This note was prompted when a newcomer to the hobby asked what angle a rudder should turn through. With no hesitation, nor any further information given or asked for, a reply of "30 degrees each way of course!" was fired back.
Now this "30 degrees" appears to have become embedded in model boating lore to the extent that some regard it as a RULE which can be dangerous. Rules inevitably have limitations, restrictions and exceptions that need to be understood if that are to be safely used. I once read a quotation (but damned if I can remember by who) to the effect that "Rules are intended for the guidance of the wise and the blind obedience of fools", sadly something repeatedly confirmed in my professional life. In the case mentioned, without knowing the type of model the original enquirer was thinking about and what he needed, just saying 30 degrees was unwise. A fast model using this maximum rudder angle might be uncontrollable whereas a sedate one could be so unresponsive as to be unpleasant to sail.
Never being happy with Rules that have no results to back them up, I carried out a series of simple tests some years ago to measure how different rudder angles affected the turning performance of scale type models. Four displacement type hulls were used in the length range of 30-42 inches. Three had a single propeller and rudder and one twin props and rudders.
Using a combination of rudder trim and stick positions, several predetermined rudder angles, up to 40 degrees, could be applied. The size of the turning circle was measured by marking the position on the bank-side when the models were sailing directly towards or away from me. Not, I admit, a highly accurate method but, by letting the model make several circles, it gave surprisingly consistent measurements.
The first graph shows the typical results for all the models. For up to 10 degrees, the rudders had a powerful effect. Between 10 and 30 degrees the effect of the rudder progressively diminishes and beyond 30 degrees it's effect is much less. This fits in with the experience that rudders can be very sensitive at small deflections and unless you apply the transmitter controls with restraint, your model is going to be all over the place.
The second graph shows how the time to make a complete circle changes with the rudder angle. As you might expect, a greater rudder angle means a tighter circle with less distance to travel and should mean a shorter time to complete the circle. In practice models slow down as the circle becomes tighter (the model traveling at an angle to the water flow and encounters greater resistance. The net result being that the turning time falls up to around 20 degrees, after that there was little further reduction and something of the proverbial "brick wall" was encountered above 30 degrees.
Playing with the results and it was found that how the models length affected the turning circle, graph 3. For any given rudder angle, the turning circle diameter was found to be directly proportional to the models length. This might be handy for the designers of scale steering courses. The actual obstacles can be as "tight" as your conscience permits but, after considering the maximum length of model expected, allow sufficient space between obstacles. This should enable models to sail smoothly between them rather than spending time shunting forwards and backwards.
These experiments have served me well over the years and can be summarized as follows;
1) Rudder angles up to 30 degrees are suitable for the general sailing of most scale models
2) For a given rudder angle the diameter of the turning circle is proportional to the models length
But, this only applies to models where the rudder is behind the propeller where the rudder blade intercepts all of the propwash. One model of a destroyer had twin props and a single rudder between them. With 30 degrees of rudder throw, it's turning circle was too large for comfort and had to be increased to 40 degrees before I was happy.
Also, for low speed maneuvering, larger rudder angles can be of benefit. I often use angles of 40/45 degrees for this reason but employ restraint in the application of rudder commands when traveling at speed.
So, 30 degree of rudder is a good staring point for many models but it isn't gospel, sometimes more or less is better, don't be afraid to experiment.
Glynn Guest