RPM Question on I/O

[Hashi]

I have a 5.0 MerCruiser MPI with Alpha drive. The recommended rpm range is 4600-5000 rpm at WOT. I had a 17p Vengeance prop and was going about 52-55 mph at 5200 rpm. I just switched to a 21p Enertia and am impressed with this prop. Now, I'm doing a little over 60 mph but, my rpm's dropped between 4400 and 4600 rpm (below the manufacturer's recommendation).

Do I need to worry about carbon build-up in the cylinders or anything else?

Thank you in advance for your advice.

Rich

 

[rndn]

Re: RPM Question on I/O

It's more of a strain on the engine than carbon buildup. Think of going uphill on a 10 speed bike in 10th gear. Your engine is designed to operate within the RPM's your manual states.

 

[Hashi]

Re: RPM Question on I/O

Thanks...RNDN. I'll get a smaller pitch prop.

 

[MikDee]

Re: RPM Question on I/O

Why don't you just try putting in a good brand of synthetic, or synthetic blend oil, and maybe some synthetic gear lube in the drive, either, and, or, both, should help you run cooler, and with less friction, instead of changing the prop. I bet you would gain a few hundred rpm's, and find no need to change the prop. I've found this to works for me, Then let me know how you make out, Good Luck, Mike

 

[Hashi]

Re: RPM Question on I/O

Hey, thanks Mike!...I'll try this! Any recommendations on synthetic brand?

 

[Bry21317]

Re: RPM Question on I/O

Hashi,
I have a 94 Merc 5.0 also, but mine is carburated. I am really looking at getting an Enertia prop also. So how does it compare to your other prop, or was the 17 pitch the only one that you had?

I can turn a 21 Pitch Rev 4 SS prop, very heavy and large Diameter up to 4750 RPMS on my motor at about 49 mph, mine is stated for WOT at 4400-4800. With my 21 pitch Aluminum I can run up to 5100 rpms with a full load of people and gas and 50 mph, no matter what I have on the boat.

I really want to try to hit the mid 50s, and these speeds are all by GPS.

Bryan

 

[Hashi]

Re: RPM Question on I/O

Hi Bryan,

From what I experienced, I ran the 21 pitch Enertia and got 61 mph from 55 mph with the 17 pitch Vengeance. But my rpm's dropped from 5200 to 4400 (200 below Merc's. specs.) So I sold the 21p Enertia and ordered a 18 pitch which should get me at 5000 rpm's with minimum load. I'll have to give up some speed in order to get in the recommended rpm range but you really don't notice the difference in speed gain or loss.

The Enertia is a great prop...It handled very nice and was very responsive on turns. The boat got up on plane very fast (did not time) and stayed on plane at very low speeds. I'm kinda sorry that I had to sell it.

The Enertia is also a heavy prop and will increase speed by only 2 mph with the same pitch. Not sure if that's what you want, but again, you really don't notice the difference in speed gain.

If you're getting 5100 with a 21p alum. prop, I'd stick with it or think about increasing the pitch on aluminum to 22 to get a little more speed and will decrease your rpm by 200 (200 rpm drop for every inch of increase in pitch and vice versa).

That's pretty good that you can get the same speed with a full load. Maybe I should switch to an aluminum prop.

I don't have GPS, so all the readings are by boat/motor gauges.

Hope that helps.

Happy Thanksgiving!
Rich

 

[Bry21317]

Re: RPM Question on I/O

Rich,
The aluminum Prop that I have is a 21 pitch Hustler, so its a performance Aluminum prop. I did try the next size larger which was 23 pitch, but the hole shot really sucked and it dropped me to like 4200 rpms, so it dropped me way down. The speed was only about 43, if that fast by GPS.

The rev 4 that I have is very very heavy also. I think I am going to still try the 21 pitch Enertia, as that should put my RPMS right where I need them. I lost some Speed with the SS from the 4 Blade, so I think if I got to the 3 blade enertia, I should beable to hit 53 at least. I am guessing. I would be happy with that, for now. Then I would have to upgrade to a 4bbl carb down the road to get more power.

Bryan