1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

[Gashette]

I don't usually use blogs as I have not ever had much response, but having read through this site at great length the last few months it looks like some of y'all really know your stuff! So here goes:

I have a "Frankenstein" Bluewave 220V CC (22 foot) we rescued from the bottom of Galveston Bay after Ike. We have completely restored the hull, and by that I mean stripped down to nothing- new fiberglass, new cabling, new fuel tanks, new gel coat, new center console- you get the idea.

The motor- 120-TLAS VRO was "given" to me by some friends who lost their boat in an unfortunate "traffic accident" (don't ever forget to put the leash and chains on when it's trailered!).

I have had the motor completely re-built and serviced by a real marine mechanic (not a shade tree guy, but the real thing with certifications and pretty papers on the wall).

The VRO system was disconnected/ bypassed during this process as we weren't sure if it was A) going to hold up much longer, B) worth the worry.

The motor runs like a champ! No problems whatsoever- BUT, at WOT I am turning MAYBE 4800-4900 RPMs on a good day. The prop that came with it is a 15 1/4 x 18 Stainless Steel Rampage, which the previous owner had re-pitched to a 16. Having read extensively through this forum, it was obvious I needed to re-pitch yet again (200-400 RPM per 2" pitch lower is my understanding?). Also, my understanding is this motor specifies 5600-5800 WOT.

With that in mind I had it re-pitched to a 15 and was told it could not go any further without compromising the strength.

I had expected to see an additional 200- 400 RPM, but no luck. And here's where it gets very interesting- prior to having it re-pitched (when it was a 16) I would turn just a hair above 5000 and sometimes 5200 with the wind at my back. So, I actually LOST RPM after I changed it?!? Prior to the change I pushed about 20- 22 knots max (on the GPS) at WOT and now even with lower RPM issue I am pushing 24-26 knots (same GPS).

In neutral, I can rev the RPM almost to the rev pack limit of 6700 RPM leading me to believe it's not the throttle linkage preventing my reaching WOT?

Should I try a new prop? Is it common and or possible the work I had done is wrong? How can I tell?

Before dropping another few hundred on a new prop (15 1/4 x 13 or 14) I would really like some "no kidding" ideas on what it SHOULD be versus what my buddies THINK (easy for them to say when they don't have to write the check!).

Re-posting here after no response in other categories.

 

[Robbabob]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Okay, you get a little confusing with the "when pitched at 16 and then 15, but originated at 18".

"The motor runs like a champ! No problems whatsoever- BUT, at WOT I am turning MAYBE 4800-4900 RPMs on a good day. The prop that came with it is a 15 1/4 x 18 Stainless Steel Rampage, which the previous owner had re-pitched to a 16. Having read extensively through this forum, it was obvious I needed to re-pitch yet again (200-400 RPM per 2" pitch lower is my understanding?). Also, my understanding is this motor specifies 5600-5800 WOT."

"I had expected to see an additional 200- 400 RPM, but no luck. And here's where it gets very interesting- prior to having it re-pitched (when it was a 16) I would turn just a hair above 5000 and sometimes 5200 with the wind at my back. So, I actually LOST RPM after I changed it?!? Prior to the change I pushed about 20- 22 knots max (on the GPS) at WOT and now even with lower RPM issue I am pushing 24-26 knots (same GPS)."

I can't tell if you hit 4800 to 4900 when pitched at 16 or 15, then the change to 500 to 5200. We would expect the speed and RPM to increase as you drop pitch, roughly 200 rpm per inch of pitch. Not sure how much altering an 18P likes when taking it down 3".

Since you are talking about a salvage/rebuild job where the motor and boat were never before prop sized as a pair, there is little concern about jumping so many inches of pitch in order to hit the right RPM band. Just make sure the engine height is proper and you're trimming correctly.

If you can get the 13P or even a 12P to test with, go for it. No point in changing gears just to have a pitch that "sounds right". You may also reduce the diameter an inch or two, giving 100 RPM increase per inch reduction.

This is all base on paper... hope you can test a prop and get back with results for a possible change if you're not hitting the max RPM area.

Good luck!

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Thanks Rob!

I was not aware the diameter would make a difference. Thank you for the info!

To clarify the previous:

When it was given to me it was 15 1/4 x 16 and turned (+/- 100) 5000 RPM. It was stamped as originally be 15 1/4 x 18, however that was before it was ever given to me and I assume the previous owners had it re-pitched for their boat to the 16.

