Originally Posted by Dockhead
I don't question their accuracy (UKHO is the gold standard of hydrography), but it's irrelevant - it's a theoretical test of the two hand methods. We'll take one hour tides built from the 5 minute data and see how well they work
compared to an electronic calculation which will approach mathematical perfection (particularly, no partial hour, which is the bane of the hand methods).
Then we'll see how well they deal with input errors. I think it will be extremely interesting.
I have one small thing to add to the SWL method. It is how to handle a partial hour at the beginning.
I included it in the email
I sent Tim Bartlett a few days ago, but I have not had a chance to go back to my other thread and include it there. This is what I wrote (the alteration to the method I posted at the beginning of that thread is highlighted):
Step 1: Mark the departure and destination
points on a chart. I will refer to these as A and B respectively.
Step 2: Measure AB (= the course distance) to enable you to start gathering data on currents. It need not be measured very accurately at all unless you need to know your predicted SMG for some reason (the time taken is computed separately and does not require this distance to be accurately known).
Step 3: Divide the course distance by the expected boat
speed. This gives time of the journey if there is no current
Step 4: You know the passage will take longer than that if there is total current
against you (and shorter if it it with you), so make an educated guess how much longer by looking at the current along the way (this may actually be difficult, but that is a whole different topic).
This gives you a starting point for how many hour lots of current you need to consider (or half hour lots if this current info is provided). For simplicity I will describe hourly data here, but the technique works equally well for any segment you care to look at.
Divide the course into an equal number of sections as per the number of hours you estimate and determine the current in each segment for the specific hour you will be there.
I know this is only an approximation as the current will not be constant over the hour, but we need to work with what we have.
Step 5: For this next step, it is important to draw all distance displacement
vectors to the same scale as the chart (I think this is actually being taught during RYA course already, as if the tidal stream is strong relative to boat speed it is difficult to accurately estimate the time taken and results can be hit and miss).
Determine what proportion of the first hour the first lot of tidal stream data will apply and work out how much the boat would be displaced by this.
Draw this amount starting from A in the direction the current will take you.
Step 6: Continue adding on hourly displacement
amounts from the tip of each current displacement vector until you can arc
off a speed displacement vector towards B that is getting close to reaching B (eg if you have marked off 2.5 lots of tides and your speed is 5 knots, then you are arcing off 12.5 nm).
If the arc
off from the tip of your subsequent current displacement vector then extends past B, almost all the hard work has been done. You know you will arrive sometime during this last hour of current.
(If the arc exactly crosses B, mark this off. This is your CTS. The time taken for the journey is the number of hours of current vectors you had to use.)
Step 7: Your arc is unlikely to exactly coincide, so draw a line from the start of this final current displacement vector to B. Arc off the distance vector for this time and mark S (for short)
Step 8: Draw a line from the end of the final current displacement vector through B. Arc off the distance vector for this time and mark L (for long)
Step 9: Look at the proportion between SB and BL. This will immediately
give you an idea how much of the final current needs to be applied.
Determine this proportion accurately by calculating:
= SB / SB+BL
Multiply this by the strength of the current for this last hour and you now know the boat displacement due to current for this period.
Mark that point on the final current vector and label it K
Step 10: Join K to B, extending the line past B
CTS = the angle of the line KB before you have made allowances for compass
variation and for leeway.
Distance travelled through water
= the length of KB
Time taken = the length of KB divided by the boat speed.
SMG (if you need to know this for some reason) = the length of AB divided by the time taken.
Step 11: Plot the expected ground track by adding the boat displacement due to boat speed to the tip of the first current displacement vector. This is your position after the first bit of current finishes applying. Mark this point.
From this point add the next lot of displacement due to current, then again add the displacement due to boat speed to the tip of that. This is your position after the next bit of current finishes applying. Mark this point.
Continue doing this, and the last point will coincide with the destination
Join all the marked points and this is your expected ground track. This can also be very easily done on a chartplotter
or using a program such as OpenCPN