Brian,
It sure would help if we knew what the rudder, keel, aperture looked
like. A picture is worth a thousand words.

"Brian Cleverly" wrote in message
...
| Mungo Bulge wrote:
|
|
| By the way lift generated by the rudder is only significant when
the
| AOA of the rudder is greater than 0 degrees, and less than the
stall
| angle (the agnle at which lift = drag). However, with increase in
lift
| comes a proportionate increase in drag, and drag increases faster.
|
|
| When sailing, the rudder is very rarely (if at all) at 0 deg and
that is why a
| properly shaped rudder can increase performance.
|
| Now, as for the prop aperture in the rudder being there to
increase
| the efficiency of the prop at the expense of the efficiency of the
| rudder, it is an over simplification. If the rudder is too close
to
| the trailing edge of a prop blade, the vibration induced would
soon
| destroy the mounting/bearing points of the rudder. Second, the
back
| pressure would decrease the effective thrust of the prop,
requiring a
| large HP rated aux, increasing the displacement of the boat, thus
| requiring an even larger auxiliary.
|
|
| Sure, and that is why there are tables to correctly place the prop
relative to
| its surroundings. Which it was on this boat and still there was
this humungous
| hole in the rudder.
|
| If the propeller aperture was designed into the rudder, so as that
| each horizontal segment, when viewed in plane view, still follows
the
| NACA foil profile of the rest of the rudder, all be it reduced in
| scale, little degradation in rudder efficiency would be
noticeable.
|
|
| Could be, but to me, logic says otherwise. We are dealing with a
very low speed
| flow and for that you need as long a chord as can reasonably be
used.
|
| Brian C
|
snip