I took the prop (15 1/4 x 16) and had it pitched to a 15 (the maximum descrease I could since it had originally been an 18 pitch) thinking it would take me to about 5200-5300 RPM (close enough for me). But instead, it had the exact OPPOSITE effect and now runs at 4800 WOT, but FASTER (about 4 to 5 knots by GPS). Weird??

Also, there is a manual 6" stand off jack plate adjusted to about a third of the way up. We adjusted the jack plate when we initially put the motor on to find the best position. Which is to say when trimmed level with the boat the Lower Unit and below is in the water- there is some additional pitch down on the trim available to help get the bow down in the choppy bay.

I've managed to find the "sweet spots" for the trim which causes the water slip to be right about center console or just behind. I constantly have to adjust this based on the RPMs, plane angle, and speed. If I exceed the maximum trim "up", and when the prop is literally half way out of the water, the RPM's increase to 5700- but that is with no load obviously, and without biting the water really.

Also worth noting, I've noticed at a third of the way up on throttle I hit 4800 RPM- the remaining 1/3 throttle to WOT does nothing for increasing RPM but DOES make the boat "cavitate" a bit. At least I think that is what it is? The engine "sounds" like it is working harder (it starts to almost shudder a little), and there is a noticeable decrease in fuel economy.

The fuel line is new, as is the filter. I run the boat 16 to 20 hours every weekend. The pee-line is shooting out a steady stream, and the engine has not over heated or had any other issues but some "ethanol laden" gas which a couple ounces of Star-Tac cleaned right up.

After your suggestion I did look around a little, and it appears most aftermarket props are aimed at the 13 - 14 diameter range for this motor? I'm not sure what the original Blue Wave spec had it paired with, but do know it is only rated up to a 150HP motor according the USCG capacities placard.

Do you (or anyone else reading) know where I can find what this boat (Blue Wave 220 V-Bay) would normally be paired with? As in what the HP/ Prop? Pitch SHOULD be from factory?

Any additional info would be useful. I really don't want to damage the engine by running it at 4800 with such a heavy boat and I have NO idea where to even find what the boat SHOULD have on it, much less with my configuration!

Thanks again!

 

[Robbabob]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Great information right here! but needs clarification,

"Also worth noting, I've noticed at a third of the way up on throttle I hit 4800 RPM- the remaining 1/3 throttle to WOT does nothing for increasing RPM but DOES make the boat "cavitate" a bit. At least I think that is what it is? The engine "sounds" like it is working harder (it starts to almost shudder a little), and there is a noticeable decrease in fuel economy."

Let us assume you meant 2/3 open throttle you hit the 4800 and the remaining 1/3 throttle does nothing for increasing the RPM; that you get what might feel like cavitation (vibration).

We've now gotten out of my realm of knowledge/comfort with this newly described issue. There are several others with better input on your condition that will probably reply within an hour or so.

If it were me, I'd ditch the newly re-pitched prop as it seems you're not getting reliable feedback from it. Yes, your RPM should have increased, along with the speed, when going from 16 to 15. So, can you trust the shop that changed it?

Wish I could give better input at this stage!

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Thanks Rob!

As to shop- it was a word of mouth recommendation and there were only two in the Houston area that came up repeatedly in those recommendations. I have to assume they do good work?

Beyond that, I'm in the same boat as you (pardon the pun) when it comes to some of the observations. I've never been in a boat that cavitates, nor have I had to rework a prop. It's all new to me! But I have to say, I REALLY enjoy the learning experience!

Who knew a boat had so much physics in it's design!

Thanks again!

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

OH! And yes, I meant 2/3 of the way up on throttle, and the remaining 1/3 causing what I "think" is cavitation.

Thanks!

 

[Mac525]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

If your motor is setback from the transom the anti cav plate should be one inch higher for every foot you go back to measure this you need a long straight edge like ten feet long held against the keel then measure up from there this would be your starting point you might have to go higher

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Awesome! Thank you!

To be sure- since it's a 6" standoff it would only be half an inch then?

