Mediaeval ships of any size were generally steered not by a wheel but by a whipstaff. This was a vertical lever, pivoted somewhere in the middle. The top came through the deck where the helmsman pulled and pushed it sideways. The other end had jaws grasping a conventional tiller. So the helmsman pushed his end of the whipstaff to port, and the bottom end moved to starboard, moving the tiller in the same direction.
This could have advantages in model yachts, particularly for those who do not want to buy very expensive transmitters able to ‘fake’ their inputs on the stick. How do we make a modern miniature whipstaff?
Suppose that we mount our servo with its shaft on the centreline at right-angles to the rudder stock (i.e. roughly horizontal, pointing aft).
On the top of the rudder stock we mount a tiller made out of a piece of brass rod, carbon rod or whatever. If the rudder stock is a piece of brass or steel rod, all we have to do is to bend it over at right-angles.
The whipstaff (i.e. the servo arm) is a similar piece of rod, which is vertical (as seen from ahead) with the servo in its central position and at right-angles to the shaft of the servo seen from the side (i.e. parallel to the rudder stock).
Both tiller and whipstaff a fitted with tubular sleeves (a good sliding fit) over them so that both sleeves can be extended to meet at a point. The two sleeves are then connected by a ball joint. If a ball joint is too complex, you can make do with a simple hole in one of the sleeves, but this must be elliptical to cope with the position when the helm is over and made quite accurately to avoid play.
Why is this a good thing? First, it is probably simpler and lighter than conventional yoke-bar steering. Second, it eliminates any possible inconvenience of a transom rudder under the Footy rule. Most important, it enables us to duplicate mechanically the ‘input faking’ features of very expensive transmitters.
With a yoke-bar, we get maximum angular deflection of the rudder per degree of servo movement in the straight ahead position. As the helm goes over further, the increase in rudder angle per degree of servo rotation gets less.
This is the exact opposite of what we want IMHO. With the boat well balanced and in the groove to windward, we want a relatively coarse input on the stick to have only a small affect on the rudder so that we do not start to zigzag all over the place. When life gets hard and we want a lot of rudder, we generally want it now and are slightly less fussy about how much we get. Coarser adjustment will do as a trade-off for speed.
A whipstaff does this, all by itself. If you want to get clever, go can do all sorts of marvellous things with the speed at which the helm goes over at different points in its travel by altering the angle of the whipstaff and the tiller relative to the rudder stock.
Any comments?
Angus
