Dr.Ing Roberto Rampinelli, Via Filelfo 9, 20145 Milano, Italy
Two years ago, a friend of mine made me almost a gift of his old Piver trimaran P123. It is an old design, but fast, comfortable and trailable enough to allow pleasant cruising for 2 or 3 people, and also as a base for running the friendly regattas we organise on the Lake Verbano, near Milano.
Only one defect I cannot tolerate in my boats - too much leeway angle. That boat had that indeed (I estimate between 7 and 9 degrees) due to the two very low aspect ratio boards under the outer hulls. That high leeway angle also reduced the potential speed of the boat because the water flow around the three hulls is misaligned, and the induced drag of such boards working at such an angle is very high.
Searching for a quick simple and cheap solution, I went back to an old idea based on the following consideration: a sailing boat needs a board of a specific area to withstand leeway, but it also needs a rudder for manoeuvring.
For top efficiency when beating, the boat must be well balanced. The rudder under such a condition should be perfectly neutral, to avoid additional resistance, though it will still have some resistance even though the boat is correctly balanced. The rudder could be eliminated, but we need it for manoeuvring (that is the only moment when we need it), and so it cannot be eliminated, but perhaps we can use it in a better way.
We know for sure two important principles.
1. A symmetrical section board develops its lift through leeway. In fact a symmetric profile does not produce lift at 0° incidence to the water flow. Therefore the boat develops the leeway angle necessary for the board to produce the windward lift needed to balance the lateral component of the wind force on the sails.
2. The efficiency of the board, that is the lift/drag ratio, improves with its aspect ratio, that is roughly the ratio of length to chord.
Some sailors use the first principle by rotating the board vertically by a few degrees to windward, thus producing the angle needed between the board section and the water flow. In such a way, the boat can eliminate its leeway, and the water flow along the hull follows the shape of the hull correctly, with the least possible resistance.
Designers also tend to increase the aspect ratio of boards and rudders up to the practical limit, but this limit is lower for cruising boats which need boards of adequate area, but which, if the aspect ratio was increased, would become too deep.
My idea was that if the rudder is controlled like a board, we could add a second small board having a high aspect ratio, such that its area, added to that of the rudder, is just less than the area that a single central board should have. If the second board can be rotated as well by a few degrees, we could have two rotating high aspect ratio boards.
But in order to balance the boat in the horizontal plane, the second board should be located at such a distance from the first one (the rudder) so that the centre of lift of the two boards coincides with the theoretical centre of a central board. This is achieved by placing the second board under the boat bow, so that it becomes a kind of second rudder at the bow.
The modification required is simpler than for any other solution, and it is very easy to make the board detachable, avoiding problems at moorings or when going ashore.
The following benefits have been achieved:
1. Leeway has been reduced by up to 3° depending on wind and sea force;
2. The boat is faster than before, even though I've not cut out the old low aspect ratio boards, now totally useless, but still a source of great resistance with their almost 2sq.m of wetted area;
3. The boat is directionally more stable than before, under sail, if correctly trimmed.
Unfortunately this last improvement, which is acceptable when going straight, becomes intolerable when the boat must gybe in a wind of some force. Sometimes the boat does not turn, and stops straight against the wind.
Therefore I slightly modified the controlling device so that it is capable of rotating the forward rudder up to 15°. Now the boat turns easily, and just after completing the turn the forward rudder angle can be reset to its normal value of a few degrees, depending on the wind strength, when going to windward, or to neutral when reaching.
A tiller arm on the rudder is connected by a screw-jack to the edge of the deck. This screw-jack is turned by means of a small rope around it which passes through a pulley near the cockpit. Moving the rope shortens or lengthens the screw-jack, thus turning the bow-rudder. The amount of rudder rotation is controlled by means of some coloured reference marks on the controlling rope near the pulley in the cockpit.
The screw-jack is linked to the tiller tip by means of a screw with a winged nut, which can be removed to disconnect the rudder which can be taken off the bow by pulling it upwards on its sliding fittings.
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