Robert Hunt said:
Can someone explain all this to me?
Well, even with all of the physics and scientific theory behind props, it truly is not rocket science, as it it more complicated than that. No manufacturer's X by y prop is identical to that of their competitor. And sometimes prop testing is you must try it proposition.
That 10" diameter by 13" pitch prop listed could give the same results as that 9.25 x 12.5" pitched one. Those in reality are so close, by diameter and pitch - individually and collectively - that they're practically the same size. Note though that that doesn't mean that they'll perform the same, if even equipped on the same motor.
Many factors bear on a prop's ability to perform, to include blade design, material, thickness, edge geometry, hub vent holes and rake ... to just name a few ... besides the physical performance factors of 'bite' and slip, etc.
The 2 stroke moved boat nicely, which will be quicker?
In short ... depends! The 4-stroke could perform better, as they usually spool up to a higher rated RPM than 2-strokes. Or it could be worse, especially in the mid-range where 4-strokes are not known for a good power/thrust performance.
To your new OB:
I personally would forget all about your former OB and that former prop and just make sure this NEW motor is properly pitched for your rig AND the load it will carry. I personally setup all my rigs to be no more than 200 RPMs off the maximum RPMs with the load in the boat carried on EVERY trip (i.e., standard gear in the boat), plus you and ~1/3rd fuel on board.
I've said it here a million times ... boats only get heavier as the season progresses. Plus there are environmental and seasonal factors that impede best performance, e.g., doing the prop test in early Spring when the temperatures are cool and the running bottom is clean, versus testing in humid July/August when normally aspirated OBs can lose us to 10% in performance due to thinner air, complicated by a fouled bottom, if left in the hull and covered with slime and weeds. Ethanol fuels also de-rate an OB power by to 10%, as the fuel has less energy per mL.
You should try this calculator: viewtopic.php?f=4&t=37433
Quick too
