WOT RPM Question

[cdnfthree2]

This is my 2nd boat and want to keep it running right, but I'm no engine expert, so I have a small question about RPMs. My 1991 150hp Johnson recently had the prop repitched to fix a cavitation issue. It worked. I can plane out and trim out a good bit and see rpms of about 5500-5600. I think thats about right!? Untrimmed I see about 4600-4900Here's my question. Do I always need to trim out to this range to keep my engine healthy? I prefer to keep my motor trimmed only to a point that gets around 5200 rpms for confidence in my impellor intake. It's also easier to stop and go without all those wide tilt range adjustments. So if you can get in a good range Its Propped Right, Right? Now, do you have to get it every time? Thanks

 

[david_r]

Re: WOT RPM Question

if youre at wot and its holding down your rpms thats straining your motor same as pulling a load with your truck. if your holding back on the throttle and running low rpms thats different but any strain could only shorten the life of your motor.

its kind of like putting a tractor on a trailor and pulling it with a s-10 and getting on the highway and holding the gas to the floor you wont max your rpms cause of the load but you wont have that truck in good running condition for long either.

i would trim her out if your runnin wide open.

 

[dajohnson53]

Re: WOT RPM Question

This is my 2nd boat and want to keep it running right, but I'm no engine expert, so I have a small question about RPMs. My 1991 150hp Johnson recently had the prop repitched to fix a cavitation issue. It worked. I can plane out and trim out a good bit and see rpms of about 5500-5600. I think thats about right!? Untrimmed I see about 4600-4900Here's my question. Do I always need to trim out to this range to keep my engine healthy? I prefer to keep my motor trimmed only to a point that gets around 5200 rpms for confidence in my impellor intake. It's also easier to stop and go without all those wide tilt range adjustments. So if you can get in a good range Its Propped Right, Right? Now, do you have to get it every time? Thanks

First, I don't know what the proper WOT rpms are for your engine and hopefully you can get that answer, because.... Your questions would have different answers if your proper WOT is, for example, 52-5500 vs 55-5800. That's the first thing you need to find out.

That said, My first thought is that "Easier to stop and go" is no reason to avoid that little button that trims up and down. That's what they're there for - it's the exact reason to have T/T - as opposed to the more basic pin adjustment - is so you can trim at various places depending on what you're doing, like trimming in a little for take off, and trimming out for cruising, etc. Like shifting a car.

Again - depending on what the WOT rpm spec is for your engine - maybe what you should do is re-prop to get the proper WOT rpms at a trim level you feel comfortable with? I certainly wouldn't keep the thing trimmed in if it weren't giving me the proper WOT rpms at that trim position.

Also, do you have any objective reason to think that you're cooling system is not working right at the higher trim levels? I've personally never had that problem.

 

[cdnfthree2]

Re: WOT RPM Question

Thanks, As for the cooling system it blows strong at a stand a stand still so I know its working. I've adjusted the direction of the tell-tale many times and still can't see that it works when the boat is moving fast(It is currently aimed up, but blows backward anyhow and gets mixed in with the rest of the raging water behind the boat). That's why I'm a little insecure about trimming out alot/often. At 5600 rpms, I can see the whale tale on top the water when I look back and I just get a little concerned about the amount of water it can recieve with the boat that far out of the water as the water intake is only a few inches below the whale tale.

My tell-tale can be adjusted 360' but the results vary only slightly. It spays about 15 degrees off horizontal in any direction. This is why I've considered replacing it with a stubby garden hose about 3 inches long, plugging the end, and drilling an 1/8" hole in the side of it to reroute spray direction and improve visibillity.

 

[dajohnson53]

Re: WOT RPM Question

Thanks, As for the cooling system it blows strong at a stand a stand still so I know its working. I've adjusted the direction of the tell-tale many times and still can't see that it works when the boat is moving fast(It is currently aimed up, but blows backward anyhow and gets mixed in with the rest of the raging water behind the boat). That's why I'm a little insecure about trimming out alot/often. At 5600 rpms, I can see the whale tale on top the water when I look back and I just get a little concerned about the amount of water it can recieve with the boat that far out of the water as the water intake is only a few inches below the whale tale.

My tell-tale can be adjusted 360' but the results vary only slightly. It spays about 15 degrees off horizontal in any direction. This is why I've considered replacing it with a stubby garden hose about 3 inches long, plugging the end, and drilling an 1/8" hole in the side of it to reroute spray direction and improve visibillity.

You might consider installing a water pressure gauge. I have the same problem as you when underway. The pressure gauge is an easy installation (usually).

 

[cdnfthree2]

Re: WOT RPM Question

I'll look into it. Thanks. That would be a good idea and make me worry less when the boat lifts up that high. I'm sure you can understand why i'd be a little too cautious. I have a good smooth, strong motor righ now. I'm still getting comfortable with this one though and am going over everything to ensure that it lasts a while (Lower unit, decarb, everything) because I like to use it alot. Powerhead rebuilds aren't cheep, and I don't want to try it on my own even though I can't imagine anything more ultimately satisfying . Maybe in a decade or so. Speaking of guages, my tilt and trim gauge just stopped working as of yesterday. Any ideas on where to look first? I appreciate all your help!

 

[Dhadley]

Re: WOT RPM Question

You're a little low on top rom but close. The target rpm for that motor is 5800-6000 with an average load. As for the water pressure, if you're concerned about it at all the gauge should be the next step. I think you'll be amazed at what you find. And reassured.

