Page 102 - Practical Design Ships and Floating Structures

2.5 Criteria
A) Significant vertical motion amplitude of the echosounder/sonar < 1.0 m for heave compensation
B) Allowable roll angle of echosounder/sonar and for launching and retrieving RIB .e 5O
C) Criteria for security / operability of the crew
Significant pitch amplitude <3.5"
At the location of the RIB I sloop, measurement analysis work spaces, deck spaces for launch and
retrieval of buoys and the bridge:
Significant amplitude vertical accelerations < 2 m/s2
Significant amplitude of lateral accelerations 4.5 m/s2 (on the bridge < 2 m/s2)
D) General criteria:
Green water over the bow < 30 timeshour, on the aft deck < 1 timehour
Slamming < 20 timeshour

3 DESKSTUDY

Extensive desk studies have been conducted in the early phase of the project. The first analysis
comprised a feasibility study of the application of an existing conventional, single-screw, single-rudder
ship for the purpcse. Compliance with the staffrequirements was verified for this ship. The desk study
showed that the existing ship could be adapted in such a way that all requirements were met.
The second, more elaborate, desk study was conducted to determine the operability of the ship and
which propulsion and steering arrangements could be applied. Pods and thrusters prove to be
promising for application to various ships and therefore the idea arose that those concepts might be
advantageous. The propulsion and steering arrangements comprised the following concepts:
Single-screw single-flap rudder
Single pod
Twin screw/twin rudders
Twinpods
Single propeller with wing thrusters arrangements.
3.1 Seakeeping

The main dimensions of the hull form resulted from operability requirements, the required deck width
and stability requirements. Operability is defined here as the proportion of time the ship is able to
successfully accomplish its missions for given combinations of sea area, speeds, and headings, see
Lloyd (1 989) and NATO STANAG 41 54. The complement of operability is referred to as downtime.
For the above mentioned ship motion criteria and wave statistics the downtime in bow quartering and
head waves is governed by the vertical motion limits of echosounder and sonar. For beam and stem
quartering waves, the vertical motions continue to affect the downtime together with lateral
acceleration limits. The limiting roll angle affects the downtime in stem quartering waves as well.
In a hull form variation study, length, displacement, C, and the draught were varied to investigate their
effects on operability. Also, a pram shaped aft hullform was evaluated and applied because of its
beneficial effect on vertical motions (Blok and Beukelman (1984) and Kapsenberg and Brouwer
(1998)). Finally, the study resulted in the following dimensions and coefficients for further model
testing.
TABLE 1

LPP 75 m A 1850 t cb 0.47
B 12.8 m LCB 36.02 m C, 0.80
LT 4m CWI 0.80
The calculations also showed the importance of effective anti roll tanks (ART) for beam and stem