain an optimal and commercial
showed an increased in speed, cavitations reduction and propeller, and calculation to design a rudder that
longer range. The applied rudder remarkably improved significantly improved manoeuvrability. This study solved
maneuverability as well as coursekeeping. a problem of poor performance of a vessel in the river due
to a partial adaptation of a towing vessel as a personnel
Keywords—Fairing, weight estimation, loading carrier with capacity of a mother vessel. This pattern of
conditions, stability criteria, squat effect, propeller study could also be replicated in the same type of vessels
efficiency, rudder selection, Schilling rudder. presenting similar problems.

2
II. 3D MODELLING OF THE VESSEL shape is passed to the program to define the cut-off to
optimize the material, since the steels have a high cost in
For the hull measurement was created a table of offsets, the construction process. To change the lines and view the
which were drawn stations from conspicuous points it applied fairing using Rhinoceros software, which worked
hinoceros
contains, which are referenced to an initial origin point that the vessel's hull as a series of surfaces. First from
's
usually corresponds to one of the ends of the hull. To that checkpoints were "pulling" those imperfections until giving
imperfect
effect it was taken as a reference point (0,0,0) point of the hydrodynamic shape of the hull.
intersection of the imaginary tip of the bow with the center
line to the height of the baseline. Most of the vessel,
specially the parallel section, keeps a depth of 1.2 m and
has a sharp rise at the bow and slight one at stern.

Once the measurements were obtained and the
information of the hull shape collected, the first table of
,
offsets was developed in order to organize information and
to take the first record of the shape lines of the stations.
s

Fig. 2. Checkpoints used to the bow fairing

After long hours of iterations station by station and
iter
point by point, was reached an acceptable and reliable
a
model in 3D.

Table I. Table of offsets format for one station
ffsets s

The information collected and entered in the produced
table offset format, necessarily had to be supplemented and
verified with different available information of the vessel.

The first introduction of data was attained with highly
as
satisfactory results in semi-tunnels shape definition of the
tunnels shape, Fig. 3. Final 3D model
m
stern section and parallel. In the bow the results were less
favourable given the muddy terrain, which led to a long
hich
process for shape lines refinement, known as fairing
fairing. III. FORM COEFFICIENTS AND HYDROSTATIC
CURVES

Form coefficients are used to show the shape of the hull
and provide an estimation of power. According to the basic
on power
features of the studied vessel is a full forms vessel, as
reflected in its main coefficients.
coefficients

126.2 m3
CB = = 0.7963 ≈ 0.8 (1)
31.15 m ⋅ 7.28 m ⋅ 0.7 m
4.976 m2
CM = = 0.9765 (2)
7.28 m ⋅ 0.7 m
Fig. 1. Reference point for the table of offsets and
erence ∇ ∇ C
imperfections details at the bow CP = = = B = 0.8168 (3)
L ⋅ AM L ⋅ B ⋅ T ⋅ CM CM
The fairing of a hull is intended to avoid discontinuities,
voids or tipping points that result in concentrating s stress, Once the coefficients were obtained, the hydrostatic
greater resistance and lack of aesthetics in design. In curves were calculated which indicated different values that
shipbuilding, is of great importance, since after the faired
pbuilding, affected the vessel’s stability at variable water lines. It is

3
customary to calculate the curves with the vessel at flat keel IV. WEIGHT ESTIMATING
(no trim), which is often shown in auxiliary curves. The
drafts range is shown from the minimum possible, when the To study the loading conditions were necessary to know
vessel fully shedding (light weight) to the highest possible, the weight of the vessel and all its components, as well as
with the boat fully loaded. the bending moment respect a reference point. Three
conditions were considered for study: lightship, minimum
Nowadays, these calculations are made by stability operation condition and full load. But, when the project
software, in general the important factor is to select the began, before using SWBS, some methods for structure
most appropriate software and enter the information weight calculation were studied satisfactory results were
carefully and make sure that the program delivers useful not achieved.
results, as well as user friendly and consistent with the
particular form to be integrated. In this case was used GHS Studied method Results
(General Hydrostatics), but before obtaining the Benford method Little displacement
corresponding values the tanks of the vessel must be edited Danckwardt method Little L/D ratio
into the model for make the calculations properly.
Lamb method Little length
Mandel method Ilogical value
Gilfillan method Just for bulk carriers
Murray method Ilogical value
Osorio method Could be use as a reference
J.L. García G. method Very little value
Table II. Weight estimating methods for main features1

Considering that no method satisfied the accuracy
required to determine the weight of the vessel, proceeded to
weigh it according to each one of its components. Detailed
procedure for weighting and CG estimation is determined in
SWBS (Vessel Work Breakdown Structure) which is a
Fig. 4. Model including the different tanks. detailed summation of weights designed by the U.S. Navy,
which considered the vessel as a set of elements condensed
Once this was done, a program was developed to obtain in seven structural groups, detailing all the components of
the coefficients of form and hydrostatic curves as shown. light ship and dead weight.

