making an inboard flatboat but can't find any aluminum props that are threaded. The only way i found to fix it was to get a stainless prop or find another shaft even though i have put many hours into searching for the right one. If anyone knows what i can do please help. The reason i want aluminum is because i would rather break the prop than the motor or shaft. It is more easily replaceable. The motor is a 18 hp duromax on a 12-15 foot flatboat

Had plenty inboard mudboats,here is what I would do and what I have done countless times.

Brass or stainless steel sub surface weedless propeller and a 3/4" or 1" shaft.

You can either use a 3/4" shaft and let that be your week point
Or rely on the coupling key to shear.

I have never broke a crank,coupling or u-joint.
Have lost props,bent rudders,broke shaft and sheared many cheap steel keys.

I'd atleast want a 1" shaft if I was going to ride alot in stumps and unfamiliar territory.

However its very hard to break a 3/4" shaft.
There will be more screw on props avalible for 3/4" shafts.

I agree with the above post,i used to run 3/4 shaft and 1" .....from 5 hp to 21 hp.
Never broke a crank,coupling,universal.....or even a key.
Tore up plenty brass wheels though,bent 1 shaft.
That was about a 10 year daily use boat in stumps and mud.
Personally id be more worried about the duromax holding up.
Read that its the cheapest of the china clones.