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<TITLE>AYRS Newsletter July 1997 July 97 Newsletter text </TITLE>
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<H1><A NAME="P187_24975"></A> <IMG SRC="Aqwvblt.gif" WIDTH="9" HEIGHT="9" ALIGN="BOTTOM"
ALT="Icon" BORDER="0"><FONT SIZE="5" COLOR="#333333" FACE="Arial"><B> The Front +
Rear Rudder Combination<IMG SRC="Aqwvblt.gif" WIDTH="9" HEIGHT="9" ALIGN="BOTTOM"
ALT="Icon" BORDER="0"></B></FONT></H1>
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<P><FONT SIZE="4" FACE="Arial"><I><B>Dr.Ing Roberto Rampinelli, Via Filelfo 9, 20145
Milano, Italy</B></I></FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">We know for sure two important principles.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">The following benefits have been achieved:</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">1. Leeway has been reduced by up to 3°
depending on wind and sea force;</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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;</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">3. The boat is directionally more stable
than before, under sail, if correctly trimmed.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
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<P><IMG SRC="797-2.gif" WIDTH="364" HEIGHT="213" ALIGN="BOTTOM" ALT="Dwg of front rudder"
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<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
<P><FONT SIZE="4" FACE="Times New Roman">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.</FONT></P>
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