Subject: Which way does a boat turn?
From: "JimB"

Shen44 wrote:
As another visual .... next time you back down (and for this
purpose start DIW)
and start going astern look at your prop wash.
You should notice two things about it.
First, on a high percentage of single screw boats, the majority
of the
disturbed wash you see at the surface will be coming out from
underneath the
stbd side of your hull (you may see little or none on the
port).

RH prop. Agreed. That say says that (for some reason) the prop is
squirting more water to the right than to the left. In its own
right (sorry!) that will cause the boat's stern to go left. As to
the reason more wash goes to the right .

I don't think it's "squirting" more water to the right as much as it's showing
more cavitation (not sure I'm using that word correctly) due to the fact the
blade is pushing the water up (into the air) and away.


Second, that disturbed wash is getting to the surface ...
lifting against air.
It's not the difference in depth between the top and bottom of
your blade, it's
the difference in what they "push" against or towards.

Once the water has been squirted, what it reacts against (air or
water) doesn't matter in terms of physics, unless it reacts
against a part of the hull

Here, I'd liken it to pushing against a door that's latched, with your right
hand, while at the same time pushing against a door that isn't latched, with
your left hand.


The fact that the boat is attop of it doesn't really matter.
Many hulls angle
up both astern and to the sides, and the wash, generally
doesn't have to go far
before it is free from the hull and can lift into the air (same
end results).

Instead, I'd explain the angle of squirt (lovely scientific term,
don't you think?) you observe as 'the result of hull/skeg
interference'. As the wash spirals away from the prop, the upper
part of the spiral (going left) is interfered with and slowed by
friction with the hull, also often by a skeg supporting the prop
shaft from above. This 'drag' and 'lift' transfers a port pushing
force to the rear of the vessel (reducing the amount of wash
going to port).

The lower part of the spiral is not interfered with so much, and
has freedom to rush off to starboard.

Understood, but I'm inclined to consider this a secondary cause and effect, not
the primary.G

Shen



Mind you, I have no studies or scientific treatise backing me
up on this ....
this is just my own observation and sense of what I'm seeing
and why, coupled
with statements from a good number of older wiser boat
handlers.

So we agree on the effect (wash goes to starboard)

We agree that the wash pushing (or not) laterally against
something is what causes the the boat to turn

Your view is that 'pushing against very little' is the agent

My view is that 'pushing against a lot' is the agent

So we can agree that differential push is what causes prop walk

We differ in our explanations of this differential push.You see
different reactions caused between hitting air or water. I see
different reactions caused between hitting boat or water.

In terms of boat handling, this difference between explanations
doesn't matter. It's whatever sticks in a pupil's mind that
matters most. I like 'paddle wheel'. It's easy.

In terms of design (attempting to add or subtract from the
effect) the difference will matter. But only if you want to add
or subtract from the effect!

Shen & JimB - mostly in agreement.

Subject: Which way does a boat turn?
From: "JimB"

Shen44 wrote:
As another visual .... next time you back down (and for this
purpose start DIW)
and start going astern look at your prop wash.
You should notice two things about it.
First, on a high percentage of single screw boats, the majority
of the
disturbed wash you see at the surface will be coming out from
underneath the
stbd side of your hull (you may see little or none on the
port).

RH prop. Agreed. That say says that (for some reason) the prop is
squirting more water to the right than to the left. In its own
right (sorry!) that will cause the boat's stern to go left. As to
the reason more wash goes to the right .

I don't think it's "squirting" more water to the right as much as it's showing
more cavitation (not sure I'm using that word correctly) due to the fact the
blade is pushing the water up (into the air) and away.


Second, that disturbed wash is getting to the surface ...
lifting against air.
It's not the difference in depth between the top and bottom of
your blade, it's
the difference in what they "push" against or towards.

Once the water has been squirted, what it reacts against (air or
water) doesn't matter in terms of physics, unless it reacts
against a part of the hull

Here, I'd liken it to pushing against a door that's latched, with your right
hand, while at the same time pushing against a door that isn't latched, with
your left hand.


The fact that the boat is attop of it doesn't really matter.
Many hulls angle
up both astern and to the sides, and the wash, generally
doesn't have to go far
before it is free from the hull and can lift into the air (same
end results).

Instead, I'd explain the angle of squirt (lovely scientific term,
don't you think?) you observe as 'the result of hull/skeg
interference'. As the wash spirals away from the prop, the upper
part of the spiral (going left) is interfered with and slowed by
friction with the hull, also often by a skeg supporting the prop
shaft from above. This 'drag' and 'lift' transfers a port pushing
force to the rear of the vessel (reducing the amount of wash
going to port).

The lower part of the spiral is not interfered with so much, and
has freedom to rush off to starboard.

Understood, but I'm inclined to consider this a secondary cause and effect, not
the primary.G

Shen



Mind you, I have no studies or scientific treatise backing me
up on this ....
this is just my own observation and sense of what I'm seeing
and why, coupled
with statements from a good number of older wiser boat
handlers.

So we agree on the effect (wash goes to starboard)

We agree that the wash pushing (or not) laterally against
something is what causes the the boat to turn

Your view is that 'pushing against very little' is the agent

My view is that 'pushing against a lot' is the agent

So we can agree that differential push is what causes prop walk

We differ in our explanations of this differential push.You see
different reactions caused between hitting air or water. I see
different reactions caused between hitting boat or water.

In terms of boat handling, this difference between explanations
doesn't matter. It's whatever sticks in a pupil's mind that
matters most. I like 'paddle wheel'. It's easy.

In terms of design (attempting to add or subtract from the
effect) the difference will matter. But only if you want to add
or subtract from the effect!

Shen & JimB - mostly in agreement.