Twin screws - which way should they turn?

[Guest]

I use a 10m twin scew boat for boat trips on inland water. I have noticed that both propellers turn the same direction. Would I notice any significant differences if the propellers turned in opposite directions? I presume I would need to buy a new propeller? If they are better turning in opposite directions, should the starboard prop turn clockwise (when viewed from behind the boat), or anticlockwise? Would any change be likely to make a real difference? Thanks

 

[markc]

Assuming that this isn't a wind up and a genuine question, and that I'm not missing the point......

The direction of the prop rotation is determined by the gearboxes. Most boats have 'handed' gearboxes meaning that the props are contra rotating. Of course the props need to be handed as well. Some boats don't have contrarotating props - the Ranger 36 springs to mind - dosn't appear to cause a problem except when climbing onto the plane - it leans a bit.

As for your boat, don't worry about it.

Mark

 

[jfm]

Re: flip the gearbox

They should counter rotate, but it's not the end of the world if they dont. Outward turning (stbd goes clockwise when viewed from back looking fwd) gives best manoeuverability, inward turning is best if you use one engine sometimes and want to go in a straight line.

The hydraulic gearboxes are usually reversible. I mean they do not care which is forward and which is reverse. In other words, you can get one new prop, then flip the gearbox control around so that it turns the other way when forward is selected. It's not like a car or outboard gearbox where reverse is a weak affair that should be used only for slow manoeuvres. That's usually the case, a mechanic can check

JFM

 

[paulineb]

It helps if they're both turning in the same direction.
PB

Pauline B

 

[jfm]

Re: Eh, Pauline?

Helps what exactly.....?

JFM

 

[Guest]

I think most of you are missing the point. The reason for handing the drives is to counteract the paddle wheel effect of turning the prop in the water. The difference will be most noticeable when going astern.

 

[jfm]

Re: Paddlewheel fully taken into account

Eh.... not missing the point at all! Paddlewheel is why I said they should be counter rotating, and why I said the inward vs outward choice is a choice between either better straight-line-on-one-engine or bettermanoeuvre-ability. All that's because of paddlewheel. ????

JFM

 

[paulineb]

Re: Eh, Pauline?

Well if they each turned in opposite directions, it would split the boat in two
PB

Pauline B

 

[Deleted User YDKXO]

Re: Eh, Pauline?

Yes, Pauline, cant fault your logic but only if they turned outwards. What about if they turned inwards?

 

[paulineb]

Re: Eh, Pauline?

DOH !!

Pauline B

 

[Guest]

Re: flip the gearbox

Yes be careful with the gearbox - case in point: I had a boat with a Volvo gearbox and it had two cone clutches and when going forward [shaft turning in same direction as engine then the clutch nearest the engine in the gearbox would be 'forced' into engagement by the prop thrust. So if I had decided to change the prop handing and selected astern the rearward clutch would have been pulled apart by forward motion ... does that make sense? You may have noticed that some manufacturers sell the same engine with a reversed rotation .. well, that's why! With outdrives it is very easy to 'hand' but not straightforward with shaft drive - identify the gearbox and ask someone who knows - Lancing Marine are firstclass in the advice dept!