Yes, I misunderstood your post.
In real life, D = S x T. 10kn x 24hrs = 240 nm
In the game, 240nm x cos (Lat) = _____
what Lat are you in?
If you're on the Equator, then, 240nm.
@ 1 degree Lat, then 239.9nm,
@ 30 degree Lat, then 207.8nm,
@ 45 degree Lat, then 169.7nm,
@ 60 degree Lat, then 120nm.
Distance x cos (Lat) is what I use in SH3...and it keeps me in the ball park.
I play time x1 and I take 5 sights per day and connect the dots (fixes). I use the measurement tool with compass to set my course between fixes and waypoints, in order to keep my boat headed for points on the ocean that the game understands.
In real life you could use the "Great Circle" formula to set your course between point A and B. On a Mercator Projection chart, a Great Circle course is a curved line. On a Gnomonic Projection chart, a Great Circle course is a straight line. If you have a desk globe and a piece of string, you can put one end at San Francisco, as example, and the other end on Japan. When viewed top-dead-center, the string appears as a straight line, and a curved chord from a side view.
What is happening in the game, is the curving of the course line which is transparent to the player.
To expand on the Gnomonic chart a bit further, absent a Navigational computer, mariners would use a Gnomonic chart to fashion a straight line course, paying sharp attention to where the course intersects with major latitudes and longitudes. These course Lat/Long crossings are then transferred to a Mercator Projection chart. Once layout is complete, it can be seen the course is curved and where course changes will occur on the Mercator chart as separate, short legs. A succession of straight, connecting courses, called Rhumb lines. (almost like calculus
) The overall effect appears as a curved course. I have not tried it yet, lacking a desk globe, but, I don't see why it wouldn't work for our purposes in the game. At least, you'd be able to see why the course changes between your waypoints.