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Hey guys-Trying to help my cousin. He has an old-non Cobalt boat with the 185 hp, 4.3 Merc motor and a three blade, 17P prop. He is wanting to replace it with a better prop-SS or not. What blade/pitch would be good for a middle of the road boating. They do not ski much. Anyone here have any experience? His existing prop has the numbers 48 78118 A40L 17P

VintageRacer wrote: Thu Mar 04, 2021 9:10 pm Hey guys-Trying to help my cousin. He has an old-non Cobalt boat with the 185 hp, 4.3 Merc motor and a three blade, 17P prop. He is wanting to replace it with a better prop-SS or not. What blade/pitch would be good for a middle of the road boating. They do not ski much. Anyone here have any experience? His existing prop has the numbers 48 78118 A40L 17P

Hmmmmm
Well as you already have a performance background I’ll give you my .03 cents.

3 blades are faster than 4 in wide open scenario. But 4 blades have less slippage. They hold a better part throttle cruise speed.

I would not put a 4 blade in that combo.

Not knowing what elevation vs max rpm vs speed currently I would look for 3 blade 14.25 -14.50 diameter 17-19 Mirage or Mirage plus ( more stern lift).

What is his current WOT RPM and speed?

Mercury does have a prop selector that can get him in the ballpark. In previous boats, I have found that when I switched to a stainless prop, I had to drop an inch of pitch.

https://www.mercurymarine.com/en/us/pro ... !/step-one

Personally, if he is ok with his performance, I would just stick with an aluminum prop.

Wow I disagree with everyone so far. Because there are variables we don't know. Like which 17p is on it now and what kind of boat it is and rpm at wot. Everytime I switched to the same number of blades to a stainless steel prop I went up 1 pitch. Because it holds better doesn't flex. So a vengeance 19 is a good cheaper SS prop. Or a more expensive 3 blade is a Enertia prop. Or you can try a 19 or maybe a 21 high five on it. 4 blade 17 or maybe a 19 offshore prop. These are all mercury props. I'm a mercury prop guy but there are plenty of non mercury ones out their but I'm not as familiar with them. I've ran plenty of 4.3's before so this is my choices. Also start with the mercury prop chart for a great base line. I personally don't like the mirage prop on a alpha drive.

Our 4.3 Volvo Penta with 190 hp had a 21 pitch on it and we never had a problem.
Seldom pulled skiers or wake boarders. Did a lot of tubing with kids.

Does he have a local dealer? One option is to get a few three blade props in his choice of metal, and buy the one just before where rpm at wide open throttle starts to drop. I don't know of another way of picking other than an estimator tool like BruceB linked, but there are lots of prop experts on here as well with years of wisdom.