so here's the deal, I got a boat on the cheap and I am bringing it back to life, it's a 1990 19 ft fisher aluminum center console with a 90 hp mariner two stroke with carburetors and oil injection, the lower unit would not shift into gear when I got it, and I got a deal on a used lower unit that came off a 115, it bolted up fine and works well, the boat comes up on plane well, and runs fine at speed, but I'm running well below wot rpm (around 1000 lower), so is the rpm drop simply a product of the gearing in the lower, or do I need to re-prop, as I said it comes up on plane easily with 2 people, coolers and fishing gear, so I don't think loss of torque is an issue (very light boat). is there any way I can calculate what prop I need or do I need to just experiment? or do I need to work on pulling the original lower apart and fixing it (it was used in salt water and not well maintained so getting apart may be tricky). I'm not sure of the gear ratio in either lower (not at home to check), the prop is original to the boat/motor I'm pretty sure it's a 13" pitch (not at home to check right now) 3 blade stainless.
Any help would be greatly appreciated.
Any help would be greatly appreciated.