Effect of changing gear ratio

I have a 25ft Scottie Craft cuddy cabin with an AQ225 linked to a 270 outdrive. I purchased the boat without the outdrive and received the 270 free. I believe it was off a houseboat. The gear ratio stamped on the lower unit is 2.15. At 3,000 RPM the boat only does about 10-11 mph. WOT puts me at 4100 RPM and about 14 mph. This seems very slow. I am looking at purchasing a lower unit with the correct V8 ratio of 1.61 to 1. My question is how much of an impact should this create? I would like to run at 3100 RPMs and at least 16-17 mph all day with 4-5 people headed offshore. Is this pheasible with the 225 HP motor? This would all be assuming I select the prop with maximum potential. Thank you for your help.

Re: Effect of changing gear ratio

In a nutshell, on boats, props work best for overall use when between 17 and 21 pitch. These pitches give you best hole shot and best high speed. In order to keep props in that range, you change the gear ratio.

If you had a gear ratio of 2.15, in order to get the proper WOT (wide open throttle) rpm, you would need a very high pitch, say 26" pitch. What would happen,with such a steep pitch is you would probably loose top end speed. and you would have a hard time getting on plane. Basically you would be using the resistance of the water to keep the engine from over revving but it would do absolutly nothing for performance.

Get the right gear ratio.

Re: Effect of changing gear ratio

Thank you, Don. All makes better sense now. I believe the prop i bought for it was an aluminum 15X21 and it kept over revving so I had them rework it to about a 23 pitch to get the RPMs down.
I plan on making the switch next weekend and will post a performance update once I get it out on the water. I am also looking into removing the trailerable bottom paint that is about 3 years old. Hopefully the upgraded ration and lack of bottom paint will yield the results I am looking for.