Group Concept
100 Hull structure
200 Propulsion plant
300 Electric plant
400 Command and Surveillance
500 Auxiliary Systems
600 Outfit and Furnishings
700 Armament
M Margins
F Loads
Table III. SWBS structural groups
Fig. 5. Curves of form, trim cero, heel cero.
There are basically three types of weight according to
our knowledge of them. The first is that which we know its
CG with certainty, and their properties. The second is a type
of weight in which the weight and centre of gravity are
likely known. The majority of the vessels weight is
collected. While there is some information available, the
weight or CG is not defined, which could cause variations
of displacement of the CG and more time and delay in
detailed design. The third are margins, which are integral
part of weight estimating and are expected to reflect the
weight of the vessel or KG at the time of delivery.

Fig. 6. Hydrostatic curves, trim cero, heel cero. 1
Taken from MIEZOSO Manuel, “Ecuación del desplazamiento, Peso en
Rosca y Peso Muerto”, ETSIN, UPM, Madrid, 1990.

4
The CG location of a combined loaded system, as a
s C. Full Load.
vessel may be regarded, can be calculated multiplying the The vessel is fully loaded, i.e., the total dead weight
weight of each component by the distance from the CG to a plus the light vessel in accordance with the characteristics
reference point (0,0,0), to find moments in the three of its design.
coordinates, which makes a final sum and are divided over
the total weight. The CG location is determined when the Once the weights, CG’s and their bending moments for
distance from each of the three planes has been established. the three axes were completely defined, and set the loading
The importance of its determination is that it exerts a conditions to study. The information was organized to
.
bending moment on the axes x, y, z that affects the trim, understand the behaviour of the vessel according to their
heel, and KG height of the vessel. Once the process of
. weight distribution. For this purpose the loading curves
or
weighing the vessel’s components finished came to define were developed.
the loading conditions.

V. LOADING CONDITIONS2

Chapter 096 of the Naval Ships’ Technical Manual
(NSTM) which deals with weight and stability define the
loading conditions for surface vessels. For th case three
this
weight conditions were selected: Light Ship, Minimum
Operating Condition and Full load.

A. Light Ship.
Combine elements of the vessel from the group 100 to
ombine
700, ready for service in every aspect. While excluding the
dead weight must take into account some weights as: fixed
eight
ballast (if applicable), basic spare parts, machinery for Fig. 7. Loading curves for min. operating condition
min
fluids in minimum levels of operation.

B. Minimum Operating Condition. VI. STABILITY CRITERIA
The vessel has the least possible stability characteristics
to survive in normal operation. The liquid cargo is included
rmal The addition of weights due to the "shield" installed,
in such a manner that seeks to keep a good stability and consisting of three steel plates of ¼" with 2 cm of sand in
ting ¼
trim, but otherwise the components of the dead weight
, the middle of them throughout the superstructure of the
hem
depend on both the type of vessel and its service. Accurate vessel which added a total weight of 17 tons, required of a
percentages for the components of deadwe
deadweight are defined initial stability criteria for surface ships using the standard
in the next table, which is presented below. This is a critical DDS-079 of the USN and 46CFR Part 170 of used by the
he
operation condition, because many tanks are empty and USCG.
most often high weights remain constant.
Standard DDS-079 begins by establishing a requirement
079 esta
Crew Same as full load for spacing between transverse bulkheads and the bow
collision bulkhead. To be considered effective, the main
Ammunition 1/3 of full load
transverse bulkheads must be spaced a minimum distance
Provisions and stores 1/3 of full load
of 10 feet + 0.03 LBP (length between perpendiculars)
Lubricants 1/3 of full load separated.
Food and drinking water 2/3 of full load
Fuel 1/3 of full load The measure for the vessel case would be:
Load 1/3 of full load Since 10 ft = 3,048 m and LBP = 31,15 m,
Ballast tanks Empty 3,048 m + 0,03 (31,15 m) = 3,98 m ≈ 4 m
Passengers Same as full load
Table IV. Percentage of variable loading for minimum
ge Moreover, one of the transverse bulkheads can be used
operating condition3 as collision bulkhead in order to limit the flooding of the
compartment closest to the bow, must be located
Note: The above table related only values according to approximately 5% to stern, measured from the forward
the characteristics of the load of the vessel case. perpendicular (FP).

