Boat design is a compromise. So too is rudder design.
If you are looking to improve the efficiency of your rudder, remove
your prop and glass over the prop aperture in the rudder. Now go
sailing and see how much that modification improved the performance of
your sailboat. If the improvement is worth not having an auxiliary
than leave it that way.

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.

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.

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.

Next thought is that due to the turbulence of an unfeathered prop,
acting on the area of concern on the rudder (prop aperture area)
probably would be more detrimental to the efficiency of the rudder in
that area than the propeller aperture.

But then, what would I know, my sailboat is a lateen rigged, 15'
half-decked canoe and my aux is a 9.5' feathered double ended paddle.
But I still have a rudder. I came across an old photograph of Harry
Chestnut standing in an 18-foot sponson canoe, fitted with a 2 HP St.
Lawrence inboard engine.
http://archives.gnb.ca/APPS/Historic...mageID=P474-24

So maybe someday I'll give it a try and see what affect the prop
aperture has on the efficiency of the rudder.


"Brian Cleverly" wrote in message
...
| Mungo Bulge wrote:
|
| http://www.yanmarhelp.com/i_propclr.htm
|
| It makes sense all around. The need for an aperture is to allow
smooth
| flow of water into the prop, to reduce cavitation. Now that you
have
| all this smooth flowing water accelerated by the efficient prop,
why
| spoil it all by bumping into a rudder or other deadwood?
|
|
| Well it eventually "bumps into the rudder" anyway...
|
| IMHO, on a sailboat, rudder efficiency is far more important than
getting the
| last ounce of effort out of a prop.
|
| Not only does an efficient rudder make handling easier but it also
gives some
| additional lift and that is precisely the reason for my question.
|
| Brian C
|
| "Roger Derby" wrote in message
| ink.net...
| | Could it be for clearance when the rudder is deflected?
| |
| | Roger
| |
| | http://home.earthlink.net/~derbyrm
| |
| | "Brian Cleverly" wrote in message
| | ...
| | Does anyone have any thoughts/theories on the reason for
extending
| the
| | prop aperture into the rudder when the rudder is hung off the
back
| of a
| | long-keel.
| |
| | I would imagine it would drastically reduce rudder efficiency,
but
| | possibly allow more of the prop to receive undisturbed water
when
| | backing-up.
| |
| | Thots anyone ?
| |
| | Brian C
| |
| |
|
|