numbers don't add up!

[rjl1914]

1988 Donzi Ragazza 25' (4600#)w/ 7.4 merc, bravo 1, quick silver 14 x 19 prop, gil manifolds, corsa exh.
With two people and 1/2 tank of fuel(35 gals) peformance is lacking, 41mph at 4200 rpm is all I can get
I think I should be able to push 50 mph, am I missing something?

 

[86 century]

Re: numbers don't add up!

What is your eng Max recommended rpm

Need gear ratio in drive
how is the eng running if you are down on a cly will kill it

 

[rjl1914]

Re: numbers don't add up!

rated max rpm is 4200-4600

I believe the ratio is 1.5

The engine starts easily and appears to run smoothly throughout the rpm range

I have not done a compression check yet

The engine is low hour (500) 330 hp

I should be able to turn a 21 or 23p prop at the 4200-4600 with that hp and torque

with the existing 19p I should be turning at least 5000 based on some of the numbers I have run

 

[Don S]

Re: numbers don't add up!

Are you using the trim, or running with the drive all the way down?

 

[rjl1914]

Re: numbers don't add up!

Yes, I have tried throughout the range to the point of cavitation
I did see 42mph once with only myself in the boat
Also, boat has been rack stored most of it's life and continuously for the last 12 years
I am the third owner
Just turned 500 hours
I have put about 10 hours on it and over 100 gals of fuel so that shouldn't be an issue
seems to run fine just no top end
previous owner was pretty good about maintenance
He only used the boat 5 or 6 times a year and rack stored it
Is in excellent condition for a 24 year old boat so I doubt it was beat on
I tried to find info on what prop was supplied from the factory but having a hard time finding any info about the ragazza.
The 19 seems a bit undersized, I wonder if it has had a problem for a while and they compensated by putting a shallower prop on
I know it isn't a speed hull, but it isn't a pig either

 

[Don S]

Re: numbers don't add up!

Time to start doing some diagnostics on the engine.. Compression test, vacuum test, full tuneup, check the flame arrestor for being plugged.