Prop design for low wattage boat

I have been printing props and they are OK for testing at my power level (well most of them, I printed my JavaProp 39 percenter, nasty looking thing, and for sure too fragile for marine use). I haven't cast a prop yet but I have done a fair bit of casting into 3d printed molds and there are some polyurethane resins which have fairly good mechanical properties (SmoothOn Onyx for example). If that isn't strong enough it might be worth cutting some carbon fiber tow into the mix.

Still hunting around on JavaProp. How do I make it do pusher props? I really like the export of .iges files, neat feature.

Is 39% @ 3000 rpm good/bad/fanciful? From what people say here, I am going for much lower rpm. I can easily gear to about 600rpm and will order a gear motor to test to lower rpm. So far I haven't been able to design any props in JavaProp which look remotely convincing, so I'm working on fairly generic designs in SW but will reduce the aspect ratio following what people here have advised.

 

I've hit 81% on JavaProp for 100W props.
What diameters were you using?

 

You want the largest, slowest turning, 2 bladed prop you can fit (AR should be very high). Anything else just wastes power. Propellers this size should be getting in the 60-80% efficiency range, not 40%.