Repower Not Enough Speed

[vette53]

I Need Some Advice. I Just Repowered My 1988 25' Donzi Ragazza With A New 350. I Am Using A Good 1987 5.0l Omc Outdrive. I Have Done The Prop Cals And It's Not Working Out For Me. The Engine Runs Up Good On Stand To At Least 5600 Rpms, Didn't Have The Guts To Go Higher Without It Being In The Water. I Was Figuring To Get Up To At Least Mid To Upper 50's With It. But I'm Not. This Is Some Of My Data. Gps Was Used For Speed. I Used Three Different Prop's.
1) With 14-1/2 X 19 I Could Only Get Up To 4100 Rpm's And 42 Mph.
2) With 15 X 17 I Could Only Get Up To 4150 Rpm's And 39 Mph.
3) With 14-1/4 X 25 I Could Only Get Up To 3200 Rpm's And 34 Mph.
I Must Be Thinking Wrong. The Outdrive Gear Ratio I Figured Overall Is 1.59. I Have A New Set Of 21:16 Gears To Install Which Would Give Me An Overall Of 1.42 But I Would Have Thought The 21:18 Gears @ 1.59 Overall Still Should Have Done The Trick.
Very Frustrating. Help!

[onesubdrvr]

I Need Some Advice. I Just Repowered My 1988 25' Donzi Ragazza With A New 350. I Am Using A Good 1987 5.0l Omc Outdrive. I Have Done The Prop Cals And It's Not Working Out For Me. The Engine Runs Up Good On Stand To At Least 5600 Rpms, Didn't Have The Guts To Go Higher Without It Being In The Water. I Was Figuring To Get Up To At Least Mid To Upper 50's With It. But I'm Not. This Is Some Of My Data. Gps Was Used For Speed. I Used Three Different Prop's.
1) With 14-1/2 X 19 I Could Only Get Up To 4100 Rpm's And 42 Mph.
2) With 15 X 17 I Could Only Get Up To 4150 Rpm's And 39 Mph.
3) With 14-1/4 X 25 I Could Only Get Up To 3200 Rpm's And 34 Mph.
I Must Be Thinking Wrong. The Outdrive Gear Ratio I Figured Overall Is 1.59. I Have A New Set Of 21:16 Gears To Install Which Would Give Me An Overall Of 1.42 But I Would Have Thought The 21:18 Gears @ 1.59 Overall Still Should Have Done The Trick.
Very Frustrating. Help!
First off, I did an engine swap on my 21' Ragazza with a BUILT 350 (360 cshp), I was running a Turbo I 22p, topping out at 60mph. I figure if your 350 is stock (80-100hp less), plan on 8-10 mph less which puts you at 50-52. ALSO keep in mind MUCH heavier boat than my 21 BR, so maybe even a little less than that. But yes, that still means you should have atleast 10 mph more out there somewhere.

Also, you will see RPM's over the span of the break in come up so, I'd say break it in for a good 10-15 hours using the 15x17 prop, change your oil and filter, then start playing with props.

Double and triple check your gear ratio and how was the coupling alignment when you put the drive back on? it is possible you're not aligned, robbing you of some hp to the prop.

Wayne

[MOP]

Wayne is right grab a 17 and get a little time on it, the motor needs to spin a little higher closer to 5K. You may need to go with a cupped 15, I think you will get where you want to go don't rush this stuff takes time.

Phil

[mattyboy]

.
1) With 14-1/2 X 19 I Could Only Get Up To 4100 Rpm's And 42 Mph.
2) With 15 X 17 I Could Only Get Up To 4150 Rpm's And 39 Mph.
3) With 14-1/4 X 25 I Could Only Get Up To 3200 Rpm's And 34 Mph.


mid to upper 50's is very optimistic with a big boat like that and SB power doesn't mean it shouldn't spin at 4400- 4700 rpm though

a 14 1/2 X 19 and a 15 X 17 is almost a push less pitch but increased diam
and a 14 1/2 X 25 needs serious ponies to push , let it break in and maybe try like a 14 X 19 and see

good luck

[MOP]

The factory spec on that engine is 42-4600 it does not hurt to spin them a couple of hundred RPM higher, peak torque on that engine is very close to 4000 and stays pretty flat to 5000. It should be kept under 4800 for longevity but it does make its power at the upper end, the lower end will give you better economy but kills the hole shot and top speed. I have propped hundreds of boats we have always propped them high and many times re propped them at the 10 hour check, with a full load of people and fuel it is important not to go below the 4200. No engine will live long if you lug it!

[Mercrewser]

14.5x17 would be my choice, 45mph would be all I would wish for. Is there bottom paint on this boat? You would need 350 hp to hit 50.

[vette53]

Well, I've got about 6 hours on the engine and I'm on my 3rd oil change. Can't have clean enough oil. I figure oil is cheep. 1st change was after 5 minute initial startup. 2nd after 30 minute run and 3rd after 5 hour mark. My 350 has about 310 HP. The boat bottom has no bio paint, just waxed it last weekend. Wayne, can you get the coupling alignment off and the outrdrive will still drive? I used a guide pin, but I'll remove it tomorrow and check it. Can't go out on the river anyway tomorrow the inlaw or outlaws are coming over. I need something to keep me busy. I do have a 14-1/4 x 21 prop I'll try next week and see what the results are. But, I'll take your advice and put more hours on her ASAP with the smaller prop 1st. It's really great being able to get such great advice from experienced people. I'l keep you guys upgated with the results.

CRAIG