I have a Navman MULTI 3100 on my boat, which records speed, distance and depth. The transducer is on the stern, the transom. The depth meter worked fine but the speed showed zero. Looking at the transducer I realised that the paddle wheel was missing. The spindle was there but no paddle wheel. A google search later I discovered that the company Navman (or North Star) was no longer trading and that all socks of the paddle wheel were gone (in the UK at least). Possibly it is this part: 46302 or AA002078. Anyway the information I found suggested it has six paddles, is 27mm across, 10mm wide and has a hole of 9.5mm for a plastic insert, which for me was still on the 2x28mm stainless axel. The paddle wheel has magnets that send a signal to the LCD device when the paddle wheel rotates.

Therefore, I designed and printed one. My fifth attempt worked well, included here. The paddle wheel has six paddles, two of which have a couple of recesses for some tiny Neodynium magnets, found here (pack of 50 for less than 5 pounds): https://www.amazon.co.uk/gp/product/B007JTKHR6/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

These magnets are 2mm diameter and 1mm thick and pretty strong for their size. They are triple coated, which is supposed to avoid oxidation. In the end I used 8 in total - 2 pairs in each of two paddles. The magnets have to be side-on, which I didn't realise initially. On reflection I would buy some slightly larger magnets, and have one each in two paddle wheels. I superglued them in, which seems to work fantastically well using some flexible (and cheap) PLA that I had.

Use at your own risk: I have yet to test it properly on the water. It seems to do the trick, and spins easily.

It's slightly curved on the bottom so you'll need a little support.

UPDATE: Six years on and it's still working perfectly!