Whats the difference with having inward turning screws?
Well, at least with a smaller, under 165’ vessel, if the screws are reasonably matched to the rudders, the vessel will walk like a sonfabitch. In pre DP days, this ability was critical for holding station in the offshore oilfield environment. For one thing. 
1 Like
You have to know if the transverse thrust from the prop running astern is helping or hindering the torque.
1 Like
Toe the rudders in or out with this setup? Starboard hard to starboard and port hard to port. Or opposite…to thrust transversely.
Inward turning wheels give a better “walking” effect.
The backing wheel (which is the only one with a significant walking effect) turns outboard swinging the stern over, in addition to the side thrust of the wheel wash from the ahead wheel flowing over the rudder.
This walking effect and rudder is necessary to overcome the “twist” which is moving the bow over.
Note: if the wheels are in Kort nozzles, there is virtually no walking effect. It does not matter if the wheels are: inboard turning, outboard turning, or all turning in the same direction.
2 Likes
With Kort nozzles what are you doing with each rudder to walk ?
Same thing. The rudders are set opposite of the twist , but you cannot use too much rudder or you block water flow to the backing engine, and the boat shakes like hell
Boats with nozzles should have independent rudders, but few do.
Over the lifespan of a boat, the cost of a bow thruster is small compared to the wear and maintenance on the drivetrain without one.
1 Like
I’ve worked with captains who toe them in and twist regularly with ahead on port and astern on stbd to move to port. But also have worked with guys who toe them out and go ahead on stbd and astern on port to walk to port. All with bow thruster so that’s the equalizer.
On 240’ supply boats but, same physics apply for the most part. Just interesting the difference in how some guys prefer it.
Interesting thread. My single-engine boat pulls the stern to port when backing. If I had two engines, I could put that one on the starboard side to make the boat twist more when backing and the opposite turning engine on the other side for the same effect, or swap sides and have the offset engine and the prop torque work against each other instead of with each other if I understand this correctly. I have only ever worked on boats that turned the “normal” way as in the first example and were easy to twist as desired.
- I am not sure at all about 1 rudder and two props, I am so used to being able to kick the stern with a blast of throttle in forward I wouldn’t know where to start with that other than pretend I didn’t have a rudder at docking speeds.
I have in a few occasions encountered old single screw boats that had been converted to twin screw, but the owners were too stupid and too cheap to install rudders behind the wheels. The USCG should adopt regulations that forbid this
I have encountered a couple of boats that had twin rudders, but they were offset from the center of the wheels so that the shafts could be pulled without removing the rudders or cutting a hole in the rudders. This compromises the performance of the boat and it’s a bad set up.
It’s much better to have the rudders centered behind the wheels. Its usually simple enough to either remove the rudder or have a hole thru the rudder to pull the shaft.
1 Like
Heck, the only boat I’ve operated with twin screw/single rudder setup was an ex USCG vessel converted to fishery work. Clearly designed by somebody who never had to drive the darned thing.
Even worse, old sailboats that had engines added after the fact (1920s/30s) or dumbass Sabre who copied that design. With one prop offset from the centerline handling is bad enough with the engine turning the correct way for the offset, but if the engine is ever swapped out for an opposite direction engine it is REALLY bad.
Those set ups were only meant to get the boat back to the harbor to a mooring or an open dock. Long before the tight marina berths of today were even thought of.
1 Like
Good thread here: Twin Screw Walking -
In my experience, if the boat has inward turning screws, it’s not necessary to know that fact in order to 'walk" the boat. It can be learned by trial and error.
By Dreyfus’ model of skill acquisition a beginner only needs a basic set of rules as to which way to move the controls (throttles and wheel) to control the boat and then some practice.
Theory can come later.
1 Like
Every fixed pitch vessel I ever worked on had outboard turning screws. Every CPP vessel I ever worked on had inboard turning screws.
I worked on one when I joined I was told it had outboard turning screws. VSM even said so. I thought this was strange, So I went and looked. Yep The two ME turned outboard. To a gearbox and shafts. Which not surprisingly turned inboard.
The first ship I worked master on ended up being my favourite.
Old direct reversible twin screw. Older than I am. No bridge control. Hydraulic steering, Big old wheel and brass telegraph. After market BP. QM used to give you shit if you used the Fwd ME. It was diesel and the anchor man gave you a dirty look as well.
Well I guess nobody could be bothered to explain it or nobody know’s. So I will try my best keep it simple.
Older vessels or smaller tended to be fixed pitch. clutches were costly and had a tendency to fail. Hence preference for CPP with larger or newer vessels. Clutch not RQD. Engine doesn’t have to stop and start again in reverse. (Direct reversible)
Most modern larger twin screw vessels have CPP’s. The advantages for maneuvering and reliability make it worthwhile spending the extra. Bow thrusters being common nowadays basically don’t need to worry.
Having the screws turn opposite. Inboard or outboard. Under normal operation. Prop walk cancels out. Steering easier. Prop walk acts going ahead or astern, not noticed much going ahead, counteracted quite easily by rudder. Rudders not effective going astern prop walk more noticeable. Props are designed to be efficient going ahead not astern. So more paddle going astern.
With fixed pitch prop.
If you go ahead the same on both they cancel out.
If you go astern the same on both they cancel out
If you go astern on one and ahead on the other. They add. So you get “twice” as much prop walk as a single prop.
If you use enough rudder to counter not just the ahead prop walk but also the ships swing. The prop walk will walk the ship sideways. It can be helpful or unhelpful. So You select which effect you want and put ahead or astern depending on which is an advantage.
Twin propellors tend to be offset. The more the offset the greater the torque or turning effect of the offset prop. Balanced in same direction not much effect. One ahead one astern. Lots of effect.
Outboard turning prop going astern added to the offset can add up to quite a useful effect or quite a hinderance so you try to use it to your advantage.
With CPP they turn the same way all the time. Design intent minimize prop walk.
CPP. Control Pitch Propellor. Truns same way ahead or astern. So no change in prop walk. The prop walk will cancel out for similar RPM.
Prop’s Offset so The Astern twist still there.
The inroad turning propellors prop walk helps to counteract the effect of the offset propellors.
So older fixed pitch mostly outboard turning. CPP mostly inboard turning.
I really liked working on an old twin screw direct reversible ship. She could turn on a dime much quicker than CPP’s. Provided I went the right way.
Length width ratio makes a big difference. Shorter wider ship these effects are relatively greater than a longer narrower ship.
1 Like