Ship-deployed boats are essential to the success of many seagoing activities such as rescue, security, inspection, piloting, and coast guard and military operations. In recent years, boat deployment systems have evolved beyond the traditional side-davit systems with dual falls, and now include systems that deploy Rigid Hull Inflatable Boats (RHIB) via stern ramps that are designed as integral parts of the transom of the mother ship.

MARINTEK has developed model test procedures, numerical prediction tools and criteria for evaluating the detailed design and operability of stern ramp deployment systems. Active operator systems (AOG) are also being developed.

Model tests, using a propelled mother ship and radio-controlled water-jet driven RHIB with an easily modified stern ramp configuration in combination with calibrated prediction methods, have provided a useful basis for design and operability studies.

Recent Projects and Developments

Comprehensive stern ramp design and operability studies have recently been performed for two Norwegian offshore Field Support Vessels (90 m FSV), US Coast Guard Security Cutter type ships and for a novel type of naval vessel.

The general objectives of these studies included:

model tests and analysis to determine the capability of the stern ramp for small boat deployment and recovery under a variety of wave, ship speed and heading conditions.
model tests to observe and quantify effects on system operability by application and optimising of detailed design features.

Field support vessel: Tethered stern ramp recovery of lifeboat.

Assessment of Stern Ramp Performance

The results of the model tests and full-scale studies clearly show that the operability and limitations of stern-ramp recovery systems depend on a combination of physical and human factors.

For the purpose of performance evaluation of stern-ramp boat deployment, we developed three sets of criteria:

mother ship motion and relative wave motion in the stern ramp sill
small boat performance in the approach and entry phases
levels of acceptable induced impact acceleration for the boat’s occupants

Coast Guard Cutter: Free speed recovery of water-jet driven RHIB.

Large Model Operability and Design Tests

US Coast Guard confirms that results obtained from model tests on a large 12 m (1 : 12.5 scale) model have proved that the MARINTEK test procedures can be employed to quantify stern ramp system operability and optimise the detailed design of stern ramps.

Model tests, especially with large-scale models, are regarded as the most effective tool for optimising stern ramp design. Model tests enable us to model both the complex hydromechanics in and around the stern ramp during RHIB deployment and retrieval, and the induced motions and loads on the mother ship and RHIB. A series of model tests that systematically vary test and stern ramp parameters has proved to may thus be the most effective and simplest way of evaluating operational effectiveness and selecting the optimal configuration of the stern ramp.

Typical parameters that can be evaluated during model tests are the limiting sea state, ship heading and speed, size of the RHIB and main particulars of the mother ship. Additional systems attached to the stern ramp, such as a water-motion control system, boat capture mechanisms, etc., and their effects on the operability of the stern ramp, can also be easily verified by the large model test.

Analysis of Operability

From the hydrodynamic point of view the process of deploying a small boat from the stern of a larger ship can be divided into different regimes; wave-induced motions of the mother ship and small boat, characteristics of the ship’s stern wake and propeller wash and local effects within the stern ramp.

MARINTEK’s Vessel Response (VERES) Program is utilised to investigate the operability of the stern ramp on various ship designs in typical ocean and coastal environments. VERES has been calibrated against a wide-ranging series of specific stern-ramp model tests and sets of criteria.

Contact at MARINTEK: Per Werenskiold

(Article in MARINTEK Review No 2-2003)

Published January 26, 2005