Prop size for 16 foot starcraft with a 65 or 70 horse motor

[ShoestringMariner]

Just wondering what prop size you 16 foot StarCraft owners are running for those with 65 or 70 horse motors...

i’m running a 1976 65 horse Merc 650. I have gotten it up to 33 mph GPS on a 10 inch diameter prop. I don’t know the pitch.

The reason I’m asking, is because I have picked up a couple of 70 horse OMC‘s from the mid 70’s. They are mothballed until I can go through the one that I think will be a runner. These motors came props that looks to be about 13 inches in diameter. Pitch unknown.

when I got my boat with the 650 on it, it did have a larger diameter prop. But the shade tree mechanic I was using switched it with another saying that I was going to “blow the motor up” with the bigger one. I’m not sure I quite understand this. Would a smaller prop not allow the motor to run faster? And possibly run it into the redline? I do not have a tachometer to tell you how fast my current motor is running.

in any case, I’m surprised to see a 3 inch larger diameter prop for only five more horsepower. They are monstrous in comparison

 

[ShoestringMariner]

Hard to capture in the photo. But they are substantially larger

 

[MTboatguy]

There is more that goes into it that just HP, the final drive ratios, power band on the motors. I have a 50 Evinrude on my 16 foot Starcraft SS right now, with virtually the same prop, at least it looks like the same one and I am getting ready to put an older Merc 650 on the boat and the prop on the Merc is in fact smaller than the Evinrude prop. You might try asking in the main forum section devoted to props.

 

[DLNorth]

It's hard to tell by your pic, but it appears that the hubs are different sizes, meaning the gear boxes are different sizes.
But you need to know the rpm and speed before you play with different props.

As a data point, we have a Merc 850 on our SS16, IIRC the prop is 13 x 19. I'd "guess" the OMC 70 props to be 13 x 15 or 17.

 

[ShoestringMariner]

Thanks guys. I’ve discovered that the pitches on these OMC props are 15 or 16 and 17.

Gear ratios make a lot of sense. The gearbox on the OMC is considerably bigger than the Merc. Actually, the whole bottom end looks beefy as anything on the OMC. I wonder if the size of the prop also has anything to do with overcoming the water friction caused by the girth of the lower unit. The one on the Merc is very slim and streamlined.