I have a feeling I won't be the first person to think of this, so all those for whom this is old hat, my apologies, however I found it useful and it doesn't seem to have been mentioned in the thread so far, so here goes.....
You can use the reverse side of the SACF to do simple TSD calculations by lining up the distance in yards with the time taken to travel it and reading the speed off the sine scale at the pointer; but lining the time and distance scales is dependent on a good eye, which I don't have. Anyway. it's a relatively simple matter to make another index pointer and mount it on top of the time scale/inner wheel, which is what I did. It's really useful for say getting speed by timing the target through the crosshairs, because all you need to do is use the floating index pointer you've just added, to line up target length and time taken, at which point the speed can be read off the inner wheel pointer.
I haven't tried it yet, but I imagine it would work just as well for points obtained from say, a radar plot, or active sonar maybe.
Anyway, like I say, I doubt it's an original idea, but it's handy and it doesn't get in the way and is worth considering, I think, for anyone planning to build one of these.
In lieu of a thousand word description, here's a photo. It's set up to show 117 yards in 25 seconds = 8.5 knots or thereabouts....
*Edit* Overstating the obvious, but because it's a logarithmic scale, this could just as well be saying that about 42 minutes at 8.4 kts will get you 11800 yards, or as close to it as makes no practical difference. Slide rules rock
