Offset centerlines for propeller and rudder

[baz]

Subject: Offset centerlines for propeller and rudder

I'm building yet another R/C USCG Motor Lifeboat. This one is a scale model of the 47' USCG MLB. The model plans show that the centerlines for the two rudders are offset inboard from the propeller centerlines.

https://en.wikipedia.org/wiki/47-foot_Motor_Lifeboat#/media/File:Coast_Guard_Boat_in_Morro_Bay.jpg

This aspect has me a little confused as I'm unsure if the plan is incorrect or that in fact there's a good technical reason for this.

The rudder centerlines are both inboard of the two propeller centerlines. Not by much mind you, but the distance is significant.

Thus when rudders are both hard over to starboard the starboard rudder's CG is aligned with the starboard propeller centerline but the port rudder's CG and port propeller centerlines are miles apart. When the rudders are hard over to port the opposite is true.

Is there a technical reason for these centerline offsets or do you think the model plan are incorrect ?

 

[bbrh842]

The rudders are offset so the propeller shafts can be removed without removing the rudders. This is common on twin engine boats.

 

[baz]

That makes a lot of sense.... even for my model boat.... thanks for the explanation. 🙂

 

[Hydraulicjump]

Our tugs are offset too for the same reason (at least mine is!). Any of us who have had the good fortune of using a gear puller to remove the dinged prop is damn glad of it!

 

[JohnH]

There is interesting information about the prototype boat at www.uscg.mil if you search for "prototype 47 mlb". Other than the already-mentioned small offset to allow the prop shaft to be removed without removing the rudder, I see no evidence of or reason for a significant offset. What you describe ("miles apart") seems larger than needed for shaft removal. The paper found at the link above indicates that they experimented with rudder configurations to improve high-speed turn stability. I think the rudders were initially canted so that they were perpendicular to the deadrise of the deep-vee hull, but it seems that they eventually went to vertical rudders. They also reduced the rudder size from the initial design. Note that I have no first-hand knowledge of any of this, and there may have been modifications after the information that I found, but I suspect that the model plans are not accurate.