How much money do you have? I found this http://www.airmartechnology.com/uplo...res/CS4500.pdf by accident and wonder if it might work. I'm also wondering if you couldn't find one used at a dock or salvager.

The brochure is light on how to hook it up but it would fit your needs almost perfectly. After a quick search I couldn't find anything on the "Airmar paddlewheel format" this is supposed to put out.