: I have a 1988 Searay 300 weekender...

[jfeichenlaub]

: I have a 1988 Searay 300 weekender with Ni-Bral, 17x15 Cup

> props and twin 5.7L Merc Inboards.  Searay indicates that the props

> should be

> 17x16 and should achieve 4400 WOT.  I have replaced the tachometers a

> few years ago.  So I believe that they can be trusted.  My WOT is

> 3600-3800; the port engine is able to run a little higher rpm.  It is

> my understanding that a prop pitch of 15 is less aggressive than 16

> and as such should allow the engines to produce higher rpm.  I spoke

> to another boater with the same boat who has 17x16 props and gets about 4200 rpm.

> Is it possible for my props to increase in pitch without causing

> noticeable vibration?  Also, both props appear to have signs of

> cavitation pitting.

[TitanTn]

Yeah, your 15 props should allow more RPMs than stock. The fact that you're getting less says that something else might be wrong with the engines (aside from something like bottom growth, bent blades, etc). Have you figured anything out yet?

[jfeichenlaub]

Not yet. I am planning to pull the props and have them checked/tuned. If my rpms are still low, I plan to use a tach to check the engine rpm against my installed taches.

[Bob_440_DA_w_Ca]

It is very hard to get accurate rpm off the old analog tach Gage's. You can get a photo tach online for cheap so I would do that first before and investment in pulling props and having them checked.

[wingless]

The Mercury Quicksilver Digital Diagnostic Terminal (DDT) is a very useful tool. I could not imagine maintaining my boat and engines w/o this tool.

The DDT reports the engine RPM as digital data on the text display.

IMO the dash tachometer gauges should not be trusted for accurate measurements unless they have been calibrated using accurate laboratory instruments.