Re: [Phys-L] floating fiasco physics

[John Denker <jsd@av8n.com>]

On 3/29/21 9:19 AM, John Sohl via Phys-l wrote:

> Another issue is prop-walk. If there are two screws that are counter
> rotating this is less of an issue, but you get an additional vector thrown
> into the mix from the "paddle wheel" behavior of props. Not only does a
> prop thrust water backwards, it also provides a sideways force (I've seen
> this in aircraft prop wash as well). This force can be welcome when going
> into or out of a dock, it can also be very annoying.

There's some interesting physics in that.

As is so often the case, it pays to keep track of the /momentum/ and the
transfer thereof (as opposed to keeping track of forces). This helps in
many situations, including fluid dynamics ... as well as certain nasty
deceptive FCI questions.

In boats, prop walk is much more significant under conditions of /reverse/
thrust, which is a big clue as to the underlying physics. Specifically:
Whereas an ideal prop would just throw a stream of water in the desired
direction, a real prop has some friction, which imparts some rotation
to the stream.
  a) There is angular momentum in the escaping water. By conservation,
   some torque in the roll direction will be imparted to the boat. The
   boat can easily overcome this, so it is not an important consideration.
  b) There is some sideways linear momentum in the top half of the stream,
   and some opposite sideways linear momentum in the bottom half. To a good
   approximation these cancel in the normal forward-thrust scenario. However
   (!), in the reverse-thrust scenario, the top half of the stream impinges
   on the hull, much more so than the bottom half. As a result, the top half
   gets straightened out. To say the same thing another way, it transfers
   some of its sideways linear momentum back to the hull, forming a closed
   loop of momentum flow, which means no net force. Meanwhile the bottom
   half escapes, so there is a net transfer of sideways lienar momentum to
   the water. This imparts the observed sideways thrust to the boat, and
   the observed torque in the yaw direction.

So we see that the oft-used analogy to a paddle wheel is not entirely apt.
If we don't look too closely, the overall effect is similar; however, the
details are different, and the underlying physical mechanism is quite
different.
 -- A classical paddle wheel depends on having half of the wheel out of
  the water. Not coincidentally, it works equally well for forward and
  reverse thrust.
 -- Prop walk depends on having half the stream impinge on the hull. It
  mostly pertains to reverse-thrust situations. (If you have a badly
  designed rudder in the propwash behind the propeller, that's a
  different story, but that's rare.)

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

The corresponding physics applies to single-engine airplanes.  The vertical
tail sticks up, not down. The top half of the propwash impinges on it
while the bottom half escapes. This is particularly noticeable under
high-power low-airspeed conditions.

This is very widely misunderstood. For decades the wrong explanation was
given in official FAA publications. Student pilots were required to give
the wrong explanation on exams. I waged a years-long campaign to fix this.

The wrongness of the wrong answer should have been obvious. For starters,
the yaw-wise torque effect is not observed in twins, even when both
engines rotate the same way. This is because the propwash mostly misses
the vertical tail. Conversely, if you taxi a single-engine aircraft over
new-mown grass, you get grass clippings plastered onto one side of the
vertical tail and not the other. That's a clue.

===========

There are a lot of things I don't understand about the Ever Given.

For one thing, it has a single main engine driving a single enormous
propeller. You would think they would want two propellers, both for
fuel efficiency and for maneuverability in close quarters.

For sure the beam is wide enough to accommodate two propellers.

One gets the impression that maneuverability was not a high priority
for the designers. According to wikipedia: "On 9 February 2019, the
Ever Given collided with the ferry Finkenwerder, which was docked at
a wharf on the river Elbe at the time. The Finkenwerder was heavily
damaged."

For such a beast, capital costs are small compared to operating costs.
You'd think they'd work harder to optimize the latter. Better handling
seems cheap compared to the cost of a crash or two.