Originally Posted by Dockhead
Andrew, to elaborate a little on this -- I'm not sure that it is helpful to think of the destination as being shifted. It's not really. Imagine this -- if the intersection point is half an hour before you reach the destination, and the last half hour of tide behaves in the way favorable to the RYA method, you just keep sailing on your same heading you exactly reach the destination.
It is not so much destination shifting as a mathematical proxy for the other leg of the of the vector triangle. It does not steer you to a shifted destination necessarily, it steers you towards the real destination but using an averaged last partial hour. Using a line other than the course line will not work.
Once we've come to a common understanding on this point, I hope you will address any residual validity in my points a) and b)
I think your explanation and terminology seeks to explain the behaviour of the RYA method, whereas mine seeks to understand the underlying concepts.
(I don't think our explanations are at odds, FWIW)
It's as if you're describing what happens to the revs when I move the 'throttle' lever on a diesel engine
whereas I'm saying "This is why it happens".
I think the latter description is more useful when the former description does not happen as expected.
- - - - -
Here's my attempt to explain the underlying process in conceptual terms, leading up to the phase you are describing:
(for simplicity, I'm assuming no errors in data or execution, a vessel motoring, and current data expressed as mean values for each hour):
The vectors for x whole hours of current are added, from the start point.
When enough vectors have been stacked that the last vector's endpoint (lets call it "C after x hrs", or "C...") is close enough that we can get within an hour of the true destination in x hours of steaming, a position "D" is marked on the rhumb line (a line passing through both the departure and the true destination.)
This mark is made at a distance from "C..." which corresponds to x hours of steaming.
(Sometimes this will not be possible, for geometric reasons, but that's for another discussion)
A line is now drawn from C... through D, and this is taken as the Course To Steer.
The length of "C...D" corresponds to the distance through the water
from the departure point to D, after a period of x hours
My explanation pauses at this point, because this is the end of the conceptually simplest phase of the process, and the information we've now got is potentially useful. (eg "That's near enough; we can eyeball it from here")
A shortened version of what we've done so far, sounds like this to me:
"Work out the point D on the rhumb line which is nearest to the true destination, reachable after x whole hours. Work out the CTS for that destination"
(optional: Work out the distance through the water
to that adjusted destination)
The next step is to get us to the true destination.
The distance through the water to the true destination is arrived at by what you call 'inflation' of the optional distance mentioned above. I don't think I need to go into that in any detail, because there is only one thing about the remainder of the process which seems relevant to my point, and that is this:
If we follow the same CTS we worked out for the adjusted destination, we will only pass through the true destination under the special circumstances you describe.
Under those circumstances, both the course to steer and the distance through the water will be correct.
Under any other circumstances, neither will be correct
But ...... IN ALL INSTANCES we will first pass through the 'adjusted' destination, at the time (and distance) we expect to.
So I think it's helpful to treat it as an interim destination. Sometimes it will be useful to use
it as such, but conceptually I think it is always valid to think of it as such.
If we can't remember the fiddly bit at the end, we've still got something solid and comprehensible (and correct), and it's back on the rhumb line.
Which (as long as we KNOW that's what to expect) is generally going to be a good thing.
Please tell me which parts
of this make sense to you, and which do not.