On Mon, 07 Jun 2004 12:33:32 GMT, (Steven Shelikoff) wrote:


snip


My two cents of fooling...

Original post has been:


We have an inboard and, of course, the prop shaft exits the hull at an
angle. I just drove by a similar (but different brand) of boat and it
appears it's prop shaft comes out at less of an angle. My first though it
that a angle that gives a more push forward and less push "up" would be more
efficient. Is that about right??




Unless specific requirement or prototype or testing purpose... the "angle" of the propeller shaft is usually "drawn" by
the boat Designer in compliance with hull shape, engine + transmission + flexible coupling + thrust bearing box + grease
box + personal opinion regarding what is better or what is not.

Many times Marine engines are "marinerized" truck or tractor or car ones.

This means else that original automotive Oil Pan has been replaced with a larger and higher one, so the "centerline" of
engine is a lot higher than automotive one.

To obtain a "straight" line from engine to propeller you "have" to make an "angle",

This angle is depending from overall dimensions of the driveline and how much free space you need aboard.

It has been done by Boat Designer using his acknowledge, choice and interests.

If angle is "too wide" engine becomes "too tilted" and you will have lubricating problems...

if angle is "too horizontal" you waste a lot of payload space...

Angle is depending else from rated propeller Outer Diameter...

So it is depending from Boat design.

The Question If is better a more or a less angle... it is "relative" and it involves personal opinions, like Political
opinion: Each one has its one.


....Submarines have horizontal straight driveline...

....Twin Engine boats have contro-rotating propellers... with any angle...

....Venetian Gondola has a straight side on RH and a curved side on LH... to go straight forward with one single oar...


No heavy or deeply mathematics... only "space" matters...

Hope this may clarify something...

(of course there are "specifically" engineered drivelines, but it is not true regarding cheap or commercial boats...)

This is my opinion, no war flames please.

Regards,

WinXP

Steven Shelikoff wrote:



That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang. This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve

The part of his explanation which is not being stressed enough, is that
you must follow the blade through it's complete revolution.
The blade starts pushing down/back (pitch of the blade) but as it
rotates it begins to push to the side/back (RH prop going ahead),
pulling/pushing the stern to stbd.
Again, the blade is more efficient during the lower portion of this arc
(solid water) than it is in the upper portion (water being lifted into
air) which causes the propwalk.
The next time you get to look at a ship in ballast or riding light, with
it's prop just beneath the surface, watch the "wash" from the prop at
the surface. You'll see it being "thrown" up/back and to the side/back.
As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.

otn

Wayne.B wrote:
On Sun, 6 Jun 2004 08:49:02 -0400, "Charles T. Low"
wrote:

The main effect is from the spiral prop wash. In reverse, where asymmetric
thrust is virtually always more pronounced, the top half of the prop wash
vortex strikes the hull, and pushes it sideways. The effect will vary
depending on the underwater hull shape, the angle of the prop shaft, design
of prop ...

So, a right hand propeller in reverse turns counter-clockwise. The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.


================================================== ====

Charles, I think you've got it, thanks.

This is one of those explanations which I feel may have an effect on
propwalk amount, but is not the "root" cause of propwalk.
Reason ... you get propwalk ahead and astern. When going ahead, the wash
does not push against the hull.

otn

Steve wrote:

That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang. This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve

Not true, in my opinion. During the lower rotation of the blade it is pushing
sideways in "solid" water .
Looking at the blade from astern (RH) and using a 360 deg circle and watching
the pich angle for angle of push.
As the blade approaches 45* it is pushing back/down, as it approaches 90* it
starts pushing back/sideways against "solid" water..... pulling the stern to
stbd.
As it rotates to 180* it is still pushing against the water, back/sideways
against "solid" water, but as it starts coming up to 270* it begins to push
back/up and loses efficiency, as it starts to lift the water up/back. This
continues through 360* where it's pushing back/sideways (and to a degree, up),
but at that reduced efficiency because it can lift that water column up and
back into the air.
I realize this is not the easiest visualization and my writing skills may not
be explaining the point at it's best.

Shen

On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:

As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.

Not on my boat. There's still maybe 5 feet of boat hull in the water
above the prop behind where the prop exits. There's no prop wash at all
breaking the surface astern of me. And since we're talking about
backing up from standing still, there really is nothing of the sort
you've described above. And yet I still get prop walk.

Steve

On 07 Jun 2004 18:46:33 GMT, (Shen44) wrote:

Steve wrote:

That theory sounds compelling at explaining a difference in the
efficiency of the blade during it's downward vs. upward travel (if
there's no overhang. This part of your explanation would create an
upward force. It would also create a listing torque force since the
upward force of your theory above is applied off the center of the prop.
But since the column of water is the same in both the sideways
directions, where does the sideways force come from to create prop walk?

Steve

Not true, in my opinion. During the lower rotation of the blade it is pushing
sideways in "solid" water .
Looking at the blade from astern (RH) and using a 360 deg circle and watching
the pich angle for angle of push.
As the blade approaches 45* it is pushing back/down, as it approaches 90* it
starts pushing back/sideways against "solid" water..... pulling the stern to
stbd.
As it rotates to 180* it is still pushing against the water, back/sideways
against "solid" water, but as it starts coming up to 270* it begins to push
back/up and loses efficiency, as it starts to lift the water up/back. This
continues through 360* where it's pushing back/sideways (and to a degree, up),
but at that reduced efficiency because it can lift that water column up and
back into the air.
I realize this is not the easiest visualization and my writing skills may not
be explaining the point at it's best.

