LOW WOT RPM (3 litre Mercruiser) EVEN with different props

[Mr. D]

Hello,

The 3 litre mercruiser (1999) (alpha: 115 HP) on my 18 foot Larson does not get up to target RPM (4400 - 4800) even at WOT. Started with a 23 pitch but only went to 3800 RPM and 42 MPH. Went down to 18 and now at 15 pitch (15.25 diameter) but still only get 4200 RPM at WOT (trimmed) and 38 MPH. On the other hand, I weight 225 lbs and can now get up on one ski within 4 seconds. Tons of power out of the hole but still will not echeive ideal RPM. The mechanic says the tack in fine and the pitch is fine...However he also winterrized my boat and I started the season with a 750$ bill because of freezing in the lower unit.

I JUST WANT TO ENJOY THE WATER. Thanks

 

[P 0 P E Y E]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

4200 rpm is close enough. I would relax and enjoy the boat.

The mechanic letting the out drive lower unit freeze? Bad mechanic or could it be newton's laws.

Many stern drive owners elect to have the drive removed and serviced by the professional. then stored inside, it costs more but usually pays off in the long run. They change the oil, impeller and grease up things with new gasket and o-ring. They will even clean and inspect the drainage passages.

Removing the prop also saves the hub.

In your case, chances are that water collected in the gear case and was trapped and froze. Not the mechanic's fault. Make sure that the drive is in the a position that minimizes ponding next winter.

 

[Don S]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

However he also winterrized my boat and I started the season with a 750$ bill because of freezing in the lower unit.

Please explain. What was froze, and how was it repaired?

 

[ziggy]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

i think i'd still be concerned about your wot. generally speaking, 1'' prop pitch change results in a increase or decrease of aprox. 200 rpm. so depending on with way ya go, more or less pitch. assuming that's true.
23p=3800
21p=4200
19p=4600
17p=5k
15p=5400
something else other than your prop may be amiss.. maybe time to go to the adults only thread and check out the won't reach wot thread...

fwiw. when i got my boat i could achieve only the bottom of my wot spec. lugging an engine ain't good for it. ideally you want to hit the top of your wot spec if possible. my boat reacted what i would consider normally. i was at 3900 w/ 21p alum. i went to a 19p alum. and now achieve 4300... my spec. is 3900-4300.

Many stern drive owners elect to have the drive removed and serviced by the professional. then stored inside, it costs more but usually pays off in the long run. They change the oil, impeller and grease up things with new gasket and o-ring. They will even clean and inspect the drainage passages.

that's the way i do it. does that mean i'm a professional? just kidding. i know i'm only shade tree.... but i try hard...

 

[Maclin]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

I ran the figures you gave thru a prop calculator to see what the drive ratio would come out to, I got 1.3 which is very much off for a 3.0 engine. Maybe the fix was a different drive with improper ratio for the 3.0, or maybe my calcs are just that far off. But, like Don S asked, give us more details on what the lower unit freeze problem actually was and how it was fixed.

I put in these 4 factors:
Engine RPM = 4200
Propeller Pitch = 15
Propeller Slip = 15%
Speed MPH = 38

This calcs to a drive ratio of 1.33. This is all only as accurate as the numbers given.

 

[Alumarine]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Here's what I know about 3 litre Mercruisers and 18 foot boats.

If you weigh 225, and it gets you up ok on 1 ski from the water with both feet in the bindingsi, I'm thinking your WOT will be greater than 4200.

I'd double check your tach.

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Thanks for the info sirs.

- Marc C, I just checked my Tack and the setting is correct (4 cylinders).
- Maclin, The lower unit Drive serial reads 2.00 R. Not sure if that is the correct ratio.
- Ziggy, I agree the RPM is way too low for a 15 pitch on this boat.
- Don S & Popeye, Winterizing was done in the fall and the receipt indicates the lower unit was removed and resealed. However that was done from my front yard. Can this be done by one person without any equipment. If not, this was a lie. Not sure what was replaced in the lower unit. But I was told the gimble was really off center and that bearings needed to be changed. Wouldn't this have been obvious in the fall if the lower unit was removed? Thinking of bringing the boat to another dealer (the only one left here) and take my chances again. CHEERS,

Andre

 

[Maclin]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

One or more of the RPM, MPH, pitch and ratio is not accurate. If you are truly hitting 38 mph I think the boat is performing well, but with a 15 pitch and the "correct" drive ratio of 2.0 (normal for the 3.0) the engine would be absolutely screaming at 6200+rpm to be able to get to 38mph.

 

[Alumarine]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

It may be set correctly, but not accurate.
Somethings not right.

- Marc C, I just checked my Tack and the setting is correct (4 cylinders).
Andre

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

I agree Marc C and Marlin, someting is not right.
Checked the RPM and it is off by 100 RPM. It was reading 4300 so it is in fact turning at 4200 at 3800 MPH. BUT this does not make sense with a 15 pitch prop. Could it be related to the diameter of the prop which has is 15 and a quarter or could something else be happening. Where and what could I check.

