Re: Refueling at sea

On Dec 4, 2:31 pm, Vince <fire...@xxxxxxxxxx> wrote:
Peter Skelton wrote:
I agree Eugene, I was referring to the need ot the smaller ship to 'drive' away from the larger, otherwise interference between the two would pull them together if on true parallel courses. How long do you think the hose would last if they steered on different courses. Lets take 10 knots as a reference - about 17 feet per second velocity. Assume the ships steer two courses that differed by 1/2 a degree (and a very, very good coxwain that would have to be!) The gap would be widening at about 9 feet per minute. Not a comfortable situation!

If the ships steer exactly parallel courses, they will collide. Bernoulli explained that. The ship taking on fuel steers to keep a fixed distance from the other. That requires rudder movements which average to a small offset.

Peter Skelton

just a question

why is it the ship "taking on fuel" as opposed to the smaller or handier ship?

The ship "taking on fuel" is the cruiser/destroyer/frigate/corvette
and is far more manouverable (both in speed and course correction)
than the tank/oiler. Despite the greater experience of the oiler/
tanker/supply vessel (AO or AOR), the receiving vessels typically have
more power, speed and manoeverability (often more shafts, rudders, and
screws). The exception is when a tanker/oiler replenishes another
oiler/tanker, such is sometimes found in major operations.

That there was a difference of log speed noticed on the film is not
surprising. The ships logs are infrequently calibrated and few people
pay close attention to them (except trivia collectors who seem
interested in "miles steamed" after a voyage or when a ship is "payed
off"). Warships navigate primarily by their charts and tracks with
"speed made good" and "speed required", or use GPS coordinates to
calculate "speed over ground" (which is similar to "speed made
good"). It's far more important to have a hyper-accurate clock than a
hyper-accurate log.

Usually the receiving vessel approaches the tanker/oiler but not
always. If the receiving vessel has gear extended, such as a towed
array or varible depth sonar, the oiler/tanker will approch the
receiver. But once the two are alongside, the international naval
standard is for the oiler/tanker to hold as steady of a course and
speed as possible (notwithstanding effects of wind and current,
usually ordered well in advance, and adjusted for local sea and swell
conditions), and the receiving vessel attempts to match the supplying
vessel's course and speed. It does this by watching the distance line
which marks the separation. Once the two courses are almost matched,
the holder of the ship's conn (OOW/OOD) orders the helm corrections by
compass/gyro headings but no longer steers a particular course, but
makes corrections only to keep the distance line steady (sort of like
a blind man dragging a cane alongside a curb to keep his bearing -
even if that curb takes a curve, the blind man will continue to keep
the same distance from the curb's edge).

The Bernoulli (suction) effect typically happens only at about 15 feet
of separation, perhaps 25 feet between two large (+500 ft) ships,
meaning something has already gone wrong and likely to get much worse.

So the end result is that the two ships (or three - as many navies
also practice 2-sided RAS/UNREPs - in which case the middle vessel
must be "the guide") hold identical "virtual" courses. It does not
matter what speed was ordered or rung on, the vital item is that the
distance line (strung between the bows of the two ships) remains
constant (typically 80-120 feet) and that neither ship gains or loses
on the other (using an identifying mark on the adjacent ship).
Because ships are steered from the rear (few bow thrusters work at the
typical UNREP/RAS speeds of 8-14 kts), gentle steerage correction is
as critical as speed corrections (some might argue more).

When complete and gear detached, receiving vessels typically speed up
(some nations more aggressively than others)and, when the stern is
clear of the supplying vessel's bow, turn outward (often with both
vessels blaring selected tunes).

Cheers (and out of breath... sorry for the long post - if I had
had more time it would have been shorter)
--CommO
.

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