I have a 17' aluminum fishing boat with a 1974 70 hp looper. WOT the boat runs 32 mph, at idle it run 4-5. When I try to troll at 8mph it wont, either it will climb to 10mph or it will drop to 5 mph. I cant get it to go 8mph. Would dropping from a 17 pitch to a 15 help this? I dont mind losing some top end. Any tips on