Thanks for this mistery diagnosis.

CHEERS

 

[ziggy]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Where and what could I check.

from the adults only section...
12. List of possible causes of low WOT (Wide Open Throttle) rpm.

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Thanks for the info Mr. Ziggy,

I've copied the 15 possible causes for low WOT RPM as suggested by Mr. Don S. and am bringing the boat BACK to the mechanic to look for the cause. - Went boating yesterday and water started coming in AGAIN from the transom (I think)...the problem the mechanic apparently fixed a week ago with a bill of 800$ tp prove it. Like I said before, I just want to be on the water ... without it costing me a summers worth of gaz on every outing.

CHEERS!

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Finally got my boat back, The mechanic went through the list of 15 possible causes of low WOT (Wide Open Throttle) rpm as suggested by Don S. and found two things: Compression is down (140-145-150 & 160). Second, needed to set timing (2 degrees) and changed the spark plugs. Went for a drive this evening and got the following result with a full tank of gaz and two people (300 lbs total): 40MPH at 4400RPM with a 15 pitch prop. The mechanic says it is what it is

Could the relatively low compression be the factor in my boat needing a 15 pitch prop to get 4400RPM and being able to get me out of the water (225 lbs on one ski)?
Thanks for your continued input.

 

[Alumarine]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Hi. Your compression is just fine. Not sure about your mechanic.

Not really sure what a 15 pitch prop is.
Do you have any other numbers on the prop?

You should use a GPS to get an accurate MPH.

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

I'm told my 176 Larson should turn a 21 pitch prop (more speed) or a 19 pitch prop (more power) and that these props would give me a WOT (Wide Open Throttle) RPM of 4400 to 4800 as suggested by Mercruiser for a 3 Lt engine. As it stands I've got to use a 15 pitch prop to get within that desired RPM range. That is what still baffles me. CHEERS!

 

[Maclin]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

40MPH at 4400RPM with a 15 pitch prop. The mechanic says it is what it is.

What it is, is a 1.41 calculated drive ratio with 4400rpm and 15 pitch and 10% slip at 40mph. If 15% slip then drive ratio calculates to 1.3.

The drive ratio should be about 2 to 1 for a 3.0 I believe. That calculates to a 21 pitch with 10% slip at 40mph and engine at 4400rpm's.

So, if the rpm and speed and prop pitch values you provided are accurate and correct then your boat does not have the correct drive ratio gearing, and it is what it is.

I'm just sayin'...

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Actually, I hope you are correct Mr. Maclin. However, I checked and the lower unit drive serial reads 2.00 R and as you suggested, that would be correct. OK so what is the consequence of having the incorrect ratio, where would I look to verify the ratio and is a different ratio than 2.00R be bad for the motor? If the wrong ratio is the problem, at least I'll know what I'm dealing with. CHEERS!

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Well, I just arrived from a full day of skiing and the boat seems to work great. It pulls me out on one ski with one foot gragging (225 lbs) within 5 to 7 seconds and the motor seems to sound fine (34 MPH at 4000 RPM) when I'm skiing. Comes out of the hole fast and has a top WOT RMP of 4400 at 40 MPM trimmed. Everyrhing seems fine...EXCEPT I need a 15 pitch prop to achieve this level of performance. I'm told by many the motor should be overreving at this pitch and that a 19 pitch is recommended. Still cannot figure this thing out. The the lower unit drive serial reads 2.00 R. Does that mean a drive ratio of 2 to 1? Thanks for the follow up. CHEERS!

 

[Mr. D]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

Well, just got back from the boat shop and the mechanic suspects the gear ratio was changed before I purchased the boat while the outer casting remained the same and thus still falsely indicates a 2 to 1 ratio. According to him it's the only explanation remaining. That being said, he suggested going from a 15 pitch 15.25 inch diameter prop to 15 pitch prop with a smaller diameter in order to get the WOT RPM from 4300 closer to 4800.

Let me know if anybody knows where I could find a 15 pitch prop with a diameter smaller than 15.25 inches. CHEERS!

 

[SeaRay182noob]

Re: LOW WOT RPM (3 litre Mercruiser) EVEN with different props

When I read you post the first thing I thougth of was.. is the 21 pitch prop you took off the same diameter as the 15 you put on and are they both alunimum or ss. Could make big difference. I have a 2002 18ft searay 3liter and my top speed is 37/38 mph @ 4400rpm. I have rapture stainless steal 17 pitch 14 1/4 diameter. My second thought was... who cares lol. If it isnt broke dont fix it!!! Just kidding here but seriously your top speed is about all your going to get and if you have enough torque to do all you water sports then you are rocken. Enjoy the water and save some for me!!