2
The measure for the vessel case would be 5% of the
NAVAL SEA SYSTEMS COMMAND, Naval Ships’ Technical Manual. total length is (0,05 x 31,5 m = 1,575 m) and the FP is 0,35
Chapter 096, pg. 96-4. 1996.
3 m behind 00 station, then, 1,575 + 0,35 = 1,925 m. This can
NAVAL SEA SYSTEMS COMMAND, Design Data Sheet 079,
Stability for surface ships of US Navy, pg II-11.
be compared with the 2D tanks distribution.
distribution

5

The value of factor P for service in shallow waters4,
defined according the standard as those that have no special
risk, such as most rivers, harbours, lakes, etc.., is defined by
the following equation:

P = 0.028 + ( L 1309) 2 (8)
Fig. 8. 2D Tank distribution and bulkhead spacing
The value A is the lateral section of the projected vessel
As it can be seen; a less spacing of bow and stern peaks, above the water line and the value H to the height from the
as recommended by the standard, also the location of the center of area A through the center of the submerged area or
machinery room affects the spacing of the cellar and the about the midpoint of draft.
water tank aft. On the other hand, the forward collision
bulkhead is 2.1 meters from the station 00, which is a The value of W corresponds to the displacement for
somewhat higher than the minimum requirement as set by each load condition and the value of T at the lower angle
the standard technique. between 14˚ or half of the freeboard.

Furthermore, the standard states warships have to bear Finally, the program was edited and were obtained
threats and external influences, which can affect stability. satisfactory results in terms of stability, which were
The main threats are: predictable, considering its high B/T ratio = 6, even in the
minimum operating condition.
1. Beam wind combined with rolling.
2. Haevy lifting over one side.
3. Towing forces.
4. People crowding over one side.
5. High speed turning. Table V. GHS run to check stability criteria
6. Top icing.
In order to not move the centres of gravity of each tank
The first and the last two pose no threat to the vessel caused by the free surface effect, the software calculates a
considering its characteristics and surroundings. The other total value of heeling moment to artificially alter the
three could represent a risk regarding the safety of the unit, position of CG.
which is an appropriate stability measure attained by
comparing the curves of upright arm with the curves of
threats of heel. Factors to be considered are the static angle
of heel with its associated arm upright as well as the
dynamic stability reserve.

To edit these threats sums were made from the three
arms of heeling simultaneously, which are described by
mathematical formulation to obtain a most critical arm, that
the multiplied by the displacement for each load condition;
calculate the most critical moments that were imposed on
the GHS program to check the set criteria.
Fig. 9. Stability curve for min. operating condition
• Heavy lifting over one side
HA = W × a × cos θ ∆ (4)
• Towing forces VII. BEAM VESSEL ANALYSIS
2
HA = 2 × N × (SHP× D) 3 × S × h × cosθ (38× ∆) (5) GHS also gives structural information provident of the
• People crowding over one side vessel by a percentage comparison of the maximum stress
HA = (W × a / ∆) cos θ (6) that the vessel will entail riding a trochoidal wave against
its tensile strength, as well as providing information for
On the other hand, standard 46CFR provides a criterion shearing and bending moments of the beam vessel.
about minimum permissible metacentric height, which is Determining the design section modulus, Z, which must
important for stability analysis which must be equal to or find continuous material in the middle section, an
greater than the following for each load condition: acceptable bending stress σad must be introduced into the

PAH 4
GM ≥ (7 ) United States Coast Guard. 46CFR. Subdivision & Stability. Part 170.
W tan(T ) Subpart E – Weather Criteria, pg. 85

6
equation. Vessels below 61m in length the strength Method studied Result
requirements are based in locally induced stresses than in Basic formula Very wide range
longitudinal bending ones. Holtrop method Low BWL/T (2.1-4.0)
Oortmerssen method Low BWL/T (1.9-3.4)
For practical purposes was created a spreadsheet of the
U. Denmark method Low LWL/BWL(5-8), 4.4
Moment of Inertia and Section Modulus (SM), adding the
USNA YPseries method Characteristics match
structural components that go along at least 40% of the
beam vessel, which are related to the master frame, Series 60 Round bilge
obtaining a value of 206251.36 cm3, equivalent to 2062.51 Series U. Brit. Columbia Unevenly shapes
cm2-m SM of the studied vessel . Once the modulus section Series Nordstrom y YP 81 High death sliver
and the values of the wave applied were calculated, this Series 64, SSPA, y Dutch Planing hulls
data and the values of modulus of elasticity and tensile Table VI. Systematic series analysis
strength of vessel material were introduced to the program.
The series chose was the U.S. Naval Academy. After
Finally, the program receives information from a beam that was necessary to complete information for using
vessel with some evenly distributed weights and other NAVCAD in three datasheets: Environment, Hull and
locals, which is subject to hogging and sagging, and is Appendices. All data were taken for the full load condition
compared in terms of percentage with the characteristics of in which resistance is greater. Some values were required to
the material of construction of the vessel indicating if it will calculate independently, but many of them are calculated by
be able to navigate properly in terms of structural the software.
resistance.

According the results obtained can be concluded that
meets the structural requirements satisfactorily, comparing
percentage of maximum stress suffered by the vessel with
the material is no more than 3%.

(5) Stress, 1 = 0.0001 T.M./cm2

(1) Weight, 1 = 0.1 T.M./m (4) Shear, 1 = 0.04 T.M.