That was a good explanation and I got what you're trying to say. The
only problem with it is the overhang. My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull. Not
only is there no noticable bulge of water behind the boat from the prop
but there isn't even a sign on the surface that there's a prop turning
at all. I just don't get that stream of bubbles behind me or a bulge in
the water that powerboats get or anything other than the same sort of
wake I get when sailing. And yet there's pronounced prop walk when
reversing.

Steve

On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote:
This is one of those explanations which I feel may have an effect on
propwalk amount, but is not the "root" cause of propwalk.
Reason ... you get propwalk ahead and astern. When going ahead, the wash
does not push against the hull.

I don't think there is a "root" cause of prop walk. It's just a sum of
many factors, some of which even counteract eachother which is why you
can't be sure of the direction of prop walk just by which way the prop
turns.

Steven Shelikoff wrote:
On Mon, 07 Jun 2004 15:52:25 GMT, otnmbrd wrote:


As for the "overhang" of the hull,issue, look again at the pitch of the
blade. When the blade is pushing up/back, it's not straight up, it's G
BACK/up, then look at the wash astern of your boat .... you'll see it
breaking the surface astern of you.


Not on my boat. There's still maybe 5 feet of boat hull in the water
above the prop behind where the prop exits. There's no prop wash at all
breaking the surface astern of me. And since we're talking about
backing up from standing still, there really is nothing of the sort
you've described above. And yet I still get prop walk.

Steve

G There's always the boat which doesn't appear to show the obvious
reaction.
Considering the angle of pitch of the prop, when ahead, 5 feet is
relatively nothing..... also, what is the shape of your hull aft of the
prop?
I've seen the same results, but my feeling is it takes very little
"lift" of the wash from the prop blade to create that unequal thrust
back and to the side, we know as "propwalk".
You are saying there's nothing of the sort that I've described. I'm
saying it's not always readily apparent, but it IS there.
Back a boat from a dead start .... you won't immediately see the wash.
Back a boat that has headway .... it will be even longer before you see
the wash .... but .... the unequal thrust WILL be occurring.
If you have a low power to weight ratio, the visual results will be
greatly lessened. However, this does not mean they aren't occurring.
Again, BG I'm no scientist, engineer, Naval architect, or prop
designer .... my opinions have developed over a good many years of
talking to individuals, handling all kinds of boats, watching wakes and
reactions .... the "root" cause is prop rotation .... everything else is
a variable, adding or detracting from the mix.

otn

Steven Shelikoff wrote:
On Mon, 07 Jun 2004 16:00:18 GMT, otnmbrd wrote:

This is one of those explanations which I feel may have an effect on
propwalk amount, but is not the "root" cause of propwalk.
Reason ... you get propwalk ahead and astern. When going ahead, the wash
does not push against the hull.


I don't think there is a "root" cause of prop walk. It's just a sum of
many factors, some of which even counteract eachother which is why you
can't be sure of the direction of prop walk just by which way the prop
turns.

G I disagree. My feeling is rotation in a water medium is the root
cause. The amount of propwalk however is dependent on many factors, some
of which can and do counteract it's effect.
On 9,999.99 out of 10,000 boats fitted with right hand fixed pitch
props, starting at DIW with no wind and current ..... the boat will back
to port and vice versa for left hand props.
However, add wind, current, headway, sternway, turning, a BARN door
rudder, etc., etc., and you are absolutely correct .... that boat may go
in the opposite direction at the least opportune time, if you are not
aware of the possibilities.

otn

That was a good explanation and I got what you're trying to say. The
only problem with it is the overhang. My prop is about 3 ft down. The
clearance to the hull above it is around 3 to 4 inches. There's still
about 5 or 6 feet of waterline behind the prop. So on the upward
stroke, the blade is pushing the water column against the hull.

First, what is the shape of your hull? Secondly, look at the pitch of your prop
- the main push is back, the "UP" angle is only slight and not directly "up"
against you hull, even WITH (I'll bet) 5-6' of overhang.
Even in reverse, with any degree of deadrise, the push to the side and up will
not be mainly impacting on your hull for the full revolution, if at all, and
even if you have no deadrise, the water will shortly leave the confines of your
hull and be allowed to push up into the air.

Not
only is there no noticable bulge of water behind the boat from the prop
but there isn't even a sign on the surface that there's a prop turning
at all. I just don't get that stream of bubbles behind me or a bulge in
the water that powerboats get or anything other than the same sort of
wake I get when sailing. And yet there's pronounced prop walk when
reversing.

Steve

My feeling on this is that the visual effects of prop thrust are not always
readily apparent, but this does not in and of itself, alter what is occurring,
i.e., you don't need a big white water wash, astern or ahead of you to be, in
reality, experiencing a lessor degree of efficiency from the prop during half
(maybe a little less, maybe a little more) of it's rotation, which is causing
the unequal pull to stbd or port, depending on it's rotation.

Shen