you need to do both. but id call bob at ultra prop and ask him. he will probly tell you to change the blocks down low before you cut'em down. but im guessing 42" and some 14* blocks would give you quite a bit more rpm. is it a 3 blade?

 

4 blade

 

I was thinkin 11* and Bob said cut down to 36"

 

good grief. yea on a 4 blade i guess that about right. just hate to see you loose that much air

 

oh. uh? well???? thats alot of drag on a 13. you may wanna get a reduction. thats the only way i see you turning that bigga blade. i was wanting to get a 4 blade 42" this week with smaller 16* blocks. but i got a new 20.8 honda im gonna direct drive. i dont suppose yours has a 1 1/8" hub? i mite be interested.

 

how big is you boat? 36" will glide you threw the water but not over much. and its gonna be loud.

 

16x56 and a 1 inch shaft

 

48" prop and a 1" shaft direct drive equals disaster waiting to happen.