 

[steelespike]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Near as I can figure a stock Bluewave 220 is 1400 lbs.
Depending on the hull shallow V or deep V etc I figure the speed should be 30 to 35 mph
likely with a 16-17" pitch.It appears from your original figures 4800-4900 you need 1000
more rpm at 200 rpm per inch thats a 5" pitch change. Putting you at 11" way less than that boat would require.Point is there is something else going on.Possibly a combination of setup and motor tune.Could be running on 3 cylinders,timed wrong etc.By the way never rev a 2 stroke in neutral as it could result in a runaway.
And if you think about it all motoprs rev pretty easily in neutral.

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Thank you for the information!

What should I be looking for to determine if it's not firing on all 4?

Relatively speaking it "sounds" ok? I don't know if something like that would be easily noticed like I'd expect to see in my truck. I did recently change the plugs as part of routine maintenance, and they were in good shape aside from regular carbon build up you'd expect to find?

I appreciate the info on revving the engine! I had no idea!

I had initially thought it may be a throttle linkage issue, but the throttle arm hits the "stopper" when at WOT (engine not running of course). So I assumed it was not an issue.

The 3 cylinder theory could explain what I think is cavitation. Although I don't really know what cavitation is supposed to sound or feel like on a boat.

 

[Robbabob]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

I've always thought of cavitation as being when the prop creates an air pocket and actually causes the engine to over rev due to lack of bite in the water. Also, if on 3 cylinders, would it rev up to the 6,700 when trimmed too high? So, what causes an engine to limit the revs to 4,800 and then the outdrive to vibrate? A spun hub would continue to rev the engine, right?

I'm thinking prop is out of balance beyond 4800 rpm. The new pitch on it should be up around 5,500, but it's reduced by 300.

I just don't have enough experience to lend actual advice. Hoping to ask the questions a different way to help someone brainstorm the answer.. or the next set of questions that find the answer.

Odd....

 

[Mac525]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Two things
1 you said that the plugs were changed are you sure the plug wires went back on the right plugs or they mey have been wrong before you took them off. still seams like you need to see why your losing power and i agree with rob you not makin what it should

2 The prop should have been balanced as a part of the repitch if it wasnt or at least checked makes me wonder about the shop if there is and issue.. The hub may have been slipping for your original numbers and possibly dryed out and reseated while it was off at the shop ,if that is the case it will start to slip again. You might try the shop you went to and see if they have a try it program and see what some other props will do for you once you get the motor sorted that is

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

99.9% sure the plug wires are in order- the mechanic originally set them up, and I changed them one at a time just to be sure there was no confusion. I also found the shop manual showing the diagram for the plugs- all matches.

Not sure on the balancing part? I don't know much about the process other than to say the shop came recommended by way of my chartering buddies (it's their guy and they make a living with their boats)?

I did notice and failed to mention earlier they put a "cup" on the ends of the blades? Not much of one, maybe the last 1/2 inch or so at the leading edge?

The old hub was in pretty good condition I am told (again, I don't know compared to what). But, as part of the process when I took it in to the shop they replaced the hub, and the seal. It's all new. No slipping, or noticeable grinding on the washer either.

The more I look at props for this motor, the more I am inclined to believe Robbs original thought on a smaller diameter? The "prop selectors" on various websites all indicate a 13 3/4 to 14 1/4 prop suggestion- which is a far cry from the 15 1/4 on it now.

Of course I don't know how accurate those "prop selectors" are? Not really looking for speed necessarily, just want to get the WOT RPM where it's supposed to be so I don't end up needing a new motor.

Appreciate all the help and advice! Thanks again!

 

[steelespike]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Wow I missed that ,15 1/4" diameter, I think that was likely intended for a high hp I/O.
Not appropriate for your outboard.And on top of that now stretched beyond reasonable modification.I think a test run with a more normal diameter 17" or 19" pitch aluminum prop may surprise.

 

[Gashette]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

Gonna try that next weekend!

I'll post back and let y'all know what I find!

Thanks again!

 

[Bamaman1]

Re: 1990 Johnson Evinrude 120 TLAS WOT RPM's and Prop Pitch??

I assume the shaft length on your 120 engine is the correct for the length of your transom.

You might want to find a good prop shop for advice. They have the training and experience to give you an answer in 30 seconds. Better yet, take your boat by. I would never re-pitch a stainless prop, as the diameter and cup come into play. Prop shops often take trades.

Some franchise boat dealers are especially good at setups, and can offer free help if you buy a new prop from them.