Shaft Turn Direction Choices

This originally was going to be posted as a response to:

http://www.boatdesign.net/forums/al...age-costa-concordia-cruise-ship-41313-53.html , but that topic spans many dozens of pages and this thread would be drowned out. Also, I didn't post it under Propulsion since the ships herein are combatants, gas-turbine powered, and twin-shaft ships, markedly different to liners, containerships, and others in size/volume/operating modes.

-------------

Hi all. This is a segue in that it is about my observations/thoughts on twin shafts that turn inboard or outboard on combatants, although two hull types may be similar.



Something that has always interested me for about 15 years is why the JMSDF Kongo's propellers turn differently than the smaller parent hull USS Arleigh Burke class.

http://www.fas.org/man/dod-101/navy/docs/swos/eng/PS9-101.html

The Kongo class has a more squared hull, and so I suppose that tank tests of various models (if not political/philosophical/aesthetic reasons were not the main things that) influenced the direction of the shaft turning. IIRC, the USN's Spruance, Kidd, and Ticonderoga class ASW, AAW, & cruiser hulls (respectively) used inboard turning.

Sometimes, the choice of turning direction may be influenced by the stern/transom shape, speed regimes, inflow and outflow wave patterns, noise and its effects on ASW operations, squat at various speeds in restricted waters, downward trip in open seas, maneuvering and self-mooring necessities, and local effects on nested/adjacently-moored vessels. Plus, erosion of the hull in the vicinity of the prop tips, the way cavitation plays into the erosion/noise/speed/etc calculus may affect choices.

I think the Kongo/Atago hulls (amidships and transom area) have some width more of a bottom-curved "D" or "U" shape while the Burke hulls appear to be more of a "C" or half- "O" shape. I guess the distance from the onset of curvature, the amount of cut up, and the props/rudders interaction also affects things and decisions. Also, the spacing of the propellers is a bit different, as is the spacing of the rudders. Generally, the rudders are more inboard for effectiveness in controlling the amount of water flow between the blades, but also to allow the shafts to be removed (usually in multiple pieces to keep their sheer mass from warping them, even though the shafts are periodically "turned" to prevent/combat warping) without needing to additionally and often unnecessarily remove the rudders.

As for merchants, reliable engines obviate the need for twin shafts and also cuts costs, thought "take home" capability is desired if affordable. As for liners and time-sensitive ships needing to maintain headway or steerage more than station-keeping, twin shafts or even twin skegged shafts would be employed if pods are not. Skegs can contribute.

As for the USN's Burke hull vs the Spruance/Kidd/Ticonderoga hulls, they are different in beam, length, and wetted surface areas as well as the exit points for the propeller shafts and the shape from bow and sonar to the cut-up and transom. I forgot the exact reasons the USN spatially flipped the Burke plant relative to the earlier gas turbine ships' plants, but I think it had to do with underwater form and water flow due to the change in length and girth, and maybe due to stability as well as reactions related to using one shaft vs two shafts at different speeds and drafts... but, I'm just guessing.

But, since the Atago/Kongo hulls have a longer underwater length, different mass distributions, and greater displacements at comparable drafts, I wonder how that plays into the choice of inboard-vs-outboard props. Cost- and design-wise, it's just flipping or swapping the location of the shafts athwartship and shortening the shaft of one to put one ahead of/abaft of the opposite shaft, since, for survival reasons, the twin gas turbine engines are each hydraulically mated to a separate reduction gear, which is many dozens of tons in mass alone, all canted downward at a few degrees. This angle, I understand, is to allow easy removal of the shafts during overhauls and groundings, whereas many civilian ships may have engines sited so far aft that they can be designed with very short shafts and therefore have horizontal or nearly horizontal shafts straight out the stern, along the centerline. But, more than ease of removal, the engines can only be so low in the hull and allow for shock and grounding protection while not wasting usable deck area in the vicinity of the engine compartments, given the relatively shallow depth of the hull (from gunwhale to keel).


As for shafts vs pods, the pods themselves are somewhat dense, and highly localized in density considering the solidness needed, the mounting locations/isolation, and the need to rotate axially. Shaft-mounted props have a relatively huger amount of axial momentum and other forces in play. Also, during turning, even at the same ordered speed, the inboard prop can and tends to lose some effectiveness unless the gas turbine prop-driven ship's engine controls computer compensates. (I cannot recall whether this remains true whether or not the twin rudders are used in the turning...).

Anyone wish to correct or add to this, from a design decision-making perspective?

Cheers!

 

pods are completely different in that they can be angled to place the propeller shaft perpinidcular to the inflow by angling the pod whereas a normal shaftline vessel has the shaft angled downward and possibly outward due to interior arrangements. For inclined shafts there may be an advantage in efficiency to rotation in the indb vs outbd due to the generally upward and inward flow at the stern, this can be viewed as a swirl that can be taken advantage of. In the case of naval vessels and possibly some very fast commercial ships, avoidance of cavitation or delay of inception may be more important.

 

I don't know in these larger vessels but in smaller rotation has to do a lot with slow speed maneuvering and single engine performance. Might also have to do with rudder placement. The rudder placement itself has something to do with water flowing around these big props. I am interested in this myself because the drive line of the Burke class is very similar to my boat, just a lot bigger.