Hi one and all
Ah, I see me name has been mentioned....the reason I use a closed loop system for the steering, in the beginning our movement originally comes from the single point of rotation from a Servo. Going to the laws of physics for every action there is a re-action, and, in the laws of engineering, with a movement of action you also have resistance to overcome, which creates wear.
In our Servo, we are converting a rotating movement into a back and forth movement and then back into a rotating movement – via a system of cranks. Also in this setup of movement we have a system of leverages. This also applies strain or friction on our single-point of movement.
To reduce wear/strain on our Servo motor, should we not balance the load equally either side of the pivot point? You have the same force pushing and an equal force pulling. The forces around the centre point you will find to be equal. In other words less strain on the servo motor for a given load. Think of riding a ‘push bike’ is it easier to turn the handlebars with one hand and arm or is it easier to turn it with two and balance the load of the front wheel – and before anyone clever says you lean a bike over to turn the corner, we are talking theoretically here.
Here then also are some thoughts which date back to when I designed and built my real boats and I had to overcome calculating sizes and positions of rudders. The rudders when moved one way or the other out of the centre line, have the effect of first of all moving the stern (assuming they are located aft) Athwartships, thereby momentarily causing the fore and aft line of the boat to assume an angle to the path along which the boat is travelling as a consequence the water ahead of the boat, as it is reached, the hull will find a lateral surface presented to it considerably greater and of different form to the usual frontal surface presented in straight running.
It is the action of this water on the hull surface presented that causes the boat to turn. It will be seen, therefore, that in order for the boat to turn rapidly and steadily the area of effective lateral surface presented to the oncoming water must be larger in front of the pivoting point than aft of it. This pivoting point is also dependent to some extent upon the hull form and trim.
In some cases a hull needs the addition of a little additional keel area forward to help turning. A small plate or fin can produce the same effect.
Given good design, therefore, of the hull from a point of view of its lateral resistance to turning and, a good balance of areas before and aft the pivoting point; there should be no need to have rudders particularly large.
Such area as is provided for rudders is, however, much more efficiently employed when placed under the hull as opposed to being hung on the transom. In the latter case the water flowing pas the rudder blade will tend to rise and spill over a deflected blade, thereby reducing the impact pressure tending to make the blade effective. If it is desired to employ an externally hung rudder, it is a good idea to have a plate formed over the rudder blade, which to some extent substitutes for the bottom.
To ensure maximum rudder effect from given blade area, the maximum effect of slipstream from the props should be made use of. This will involve placing the rudder blade near to the propeller and of such dimensions that nearly all of the disc area of the propeller impinges on the rudder.
It will be noticed very clearly, if, for instance, a large prop involving greater shaft angle has only a small portion of its disc area of slipstream impinging on the rudder. A considerable fall off in rudder effect results.
To assist in operating the rudder through the normal form of tiller gear, power-operated servo or similar systems are sometimes employed for the larger fast types of craft, but a substantial reduction of turning effort can be achieved by ‘balancing’ the rudder.
This involves locating a certain proportion of the area of the rudder in front of the vertical or stock axis which helps to balance the effort required to force the rudder blade into the path of the oncoming water and slipstream.
In practice there is a limit to this amount, as although quite a large degree of balance can be used to help in the initial angles, after a certain point ‘reversal’ can and does take place, which has the effect of helping the rudder too much too hard over, the oncoming water then holding it thereby a considerable force.
About 22/23 percent of balance is the maximum which has been found to be safe to use.
The shape of a section of a rudder suitable for high speed craft is a subject which has been debated rather widely by the various authorities concerned. There are at least two considerations which have to be taken into account here.
There is the case where the rudder is amidships when the optimum shape will be that which offers least resistance to forward motion. Here the situation could be expected to be similar to that described in the case of the strut, where optimum section beyond a certain speed will probably envisage maximum chord about midway between leading and training edge. Entry and exit should be sharp.
The other consideration refers to the qualities possessed by the rudder from the point of view of turning. Here the situation is somewhat different, in that the flow of water will impinge on the leading edge of the rudder at an angle. For maximum effect here a rather blunt rounded section will probably prove most effective.
Equally, when considering the efficiency of the rudder section from the point of view of turning, it will be found that it is undesirable to reduce the trailing edge to a point; some reasonable width here greatly assists the effectiveness of the rudder.
Aye
John e
Bluebird
I wonder should I have posted this on Nautical strange but true ::)