(3) Buoyancy, 1 = 0.02 T.M./m

(2) Point weight, 1 = 0.7 T.M

Fig. 11. Hull Data in NAVCAD

The wetted surface required a special report on GHS for
Section Modulus = 2062.51 cm2 - m its importance in the frictional resistance:

RF
CF = (9 )
1 ρ SV 2
Fig. 10. Beam vessel longitudinal resistance 2
With regard to the environment in shallow waters such
some sectors of Caquetá River, the most significant effect
VIII. RESISTANCE on resistance is squat effect. First of all, there is a
significant variation in the water flow around the hull as the
For this case, the resistance curve was made using water passing under and going sideways faster than in open
systematic series of the software NAVCAD. It has series of waters with a reduction in pressure and a sinking of the bow
models ran in certificated towing tanks. The engineer or the stern (squat), as well as an increase in trim and
ability is not just to match the series consistent with the therefore the resistance, determining the maximum
vessel, but read and obtain useful information of the results allowable velocity without bottoming.5 Taylor and Tuck
presented. define squat as the change of draft and trim of a vessel that
is the result of variations in hydrodynamic pressure on the
The most appropriate series is selected according form hull, in its movement at any water depth.6
curves, main ratios and Froude number. To make the best
selection were studied the displacement hulls similar with 5
LEWIS Edward, “Principles of Naval Architecture”, The Society of
characteristics of ARC Sejerí. Naval Architects and Marine Engineers, 2nd Revision, Vol. II, Ch. V,
Section 5, Pg. 42, 1988.
6
HERREROS Miguel, ZAMORA Ricardo y PÉREZ Luis, “El fenómeno
squat en áreas de profundidad variable y limitada”. XXXVI Sesiones

7

After entering the characteristics of the hull of the The previous graph shows clearly the peak of the
vessel that provide resistance in NAVCAD and studied critical region.
how they affect the speed of the vessel, resulting in
t
resistance curve presented below. To understand better how the change in the river deep
o
affects the squat effect prediction the software NAVCAD
prediction,
Resistance Curve was used to compare squat effect at various depths, and
24000 increasing speed step by step, determine the minimum
allowable depth to be able to navigate the subcritical
llowable
21000
16.2
region. On the other hand will be seen, graphically, the
15.84 drastic increase in the total resistance when the depth
18000 15.48
15.12 decreases to critical levels.
14.76
Resistance (Newtons)

15000 14.4

12000
12.6

9000
10.8

6000

7.2
3000

3.6
0 0
0 2 4 6 8 10 12 14 16 18
Speed (kph)

Fig. 12. Vessel case resistance curve

This graph shows the increase of resistance as well as
the speed. The vessel will respond readily to low speeds
without significant opposition, but from 14 m the slope
ion, mph
rises sharply and the vessel will require a much larger
machine or an optimal propulsion system that allows
maximizing the actual configuration.

IX. SQUAT EFFECT INFLUENCE
Fig. 14. Squat vs. Spe curve at four depths
Speed
The vessel must be designed for the subsubcritical region in
cases where there is displacement of ships and the The previous graph shows the squat effect variation as
supercritical region in the case of planning hulls. 7. increasing the speed of the vessel for different river depths.

The curve corresponding to one meter deep, has the
three critical regions and a 0.32 m peak in the critical area
that added to the full load draft, 0.87 m, causes the vessel
bottoming, being impossible the navigation is in such
conditions.

The problem of the squat effect and difference in the
curves for one and three meters, raising the need to know
the minimum depth to avoid this undesirable peak for safe
navigation, for which some iterations were made with
curves between 1 and 3 m deep.

The first iteration resulted in the critical region even
show up to 2.5 m deep, but the vessel will bottom with only
Fig. 13. Design regions
values less than 1.7 m, nearly the double the draft for full
load vessel, 0.87 m, which shows the true importance of the
técnicas de Ingeniería Naval. ETSIN Universidad Polítécnica de Madrid, squat phenomenon.
Pág. 2, 2000.
7
HOFMAN M. and KOZARSKI V. Shallow Water Resistance Charts For
Preliminary Vessel Design. International Shipbuilding Progress. Volume
47, Number 449. Pg. 63. 2000.

8
The curves have similar characteristics, but between
three to six meters deep curves is a difference of 4000N,
which affect the performance spe of the vessel with the
speed
installed power.

