NPT Propeller delivers impressive results

MARCH 25, 2013 — The latest Grontmij/Schmidt Maritime Seahorse 35000 design bulk carrier, built by China’s Yangzhou Guoyu Shipbuilding Co. for Nordic Hamburg, recently completed sea trials in which it exceeded all the performance requirements built into its contract.

Powered by a Wärtsilä engine developing 6,350 kW at 99 rev/min, the ship is fitted with a Becker Mewis duct and a 5.9 m diameter Stone Marine Propulsion NPT propeller.

The Mewis duct sits in front of the propeller to straighten and accelerate the flow into the propeller, while an integrated fin system reduces rotational losses. The NPT (new profile technology) propeller, incorporates cavitation friendly section profiles to reduce the blade surface area and increase propeller efficiency. Additional benefits of the NPT design include reduced propeller weight and inertia.

The Seahorse 35000 is a 180.0 m LOA, 30 m beam handysize shallow draft bulk carrier with a dwt of 35,000 t on a scantling draft of 10.1 m.

The contract for this latest Yangzhou Guoyu newbuild originally required 14 knots at 5,700 kW in 10.1 m draft conditions. With a Becker Mewis duct fitted, model tests, conducted at FORCE Denmark and SVA Potsdam Germany, indicated that the power required would be reduced to 5,440 kW.

An analysis of the sea trials results shows that the actual power demand to achieve 14 knots was only 5,252 kW, 3.5 percent better than the model tests showed and an impressive 8 percent better than the contract power requirement.

The number of vessels fitted with Stone Marine Propulsion’s NPT propeller is growing rapidly as more naval architects and ship owners appreciate the benefits of fitting the propeller, not just to vessels like the Seahorse 35000 where the engine and propeller have been chosen to provide optimum performance but also to other vessels adopting a slow steaming policy where significant improvements can still be obtained.

Generally, an NPT costs no more than a conventional propeller but brings with it lower weight and inertia and delivers its considerable power with reduced noise and vibration.

Nordic Hamburg now has three more ships of this class on order, all of which are being fitted with NPT propellers and, in total, NPT’s have been supplied, or are on order, for 20 ships in the Seahorse 35000 class.