I'd guess all three ideas have merit. Consider the helicopter. That
WHOP-WHOP-WHOP you hear is the downwash from the rotor blades striking the
tail. The more space, the quieter.

Room to pull a prop probably includes space for the puller and clearance for
the blades when the rudder is deflected.

Roger

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

Hi Matt,

Matt Colie wrote:

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.


Not on this boat... The prop has been pulled, as has the shaft, and there was
more than ample room for the puller.


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....


I think people have lost sight of the original question..

On this boat, the prop is placed in the aperture pretty much according to all
the expert's calcs as to what is a good situation.

My question was re the excessive continuation of the aperture into the rudder.

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.


A service I myself provide to others.

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).


Now that *is* a thought, but in this case it would only have to be large enough
to provide clearance for shaft removal.

I still go back to my original possibility that a large rudder aperture might be
there to provide better water flow to the prop when backing down. After all,
long keel boats are a bear to backup at the best of times.

But, again, a sailboat is primarily a sailboat, not a motor boat, and to my mind
rudder efficiency should take priority over prop performance.

Brian C

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

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

Well Brian,
If there is more than enough room to work and the prop is where it
belongs, then I can't guess. When I did work at an NA firm that did
some sailboats, I was told by one person that large aperture made for
better efficiency ahead, but poor backing control. But, about a day
later another told me that another said that it almost didn't matter if
you kept the edges of the aperture as thin as practical. As though not
to be outdone, another associate told me that he had put a plug in the
trailing edge hole - leading of the rudder - to improve backing
control, It did and he could detect no loss in ahead performance. He
was also an NA so we discussed this in terms of naca sections.
- Now that we have everybody completely confused.
Matt

Brian Cleverly wrote:
Hi Matt,

Matt Colie wrote:

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.


Not on this boat... The prop has been pulled, as has the shaft, and
there was more than ample room for the puller.


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....


I think people have lost sight of the original question..

On this boat, the prop is placed in the aperture pretty much according
to all the expert's calcs as to what is a good situation.

My question was re the excessive continuation of the aperture into the
rudder.

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.


A service I myself provide to others.

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).


Now that *is* a thought, but in this case it would only have to be large
enough to provide clearance for shaft removal.

I still go back to my original possibility that a large rudder aperture
might be there to provide better water flow to the prop when backing
down. After all, long keel boats are a bear to backup at the best of
times.

But, again, a sailboat is primarily a sailboat, not a motor boat, and to
my mind rudder efficiency should take priority over prop performance.

Brian C

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