40000
PREDICCIÓN MANACACÍAS-3m.nc4
SEJERÍ SQUAT PREDICTION 3.0 m. nc4

30000

Rtotal N
20000
RT difference of 4000N
at max. speed: 8.4 kts

10000
Fig. 15. First iteration between 1.5 – 3 m
.

The second iteration also presented in all curves the
peak values, in other words, up to 2.9 m deep will be a
spontaneous effect of trim, affecting safe navigation of the 0
0 1 2 3 4 5 6 7 8 9 10
unit. For that reason only for depths equal to or greater than Vel kts
3.0 m, the vessel will sail in a subcritical zone. Fig. 17. Resistance curves for 3 – 12 meters deep
Moreover, the following graphics shows the squat effect
importance on resistance of the vessel. In the first one, the
.
X. SELECTION OF OPTIMAL PROPELLER
curve of 1 m deep shows an important differe
difference with the
following three, with two meters apart, while it is observed
One of the initial important requirements was more speed,
a minimum difference of total resistance from 3 to 9 m, six
but not changing the current configuration by a larger
of difference.
motor or gearbox due to budget problems.

On the other hand the pronounced slope of the resistance
curve from 14 kph away, discard the changing idea and the
solution is to work out with reference to the existing diesel
engine and gearbox in order to determine th most the
appropriate propeller with the help of NAVCAD.

Propulsion system technical data of ARC Sejerí:

• 01 main diesel engine DD671L, 180 BHP@1800 RPM
DD

• 01 gearbox Twin Disc DD
DD-5091V, reduction ratio:
2.45:1.

• 01 three blades fix pitch propeller, B Series,
36”diameter, 32”pitch.

With the engine performance curve available to work with
NAVCAD, we can combine it with the vessel resistance
curve and compare the speed of the machine with the
Fig. 16. Resistance curves for 1 – 9 meters deep power supplied to the axis and the maximum speed to
achieve by the vessel in its current state as can be seen in
state,
Second graph is a zoom of the first, rejecting the one
econd the next graph.
meter deep curve in order to see the difference in resistance
when the vessel is in the subcritical region.

9
The results show an excess of cavitation as the speed
increases, obtaining a value of 11.2%, above the 5%
acceptable, according to the criterion applied.

According the report the current propeller has three
problems: First, the engine is underutilized; its current
irst,
maximum output is less than 150 HP to 180 HP available ata
revolutions from 1200 to 1500 average rpm, which causes
carbon in combustion chambers of engines. Second, the
engines
current propeller has a cavitation problem that is wearing
the blades and reducing their thrust. Third, the current
maximum speed of the vessel is 8.4 kts, which is likely to
increase with an optimal propeller.

NAVCAD on the menu of propeller data allows a
comparison up to three different propellers and have a
selection for optimize the pitch.
ep

The diameter is optimal due to the semi-tunnel
he
restrictions in geometry. The first iteration was done for 3
and 4 blades propellers.
Fig. 18. Resistance and engine curves comparison

This curve shows an abnormal performance of the main
engine, which is working below the nominal speed,
reaching only up to 1500 rpm, as was evident in the records
of the engine. These low revolutions affect the engine's
performance promoting carbon in combustion chambers.

In addition, the propeller expanded area ratio (EAR)
was obtained involving it in a paper and passing the
drawing to AutoCAD for the necessary calculation.

Among other parameters to select is the application of a
cavitation criterion for which was determined to choose the
Keller equation.8

AE (1.3 + 0.3Z )T
= +k (10)
AO ( p O − p v ) D 2 Fig. 19. Optimum pitch selection for 3 and 4 blade
propellers with the same diameter
This criterion may be implemented by the software and
gives an indication to establish if EAR allows an acceptable The next graph has a grid that shows that lowering the
gri
pressure differential. However, the equation leaves aside
he pitch and therefore the value of the P/D, the speed curve
variables that affect cavitation such as the influence of the can be moved to the apex of the curve of the engine
wake and blade geometry. performance.

These variables are absorbed by the software that The current propeller is 32" pitch and 36" diameter, for
he
involves these factors and calculates an overall percentage a P/D of 0.889. If this value is decreased to obtain an ideal
of cavitation in the current propeller as well as the other P/D, keeping the same diameter, the software recommends
proposals. a new pitch of 0.5545 m and consequently a P/D of 0.607.

To study the laws that govern the propeller behavior it In reporting results also identified a reduction of
is tested with no hull in front which is known as "open cavitation due to pitch change from 8.4% to 3.6%, thus
water". meeting the criteria set, but kept the value of the differential
teria
pressure between the two sides of the blade, which is
within acceptable values.
8
Naval Architects and Marine Engineers, 2nd Revision, Vol. II, Ch. VI,
Section 7, Pg. 183, 1988.

