I was going to add to the above and the time expired on a change. So here is the whole shebang.
Can't be a bad thing...........tons of variables here but basically you have a gear train and the engine RPMs, aka drive shaft RPMs vs the lower unit gear ratio and the prop gear ratio.......pitch all play into that equation. Prop diameter has to do with HP available to turn that wheel under the conditions mandated by the situation at hand and a small diameter prop running fast was expected to do what a larger diameter prop turning slower could do.....not necessarily so ask history shows us.
My non professional comment about mid range Mercs (40-60 hp triple cylinder loopers) of years ago, where they were turning a 10" diameter prop, had to turn them fast to get the thrust that was expected of that engine. They were good on lighter, faster hulls.4 Later on, running the larger diameter prop with a 13" diameter, at a lower RPM, did a lot better job of not only taking care of top end, but also doing what it needed to be done when on the top end running unloaded. Today's Command Thrust lower units are a testiment to that and those with the CT lower unit have the data to prove the point.....Q'Buoy from Great Britian is an example of a satisfied customer with his 115 CT and what it can do that a normal 150 HP is required to do.
On slow revolving props with a large diameter to get some push under heavy loaded conditions, My last 2 summers in high school were spent on tug boats relieving the crew for their summer vacation. Was a thrilling experience for a teenager. On my first year the tug to which I was assigned was in dry dock getting a replacement 13 foot diameter, shallow pitched (didn't find out the pitch but not much) prop replaced as it had a bent blade from hitting something in the Intercoastal waterway. The engine was an Atlas brand, 400 HP diesel but the torque was way up there to turn that wheel. It could push 4 ea. 135x 60', 8' draft. barges a few MPH all day and all night.
On the flip side, growing up, had a friend that, with his dad, were into light weight racing hydroplanes (10' roughly) used direct drive lower units (Quicksilver direct drive racing LUs as I recall) on small engines and light boats, turning 7-8" diameter 2 blade props, very high pitched at a 1-1 ratio with the engine RPMs which just screamed.......Too get on plane the driver would squeeze the throttle and the prop would ventilate, releasing the spring loaded throttle and reapplying got a little more MPH before ventilation...etc. etc. etc. finally on plane an away they went.......Just something of a comparison as to large diameter, slow revolving vs small diameter fast revolving props.
My 2c and worth every penny you paid for it....grin.
