The idea was actually born in the in the ice-strewn reaches of the Great Lakes during the late 19th century when the car ferry ‘St. Ignace’ was built with bow propellers to enable it to move more effectively through ice-covered waters. With the advent of modern podded propulsion units, or azimuthing thrusters, ships have now been developed to operate effectively both in a bow-first configuration in open waters, as well as in a stern-first “icebreaking” mode.
Concept and Background
- by Lloyd’s Register’s Robert Hindley and Robert Tustin:
Bow propellers have been installed on ships operating in ice since the 1880s with the following practical effects observed:
- Encourages the ice to break ahead of the ship’s bow
- Ability of propeller to further break-up ice into smaller pieces
- Washing of ice pieces away from the ship’s hull
- Improved maneuverability of the ship in the ice
- Assistance for freeing of the ship in trapped conditions
The practical effects of bow propellers in ice can also be achieved with podded propulsion units, or azimuthing thrusters which, when located at the stern, can be rotated to pull the ship ‘stern first‘ with the additional benefits of directional thrust for maneuvering in ice and flushing of ridges.
Ice interaction with propeller and hull operating stern first
Beneficial effects of stern first operation from propeller and hull ice interactions are shown above and described below:
- Ice sheet bending and breaking – the water flow into the pulling propeller causes a pressure drop under the ice sheet ahead of the stern; which reduces buoyant up thrust on the ice sheet and promotes ice breaking by bending.
- Flushing of ice ridges – by azimuthing the pod adjacent to the keel of an ice ridge the variable flow of water promotes the erosion, or flushing, of the ice ridge keel.
- Lower frictional resistance in ice – with the propeller pulling, the water flow from the propeller washes along the length of the hull. The water flow reduces the friction between the hull and the ice by lubrication.
You may wonder, so does this present a curious rules of the road situation while at night?
Not to worry, “Stern First Ice Class Ships” will have mast head, side, and stern lights for both bow first and stern first operations.
Lloyd’s Register announced today that the first dedicated set of rules for stern-first ice-class ships has just been published, answering the demand for technical support as industry continues to explore the potential of polar transportation routes and the new energy reserves in the far north.
“These practical rules are answering a growing demand in the market and include the use of standard operational scenarios to provide designers with a basis for prescriptive rule applications that have been validated with designers and operators of these specialist ships,” said Robert Tustin, Lloyd’s Register’s Technical Manager for New Construction in Asia.
Lloyd’s Register has had a long involvement in the development of this class of ships. Mastera and Tempera – two 106,000 dwt “double-acting” tankers owned by Neste — were built to its class in 2002 and 2003 at Sumitomo’s yard in Japan. The ships were deployed to the Baltic, where they often operate stern first in heavy ice conditions, independent from icebreakers.
Other tankers, such as Sovcomflot’s Mikhael Ulyanov and Kiril Lavrov, were designed and built for operating stern first in ice in the Arctic. These ships, dual classed by Lloyd’s Register and the Russian Maritime Register of Shipping, were designed to eventually shuttle crude from the Prirazlomnoye platform in the Pechora Sea to a floating storage and offloading unit moored off Murmansk.
The development of Lloyd’s Register’s new rules were supported and validated by leading ice-class tanker designers, key regulators and operators.
The interpretation of regulatory and other rule requirements — and validation of strength levels for the hull and propulsion units — were confirmed by a review of the current fleet of double-acting ships, ensuring that practical experience supported the rules’ development.
They offer the following key interpretations:
§ The ship is considered as a bow-first and stern-first vessel for application of ice-class rule requirements for hull and machinery
§ It is also considered as a stern-first ship for the application of navigation-related rules and regulations
§ In other cases, the rule applies to bow-first ships only
The rules also include a framework for alternative-load scenarios when unusual operations are envisaged, as well as interpretations of international regulations and classification rules based upon industry precedents, said Tustin.