I have a 1967 Stevens 18ft Silhouette. The original engine was a bored and stroked Chevy 409 which was reverse rotation, 480 cubic inches. I estimate the hp at 450. When the boat was ordered this information was supplied to Stevens. They installed the Casale V-drive with 17% overdrive. The unit was a right hand turning V-drive. I have installed the 409, after changing the rotation in my 55 Chevy. I have built a Big Block Chevy to go in the boat. It turns left hand, and is 510 cubic inches, and is rated at 750hp. I am getting ready to put the V-drive in a crate to send to Casale to change it to LH rotation and change the overdrive ratio. Not sure of optimum ratio. I'm thinking 30%. I just purchased the new LH turning propeller, a Signature 11x16. The original prop was a 12x16 with the 409. Any suggestions on the new OD ratio would be appreciated. This will be the third engine I've built to the same specs but they have gone in cars. I constantly twisted them 7,500 RPM but I'm thinking 7,000 in the boat.