10
0.50
BS-3: 0.914x0.555x0.450
BS-3: 0.914x0.546x0.800
GA-3: 0.914x0.503x0.800

0.48

0.46

PropEff
0.44

0.42

0.40
1 2 3 4 5 6 7 8 9
Vel kts

Figura 22. Comparative curve between B-Series and
GAWN geometry propellers

Finally, the designed propeller was consistent to a
Fig. 20. Optimum and current P/D comparison showing the commercial one, Aquapoise 45 of Teignbridge Propellers
performance area at max engine RPM Ltd., found to be a better fit, which presented the highest
efficiency and acceptable parameters of cavitation
From the reports, analysis concluded that the optimum according the criterion established.
propeller is one of three blades with optimum pitch, as can
be seen in the graph below: Once selected the optimum propeller, a comparison
against the previous showed a moderate increase in speed
0.50
but a significant reduction of cavitation.
BS-3: 0.914x0.813x0.450
BS-3: 0.914x0.555x0.450 Moreover, although in the current configuration the
BS-4: 0.914x0.530x0.610 engine is only reaching a maximum of 1500 RPM, is
consuming more fuel than if it develops its maximum
speed. The results of the runs show a difference of nearly
half a gallon per hour for the maximum speed of 15 kph,
0.48
finally running over several days are three days of
operation, which is a significant additional cost, avoided by
installing an appropriate propeller.
PropEff

XI. RUDDER SELECTION
0.46

Optimizing the rudder includes three basic aspects:
governability, which is the ability to maintain the desired
course; maneuverability, defined as the controlled change
in the direction of movement; and change of speed, which
is the controlled change of speed variation including
0.44
2 3 4 5 6 7 8 9 10 stopping and reversing9.
Vel kts
Fig. 21. Comparative curve of propellers efficiency at open Two criteria were used to meet the rudder area
water requirement. The first, Lamb & Cook, establishes a general
rule of 2% of the area product of the length at water line by
An additional consideration is geometry comparison the average draft, and to this type of vessels particularizes
between a GAWN and B-Series propeller. Normally, these by 2.5%. The second is a general formula of Det Norske
applications use a propeller with a larger EAR, and GAWN Veritas, that criterion is based on the following formula:
used to be bigger, slower and may become more efficient.
However, the results of the run widely favored B-Series.
The reason is that this optimization is just of the propeller, 9
the motor and the gear box remained the same. Naval Architects and Marine Engineers, 2nd Revision, Vol. III, Ch. IX,
Section 1, Pg. 191, 1988.

11

T × LBP   B  
2

AR = 1 + 25   (11)
100    LBP  

Replacing with known data of ARC Sejerí plus the
barge that pushes in certain operations:

0.75 m × ( 26 + 26) m   5.5  
2
2
AR = 1 + 25   = 0.5 m
100 
  26 + 26  

Therefore, 0.5 m2 is the minimum required rudder area.
area
However is required a little increasing for stability in
direction (concept of governability), particularly for small
), Fig. 24. Increasing of lift coefficient for different aspect
vessels, so the minimum required rudder area of the vessel
, ratios at different rudder angles Source: PNA, Vol III.
angles.
is 0.6 m2.
The rudder of ARC Sejerí had a height of 65 cm over a
chord of 138 cm for an aspect ratio of 0.49, i.e., outside of
However, the reality of ARC Sejerí was different Each
different.
curves of the previous graph, which gave a clear indication
revious graph
rudder had a total area of 0.68 m2, and with the two rudders
of a defective rudder.
configuration a total of 1.36 m2, too much area without a
benefit for the vessel, so the vessel can operate with just
Adjusting the height of the rudder to the diameter of the
one rudder without affecting the area requirement
requirement.
propeller in order to absorb all its turbulent flow, it was
determined a height of 88 cm. On the other hand, as the
cm
rudder area had been determined at 0.6 m2, the chord was
deduced to 68 cm, to a final aspect ratio of 1.3, increased
significantly the lift force.

The position of the rudder was another critical aspect,
since there was much separation between the rudder and
propeller.

Instead of one-design considerations of rudder is its ease
of fabrication, installation, i.e., rudders were hung from the
transom, the distance between the start of the rudder, and
propeller core was 92 cm. T . This distance must not exceed a
propeller radius, in this case, no more than 44 cm, so that
the rudder blade can absorb as water flow as possible.
Fig. 23. Comparison of one of the two old rudders against
.
the installed Moreover, the trim towards the bow allowed to show a
small portion of the upper rudder above waterline, causing
Because of a single rudder could be placed at the semi-
a vibration, erosion and wear, and undermines the stability of
tunnel middle, behind the propeller, the concept of the direction due to external interference. The practical solution
T
highest possible rudder improved its aspect ratio, the higher
ra was installing the rudder stock, supported by three bushings
the greater lift to increase the rudder angle involved. The and nut up at the top to easily absorb the propeller flow and
next graph shows the evolution curves of the lift coefficient is completely protected and water covered.
depending on the angle involved with rudders for different
aspect ratios, allowing understanding the concept
importance.

12

Fig. 25. Previous position of two rudders layout vs. current
position of one rudder layout

Furthermore, a system of two rudders behind one
propeller must be avoided10, unless the requirements of
minimum rudder area require it. This layout generates Fig. 26. Tactical diameter improving using the Schilling
actical
effects of interference between the rudders when the ship is rudder. Source: Japan Hamworthy & Co.
.
turning, especially with compensate rudders The same
rudders.
way, the propeller flow is lost amid the rudders, which does
not take advantage of the turbulent flow from it.

