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
| |
| |
|
|

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

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
| |
| |
|
|

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