Ship Design Efficiencies

Posted on by

GCaptain has published a nice two part presentation by Wartsila discussing ways to make ship propulsion more efficient. The target audience is merchant ships, but much of it is applicable to Cutters.

Makes me more convinced than ever that, considering life-cycle costs, an integrated propulsion and ship service electrical system, like that already being used successfully on the Lewis and Clark class T-AKEs (and planned for the DDG-1000), combined with Azipod propulsion should be seriously considered for the Offshore Patrol Cutter.

14 thoughts on “Ship Design Efficiencies”

  1. i may be wrong but isn’t the nav either installing or considering installing this system in one of it’s amphib types? thought i read something awhile back in proceedings.

    Reply ↓

  2. One of their big deck amphibs is installing a “Hybrid” power plant. It will have electric motors that can be geared to the shafts for loitering and will use gas turbines geared to the shafts to provide high speed.

    In an integrated system the same engines that supply the main propulsion also supply the ship service electricity. The system the US is using for the T-AKEs and the DDG-1000 comes from Converteam, a division of GE.

    http://en.wikipedia.org/wiki/Converteam

    It is also being used on Britain’s new Type 45 Daring class destroyers and their new aircraft carriers. France uses the system on their Mistral Class LHDs which are also being sold to the Russians.

    Potentially the OPC might have only four diesel engines on board, but yet have ship service electricity and ship propulsion supplied from three different compartments. Putting an emergency generator forward capable of supplying ship’s service power and driving a drop down trainable azipod in bow; two primary generators in the main machinery space capable of providing both ships service and full speed power to the main prop motor aft which might be two or even only one azipod; and a secondary generator in an auxiliary machinery space aft capable of providing both ship’s power and cruising power to the azipod(s) aft, the ship could minimize manning, have great redundancy, and have a very flexible and efficient propulsion system. During normal transits the ship might shut down the main machinery space all together and run exclusively on the generator in the aux space. My very rough estimate would be that the emergency generator forward would need 1500 to 2000 HP to give the ship about 10 knots on the forward azipod; the aft generator about 3,000 HP to allow a 14 knot cruise, and the two main generators about 10,000 HP each to allow a 20-22 knot high speed cruise on one engine and 25 knots on both.

    Reply ↓

  3. this kind of advanced d/e propulsion and azimuthing thrusters have been in the USN since the Pathfinder class of T-AGS were introduced in the early 1990s. It is NOTHING new to MSC just blue water officers~

    Reply ↓

  4. Chuck, I think you’ve fallen for the common misconception that electric drive is inherently more efficient. It ISN’T.

    There’s a cost to converting from mechanical to electrical energy (and back). Add in the cost, weight & volume of the extra equipment needed for this conversion (generator + switchboard + electrical motor) and the impact on OPC will be a non-trivial INCREASE in purchase & lifecycle costs. (Even though there’s a small offset from eliminating the reduction gear)

    That’s not to say that electric drive doesn’t sometimes enable other interesting things. But it’s these “other” things that are inherently valuable, not electric drive per se. For example, those “other” things could be:

    1. Eliminate engine (especially gas turbine) operation under partial load, during loitering (LHD-8)
    2. Move engines (especially gas turbines) high up or out of the way to reduce trunking & gain payload (CVF, cruise ships)
    3. Replace shaft lines with more hydrodynamic, space-efficient & maneuverable pods (European amphibs, oil rig support vessels)
    4. Use the huge increase in disposable electrical power for something (CVN-78, DDG-1000)
    5. Decouple engines from shafts to reduce noise (ASW frigates)

    The Pathfinder T-AGS ships benefit from all of the above. A simple OPV such as the OPC would benefit from none. At most, you might add a secondary low-power electric motor for low-speed loitering, but that’s quite different from full electric drive.

    As for Azipod/Azipull, they might bring long-run cost savings, but I note that no other OPV has gone down this route (due to the upfront cost premium?). They also don’t automatically require electric drive – you could use a diesel prime mover coupled to a Z-drive.

    Reply ↓

    • HK right you are. The Lipps Z-drives allowed for some specailized functions relating to hydrographic survey & tracking to include hull noise. The engines, SCR controls and Z-drives were called for from the start and were a very tight installation

      Reply ↓

    • There is now a third post in this series:
      http://gcaptain.com/part-marine-engineering-technology/?36541

      Looking at the section on propulsion (http://gcaptain.com/part-propel-efficient-ship/?36494) suggest that there are savings that exceed 6% that can come from the greater streamlining made possible by eliminating the propeller shafting and the attendant struts. Gains for pulling thrusters alone are estimated from 10 to 15%. An integrated system would also likely take up less space because of the elimination of shafting and the reduced number of engines. Requiring less volume, the ship could be smaller and lighter resulting in additional savings.

      Other benefits of going to integrated system would include lower manning and avoiding the necessity to run large main prop diesels at little more than idle to slow cruise or loiter. An integrated system could use the smallest (and consequently most efficient) generator that will supply the power needed.

      The simplest solution, two main prop diesels geared to twin props requires that engines frequently are run at inefficient low RPM and power settings. Unless the two engines are geared together, it may be necessary to trail a shaft, which also causes inefficiencies.

      Variable pitch props are only optimum for one speed, resulting in inefficiencies at other speeds, while electrically driven fixed pitch props are much more nearly optimum at all speeds.

      Reply ↓

  7. Another class using hybrid propulsion, UK’s Type 26 surface combatant (expected to be about 5,400 tons):

    http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_10_15_2012_p17-501122.xml&p=1

    “The new ship’s propulsion departs from RN practice. Defined as Codlog (combined diesel-electric or gas), it will comprise three elements: a set of four high-speed diesel generators; two propeller shafts with integrated electric motors; and a single gas turbine, most likely a Rolls-Royce MT30, geared to both shafts. The diesels—isolated from the hull and shafts to reduce noise transmission to the water—provide efficient and quiet propulsion at patrol speeds up to 18 kt. Diesel-electric drive also eliminates long power shafts and allows the installation to be more flexible.

    “The MT30, with its high power/weight ratio, can be installed aft in the hull and provides a sprint capability to 28 kt. Because the turbine is only used at or near full power, there is no need for a complex recuperative turbine system like the Type 45’s WR-21, which is needed for part-power economy.

    “Another factor that makes Codlog attractive is the steady improvement in diesel performance—including power from a given weight and volume. The diesels will most likely come from MTU, part-owned by Rolls-Royce. ”

    An earlier post on the Type 26 here: https://chuckhillscgblog.net/2012/08/26/innovative-features-in-britains-new-frigate/

    Reply ↓

Leave a comment