Posts: 410
Location: With my son on the water | From my own personal experience I had a rig with a motor in the mid size and with a lighter load it worked fine. However when I started fishing with two more guys my hole shot was terrible so I reduce the prop size to correct the hole shot. Smaller props need to run a higher RPM to maintain the same speed as even a prop one size larger. On that rig when I did that the reduced prop size/increased RPM reduce my gas mileage significantly.
Now if you look at the torque charts of say the old Merc Optimax (75,90,115). It is the same block for each motor, the only difference is the fuel mapping. On those three motors the low end torque is exactly the same so the hole shot will be the same, the only difference will be top end. This is very common with motors that use the same block.
So if you want a true difference (increase) in low end torque of an engine you need to jump block size. So if you compare something like a Merc 115 and a Merc 150, the Merc 150 will actually be working less at low to mid RPMs than the 115. You will also have situations where the speed is the same but the RPMs of the 150 is measureably lower than what the 115 would be, to the point that the 150 will actually get better gas mileage than the 115 because it is running at lower RPMs.
One of my best friends was the lead engineer on the Opti package at Merc and he has shown me the charts, they are on line too. For me this made a good case to max out the HP and I have never regretted it.
Everybody spends their money differently, if a guy is buying a new boat he has already committed to spending too much money on a toy so don't get cheap now. It's like comparing a Legend to a TI, both will do the job but one costs a bunch more.
Have a great weekend everyone. 
Edited by btfish 2/21/2015 10:34 AM
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