You don't have enough power in that motor for a submerged prop of 40 mm. You did good with that prop shaft though. That shaft allows you to use the Graupner K series 4mm threaded props. They have a lot of them between 29mm and 45 mm in size. You would be better served in using a K33 or K35. Check with Offshore Steven should have a good selection. I will check my own stash of K series props. I used them a lot. There is a better motor for what you are doing. Kyosho used to sell a 19turn long can motor called a Magnetic Mayhem. It has good magnets and replaceable brushes like the car motors. You could likely get one on ebay. I used them on somewhat larger or heavy boats. Easy to cool the brush hoods too.Really helps with the burnout.
n.h.schmidt