Re: Ocean Stream Speed - To Inger.


"Dylan Sung" <dylanwhs.tsktsktsk@xxxxxxxxxxxxxx> wrote in message
news:e7ojgm$5lr$1@xxxxxxxxxxxxxxxx
Hello Inger,

You mentioned that knots/hour in your thread about "Ocean stream speed".
I've told you that technically, what you've written is an acceleration.

Correct and that was what I meant when writing on top of the other speed.
The acceleration that occured must at least be assumed to have been the new
shipspeed - the old shipspeed. I didn't discuss anything but that from what
I learnt, I have sailed old sailingships as well as modern one. This one I
only sailed three days back in 70's when SS Gratitude wasn't a owned by a
sailing-school. I also been on board it's sistership Leader. The later was
owned by fishermen in Sannäs who had placed a motor in it before some of the
younger men in the village bought it and changed it.
http://www.sxk.se/seglarskolan/GRU_eng.html

Now, after the boat has entered the stream, does the boat continue to
speed
up, until it reaches it's destination, or does it reach a steady sailing
speed?

Until it reach the speed of wind in sail + stream that's what I learnt but
only as long as you are sailing with wind taking you exactly same way as the
stream is heading. Don't ask me how to calculate the deviation/leeway.
Haven't done that for the last 15 years. I love the sea and so does my
daughter. BUT the elderly say that she and Ran must have a special
relationship. When she is on board, small boat or larger ship the wind
fades. No matter where we go so forget sailing with her on board. We have
gone over the North Sea, from Gothenburg to two places in southern Norway,
from northern Bohuslän over to Agder and from Stockholm over Åland's hav(the
part of the Baltic sea between Sweden and Aland island). At least once a
year we been on board sometimes more..... but my daughter hasn't seen more
than 3 hours all in all with wind over 9 secondmeter....... Old workmates of
mine from the time in 70's when I worked on board a ship one summer say that
if she comes on board one can forget what the weather forcast have said.
Hadn't I seen it over and over - storm only hour before we left the harbor
and so on, I wouldn't have believed it.

Remember, I gave a few definitions earlier.

Speed is the change of distance with respect to time.
Acceleration is the change of speed with respect to time

Correct and that's what I have been trying to say over and over but you and
others doesn't seem to understand what I am saying.

Inger E

Let's put velocity to one side, for the moment.

Now, if a boat is travelling from the north towards the south at a
constant
speed, and then hits the gulf stream, which for our purposes is travelling
west to east

(BOAT)
. |N
. |
. |
. |
. |S
.\|/
. V

-------------------->
W (Gulf Stream) E


The boat over time will do this


(BOAT)
.B North (N)
.B |
.B South (S)
.B
.B meets gulf stream here
. B
. B
. B
. B
. B -----> drag of the gulf stream current west to east
. B
. B
. B
. B boat seems to move in a south easterly direction

Assuming that the boat speed and current speed is constant the only thing
which changes is the direction of the boat which goes south easterly.

The boat experiences an impulse which nudges the boat towards the east.
The
impulse is a force applied by the current on the boat for an amount of
time.
And as you know, from Newton's laws, F(orce) = M(ass) x A(cceleration) or
F
= ma

Given a constant acceleration, the more massive something is, the greater
the force is needed to move it. Applying a force like the current on the
boat, it will move the boat from a north south direction to a south
easterly
direction. Here's where the vectors come in. Lets assume that the boat
continues going southwards at the same speed.

Before the boat hits the gulf stream, the boat is not moving west to east,
so the component of its velocity in the east west direction is zero. Once
the boat get into the gulf stream current, it feels the impulse of the
current on it, and the force acts on the boat for an amount of time t (the
impulse).

The initial velocity in the east west direction is zero, therefore the we
let's call that v(WEi) = 0. Let us assume the boat is headed south and
doesn't alter course to follow a west-east direction along the current.
The
boat therefore reaches a point where it matches the speed in the west-east
direction due to the current. Let's call that the final WE velocity
v(WEf).

The impulse = m[v(WEf) - v(WEi)] = m[v(WEf) - 0] = mv(WEf). That is, once
the boat matches the gulf stream current, as it moved north to south, it
will drift with the current west to east with the velocity v(WEf) and so,
without any other external forces on the boat going west to east, it will
have a west to east momentum mv(WEf). Since the momentum in that direction
due to the drift the boat experiences under the current is constant, it
(the
boat) experiences no acceleration. That is, the speed doesn't alter in the
WE direction so acceleration in the WE direction is zero.

The time taken for the drift from zero velocity in the West-East direction
to the velocity v(WEf) is short compared to the overall length of the boat
journey. Therefore, any acceleration caused by the drift in the West-East
direction is shortlived. Had there been a constant acceleration along the
entire length of its journey, the velocity of the boat would have
increased
until it reached land. In which case, it would have been at fairly high
velocity to cause a catastrophic impact. To be able to sail at a constant
speed, the rate of change of the speed or velocity (if you want direction
thrown in), must be nil. That is, no acceleration for a constant speed.

I assume you drive. If you applied a constant acceleration your speed will
go up and up and up. If you applied an acceleration for a small amount of
time, then reached a constant speed, your acceleration is zero. So,
knots/hour which as I have mentioned earlier may endanger the crew of the
vessel, unless it is for a short duration. So I think the answer to the
question I asked above must be that the boat would have travelled most of
its journey at a fairly steady speed. Had it accelerated all the way,
well,
I told you above.

Dyl.



.

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