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 Cleverly wrote:
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.


Makes sense in front of the prop, but behind?

Lew

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,

I was reminded in a design class that it was a resonable idea to allow
enough room to remove the prop without removing the rudder or engine.
Even on long keel boats, propellors sometimes take damage.

Matt Colie

Brian Cleverly wrote:
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

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?

"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
|
|

Roger Derby wrote:
Could it be for clearance when the rudder is deflected?

Roger

http://home.earthlink.net/~derbyrm


Not in this case... The rudder would have to go way past 90deg to interfere
with the prop.

Brian C

"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

Matt Colie wrote:

Brian,

I was reminded in a design class that it was a resonable idea to allow
enough room to remove the prop without removing the rudder or engine.
Even on long keel boats, propellors sometimes take damage.

Matt Colie


I agree, but that wouldn't require the approx 6" additional clearance this boat has.

Brian C

Brian Cleverly wrote:

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:

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,

This might seem like a lot, but when you have to get a puller in there
to get the prop off the shaft, that may even be a little tight.

There is also shaft and bearing service to consider here.

I also knew of two hull of the same class and one had the propellor
about centered in the aperture, the other had less than an inch forward
clearance. They had both had shafts replaced....

Then there is still the issue that the builder may just have choosen to
make the aperture large (it was written many places that this makes for
less rudder vibration under power - I don't agree). Builders (by
enlarged) make what is most effective for them to build and sell.
Seldom does competitive performance enter into the equation. This is
why service is available to re-fair keels and rudders.

If you wanted to plug it to improve the rudder effectiveness, go ahead.
Just make the piece removable (and do your best not to leave hard
edges at the leading edge that will cause turbulance on the low pressure
side).

Matt Colie

Brian Cleverly wrote:
Matt Colie wrote:

Brian,

I was reminded in a design class that it was a resonable idea to allow
enough room to remove the prop without removing the rudder or engine.
Even on long keel boats, propellors sometimes take damage.

Matt Colie


I agree, but that wouldn't require the approx 6" additional clearance
this boat has.

Brian C

Brian Cleverly wrote:

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

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
| |
| |
|
|