Not sure what size of props for new bh 1:50 ratio 27,29,31.I have 18 donzi with 400+hp wanting to buy props.thanks craig

What area are you in? You may find that someone will help you out with borrowing a set before you buy that set, or borrowing a set that will help you determine a good starting point. There are also prop stores that offer a "trial" period for a price.

Is your boat a small block or big block?

my motor is a small block 350 brand new.Irun high 70 with my alpha 25 labbed mirrage plus.Ithink i should start with the 29.But not sure.