As for the trim, no matter what rpm you're at, 2500 or 6000, trim to the point that you get the most rpm & speed. If you trim too far the speed will drop as the r's climb. And the boat may start to porpoise.

On the trim gauge, I'd start with the wiring (make sure the wires still carry a signal and are connected) and work toward the sender.

 

[cdnfthree2]

Re: WOT RPM Question

Well that stinks about the rpms. I'm not sure how to handle that news. When I bought the boat I had a SS 14.25" 22 degree prop. Rarely could get it to hook up from a stand still with full throttle. It would begin to move boat and then just break lose. Had it re cupped and it was same song second verse. added another half degree a week later and it performed well. As I'm writing this Iam seeing the real value in the water pressure guage because my only idea now before reproppin' is to keep trimmin out further tryin to hit 5800.

 

[Dhadley]

Re: WOT RPM Question

Prop pitch is measured in inches, not degrees. Prop rake is measured in degrees but we'll assume they weren't changing rake angle, just pitch. What brand & model prop is it?

 

[cdnfthree2]

Re: WOT RPM Question

Turbo brand ... reads M14 1/4 X 22. They told me they were roling over the edges and that it was essentially like adding half a degree. True, they did not change the base angle of any blades, only rolle the tips.

 

[Dhadley]

Re: WOT RPM Question

That's a Turbo hubbed for Merc. Are the vent holes actually holes or slots?

 

[cdnfthree2]

Re: WOT RPM Question

Fortunatley, I keep it sitting on my desk. There are 1/4" holes behind the leading edge on each blade. and 15 splines

 

[Dhadley]

Re: WOT RPM Question

It's either a Turbo I or Turbo II. If it's in the original configuration it should be fairly hard to get it to break lose on takeoff. Once someone rebuilds it there's no telling what might happen. If the tips of the blades are worn it'll do as you describe. Another thing that causes it to break lose is height but if the prop is in good shape and as produced the prop would have to be fairly high to vent that bad.

If it's venting too much on takeoff I'm surprised the prop shop didn't mention closing the vent holes. It would be a fairly odd situation that requires that but it could be. If you want to try that you might try putting a short metal screw in the holes to temporairly close them for testing. You could try epoxy too.

 

[cdnfthree2]

Re: WOT RPM Question

Dhadley, I appreciate the update on what my r's should be for WOT. The prop was slipping and motor height is correct. It doesn't slip anymore so before I reprop I 'd better install that water pressure gauge and see how far I can actually trim out. It sounds as if I might see 5900, or at least 58 without porpoising if I just push it a little harder. I think my next project is going to be gauges, oil and water. Thanks alot for all the help -past and present.

 

[Dhadley]

Re: WOT RPM Question

If you can trim that far you should be able to raise the motor slightly. That should help across the entire rpm band. Maybe you won't have to buy another prop. The 2 gauges we use the most are the tach and water pressure when setting up a rig. On mine I find myself staring at the Flow Scan.

 

[dajohnson53]

Re: WOT RPM Question

... think my next project is going to be gauges, oil and water. Thanks alot for all the help -past and present.

If you're thinking of an oil pressure gauge, your outboard doesn't have oil pressure.

If you can trim that far you should be able to raise the motor slightly. That should help across the entire rpm band. Maybe you won't have to buy another prop. The 2 gauges we use the most are the tach and water pressure when setting up a rig. On mine I find myself staring at the Flow Scan.

I had a fuel flow meter (a Standard Horizon). Fortunately (unfortunately) the thing failed a couple of times (heavy condensation in the gauge twice, sensor failure) and after it ran out of warranty, I've abandoned it.

I never liked that thing and happy to see it go! Being facetious of course since I actually did like being able to keep track of fuel usage without dipping the tanks, and finding the sweet spot in terms of MPG for long trips. However, I never liked the GPH numbers! I might even try another one next year unless I have something more important to spend money on.

 

[cdnfthree2]

Re: WOT RPM Question

If you can trim that far you should be able to raise the motor slightly. That should help across the entire rpm band. Maybe you won't have to buy another prop. The 2 gauges we use the most are the tach and water pressure when setting up a rig. On mine I find myself staring at the Flow Scan.

What do you think that monster weighs? I know on my old Mariner you could intall a ring on top the powerhead and hook it up to a chain. I 'm not sure about this one. Your most likely correct though because the water line is rarely lower than a few inches below cowling, so it could most likely benefit from a raise.

 

[Dhadley]

Re: WOT RPM Question

Yes, the correct method is to do it the same way, a lifting ring on the flywheel.

If the water is up that high on (in) the mid section that may hurt the idle quality. What kind of boat is this, a bass boat?

 

[cdnfthree2]

Re: WOT RPM Question

Not a bass boat. It's a 1985 Liberty 19' CC made by a defunct company in Grand Prarie, Texas callled Arlington Special Editions. (MIC) ARI . They split to Florida, made air boats for as while and went out of business in '99. It is pictured below. Sorry I don't have a better picture of the motor position(camera being serviced). As for Idle quality, yes the engine sits very low and it sounds alot better when the boat is moving. Sitting still gives a slightly uneven bouncy sound. But I've heard alot worse on alot newer motors. Not 100% sure on the weight but I know it's heavy.

 

[cdnfthree2]

Re: WOT RPM Question

Siiting still, water is about an inch or two below swim platform. Examining the picture, not real sure the motor can move up.