Are you designing the sub. to meet performance specifications?
Does weight matter or does it interfere with the indented performance.
Some time, you may have to cut the weight down from the start.
If this is the case the weight and size of the shaft will matter and you may
have to select the proper material to maintain the weight requirement.

wrote in message
ups.com...
Michael wrote:

I'm making a large scale (13ft) model submarine and am currently
designing
the propulsion unit for it.

Try going to psubs.org. They're building manned ones, but as small as
your model. Someone there can lead you to the answer.

M