1.
Details of the Design Project
a.
Proposed Study Title
:
Design of hoisting and lowering
arrangement for SLNS Samudura to
accommodate newly acquired Rigid Hull
Inflatable Boat (RHIB).
b.
c.
Name of the Designer
:
HS Amarasekara
(1)
Institutional Affiliation:
Directorate of Naval Design, SLN
(2)
Telephone No
:
0715468436
(3)
Email
:
hasitha803@gmail.com
Name of Supervisor
:
Cmde (E) RVP Ekanayake
(1)
Institutional Affiliation:
Directorate of Naval Design, SLN
(2)
Telephone No
:
0718320688
(3)
Email
:
rvpekanayake@gmail.com
1
2.
Introduction
a.
Background.
SLNS Samudura is an Offshore Patrol Vessel (OPV) of the
Sri Lanka Navy (SLN), primarily tasked with conducting deep-sea operations to
safeguard the country’s territorial integrity. A key challenge for the vessel is preventing
illegal drug and human trafficking, which requires the deployment of high-speed,
reliable small boats for independent operations in high seas. Recently, SLNS Samudura
replaced its slow-speed Motor Surf Boat (MSB) with a Rigid Hull Inflatable Boat
(RHIB) to enhance operational capabilities. Consequently, a new requirement has
emerged to design or modify the existing boat hoisting and lowering mechanism,
originally designed for the MSB, to accommodate the RHIB effectively.
b.
Design Problem.
The existing hoisting and lowering arrangement for the
Motor Surf Boat (MSB) is achieved using a two-point davit system, consisting of two
independently mounted davit arms connected to Fwd and Aft sections of the boat via
wire ropes. However, due to dimensional differences between the MSB and the newly
acquired RHIB, the existing two-point davit system is incompatible with the RHIB.
Therefore, a new hoisting and lowering arrangement to be designed to accommodate
the RHIB while ensuring compliance with prevailing safety standards and operational
efficiency.
c.
Rationale.
The need for a new boat hoisting and lowering system arises
from the incompatibility of the existing two-point davit with the newly acquired RHIB.
This necessity is driven by several key factors, primarily the dimensional and structural
differences between the RHIB and the previously used craft. These differences include
variations in principal dimensions, weight and the hull shape as indicated below.
Sr
No
1
Characteristics
2
Maximum beam
2.41 m
2.27 m
3
Maximum draught
0.56 m
0.49 m
4
Weight
2268 kg
930 kg
5
Hull shape
Displacement Hull
Planning Hull
Overall length
Existing boat
(MSB)
7.82 m
2
Newly acquired boat
(RHIB)
5.72 m
The existing two-point davit was specifically designed for the MSB, with two wire
ropes secured to the lifting points at the forward and aft sections of the craft. However,
these cables cannot be used for the RHIB due to its shorter overall length, which may
lead to instability, excessive stress on the attachment points, and unsafe launching or
retrieval operations. Furthermore, the existing two-point davit is overcapacity for the
RHIB, given the significant load (weight) difference (930 kg for the RHIB compared
to 2268 kg for the MSB). According to Stastny and Motycka (2019), the load is a
decisive factor for the design and selection of a lifting device.
d.
Policy context.
The policies governing boat hoisting and lowering
arrangements for ships are primarily based on classification society rules. In this design,
the Lloyd's Register Rules and Regulations (July 2018); the Code for Lifting Appliances
in a Marine Environment (Chapter 3, Section 1) will be incorporated.
3.
Objectives
a.
The primary objective of this design is to provide a technically and
economically viable arrangement for hoisting and lowering the newly acquired RHIB
onboard SLNS Samudura, in compliance with classification society rules.
b.
The specific objectives of the design are as follows.
(1)
To identify the optimum load applied to the davit for hoisting/lowering
the new RHIB at a single point by analysing forces.
(2)
To calculate and verify the design of the pedestal base, sheave, selection
of wire rope, load hook, blocks and shackles.
(3)
To select appropriate materials for the davit arm, base, and other
components, and identify the most suitable fabrication methods.
(4)
4.
To verify the design using computer simulations.