Another design factor to consider is the balance ratio or
degree of compensation11, which is given in terms of the
coefficient block, equivalent to 0.8 for ARC Sejerí at full
load condition, so its range of balance ratio should be
r
between 0.265 to 0.27012. This concept refers to the r ratio
between the blade area ahead of the rudderstock over the
total blade rudder area, to facilitate turning of the vessel
(concept of maneuverability).

Once defined the dimensions of the rudder, the most
appropriate profile must be selected. In the Colombian
.
jungle, the available technology just allowed building flat
plate rudders, but in meeting and training with the welding
team, this process improved significantly with the Fig. 27. Lift coefficient comparison in turns going ahead
introduction of a recent technology rudder building the
building, and astern. Source: Japan Hamworthy & Co.
Schilling rudder.
Considering the advantages of the Schilling rudder andan
Just try to explain the features of this rudder, would
th in order to make a significant innovation in vessel
deserve another paper, but let us summarize the most maneuverability, the rudder was built according to this
important advantages. The rudder major innovation is the i profile. In the tests, the tactical diameter decreased from
stall angle is bigger than 35°, as is used in others, rising to
, four to just two lengths, as well as an excellent steering,
,
70° without loss of water flow, due to its cross section in
it maintaining a steady heading.
headi
the form of “fish-fin”. The manufacture, like the NACA, is
. NACA
in one piece, so do not require additional maintenance. It
has great stability in direction, which benefits fuel
consumption. The lift coefficient is also high when the
vessel going astern (concept of speed changing).
concept changing

The following graphs show the significant difference in
the tactical diameter as well as the higher lift coefficient of
a vessel using Schilling rudder, comparing with a
,
conventional NACA and a movable flap rudder.

10
Naval Architects and Marine Engineers, 2nd Revision, Vol. III, Ch. IX,
Section 17, Pg. 365, 1988.
11
PEREIRA Heber, “Teoría del Buque”, Timones: Teoría y sus efectos
evolutivos sobre el buque. Pg. 261, 1984.
12
Ibid 9.

13
The difference that can have some benefit of this non-
standardized procedure is the installation costs. One square
meter of certificated steel installed in a riverine support
vessel built by Cotecmar requires 37.5 kg of steel at a cost
of $ 40.000 pesos kilogram installed, for a total of $
1'500.000 pesos per square meter. On the other hand, the
square meter, with the replacement of sand by injected
polyurethane, worth $ 700.000 pesos.

However, this difference, slightly more than double,
offset certified security costs and the remarkable difference
benefit the stability of the vessel.

On the other hand, the steel with the sand assembly have
additional costs associated with lower load capacity and
increased fuel consumption which affects directly the
operating costs, thus offsetting the installation costs.

XIII. CONCLUSIONS

The vessel keeps a good initial stability instead of the
Fig. 28. 3D view of Schilling rudder and the built one for almost 18 tons added by the superstructure shielding,
ARC Sejerí, according corresponding model. revealed in the stability evaluation, according criteria
applied to the calculations, as was expected, considering is
high B/T ratio of 6.
XII. IDENTIFICATION OF NON-STANDARD
PRACTICES The engine installed is operating below its rated speed,
that cause carbonization of the combustion chambers, speed
Current shielding is made of a combination of three ¼” lost and higher maintenance costs.
naval steel plates with two layers of sand of 2 cm thickness
between the plates, with a composed specific weight of 222 The propeller installed aboard exhibits relatively good
Kg/m2. performance characteristics, however the optimal
recommended will reduce cavitation, improve efficiency,
Comparing this shielding with the ballistic steel used for and such, will reduce fuel consumption, extending the
the construction of riverine support vessels build by vessel range.
Cotecmar is nearly six times less than the weight of the
current assembly. This certificated steel is just one 3/16” The reduction of cavitation at an optimal level will
ballistic steel plate with 50 Rockwell C hardness and a prevent blade erosion, loss of thrust and generation of noise
specific weight of 37.5 Kg/m2. This means that if the and vibration in the hull.
calculated weight for shielding, as detailed in SWBS was
almost 18 tons, with the application of ballistic steel would The rudders of the vessel were inefficient due to its low
be only 3 tons. aspect ratio (0.43) and a low balance ratio (0.12), as well as
its wide distance from the propeller (1.5 times propeller
Moreover, the combination of three steel plates in diameter), which leads to design a rudder more efficient and
addition to two sand layers estimates that it could give better placed to improve the maneuverability and general
better protection than ballistic steel. However, Cotecmar performance of the vessel .
test is made by firing a rifle AK-47, 7.62 calibers at a
distance of 15 meters, with satisfactory results, while The type of shielding installed does not affected the
Riverine Brigade tests, with the same rifle and distance, the initial stability seriously, but has done some damage
bullet never reached the third plate, but did damage even in associated with an uncertified armor, less stability, less
the second one. This would weaken the structure and cargo capacity and higher fuel consumption.
allowing for the passage of moisture into the arena, gaining
more weight, and thus less stability. The lack of an appropriate methodology and an
investigative process in the project of vessel conversion
Today, the procedure have been improved using lead to a non optimal result, thus economic damage in
injected polyurethane instead of sand which has allowed operating costs, instead of at the beginning there is an
better results in the ballistic tests, as well as weight apparent saving in repairing costs.
reduction.

