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