Propeller advise due to RPM and engine Performance

[Oliverlm]

I have an 225 hp Optimax ( 2007 ) on a 22.5 feet boat.

I have test with a 17" pitch propeller and it runs 50.5 mph at 6,000 rpm ( discarded, too much rpm ),
Then i use a 19" pitch it runs 49 mph at 5,650 rpm ( good rpm but low speed, don't understand )
My last test was with a 21" and it runs 54 mph at 5,200 rpm ( it sounds great, less rpm and more speed ) but some peolple have told me that i can blow the engine because it should gives around 5,700 rpm at maximun acceleration. For me it doesn't make sense if some can explain it to me please it will help me a lot.

Note: the 3 propellers were the same brand and style. ( only change of pitch )

 

[steelespike]

Re: Propeller advise due to RPM and engine Performance

My first thought is the 19 is different in some way.Are all props new out of a box not loose on the shelf?I sure would try another 19.If the 21 has some cup you could have a prop shop remove a little.I've read here both an increase in speed and rpm with less cup.

 

[Oliverlm]

Re: Propeller advise due to RPM and engine Performance

My first thought is the 19 is different in some way.Are all props new out of a box not loose on the shelf?I sure would try another 19.If the 21 has some cup you could have a prop shop remove a little.I've read here both an increase in speed and rpm with less cup.

No, they are not new. Today i'm going to test with a different 19 pitch maybe other brand. The one i used was a mercury mirage plus or maybe another 21" just mercury mirage.

Because i remember i test a 21" mercury mirage in other boat and it was around 5800 rpm and when i change it for a 21"mercury mirage PLUS it increase 3 mph and reduce rpm to 5,500.

 

[steelespike]

Re: Propeller advise due to RPM and engine Performance

When you jump around to different models/brands You can't get a base line of what works.
You need to procede in a logical fashion.You may be able to find a dealer that will let you test props till you find the one that works.Expect to leave a sizeable deposit and purchase your prop from the dealer.
It appears the 21 was close but not enough rpm.A known good 19 in the same model/brand should add about 300 to 400 rpm. The motor is rated at 5,750.
Again you may be able to have some cup or pitch removed from the 21.