Question: What is the net gain, if any, for inboard turning wheels (twin screw vessel), versus outboard turning wheels when calculating bollard pull of a vessel with Kort Nozzles? Not looking for scientific answers - just in layman’s terms. I always worked under the assumption that inboard turning produced more pulling power and outboard turning traded off pull for better handling.
[QUOTE=anchorman;30569]Question: What is the net gain, if any, for inboard turning wheels (twin screw vessel), versus outboard turning wheels when calculating bollard pull of a vessel with Kort Nozzles? Not looking for scientific answers - just in layman’s terms. I always worked under the assumption that inboard turning produced more pulling power and outboard turning traded off pull for better handling.[/QUOTE]
I have not read any literature on the difference. However, as far as maneuvering, most twin screw vessel fixed bladed (non-nozzled) turn outboard. When going astern on port only, vessel backs to starboard and vice vera. With nozzles or tunnels the sideforce is, for all practical purposes, negated by the tunnel. Depending on the propeller tip clearance, tip vortex and side force is mostly converted into direct thrust parallel with the tunnel or “straight out the back” when going ahead with little or no side force. With that said, whether using CP or fixed propellers, it does not seem like maneuvering or bollard pull would be appreciably discerned either way. A company I worked for has a 630 ft. bulk carrier with twin screw Kort Nozzles with 4 flanking rudders (actually 8). The vessel was specially design to work the Cuyahoga River in Cleveland, Oh without the use of tugs. It is VERY maneuverable and tugs are NOT needed. It is a one of a kind vessel. What brought you to the assumption that inboard turning wheels produced more bollard pull? Just curious.
I was under the impression that the kort nozzles eliminated or nullified the difference by taking loss due to side force and/or “walking” out of the picture.
Generally speaking: And this varies with each particular vessel, since some have quirks and vary from the norm. And this applies only to tugs, since they have larger rudders than traditional vessels.
It has been my experience that inboard turning wheels have better thrust ahead but aren’t as good backing. They don’t twist as well. However they walk like crazy.
Outboard turning wheels back like crazy, and turn better going ahead. But they don’t walk at all. (Some times you can overpower the backing wheel and sort of crab sideways ahead though.)
The conventional thought is to use your outboard wheel when turning to ‘help’ you through a turn. Sort of like having the ‘lever’ further outboard to help the vessel turn.
The evidence on this is the outboard turning wheels allow you to use the outside wheel (E.G. port wheel when turning to stbd) so the traverse thrust of the wheel will help ‘walk’ through the turn. On an inboard turning vessel when turning to stb (previous instance) using the stb wheel is sometimes better to turn with.
To check this out, in either type vessel, when going ahead back one wheel full, just so the wash is coming out the side. Go to each side quickly and watch the results. On an outboard turning wheel all the wash will come out on the side you are backing on (sometimes at about 45o angle to the hull. On an inboard wheel some of the wash will actually cross over to the other side of the boat.
This is why some boats will back well when put astern, and some will immediately start to twist as you back.
Then other mention that k-nozzles negate the effect is not true. K-nozzles may reduce the effect, but it is still there.
My experience on this is twofold. I worked on a triple screw tug. port and center right turning (inboard), stbd left turning (inboard.) Backing both port and center screws the boat would go straight back, back the stbd would pull the boat to stbd. Backing just the port would pull to port.
Also at another company they had a k-nozzle boat. While re-engining the resident rocket scientist in the office ‘heard’ that since it had nozzles it didn’t matter which rotation the props had. So he ordered three (port, stbd, and spare) right wheels. The boat was a death trap. On her first job with a loaded barge the operator took out a dock, doing an easy, often done right turn out of a berth. Didn’t twin screw for crap! When he backed it backed to stbd, pulled the bow further into the dock too! They lifted the boat out the next morning and had a left wheel put on ASAP. Problem solved!
Your other question about bollard pull is negligible either way, however it is the K nozzle that provides the great increase in pull. Maneuverability is sacrificed by K nozzles though. That is the trade off, nozzles = more bollard pull, but open wheel are more maneuverable.
Anyway, I hope I haven’t further clouded the issue, but I am sure there are others with different opinions, and I am sure some who aren’t even aware of these small differences. Sorry for the long ramble.
Notice how the rudders are hung. If you have offset rudders and your combined thrust (inboard) is channeled more between the rudders and dead astern, it seems that you would have most effective bollard pull. combining wheelwash (inboard) just makes sense for minmizzing friction loss and drag. definitely not scientific. Net Gain may be calculated from difference in drag coefficient, given water flow outboard of the rudders is different with IB vs OB.