Do you have any thoughts on whether it is better to offset a rudder outboard or inboard of the propeller shaft (to allow for the removal of the shaft without dismantling the rudder) for twin-screw applications? Does this position affect any of the drag predictions in NavCad?

The easy answer first – my research into your question indicates that rudder drag is not measurably influenced by the offset position. Now onto your more interesting question. Which position is better – outboard or inboard?

My conclusion is… I really cannot say that one is better than the other. After reviewing a number of references in our library, I can summarize that a) placing the rudder behind the propeller outflow jet is essential for rudder effectiveness, b) that locating the rudder shaft directly aft of the propeller shaft is not a good idea, and c) there is no firm recommendation about transverse offset position.

Saunders [1] – the bible for all such things – makes little comment on this topic. He notes that interaction of the propeller hub vortex on the rudder can contribute to “buffeting, hammering, and erosion”, and that rudder offset “appears not to appreciably reduce the beneficial effect on lift and lateral force of the augmented velocity in the outflow jet” (Sec. 37.12). One graphic shows an outboard turning propeller with the rudder outboard of the shaft, with text that states “rudders may be inboard or outboard of the shafts” (Sec. 74.2). He also shows the propeller outflow jet at the position of the rudder in this graphic, and the jet at the rudder is slightly higher than the propeller disc (as is expected to follow the rising run of the stern), but also slightly inboard (which is also expected, but which often is a surprise to some people).

I would recommend finding a copy of a really useful paper by Don Blount [2] entitled “Tuning a Twin-Screw Rudder Installation”. It is full of great information, but this reference also does not offer much regarding offset position. The one graphic showing a rudder offset is also for an outboard turning propeller with the rudder stock centerline positioned outboard (at a recommended distance equivalent to 10% propeller diameter). Likewise, he notes that rudder offset “has little effect on the boat’s turning performance”.

Anecdotal references can be found that suggest that you should locate the rudder outboard for an outboard turning propeller, and vice versa for an inboard turning propeller. However, I can find no solid hydrodynamic corroboration for this suggestion, but the graphics from the Blount and Saunders references do conform to this. Of course, when deciding on an outboard offset, it would be necessary to insure that the rudder stays well beneath the hull to avoid any possibility of ventilation. Other than these practical considerations, it would appear that either location – outboard or inboard – is appropriate.

[1] Saunders, K.E. Hydrodynamics in Ship Design, SNAME, Vol. 2, 1957.

[2] Blount, D.L., “Tuning a Twin-Screw Rudder Installation”, Professional BoatBuilder, February/March 1997.