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adamac22
06-10-2011, 03:10 PM
Ive got a 1989 Donzi Ragazza 21' Cuddy, it has a 5.7L OMC. I have a 15X17 3 blade prop now. it will run 40mph at 4,200rpms pretty slow i know. Can anyone recommend a better size for speed?

Crystal Pro
06-11-2011, 09:23 PM
There are lots of good reference shops out there. I just got a new 14" 4 blade 18 and had to go up to a 20 after a short trial. Only way to be sure is to try it on the boat.

Tacoma propeller. 253-272-5065
www.tacomapropeller.com (http://www.tacomapropeller.com):popcorn:

Walt. H.
06-13-2011, 10:29 AM
Ive got a 1989 Donzi Ragazza 21' Cuddy, it has a 5.7L OMC. I have a 15X17 3 blade prop now. it will run 40mph at 4,200rpms pretty slow i know. Can anyone recommend a better size for speed?
I would say right off the bat I would drop 1 inch on the dia and 1-inch on the pitch to get your rev's up to 4700 to 4800 rpm's because right now you are lugging your engine costing you gas mileage besides a few mph.

Donzi_Dude
06-13-2011, 04:53 PM
your in the correct rpm range (4200-4600) so if the boat is not laboring too much you will gain little by going to a smaller prop.

Walt. H.
06-13-2011, 11:32 PM
your in the correct rpm range (4200-4600) so if the boat is not laboring too much you will gain little by going to a smaller prop.
Sorry D.Dude,
But your recommendation is correct only for the 4-cylinder variety, and is completely wrong for his V-8. So I have to say politely as possible that you're incorrect with your contradiction to my post, and even though I didn't need to check what I recommended earlier I dug this link up for both you and adamac22 to avoid any doubt or hurt feelings.

http://www.perfprotech.com/store/articles/mercruiser-v8-engine-specifications.aspx
Hopefully you'll be in complete agreement now...
WH

Donzi_Dude
06-14-2011, 05:28 AM
Ive got a 1989 Donzi Ragazza 21' Cuddy, it has a 5.7L OMC. I have a 15X17 3 blade prop now. it will run 40mph at 4,200rpms pretty slow i know. Can anyone recommend a better size for speed?

Sorry D.Dude,
But your recommendation is correct only for the 4-cylinder variety, and is completely wrong for his V-8. So I have to say politely as possible that you're incorrect with your contradiction to my post, and even though I didn't need to check what I recommended earlier I dug this link up for both you and adamac22 to avoid any doubt or hurt feelings.

http://www.perfprotech.com/store/articles/mercruiser-v8-engine-specifications.aspx
Hopefully you'll be in complete agreement now...
WH


maybe!



http://www.piranha.com/motor_specs.php?id=2894



Manufacturer: OMC
Model: "5.7L Cobra; 5.7 L Cobra"
Year: 1989
Horsepower: 260.0
Cylinders: 8
Type: 4 stroke
Gear Ratio: 1.40:1
Min WOT RPM: 4200
Max WOT RPM: 4600
Spline Count: 15
Drive Type: Sterndrive

Walt. H.
06-14-2011, 12:56 PM
D.D,
You will not get the best performance running at the bottom of the rpm range, and other then OMC painting that Chev 5.7/350 engine gray rather then Mercury black or Volvo red it is still the same identical engine with all the same spec's.
So I still stand recommending starting with achieving as close to 4800 rpm with a minimum load on board so when that 21-ft cuddy cruiser is loaded with 3 to 5 people and a weekend supply of food and fuel that will usually drag the rpm's down by two to three hundred rpm's he won't be @ 3900 straining itself, but rather @ 4500 and getting up on plane will be achievable instead of nearly impossible.........

VetteLT193
06-14-2011, 01:32 PM
The rpms are a little low but the prop seems right. have you done a compression test and general maintenance to make sure it is running properly?

The engine could just be tired from age, but it could be mis firing or fuel, or other.

It wouldn't make a whole lot of sense to prop down because you have a problem elsewhere.

Donzi_Dude
06-14-2011, 04:35 PM
D.D,
You will not get the best performance running at the bottom of the rpm range, and other then OMC painting that Chev 5.7/350 engine gray rather then Mercury black or Volvo red it is still the same identical engine with all the same spec's.
So I still stand recommending starting with achieving as close to 4800 rpm with a minimum load on board so when that 21-ft cuddy cruiser is loaded with 3 to 5 people and a weekend supply of food and fuel that will usually drag the rpm's down by two to three hundred rpm's he won't be @ 3900 straining itself, but rather @ 4500 and getting up on plane will be achievable instead of nearly impossible.........

Walt, i would tend to agree but its not written in stone. if the boat does not labor i still see little benefit in a smaller wheel. like if it can pull a skier up then its not too big. if its a total dog and is hard to plane ect, then you open a can of worms.



The rpms are a little low but the prop seems right. have you done a compression test and general maintenance to make sure it is running properly?
The engine could just be tired from age, but it could be mis firing or fuel, or other.
It wouldn't make a whole lot of sense to prop down because you have a problem elsewhere.


i agree 100%.