14
REFERENCES
[15] LEWIS Edward, “Principles of Naval Architecture”,
[1] American Bureau of Shipping. “Steel Vessels for The Society of Naval Architects and Marine
Service on Rivers and Intracoastal Waterways”. 2003. Engineers, 2nd Edition, 1988.

[2] ASTM. A 131/A 131M – 04. Standard Specification [16] MARTIN DOMINGUEZ Ricardo, “Cálculo de
for Structural Steel for Ships. Estructuras de Buques” Vol. I, Escuela Técnica
Superior de Ingenieros Navales, 1969.
[3] AVALLONE, Eugene A. y BAUMEISTER III,
Theodore. “Standard Handbook for Mechanical [17] Massachusetts Institute of Technology. “Lectures of
Engineers”. 10th Edition. 1997. Projects in Naval Ships Conversion Design”.

[4] CHRISTOPOULOS, R & LATORRE, R., “River [18] Mc Pherson, D.M., “Ten Commandments of Reliable
Towboat Hull and Propulsion”. SNAME. Great Lakes Speed Prediction”, Small Craft Resistance &
and Great Rivers Section, January 1982. Propulsion Symposium, May 1996.

[5] FAIRES, Virgil M. “Diseño de Elementos de [19] MIEZOSO FERNÁNDEZ Manuel, “Ecuación del
Máquinas”. Tabla AT 7: Propiedades típicas de desplazamiento, Peso en Rosca y Peso Muerto”,
materiales ferrosos forjados dulces, 2001. Escuela Técnica Superior de Ingenieros Navales,
1990.
[6] GHS Manual. Commands based on Navy stability
criteria. [20] NAVAL SEA SYSTEMS COMMAND. “Naval Ships’
Technical Manual. Chapter 096, Weights And
[7] HERREROS Miguel, ZAMORA Ricardo y PÉREZ Stability”, August 1996.
Luis, “El fenómeno squat en áreas de profundidad [21] PEREIRA Heber, “Teoría del Buque”, Timones:
variable y limitada”. XXXVI Sesiones técnicas de Teoría y sus efectos evolutivos sobre el buque, 1984.
Ingeniería Naval. ETSIN Universidad Polítécnica de
Madrid, 2000. [22] SAUNDERS Harold E., ”Hydrodynamics in ship
design”, The Society of Naval Architects and Marine
[8] HERREROS Miguel y SOUTO Antonio, “La Engineers, 4th Edition, 1985.
influencia de los fenómenos "wake wash" y "squat" en
el diseño de buques rápidos: límites aceptables y [23] STRAUBINGER. Erwin; CURRAN, William;
métodos de predicción”. XXXVII Sesiones técnicas de FIGHERA, Vincent. Fundamentals of Naval Surface
Ingeniería Naval. ETSIN Universidad Polítécnica de Ship Weight Estimating. En: Naval Engineers Journal.
Madrid, 2001. Mayo 1983.

[9] HOFMAN Milan y KOZARSKI Vladan. [24] Toutant, W.T., “Mathematical Performance Models for
“SHALLOW WATER RESISTANCE CHARTS FOR River Tows”, SNAME. Great Lakes and Great Rivers
PRELIMINARY VESSEL DESIGN”. International Section, January 1982.
Shipbuilding Progress. Volume 47, Number 449,
2000. [25] United States Coast Guard. 46CFR. Subdivision &
Stability. Part 170. Subpart E – Weather Criteria.

[10] HYDROCOMP. NAVCAD Manual. 2001. [26] United States Navy, NAVAL SEA SYSTEMS
COMMAND Design Data Sheet 079, 2003., Stability
[11] HYDROCOMP. “Propulsor Data Form”. 2001 for surface ships of US Navy.
HydroComp, Inc.

[12] IGLESIAS, Santiago, LÓPEZ, Pablo y MELÓN,
Enrique. El timón Schilling, mejora relevante en la
maniobrabilidad de un buque. Escuela Técnica
Superior de Náutica y Máquinas de Coruña. Revista
Marina Civil.

[13] Instituto de Hidrología, Meteorología y Estudios
Ambientales. “Manual de ríos Navegables”, 1992.

[14] LANDSBURG Alexander, “Design Workbook on
Vessel Maneuverability”, The Society of Naval
Architects and Marine Engineers.

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