number of windings to get a desired voltage?
This question does not have an easy answer. It will depend on how strong
the flywheel's magnets are, how fast it is turning, and how well the field
is coupled to your coil. Are you planning to make an iron core, with shaped
polpieces to fit closely to the flywheel?
Also, if no load is put across the windings, how much drag will it produce?
None, if there is no load.
Just looking for a way to power lighting on a small boat that doesn't
require lugging a big battery around
This contradicts the no-load stipulation in the previous question. In order
to produce light, you have to produce power. If you know how much lighting
wattage you are going to need, and then divide by the expected efficiency of
the coil/light setup, you will have a ballpark answer.
In my opinion, this is a very challenging (difficult) way to get some light.
It's a cute idea, and if you can make it work, your fortune will be made.
Read another thread on whether to apply for a patent. :-)