Inboard Repower Checklist: Gear Ratio, Shaft RPM & Prop Size Explained

Free Nav Rules Course - https://www.nauticed.org/powerboating-courses/view/navigation-rules-for-powerboats?utm_source=youtube&utm_medium=video&utm_campaign=zows0zMxJ6o https://neprop.com Thinking about repowering? Do the prop math first—before you buy an engine or transmission. Gear ratio and engine RPM set shaft RPM, which determines the propeller diameter your boat needs. If your hull only fits a 13" prop but your new setup requires 16–17", you’re stuck—and you can’t compensate with pitch. This video explains how to choose engine + transmission combos that match your prop diameter limits so you don’t end up pulling the gearbox back out. What you’ll learn - Gear ratio → shaft RPM → prop diameter (why slower shaft RPM demands a larger prop) - Why you can’t fix a diameter problem with pitch - Common failure cases (e.g., switching from 1:1 to deeper reduction and exceeding prop clearance) - Real-world example: replacing 2.88:1 with 2.5:1 and the maneuverability hit - How to plan a repower so engine, transmission, and prop all work together - Repower Checklist (use before you buy anything) - Measure prop clearance & max diameter your hull can accept - Get target WOT RPM from the engine manufacturer - Decide on gear ratio to hit a workable shaft RPM for your max diameter - Confirm prop options (diameter, pitch, blade count, cup) with a prop shop - Validate dockside handling needs (thrust at idle, gear engagement feel) -Lock the engine + transmission only after the prop is sized on paper - After install: sea trial to confirm WOT RPM, speed, and maneuverability 00:00 - Repowering a Boat: Plan Everything First 00:30 - Engine + Transmission Choice Impacts Propeller Size 00:58 - Gear Ratio / Gear Reduction Explained (1:1 vs Reduction) 01:51 - Common Repower Mistake: Installing Wrong Gear Ratio 02:49 - Repower Checklist: Talk to a Propeller Shop Before You Buy Pro tip: Talk to a propeller shop early. They’ll ask the right questions and size the prop to your hull limits, engine RPM, and gear ratio—saving you from costly do-overs. #sailing #powerboating #powerboat