Thanks guys - it's becoming clearer.
As mentioned I sail on a Viper 640 sports boat (really a 21ft open transom dinghy but with a 120kg bulb keel) where all three on board have their hands full, so being able to do the lat/long math's and key into the GPS in the short time available - from course laying to warning signal - will be a tad difficult. (I'll need a calculator as well!!) But an example may be useful for other times, so if you can provide one, that would be appreciated
By round-the-bouys races I mean our regular club races (4 courses of 10-15 Nm from which RC choose one just before the warning signal based on wind direction & strength) around a combination of 10+/- fixed marks (navigation pylons and permanently tethered bouys), sandbanks, headlands, etc. - so finding the next mark, and knowing the current angle to it, are useful bits of info when you can't actually see it. Hence the point at the end of my last post which danielt63 stated as:
...few GPS units will give you VMG toward two different waypoints at the same time.