Ocean Stream Speed - To Inger.

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.

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?

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

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.

.

Relevant Pages

  • Re: sliding riggers
    ... Please see Walter's very clear graphical depiction of velocities of boat, ... for the 1st 2/3 of the stroke the rower's CofG is increasing its velocity - it is accelerating. ... Moreover to produce an average mean velocity per stroke of the bow ball with respect to the finish line, any acceleration during the leg drive must compensate for any deceleration during the recovery. ...
    (rec.sport.rowing)
  • Re: Ocean Stream Speed - To Inger.
    ... You mentioned that knots/hour in your thread about "Ocean stream speed". ... what you've written is an acceleration. ... Now, after the boat has entered the stream, does the boat continue to ... Let's put velocity to one side, ...
    (sci.archaeology)
  • Re: Ocean Stream Speed - To Inger.
    ... You mentioned that knots/hour in your thread about "Ocean stream speed". ... what you've written is an acceleration. ... Now, after the boat has entered the stream, does the boat continue to speed ... No, in addition the southward speed is reduced as the boat leaves the southward current;-) This would also be a short period of deceleration but resulting in a markedly lower speed over ground assuming the boat is to continue sailing southwards, with a further reduction to overcome the eastward drift you show above. ...
    (sci.archaeology)
  • Re: How I decided which new sculls to purchase
    ... moving Mass, reacting against the hull, is already up to speed and the ... acceleration" of the hull due to the increased drag. ... "Ideal" is a constant hull speed, impossible in a rowing shell by the ... In my opinion, to make the boat go fastest, you want maximal early ...
    (rec.sport.rowing)
  • Re: which gadgets?
    ... As soon as a boat yaws it is ... but *this* provides the desired acceleration immediately. ... > No, there is no lag. ... Acceleration creates velocity immediately. ...
    (uk.rec.sailing)
Loading