Old props vs. new props with interchangeable hub?

[Sundancer]

I've been running our boat for the last two years with the same props. Performance was great and I had no reason to change them. Prior to our last outing, I dinged one on something and had to pull both props off to get the damaged prop fixed.

I put the spare props on and noticed some differences. The props I've been running use the interchangeable hub that are used on boats nowadays. The old props that I put on are specifically for the Alpha I and don't have the interchangeable hub inside. The old props seemed noticeably quieter and I seemed to have picked up some mph according to the GPS and tach? Actually, it was my son that noticed the speed increase while I was noticing the lower decibels.

All props were supposed to be 14 x 17, but I'm wondering if the old set was reworked to a 14 x 19 now. I'll figure that out next Spring. The fact that they were quieter seems odd? Any suggestions why one set would be be quieter?

 

[Lazy Daze]

Quieter? I don't think I want to know where you had to put your ear to hear the props!:smt001

Maybe the older props where just more "in tune"? Meaning each blade was more like the other.

Really, though, I don't know. :huh:

 

[jhild]

There is a lot more to propeller design that pitch and diameter. Even within a specific brand (Mercury, for example) not all props of the same size will perform the same way. It’s certainly possible that both propeller sets are 14 x 17, but one design is better suited for your application.

I can only guess as to the noise issue. Maybe the spare set was balanced properly, thus reducing vibrations in the outdrive? Maybe the hubs on the set you took off had become compressed and were allowing for too much play on the shaft?

 

[Go Aweigh2452]

Bring both sets to your local prop shop and explain your issues and see what measurements they come back with. Having this conversation without the benefit of a prop shop expert is little to no help. JMTC.

 

[Sundancer]

I probably should have them checked. They look similar, but I know I couldn't tell if they were off slightly or one cupped more than the other. I have been stewing about this for a few weeks and figured I'd toss it out there to see what responses I got. I know it's an odd one and without a decibel reading and knowing how accurate tachs are, there is no "science" behind it. The fact that both my son and I noticed it is what had me scratching my head. I wondered if the interchangeable hubs had a larger opening for the exhaust or maybe a smaller one sending some of the noise out another path? I know, grasping at straws!!