A .15 should be running a 7x6 to 8x4 range of sizes so for starters you need a bigger prop.

And you're right. That's not enough threads engaged for safety or for wear and tear on the crankshaft threading. How the hell they figure we're going to mount a prop and spinner on there with so short a length I'll never know.

You'll want to find a prop brand with a thinner hub or get one of the nuts that extends down into a washer with a larger hole so that more threads are in engagement. Generally the rock bottom minimum number of turns of full engagement is considered to be 3. But not for big loads. This isn't a big load however wear comes into the picture as well. The threads in a nut with only three turns of engagement is going to wear out sooner from being put on and taken off numerous times.