By tide is Vector triangle .... simple geometry drawn on a north-up compass representation - Hypotenuse is distance thru' water (logged), Adjacent is distance over ground made good (of chart), Opposite is drift. Make sure that H is drawn on the compass representation on the course steered, A is drawn as per course taken of the chart actually made good. Scale must be same for all lines, and does not matter on ratio, as long as all are same ratio.
EG. Speed log reads : 5.0 nmiles, course steered 035 Charted distance over same time : 4.5 nmiles Charted course made good : 047
Now draw a line 5 inches / cms long at angle of 035 Draw second line starting at same origin, 4.5 inches / cms at angle 047.
Join up the two ends of these lines and that is the tidal / leeway drift ....
If you want to 'pre-calculate' expected course made good etc. or any other of the lines, you draw two of the lines and measure angle / length of the third.
By leeway is estimated based on observations / experience is only usually applied as offset to course steered.
Hope that above helps ..... I'm sure that another may explain more eloquently / simply !
Good nautical book-shop is darn sight cheaper, and its only a 'beer-mat' vector diagram ..... Invest in a good quality Coastal Nav book / tutorial .... get an old chart, Chart Agents will sell 'Cancelled charts without title block' to practice on ...... and let loose with your compasses, pencil and protractor !
Simple geometry covers 90% of nav, spherical trig covers the rest, along with common sense.
Having navigated professionally all my life ......
Yep, if you need to ask this (and I CERTAINLY would have 12 months ago, so no criticism here) - do the Day Skipper, much cheapness and you'll know this type of stuff inside out, as well as meeting (and boozing with afterwards)other "boaty" folks with widely differing experience - on mine were everything from a commercial fisherman to couples who'd never been to sea once, boat owners/non owners, the whole gamut in fact - really good mix of folk on these courses, so no one feels the odd one out. Cannot reccomend it enough to you.
