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[destroyer42]

Hi All,
What's the best prop rotation on a twin prop in fixed nozzels with rudders for best slow steeing and reverse steering as my tug has contra rotating props, but when I go in reverse it will not go straight but starts to turn.
Does any member know what i'm doing wrong?
I have considered swapping the props round.
Destroyer42

[Colin Bishop]

Very few models will steer a straight course astern whether single or twin screw - it's to do with the shape of the hull. You just have to get used to the characteristics of the boat and allow for it.
 
Colin

[Umi_Ryuzuki]

Try backing using one prop, and using the second porpeller to control the turn, or boat rotation.

 

[ray123]

 hi colin  when looking at the props from the stern   do's the port side one turn clockwise? & the starboard one  anti clockwise?   thats how mine are set up  my ken mackenzie dont steer that well going astern  & thats fixed nozzells  with rudders   talking of rudders are you rudder's big enough??
 regards ray  p.s could you post a picture of the stern of your boat   'i take it the boat in question is the 'ace?

[condeh]

Which way does it turn? i.e does the bow go to port or to stbd. A fixed pitch right handed prop will send the bow to stbd when run astern, and a fixed pitch left handed prop will send the bow to port, so one of each should cancel each other out, assuming run at same rpm etc...




The direction of rotation shouldn't really make a difference,  so long as they are moving in opposite directions.

[Colin Bishop]

Basically the problem is that when a boat is going forward the sharp end divides the water in front of it pretty much equally and there is a tendency for the hull to run straight. However, when going astern, the water will pile up against the stern and the slightest influence will cause it to swing slightly off centre and slew round to one side. The rudder(s) are usually not powerful enough to offset this.
Of course it's a bit more technical that that but you will see the principle.
 
People often wonder why the old sailing ships had such small rudders. The reason is because that is all they needed. Putting the rudder to one side or the other was sufficient to alter the hydrodynamic flow around the hull and the hull itself then amplified the imbalance. Not dissimilar to extending flaps at the trailing edge of and aircraft wing to increase lift etc.
 
Colin

[condeh]

Yes, I have copied below some of the notes I was given on ship handling at college

The helical discharge, or flow, from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so it behaves quite differently. On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force in tonnes that is capable of swinging the stern to port giving the classic 'Kick Round' or 'Cut' of the bow to starboard.

I've also copied some other info it gives on transverse thrust, for information.

Shrouds


For economical purposes, propellers in shrouds or tunnels are growing in number, even on large VLCCs. This ultimately has some bearing upon transverse thrust because they alter significantly the flow of water exiting the propeller area. It may be more concentrated and is likely to impose an equal thrust upon both sides of the hull thus resulting in little or no transverse thrust.


Hull Design


Finally, hull design features may also play a significant part in altering this simplistic and traditional concept of transverse thrust. It is possible, for example, because of a different hull shape or length to breadth ratio, for the point of impact of water flow to be much closer to the position of the pivot point when going astern. In such a case, transverse thrust, although relatively pronounced with headway, may be surprisingly weak with sternway, to the extent that the bow may literally fall off either way, particularly if influenced by wind or shallow water.

A common system I see on twin prop ships is to either run one prop astern, and use the other ahead as req'd using the rudder to correct the heading (works best with twin independent rudders), or to run both astern, and use a bow thruster to control the swing of the bow as/when it occurs.

[Netleyned]

AKA  Paddle Wheel Effect

Ned

[tobyker]

And of course old sailing ships were designed with hollow buttocks so that nice clean water (not nasty noisy stirred-up water) was accelerated past the rudder, probably forming a virtual extended rudder made of water - and of course nobody expected them to steer astern!

[Peter Fitness]

One of my models, a Grand Banks pleasure cruiser, can be run backwards almost as accurately as forward, and I don't know why. All my other boats turn when reversing. The GB has twin screws with rudders behind each prop, and both motors operate through one ESC. Other models have similar setups yet turn when going astern. There is probably some hydrodynamic reason for it, but I can't explain it   I assume I have unwittingly hit upon the answer


Peter.

[condeh]

The other thing to consider is where the pivot point moves to when moving astern, its movement aft can make the effect of wind etc more pronounced than when moving ahead. Vessels with more windage will be more prone to this of course.

[destroyer42]

Hi All,
Thanks for the interesting and informative reply's.
The tug in question is the attached Sea Span ACE.
Do you think it needs bigger rudders as when moving slow the turning circle is quite large for a 14inch tug, but when at speed the turning circle improves.
 
Destroyer42