My dad has an 18.25 foot 2006 ranger fisherman tiller with 90 horse yamaha. The engine vibrates very badly at very low rpms and we like to troll and backtroll so this is a problem. The motor has a four blade mercury trophy plus prop (stainless steel, 17 pitch). We took it to the shop and they said it was the prop. I was wondering if a three blade yamaha alluminum prop would solve the problem. If anyone has a 618T please let me know if you have had similar issues. Or if anyone has a similar boat or similar problems or any input at all it would be greatly appreciated.