RPM Concerns!!! Prop Issue maybe?

[Pescado48]

Hey All hoping I can get some help here...

Ran the boat for the first time this year with a new tach, replaced as was not working last year.

My concern is the high RPM's I can get to without the hammer down...I'm at 4800rpm and doing about 28mph with still quite a bit of throttle left. Last year I was reaching speeds of 36-37mph but the tach was not working so not sure about the rpm's. It seems to me that I could easily go over the rated RPM at WOT. Not sure what to do here...I'd like to get back to 36mph...I don't want to risk damage to the engine to see what my RPM's are at with the hammer down...any thoughts?

Here are some spec's

-2004 Merc I/O 3.0l (can't make out the numbers on the air filter cover)
-prop quicksilver 1916P19
-alpa1 gen 2 drive ratio 2:00 serial number 0W518566
-21' cuddy 2250lbs

 

[mfkadz]

Is there a calibration adjustment on the back? You may need a good true tach to calibrate your new tach to.

mike

 

[Pescado48]

Only adjustment on the tach is the cylinder quantity selector...checked this, all good.

 

[Sea Rider]

Which are the wot min-max rpm range for than engine ?The only possible way to check if that engine could over rev is going full throttle, it's only a short wot run, the engine won't even notice it. Test combo lightly loaded on flat calm, no wind waters if possible. If engine is over revving as soon you begin to add more weight or passengers wot rpm will decrease accordingly.

Happy Boating

 

[Dukedog]

have you changed "anything" on tha boat other than tha tach? is it tha same as it was before your tach problem? did it over rev before your tach problem started?
or has tha tach been a problem since day one of your ownership??

 

[steelespike]

Your tach is wrong. You can't go 4800 rpm and only 28 mph unless the trailer is still attached
Your saying your at 28 because you don't want to go over the max rpm?
Many universal tachs the selector is for the number of poles in the stator not the cylinders.
Are you sure the switch is on the right setting. 4 stroke 4 cylinder I/O Is there a setting specific to I/OsIs it intended for an I/O?
28 is just about cruising speed if we calculate for rpm using 15% slip we get about 3700 rpm a fairly normal rpm for cruising speed.
The tach is either set wrong or defective.

 

[Pescado48]

Thx for the fed back guys...

-I don't see any other possible adjustments on the tach ( I ordered a factory replacement tach )I'll try and post a picture of the old tach
-I don't think the new one is defective...I cross referenced it with a Tiny Tach (TT) and the TT varied but no more that 100rpm
-I haven't changed anything else on the boat and yes the gauge has been defective since owner ship
-WOT for this engine is 4400-4800 rpm

Thx for the help so far ... I know I could get a new prop with a higher pitch value to reduce the rpm's but not sure if pitch is the real problem....what else could cause the engine to reach higher than normal rpm's.

 

[Sea Rider]

What else could cause the engine to reach higher than normal rpm's.

If tach is accurate, engine has spot on timing advance and carb sync, I'll bet on non ideal prop pitch or prop on its way becoming spun while at wot if happens to be a veteran prop. Make a prop slip test and check.

Happy Boating

 

[steelespike]

So you say you have made no changes other than a new tach.We have a 3.0 motor a 21' boat that when new with a perfect prop
and ideal setup might do 40 mph and commonly run 19" props for slightly less speed.37 or so.
You changed the tach and now you have 41% slip (normal slip is usually low double digits).
If the tach is right your saying you routinely over revved to get to 37.
If we assume the 41% slip is correct on the prop from last year
​ If we take it to 37 with 41% slip we get 6900 rpm If we improve the the slip to 30% we get 5800 rpm.
This is last year same prop no tach.I don't think a 3.0 can do that.
Now if the tach is right then it's a prop problem.Then based on 3.0 history if the prop is defective a
known good one or new will solve the problem. If its just a pitch problem then at 30% slip we have to
make up 1000 rpm thats about a 5" pitch change that would make it a 24" pitch Way too much for a 3.0.
Maybe you have a friend that has a 3.0 you could borrow their prop 17 or 19 for test.

 

[Pescado48]

Not sure about the slip factor...all I know is last year I could get up to 37mph (and should still be able to this year)but had no working tach so don't know the rpm's at 37mph. This year i have a working tach and at 28mph I'm at 4800 rpm's with still some throttle left.... i guess the best thing to do is drop the hammer to see what my WOT rpm's are..how far can i push her before i should ease up?

 

[steelespike]

Exactly,wot long enough to get max rpm and gps speed. If its really over revving
it may not let you do it.If your actually at about 3600 now You will have lots of room to get to a true 4800.
Think of it like in a car when you drag race or check top speed.I think the red line in a modern car V8 is like 5000 - 6000..

 

[Pescado48]

So finally got out on a flat lake Ontario.....GPS said 34.8 MPH and RPM was 5250...hammer down and trimmed...so I need to reduce some rpm's here but would love to improve my hole shot...Im thinking a 4blade 21P prop ...any thoughts?

 

[steelespike]

The 3.0 in a 17 to 21' boat is about the easiest set up to trouble shoot.typical prop sizes are 17,19,21.
Your 21 is likely slower than the smaller boats but results still are around the 3 prop sizes.
34.8 at 5250 with a 2.00 ratio,19"prop calculates at 26% slip. About twice of normal slip.
If we assume the tach is accurate then high slip is a fault of the prop or perhaps the hull.
I feel that a 21" 4 blade will definitely lower your rpm will likely ruin what little hole shot you have now.
Does the boat respond to up trim? Is the bottom clean and smooth?Is there a hydrofoil on the outdrive?
If the hull responds well and bottom is clean then the prop is suspect not efficient.
Perhaps it is old and worn?A new 19" should improve the slip numbers and reduce rpm.
Maybe you could borrow a known good 19" prop.
A good 19 at 34.8 mph 12% slip produces about 4400 rpm.
If we go another way a 19" at 4800 12% slip produces about 38 mph.

 

[Sea Rider]

Are you testing wot numbers with you alone, lightly loaded combo on flat calm no wind water cond to achieve 5250 revs ? If so 450 + revs from max 4800 wot rpm isn't a wow number. As soon you start to add mnore weight 5250 rpm will drop acccordingly. If combo takes its time to plane add trim tabs or doel fin to plane faster.

Deck weight even distributioin along being well trimmed is a must go for. If boating with you alone throttle less, have an eye on tach so not to over rev engine at 4800 rpm. It's better to achieve slight more wot numbers than slight less, will achieve better hole shot.

Happy Boating