More RPM after break-in??

[Tahoe 55]

Should I expect to get a few more RPM out of my engine once it breaks in and "loosens up"? I don't want to re-prop now and then exceed my RPM limit once the motor gets good and broke in.... Should I wait to re-prop? I'm about 500 RPM low with the stock 19p prop.

 

[achris]

Re: More RPM after break-in??

By 500 low do you mean out of the range or 500 low from the centre of the range?

Revs low is overloading your engine... I prefer to under prop than over prop.... A 2" shift in prop usually equates to about 400 rev at the top end... If a 17" would bring you JUST into the bottom end of the range I would DEFINITELY change now... and I would probably even opt for a 15".. you using ali or Stainless?

Will you pick up a few revs as the engine loosens up? Probably.... but not 500...

I'm currently running a 17" and will bang on the rev limiter at WOT with a light load, but it performs like magic.... Great fuel consumption (1.3 litre per nm, all the way through the rev range) and the engine is never overloaded, even with 2 big guy, 10 SCUBA tanks, a fridge and 3 days of supplies....

Chris.............

 

[Tahoe 55]

Re: More RPM after break-in??

Well, I'm at 4500 now and the range is 4600-5000. I figure I should shoot for close to max after re-propping. The Merc, prop selector suggests a 17p in stainless and a 18p in the newer Enertia prop (because it turns 200 RPM faster) That's what I was going to do, 18p Enertia. I'll put me at 4800-4900 and then if I get another 100 after break-in, I should be right on... Right??

Thanks for any help, I'm new to all this boat stuff and trying to learn....

 

[achris]

Re: More RPM after break-in??

Well, I'm at 4500 now and the range is 4600-5000. I figure I should shoot for close to max after re-propping. The Merc, prop selector suggests a 17p in stainless and a 18p in the newer Enertia prop (because it turns 200 RPM faster) That's what I was going to do, 18p Enertia. I'll put me at 4800-4900 and then if I get another 100 after break-in, I should be right on... Right??

Right!!!

 

[Tahoe 55]

Re: More RPM after break-in??

Thanks Chris!

 

[HT32BSX115]

Re: More RPM after break-in??

Well, Alrightee then.......this IS the prop forum...


It appears that you have a new boat and you just didn't rebuild the engine?

If that was a rebuild I would want to know what it did before the rebuild (when it ran correctly)

If it's a new boat then the dealer probably just didn't prop it correctly. (This is IF you mean 500 rpm low is 500 RPM LOWER Than the range recommended by Mercruiser)


Your engine has a top rated RPM of somewhere around 5100 or so (rev limiter set at about 5200 or so) your owners manual should indicate the range..... 4800-5100 rpm etc....you want to prop for the top of the range at WOT, not quite into the rev limiter....

Your drive is probably a 1.5:1 ratio....it's usually stamped on the side.

If you know these numbers you can use a prop slip calculator to determine what you need.

Go here for a good prop slip calculator....single prop slip is usually 12-15%

You'll need to know accurate RPM (some tachs can be as much as 500 rpm OFF at the top of the range so they really need to be verified) and speed (GPS)

Once you know all this stuff you can make a decision on what prop you need. Your best prop for all around performance will probably be a 3 blade SS prop.


Cheers,


Rick

 

[Randybeall]

Re: More RPM after break-in??

Sounds like you have the right idea, but I have questions. How many hours on the engine. Let it break in easily, if it is over proped then by all means go down to what gets it right. Don't take advantage of WOT but for very short periods till you have 5 to 10 hours on it. At least the first oil change in a 4 stroke. Even the finest engines need break in if they are to last a long period. Race engines are different, for example, NASCAR engine, 600 miles. Take it back apart and rebuild it again. Yes, they have mule engines which are run in practice again and again but the winner is fresh.

 

[Tahoe 55]

Re: More RPM after break-in??

This is a new boat with 13 hours on the engine. At 11 hours was the 1st time I opened it up and even then just for 30 seconds at a time. Just long enough for things to settle in at max speed/RPM trimmed out. It came from the factory with a 19p Vengeance prop. It seems to be pitched too high for the boat. The 5.0 MPI has a RPM range of 4600-5000 per my Merc. owners manual. I'm at 4500 WOT trimmed out @ 50 MPH on the speed O'meter. This is with my most common weight/load on board. 54 gal fuel, 20 gal water, 2 adults, 2 kids (90lb & 30lb), and all out other normal gear....

 

[mtnrat]

Re: More RPM after break-in??

Use a remote tach and GPS for rpm and speed. Boat gauges are notoriously inaccurate. I use a timing light with rpm readout.
That will let you know truly where you are at.