Prop comparo, SS vs Alum on my boat.

[ParallaxBill]

My 18' 1972 Glassmaster Tarpon 165 hp Mercruiser will run 41 mph (gps) at WOT 4,300 rpm with my new condition 14.5" x 19" pitch Mich Wheel aluminum prop.

With my older but excellent condition Mercruiser SS prop of 13.75" x21" prop I am getting 43 mph at WOT 3,900 rpm.

Both tested with two adults, some fishing gear and half a fuel load on smooth water, no wind, trimmed out for max rpm and ride. WOT rpms on my boat should be 3,900-4,300 rpm.

Which should be preferred? Both seem to come out of the hole quickly with the edge to the SS prop and the SS prop seems to be running a little smoother on the top end. Could be the rpm's being less but I'm not sure.

 

[ParallaxBill]

Re: Prop comparo, SS vs Alum on my boat.

Everyone must still be at work or got out early on the water.

My feeling is the SS prop may be my best choice since it does everything the aluminum one does at less rpm meaning I don't have to turn the engine as hard to achieve basically the same speed. Running at the bottom of the recommended WOT rpm range or 3,900-4,300 shouldn't hurt a thing.

This is still open for other opinions however.

Also, this may help someone with a similar boat/engine configuration as well.

 

[studlymandingo]

Re: Prop comparo, SS vs Alum on my boat.

Is this on an I/O?​

 

[ParallaxBill]

Re: Prop comparo, SS vs Alum on my boat.

Yes, an L6 250 cubic inch GM.

 

[ParallaxBill]

Re: Prop comparo, SS vs Alum on my boat.

I should also note that with the SS prop I was able to trim up much higher than with the 19 pitch aluminum prop. That could be why the ride felt so much smoother.