Contra-rotating propellers and containers

07 Aug 2017

Our predecessors, writing in the September 1967 issue of The Motor Ship, were considering the dilemma facing ship designers following the demand for higher speeds, which in turn had led to a horsepower race.

They felt that technology of the time was approaching its practical limits, particularly in terms of the power that could be squeezed out of a single engine and in propeller design. Therefore, they believed, the industry would turn to twin screw power units, which would allow both higher ship power and smaller diameter propellers – the latter being necessary due to draught restrictions. The move towards containerised freight would lead to still higher speeds, and thus greater power requirements.

One solution to the propeller problem that seemed to be emerging was the adoption of contra-rotating propellers. It was known that the concept was being seriously studied in Britain and Sweden, but the marine engineering world had been taken by surprise with the announcement of three large barge-carrying ships to be built in the US, which would have CRP propulsion. This was thought to indicate that US shipping, where the steam turbine and other so-called outdated technology still reigned supreme, for once was ahead of Europe. The ships were expected to employ two 18-cylinder Fairbanks Morse engines, for a total power of 36,000 bhp.

The British approach to CRP technology was outlined in a technical article submitted by Stone Manganese and Newcastle University. The paper examined design considerations for CRPs turning at equal and unequal rotational speeds, powering a high-speed, restricted draught cargo ship of 32,000 bhp. Despite the added complication in gearing and transmission, and the slightly higher shaft losses, the CRP arrangement offered a “substantial” (about 5%) gain in propulsion efficiency over an equivalent conventional single propeller arrangement.

Ship design was becoming ever more sophisticated. Despite the growing popularity of containerisation – which had spawned a three-page news section on containers and unitised cargo handling – more efficient general cargo ships were still being developed. One such vessel was the first of new 7,500 dwt ships built at St Nazaire for the French CGT company. Designed primarily for Transatlantic service between Le Havre and New York, but also able to navigate the St Lawrence Seaway and Panama Canal, the 149.5m ships’ capacity of 15,000m3 of cargo was considered to be high for the deadweight. Maximum cargo access to the five holds, with a comprehensive outfit of cargo handling equipment, was an important design aspect. The large hatch openings, equipped with remotely-controlled sliding flush covers, dictated the use of high-strength steel. Indeed, all the major items of cargo equipment, as well as the main and auxiliary machinery, were able to be controlled directly from the bridge. The two SEMT-Pielstick medium speed engines, rated for a total of 14,180bhp, were connected via a reduction gear to a single Kamewa CP propeller. Comprehensive alarm and monitoring arrangements, including a 398-point data logger, qualified for BV unmanned operation class notation.

Although the fully-containerised cargo ship similar to those of today was still to make its appearance, a number of prominent shipowners, including Wallenius, CGT, Holland-America Line and Swedish American Line, had jointly formed the Atlantic Container Lines, or ACL, alliance. The British representative was the Cunard Steamship Co, which had ordered two new ships from Tyne Shipbuilders as part of the second phase of new ACL vessels – the other four being built in France. Although the first phase of four ships were all diesel-powered, the two Cunard vessels would employ twin-screw British-designed AEI steam turbine power. As well as containerised cargo, the ships were equipped for ro-ro and unitised shipments.