Twin screw, rudder placement

I am curious about this please.

When we have twin screws, contra rotating, do we have the rudders set inboard, centre or outboard of the shaft alignment. I have seen all three types, but would like to hear some discussions please regarding what is deemed "correct" and why.

ta

 

If twin Rudders are used, they are usually placed slightly outboard of the propshaft centerlines (an inch or so). This is done to facilitate propshaft removal for bearing servicing etc.

 

Most guidelines I've seen on the subject agree with Jango's observation. Twin rudders are most commonly set far enough outboard of the propshafts for the shaft to be pulled without removing the rudder.

 

Yes fellas, I realise that, we do the same, but should they be inboard or out board and why?

 

It shouldn't matter, Inboard or Outboard. Although, I believe in extrenely slow vessels, further outboard the better.

 

I have yet to see any evidence as to whether it is better to go 2" inboard or 2" outboard of the prop shaft. Gerr's The Nature of Boats recommends mounting rudders inboard of prop shafts, if the screws are outward-turning. Other sources suggest the opposite. I think it's more a matter of hull geometry and where they will fit best.

 

Thanks fellas, I really would like someone that can give a scientific reason, I have my own thoughts of course, I believe outside the shafts because that is where the maximum thrust would be coming from the props, this may be correct, but so far no one has actually stated why. Thanks anyhow.

 

You're right, there isn't a lot written in easily accessible sources. I looked, couldn't find any references, so here's my take, for what it's worth.

My understanding is that rudders will be placed slightly outboard of the shaft on boats in which low speed maneuvering is a priority. The reason is that small forces have more effect (leverage?) if they are further from the centerline.

If maneuvering at low speed is not important, they may be placed outboard or inboard. I remember a photo of a US Navy DDG class destroyer in drydock showing the rudders mounted just inboard of the propshafts. I wondered about this, but it seems that naval ships use tug assistance nearly all the time when mooring to a pier.

 

If your rudders are inboard in relation to your props, fit your counter- rotating props such that both may turn inwards for forward motion.

If your rudders are outboard in relation to your props, fit your counter-rotating props such that both may turn outwards for forward motion.

This setup should give you maximum rudder effect.

Reverse motion will simply follow suit, for both the above scenarios.