Slow Sulzer for fast vessels

The latest design for a low-speed engine aimed at smaller, but relatively fast, ships has led to wide consultation.

Wartsila NSD has added the RTA60C to its low-speed engine range. The 60cm bore engine comes in five to eight cylinder version spanning the range 8,250 —18,800kW (11,200 — 25,800 bhp) and is aimed at smaller, faster types of vessels like container feeder ships, car carriers, roros and reefers. With a stroke of 2,250mm, the engine has a speed range of 91-114 rev/min and a mean piston speed of 8.55 m/sec. Maximum cylinder pressure is 155 bar with a mep of 19.5 bar to produce 2,360 kW/cylinder (3,210 bhp/cyl). Fuel consumption at full load is put at 170—164 g/kWh.

In selecting a 60cm bore Wartsila NSD is in direct competition with MAN B&W’s product range instead of choosing bore sizes between those offered by the market leader. Power output of the RTA60C is slightly ahead of the S60MC-C, as is the speed (79-114 against 91-105 rev/min). The lower power L60MC has a speed range of 92-123.

Kaspar Aeberli, vice president of technology for Wartsila NSD, says the company had initially decided on a different speed range but changed its mind following consultation with shipowners and ship builders. While pure cost and manufacturing considerations would have dictated a speed of 125-130 rev/min, one study showed the ideal shaft speed for a containership of 1,500—2,000 TEU was 109 rev/min.

However, the RTA60C is not just a different sized version of the existing engine range. It has a reduced height and is said to be the shortest and lightest engine for the power range, with the 18.9MW 8RTA60C weighing 415t (about 50t lighter than an 8S60MC-C) and measuring 9.7m in length. To accommodate the low deck height requirements of the target vessels, the distance from the shaft centreline to the top of the engine has been kept to 8.52m. A hook height of 10.4m is sufficient to withdraw the liners and special tooling can be used to keep the piston drawing height even lower.

While the new engine retains the RTA-series box-type columns and fully bore-cooled combustion chamber components, input from licensees has produced a design that is more economical to manufacture, says Mr Aeberli. Self-supporting cylinder liners are used – allowing the liner support ring to be deleted. In common with medium-speed practice, the RTA60 uses a four stud head (instead of eight) leading to a more rigid cylinder cover and improved uniformity of clamping. This type of head and attachment will be extended across the RTA range.