Crunched my prop so I am trying to figure out what to replace it with.

[Spence340]

Did my first full day of boating out on the ocean the other day and make a lot of rookie mistakes including one disaster when I drifted into a jetty and ruined my propeller. I was thinking it might be a good time to try another size prop since my boat was having trouble getting level and would only plane when I put a lot of weight on the bow. It is a 86' Sea Ray MC21 Seville that was repowered in 2008 with a new Merc 4.3L engine and new Alpha 2 with 1.81 ratio D and I will be running it heavily loaded with people most of the time and I use it for cruising and tubing. When I hooked up a Big Bertha to it the other day with two college kids on it I must say the boat did pull it without any problem. I think my WOT should be 4400 to 4800 but I am often in the 5000 range now. The propeller on it was a Merc 14.25 x 21 and from what I have seen the heavier you run the lower the pitch you want. Should I try a 14.25 x 19? I did notice that when I would get on plane I had to back off of the throttle because the boat would really take off then. I have no idea what mph I am doing since the speedo does not work and I don't have GPS but she was flying right along at some points.

 

[Chris1956]

First of all, if she runs 5000 RPM, you should have no issue getting on plane. You should not need to put weight in the bow. Is your motor tilted all the way down, when you start to plane?

I had a similar boat, with the same power as you. I ran a 21P prop. However, since you are overreving the motor (if tach is accurate) you would need to increase pitch.

Right now I would stick with the same pitch prop, until you get the conflicting issues sorted out. I would recommend a 21P Turning Point Hustler in 3 blade. I ran a similar prop on my '88 SR 18 footer with a 4.3LX MerCruiser, and she ran about 42MPH at about 4500RPM. and had good power.

 

[QBhoy]

If you are running 5000rpm and tach is right, you are already too high up the revs. If you put a smaller pitch on it, you will most likely blow the motor up.
Strange that you have problems getting on home plane and you evidently have a boat that's under propped.
I would check the true rpms. Take it from there.

 

[Spence340]

I have been playing around with the tilt but I have kept it down for the most part. I agree I should probably just stay where I am.

 

[Spence340]

One of my problems is probably too much weight in the boat. There were seven people in the boat the other day so I probably had over a 1000 lbs of extra weight. I picked up a used stainless 14 x 22 prop today but I can't believe how much more it weighs then the aluminum one. Does all the extra weight on the SS prop put stress on the drive system?

 

[Chris1956]

The extra weight of the SS prop is no issue. The SS prop will put a lot more stress on the drive system, if you hit something, as it will not deform or break the way an aluminum prop does.

If the bottom of the body of water you boat in is sand or muck, SS is not an issue. If you boat in a place that has rocks to hit, a SS prop could really damage the drivetrain. if you hit one..

 

[Spence340]

Thanks, the guy who sold it to me said the same thing so I guess I will be extra careful while it is on the boat. I just ordered a new aluminum Mercury one also so I will see what the difference is on the water and use the aluminum one in tricky waters.

 

[NYBo]

The extra weight of the SS prop is no issue. The SS prop will put a lot more stress on the drive system, if you hit something, as it will not deform or break the way an aluminum prop does.

Doesn't the rubber hub absorb the impact and sacrifice itself when the prop hits something solid?

 

[Chris1956]

Yes, the rubber hub absorbs some of the impact. The aluminum prop also absorbs some of the impact, when it bends, like the crumple zones in your car. The stainless steel prop doesn't absorb much impact due to it's strength.