Don't know exactly Thom... but I believe it has something to do with wakes,
cut of the blade and Hydrodynamic stuff that is supposed to deliver better
power to twin outboards. Maybe some of the engineers here can enlighten us.
I know the boat I drove with the counter-rotating props was a dream to
maneuver.

CM

"Thom Stewart" wrote in message
...
| CM,
|
| You must be over doing it on the "Overproof!" Jeff has "Zero" prop
| walking.
| Why in the world does he need a counter rotating engine?
|
| OT
|

Capt. Mooron wrote:



| Quite possibly correct, but when going astern, there would be no prop
| wash past the rudder and I would say most would see the affects of
| propwalk to a greater degree when backing. G You don't have to
| completely agree with me any more than I do with you. We're talking the
| possibilities, here.

Gothcha... I'm just not correctly differentiating between flow and wash on
the prop in my original reply. My prop actually becomes narrower further
down. The prop is angled so that the flow would be strongest at the point
where it begins to narrow the most. I believe that's the reason I have such
a bitch of a time with propwalk to starboard in reverse.

CM

So, you've also got a lefthand prop (There are more of them out there
than people realize).
I really can't say, whether the above is the reason, or not. In "most"
cases, single screw, no matter what the hull design (inboard, fixed
pitch prop, right or left) when backing down, you are going to get a
serious condition of prop walk, which can be a pain and a boon to boat
handling (exceptions noted). However, there are many boats/ships out
there, that 8 out of ten times will torque as expected .... the next two
times they'll go in the opposite direction, for no reason, or at least
none that you can figure out at the time.

otn

ubject: Inherently beautiful.
From: (Thom Stewart)
Date: 10/23/2003 14:25 Pacific Standard Time
Message-id:

Shen,

You're way off about two bladed and folding. I have a single prop mono
hull and I use both a fixed Martec and at times a Martec Folding, Both
13x13. They both walk. The Folding much worst than the fixed, due the
the need of higher RPM's initially to open the blades.

You may very well be right, but for your application, not his, or possibly,
also his.
BG I have no experience in that area (folding props) and have said so, but
there are other points he has brought up, that make me think "prop walk", in
his case, is not a serious maneuvering factor ( don't know why, but think the
folding prop/pitch/efficiency may be inclusive as a factor).
If nothing else, I'm getting the sense that we need to throw out preconcieved
notions of twin screw boat handling, when discussing "cats" in general and
Jeff's in particular.

Jeff I don't have the answer of why you don't walk but I would suspect
it has to do with the Saildrives. Walking occurs on bare props. The
Saildrives have cavition plate and shaped housing forward and shedges
below which control the water flow to aid forward and stern direction

Can't agree with this, since I feel "propwalk" happens AT the prop and is not
related to G appertenances surrounding it (though they might have a very
slight effect on end results)

You, my friend, are BLESSED with a vessel that goes forward and back in
a straight line. Give thanks!!

"I'LL DRINK TO THAT!"

Ole Thom

As an addendum to your other post to Mooron (I believe) in the twin screw
world, there are three setups:
twin screw both same rotation - to be avoided at all cost (Jeff's exception
noted and reason for many of my questions).
Twin screw, inboard turning - a favorite of many "Supplyboats" and the Navy -
has it's advantages, but not my overall favorite.
Twin screw, outboard turning - what you'll find on the majority of twin screw
boats and my overall favorite.

Shen

Subject: Inherently beautiful.
From: "The Captains Nemesis
Date: 10/23/2003 13:45 Pacific Standard Time
Message-id:

On Thu, 23 Oct 2003 18:11:19 GMT, otnmbrd
wrote:

G Also interspersed

Jeff Morris wrote:

A simple test, would be both engines astern, equally. Since they are
both "left hand", your boat should immediately start backing to stbd.
How quickly and at what "turn rate", would indicate the degree of prop
walk.
I'm guessing here, but I'm betting you can steer your boat astern by
just increasing/decreasing throttle on one or the other of the engines?



I don't recall the boat ever doing anything other then going straight back
as it first
started up.

Interesting, G another test for next year?

Cats don't prop walk to the same extent as monos.
Probably the same reason they don't tack as easily.

I wonder how much of this is attributed to hull spacing and length.


Shen

Subject: Inherently beautiful.
From: =?ISO-8859-1?Q?The=5Fnavigator=A9?=
Date: 10/23/2003 13:33 Pacific Standard Time
Message-id:

The biggest factor is the interaction of the prop with the hull.

Cheers MC


Why?

"otnmbrd" wrote in message
| So, you've also got a lefthand prop (There are more of them out there
| than people realize).

Yup...

| I really can't say, whether the above is the reason, or not. In "most"
| cases, single screw, no matter what the hull design (inboard, fixed
| pitch prop, right or left) when backing down, you are going to get a
| serious condition of prop walk, which can be a pain and a boon to boat
| handling (exceptions noted). However, there are many boats/ships out
| there, that 8 out of ten times will torque as expected .... the next two
| times they'll go in the opposite direction, for no reason, or at least
| none that you can figure out at the time.

I try and use it to my advantage whenever possible. I have found that with
short bursts of high RPM I can compensate for most of the walk... some of
the time. For a full keeler I can spin it on a dime in quite water by short
bursts of forward and reverse.

CM

Every boat, operator, and set of conditions differ. I try never to use
any more power than is necessary, but, what works for me, may not work
for the next guy. A nice full keel sailboat, is one of the best learning
experiences for single screw powerboating, as they tend to be very
predictable, and relatively responsive to gentle prods.


otn

Capt. Mooron wrote:
"otnmbrd" wrote in message
| So, you've also got a lefthand prop (There are more of them out there
| than people realize).

Yup...

| I really can't say, whether the above is the reason, or not. In "most"
| cases, single screw, no matter what the hull design (inboard, fixed
| pitch prop, right or left) when backing down, you are going to get a
| serious condition of prop walk, which can be a pain and a boon to boat
| handling (exceptions noted). However, there are many boats/ships out
| there, that 8 out of ten times will torque as expected .... the next two
| times they'll go in the opposite direction, for no reason, or at least
| none that you can figure out at the time.

I try and use it to my advantage whenever possible. I have found that with
short bursts of high RPM I can compensate for most of the walk... some of
the time. For a full keeler I can spin it on a dime in quite water by short
bursts of forward and reverse.

CM

Because the hull impedes the flow of water.

Cheers MC

Shen44 wrote:
Subject: Inherently beautiful.
From: =?ISO-8859-1?Q?The=5Fnavigator=A9?=
Date: 10/23/2003 13:33 Pacific Standard Time
Message-id:

The biggest factor is the interaction of the prop with the hull.

Cheers MC



Why?