Proposed Design Methods
a.
Methodology.
The ‘Weighted Scoring Method’ is chosen to select the best
option from the available alternatives for the designer. Based on the analysis, the design
of the ‘Slewing Davit’ is identified as the most technically and economically viable
option. The relevant analysis is provided as an enclosure. The methodology involved
includes the following steps as highlighted by Kulkarni (1997).
3
(1)
Defining the Design and Regulatory requirements – Includes load
capacity, operational parameters, SWL and relevant maritime regulations.
(2)
Structural Design of main components – Covers the davit arm, pedestal
base, slewing mechanism, sheaves, wire rope, and other critical components.
(3)
Material selection – Choosing marine-grade materials to ensure
durability and corrosion resistance.
(4)
Verification of manual calculation of the design computer simulations
tools to optimize performance, strength, and safety.
b.
Design Problems to be answered.
When designing a slewing davit,
following design problems must be addressed be the designer.
c.
(1)
Load and Structural Integrity.
(2)
Slewing and Hoisting Mechanism.
(3)
Material Selection and Corrosion Resistance.
(4)
Safety and Regulatory Compliance.
Expected results of the design.
A well-designed slewing davit should
guarantee structural integrity while maximizing the use of existing materials to enhance
efficiency and cost-effectiveness. The hoisting and lowering system must operate
smoothly, ensuring safe and controlled movement of loads while complying with
maritime regulations. Importantly, the davit should not compromise the ship’s stability.
5.
Timeline
Sr
No
a
b
c
d
e
f
Activity
01
Literature survey
Identify and define design
requirements
Structural design of components
Material selection
Verification of the design using
computer simulations
Report writing
4
Duration (Months)
02
03
04
05
06
6.
References
a.
Kulkarni, S.G. (1997), Machine Design, Tata McGraw-Hill, New Delhi.
b.
Stastny, J. & Motycka, V. (2019), Design Optimization of Lifting
Mechanisms, IOP Conference Series: Materials Science and Engineering, 471
062032.
c.
Moss, D. (2004), Pressure Vessel Design Manual, 3rd Ed, Gulf Professional
Publishing, USA
5
ENCLOSURE
USE OF THE WEIGHTED AVERAGE METHOD
The following alternatives were identified by the designer as potential arrangements for
hoisting/lowering of the RHIB.
Option 1 -
Modify the existing two-point davit, utilizing both davit arms.
Option 2 -
Use only one arm of the existing two-point davit with modifications.
Option 3 -
Modify one arm of the existing two-point davit to function as a slewing
davit.
Option 4 -
Design a new slewing davit using the material from the existing twopoint davit.
The weights were allocated to above option as indicated below.
1.
2.
3.
4.
5.
6.
Cost involvement
Ease of implementation
Safety
Operational efficiency
Compliance with standards
Time to implement
-
0.25
0.20
0.20
0.15
0.10
0.10
For each alternative above, ratings were given based on the designer's perspective on a scale of
1 to 5 to assess how well each criterion is satisfied, where higher values indicate better
performance. Accordingly, the following table was formulated.
Criterion
Weight
Cost involvement
0.25
Ease of implementation
0.20
Safety
0.20
Operational efficiency
0.15
Compliance with standards
0.10
Time to implement
0.10
Total Weighted Score
Option 1 Option 2
4 (1.00)
4 (1.00)
3 (0.60)
2 (0.40)
1 (0.20)
3 (0.60)
1 (0.15)
2 (0.30)
1 (0.20)
3 (0.30)
3 (0.30)
4 (0.40)
2.45
3.0
Option 3
3 (0.75)
3 (0.60)
2 (0.40)
4 (0.60)
4 (0.40)
3 (0.30)
3.05
Option 4
2 (0.50)
2 (0.40)
5 (1.00)
5 (0.75)
5 (0.50)
4 (0.40)
3.55
Table: Evaluation of davit alternatives using the weighted average method
This analysis highlights that Option 4 (designing a new slewing davit using the material from
the existing two-point davit) scored 3.55, indicating its viability. It ranks higher than the other
alternatives in terms of safety, operational efficiency, and compliance with standards.
6