Twin-screw manoeuvering

[Sandrena]

As well as producing thrust forwards or backwards, a spinning propeller also produces a sideways force -- often called "prop walk".
This is why a single-screw boat usually turns more tightly in one direction than the other.
If you imagine yourself in the water behind the boat, looking at the prop, if the prop is turning clockwise, prop walk tends to push the stern to starboard. If the prop is spinning anticlockwise, the stern goes to port.
In a twin screw (shaft drives, not outdrives) boat, the props and gearboxes are usually configured so that when both engines are in ahead gear, the props turn in opposite directions, with the starboard one turning clockwise, and the port one anticlock.
The propwalks cancel each other out.
When you put the port engine astern, both props are turning clockwise, so both propwalks are pushing the stern to starboard.
If you use a bow thruster to push the bow to starboard at the same time, the whole boat moves sideways.
It works better on some boats than others.

 

[Leighb]

Regardless of where the props will put her, ALL boats sterns will seek the wind when running astern, so use the effect to advantage, and never ever fight it.

You have not dealt with the vagaries of a long keel yacht with the prop in a rudder aperture, there are NO rules that apply, if it decides to swing when reversing the only thing you can do is to go ahead. It is - nearly - always possible to turn round and get out forwards. The one thing they do really well is turn short round.

 

[chrishelen]

We had a twin engine ferry boat and the first bit of useful info the skipper gave me on my first day was when manoeuvring the boat put the rudders midships and one hand in your pocket use the engines alone. We could make that boat jump through hoops.These days I'm driving a single screw passenger boat without a bow thruster and find no problems to manoeuvre.

 

[alant]

Theoretically but each boat differs:
Going to starboard example without bow thruster. (Apparently works better on shafts I've never managed it on legs but havent tried too much)
Turn steering 45 degrees to port.
port engine forward / starboard engine reverse. Manage throttles to maintain no forward or aft movement. The boat should slowly move to starboard.

With bow thruster.

turn rudder hard to port. Use port engine and bow thruster in synchrony.

You can see how the theory works using this simulator. http://theboatdocker.com/

Having said all that my close quarter handling skills are legendary. Harbour masters sell tickets when they hear I'm coming. Punters always have a hoot and laugh as I yell at the crew and slowly wilt in embarrassment

I've always done it, for example moving to starboard, just starboard engine in forward, wheel to starboard, port engine in astern. Adjust throttles & wheel position as appropriate & vessel will move sideways to starboard. Works with shaft & jet drives.

 

[BruceK]

I've always done it, for example moving to starboard, just starboard engine in forward, wheel to starboard, port engine in astern. Adjust throttles & wheel position as appropriate & vessel will move sideways to starboard. Works with shaft & jet drives.

That's good to get confirmation then Alant :encouragement:

 

[alant]

That's good to get confirmation then Alant :encouragement:

Not sure there is confirmation, your post suggested port ahead & starboard astern, to move to starboard, the reverse of mine.

 

[BruceK]

I did too. And indeed the simulator runs that way too as does the going advice. The mystery deepens. I'd love to see it in action if you ever have the inclination to video it.

 

[oceanfroggie]

Idle curiosity from a sailyboat person here

In another thread, someone mentioned that while bowthrusters are very useful, stern thrusters are unnecessary because you can generate the same effect using just the props.

I'm reasonably competent at manoeuvering boats like mine - keel in the middle to pivot around, big rudder that works at low speed, single propeller sticking out of the bottom halfway between the two - but I've never driven anything with twin engines.

I'm sure whoever it was in the other thread is correct, but I'm curious how it works. How do you shove the stern of a boat sideways using just its props (and possibly rudders, though I suspect not)? Are there other unexpected tricks you can so?

Cheers,

Pete

It is a long time ago but I still remember the huge transition it was from single engine to twin engine handling. Gone is the barn door sized rudder replaced by two midget sized cat flap rudders.

Put the helm amidships and forget about the steering wheel. Manoeuvre using only the throttles and if it helps the bow thruster. Instead of shifting the boats ass sideways with squirts of power against the barn door rudder completed with a dart of reverse to stop forward motion, use asymmetric throttles to swing the stern to port or starboard. You will use far less power than you did on the single, and often just gentle notching throttles in and out of neutral is enough. Sometimes the wind or current can negate the need for bowthruster, or increase its need.

Gentle clicks in and out of neutral does it nicely.

PS: You can actually use the rudders when using asymmetric throttles to shift the pivot point off the vessels centre line which can reduce or increase the vessels rate of rotation during asymmetric throttle operation.