Anyone familiar with the Mercruiser 3.0 props?

perna00

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I've got a 2007 Bayliner discovery 195 with the mercruiser sterndrive 3.0 (135hp). It has the 14.5 prop at 21 pitch. It is the basic black painted aluminum 3 blade. My engines WOT is 4400-4800. With this prop, it will rev up to 5,150 rpm if I test its max in the water. What can I use to gain some usuable speed (understood I lose some hole shot) so it will let me ride at a higher speed at regular rpm?
 

airshot

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First thing I would do is to check your tach, sometimes they don't read correctly and that is something you really need to be accurate. If you truly are reaching that kind of RPM then that is way to high and your engine will be short lived. I have the 140 hp version of the 3.0 and mine redlines at 4600 with two adults and fishing gear but a larger boat than yours. If your numbers are accurate then I would give serious consideration to a 4 blade prop. However your RPM does sound high for the boat you have so....I would seriously look at the tach to be sure it reads correctly before anything else.
 

airshot

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Forgot to add something....I would have your current prop checked out to be sure the hub has not spun, if the hub blows out it will read higher on the tach but you won't get the bite and speed you need.
 

perna00

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Sounds like I have to double check the rpm's. i can get a few speed readings off the gps next trip out. I'll take the prop off and check the gearing to be if it is worn down. It will be more apparent on prop or the hub? Finally, that's the best way to double check rpm?
 

Bondo

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Finally, that's the best way to double check rpm?

Ayuh,... By hookin' up a known good shop tach, runnin' the motor, 'n cross-checkin' the 2 tachs,....
 

steelespike

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While a good tach can accurately tell us about the rpm An accurate lightly loaded wot gps speed can get us started
toward solving the problem/s.
It has been my experience that a truly spun hub won't drive the boat. I have seen a prop spin and then work normally
for an indeterminate time but will fully fail eventually.I don't believe a prop will creep And if it does creep it won't be enough to affect
rpm it will fully fail when it affects the rpm.
 

b.gagnon

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I am shocked that you can turn a 21p.... That size boat with a 3.0 I would think a 17-19p.
I think you have a bad Tach or a Spun hub on your prop! A friend of mine had a 19 with a 3.0 and a 17p worked well but the 19p had a bad hole shot.
 

Scott Danforth

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I ran a 19p on my 3.0. Tries a 21p and the motor RPM dropped to 3900 from 4700. I also ran a 17p. And 15p for water sports.

Did the OP verify the tach yet
 

steelespike

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A "18 ft" with a 3.0 is one of the easiest to prop for basic performance.21" usually is fastest but suffers with a lot of added weight or in water sports.
A 19" usually improves hole shot a does better with weight and water sports it may over rev.
A 17" will usually do water sports with a fully loaded boat.But will over rev with a light load.
A older 3.0 will often be a little tired and require some fiddling for best performance.
And there are particular props and boats that may solve certain issues.
 

perna00

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A few more days I will have the RPM reader in hand. I'm waiting for the mail. I'm assuming that the gauge probably is off. When I tried tuning the timing and RPM last time after a full tune-up, I set the idle off the gauge and the engine ran rough and I had to increase the RPM slightly above for a stable idle. I've already swapped a few other gauges that had had problems (oil, temp). I'll post results when I find them out. I'm thinking of maybe going to a SS prop to increase performance slightly as well.
 

airshot

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Just going to a SS prop won't do squat, still need all the other info to select the correct pitch. You don't really have enough motor to worry about a performance prop.
Also keep in mind that a SS prop is tougher, so if you strike something underwater rather than a blade shearing off something inside your outdrive will probably give way at a much higher cost. Bigger performance engines can benefit from a SS prop but a little 3.0 is probably not going to gain you any performance for the cost.
 

perna00

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Thanks for the info. My last engine was a 5.0 omc so most of my knowledge was bias off that engine. I'll get the correct numbers up for this one and see what you think.
 

b.gagnon

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Going with a stainless prop will require minus 1-2" of pitch (depending on the cup)compared to an aluminum one....
 

perna00

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I got out the meter. I didn't get a chance to run it on water but doing various rpm tests it was about 10% off. That puts my 5200 down to 4680. Right where it should be. (Plus explains why I had to set the rpm's higher for stability based on the gauge)
 

perna00

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So Gagnon, if I go with a stainless 14.5 21p like I have now I will need to adjust the pitch?
 

steelespike

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I wouldn't do any prop changes until you have a base lightly loaded wot rpm and gps speed.
Near as I can tell we don't have anything to start from.
 

airshot

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Yea...nothing beats an actual on the water test with proper instrumentation to know for sure. Hate to see you drop big bucks on something that doesn't work right. Who knows... once you get your gages working correctly and a good gps you may be happy with what you have. Just keep in mind that while that 3.0 is a very good motor....it is not a speed demon and does not like to be run at wot for long periods of time. Anything over 3800-3900 rpm for long periods of time will be asking for trouble as far as long motor life goes. You may well find you need a couple different props to make the boat/motor combo do what you want it to. I carry two props in my boat, one for normal crusing and one for heavy loads/tubing. My 22' with a 3.0 uses a 19" for normal crusing and a 17" for heavy loads and tubing.
 
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