Top speed and RPM

[Axhammer]

I have a 1998 Ranger R-91I, a 20’ BB with a 1998 Mercury EFI 200. I bought the boat three years ago, and the previous owner said it would do 70 at WOT. I believe I have had it up to about 68 at around 5800-6000 RPM, after I first bought it, once. I understand my engine has a 5800 RPM redline. The engine was overhauled by a reputable shop about ten years ago, and I was able to verify it because I used to drag race with the guy that owns the shop that did the work. Before I bought the boat I verified that it had in fact been overhauled, and the fact that this same shop did all of the routine maintenance on it while the PO owned it.

I didn’t get the boat out on the water once in 2020. I have had the boat out several times this year, and it runs very nice, I have no complaints. I have noticed this year that my top speed at 5800 RPM is about 58-60 MPH. I have to back off the throttle or it will run past 5800 RPM. It seems like I have lost some top end MPH, for a given RPM. Yesterday I decided to just run it wide open long enough to see if the top speed will level out, and stop increasing with the RPM’s.

I had less than a half tank of gas and the normal fishing gear that I carry. I doubt I carry near what others do, I am a casual fisherman, and I normally fish for catfish or strippers with my wife. I believe my boat is relatively lightly loaded.

When I opened the throttle all the way to WOT, and trimmed the motor up, the boat accelerated to 65 MPH at 6500 RPM, and it would have continued to accelerate I believe, but I figured 6500 RPM was enough, and I learned what I wanted to know. I guess my prop is a little flat for my boat and motor combination.

My prop is a Tempest 48-825866 A45 25P, and it looks to be in nice condition with minor nicks hear and there on the leading edges. It appears to have some cupping in the blades. I don’t know if a Tempest A45 25P is cupped from the factory, or if mine has been worked. I never asked the PO.

My hole shot seems to be plenty fast to get on plane, and the boat & motor seem to perform well in most all situations. I checked the prop depth years ago, but I do not recall the exact numbers, I do remember it was an acceptable depth. I am not running a Jack plate, although I do have a 6” manual Jack plate attached to my spare engine. I have thought about installing it, but I’m not sure it’s worth the time and the effort.

I have a four blade prop on a spare older 1989 2.4 Mercury 200 engine, that I plan on installing and then see how it performs at WOT, and compare the two props.

My question is, should the prop be pitched or worked in such a manor, that the boat stops accelerating at a desired RPM? Which for me is my motors 5800 RPM red line.

[mikes733]

Maybe your tach is off. I can’t see loosing 10 mph at 5800. But I’m not well versed on the subject

[Axhammer]

Yes, I forgot to throw that into the possibilities, but it’s something I have considered. I wonder if there is a easy way to verify my tech’s accuracy?

[mdtritn21]

The tach is off and it can be checked to see if it will work by removing it from the dash turning it over and finding the dip switch. It should be round with numbers around it and you can turn it the back and forth a few times to get the corrosion off the contacts. Just be sure to mark where you start and put it back to the same setting number. If that don't fix it you need a new tach. Red line is 5800 but that motor can run at 6000 all day but with out a plate you won't see 70. You can buy a tiny tach to check it with too.

[Axhammer]

Ok, so I’ll buy a mini tach, and try it out, and maybe I can fix my old tach by rotating the knob on the back. Thanks for posting this info.

Now about my question…

When I run my boat at WOT, should the top end MPH level off, and basically stop accelerating when red line is achieved?

I’m thinking with the correct prop, it should stop accelerating, and pretty much max out RPM & MPH when the engine is at red line, or close to the red line.

My 1648 Jon boat with a 25 Evinrude will run at full throttle all day long, I’ve been running it at WOT for long periods of time, for years. I can’t kill that motor.

I have never been able to run my 200 EFI at WOT for a long time because the RPM’s climb past the 5800 RPM red line. The PO blew it up, and had it overhauled. I like to go fast, but not at the cost of loosing the engine.

I guess I’ll get the Tach where I trust it first, and go from there.

I have that 4 blade prop to try, and the results may differ.

Thanks for all the input!

[Axhammer]

I took the boat out yesterday after work, with a mini tach installed. According to the mini tach my tachometer is off by about 600-700 RPM. I am not sure I trust the mini tach, based on the way the instructions read. While I was fishing I noticed the tachometer in the dash was reading about 700 RPM when the engine was not running. I will try removing the tachometer and check/rotate the dip switch on the back. Hopefully that will fix my problem.

On a side note, I caught four small Striped Bass. Hopeful they will grow more, so they will be keepers this fall.

[Axhammer]

I got under the console inside my Ranger R-91I and removed the two nuts & washers that secure the tachometer to the console face via the aluminum C-bracket. After I removed the bracket, I repositioned the tach so I could see it better for a visual inspection of the wires, the switch and light bulb holder, on the backside of the tachometer.

The wires and connections are all secure and seems to be good condition, so I didn’t disconnect any wiring. I did rotate the dip switch back and forth several times. It turned smoothly and I didn’t feel any detents, that I was expecting to feel. I did place it back in the same position it was in before I rotated it, but since it doesn’t have detents, it’s probably not exactly the same spot, but very close. I reinstalled the bracket, nuts & washers, securing the tachometer in place.

I have a question. Should this switch have detents so it stops in specific locations, or is it designed so it can be stopped or placed in any position required to fine tune the tachometer’s accuracy?

I also rewired the mini tach so I could read it better while driving the boat at WOT.

I added a gallon two stroke oil to the small tank under the battery access cover.

I drained and serviced the lower unit gear lube as well.

I ran the motor with water muffs on the lower unit for several minutes to make sure everything was OK, before driving to Sawyers Creek boat ramp. The tachometer was reading about 700 RPM after the thermostats opened and it settled down into a steady idle. The mini tach was reading slightly lower, but it bounces around more that the tachometer. The mini tach seemed to be reading about 650 after averaging the variation.

I launched in a creek that twists & turns for a while before opening up into Tulls Bay, where I was planning on making the WOT runs, because it is a large open body of water with a lot of room to operate at speed.

There were many folks out on the water yesterday, in small boats and plastic kayaks, so I had to slow to idle or no-wake for their safety. Each time I opened the throttle to get on plane, and throttled back to around 3500 RPM to stay on-plane and keep my speed down, I checked my tachometer against the mini tach, and it was much closer than the day before. I believe they were within 100-200 RPM, but I can’t say for sure because every time I turned a corner, I had to slow to no wake speed. Moving the switch back and forth, and then resetting it where it was before, definitely made a change in the tachometer reading.

Unfortunately a solid tone audio warning started going off in my boat. My WP was low, but the tell-tail was flowing water out of the engine. If I turned it off, and restarted the engine it would reset, with no tone, for a couple minutes, but eventually the steady tone came on again, so I turned around and went back to the boat ramp. I stopped to cool for a couple minutes several times, and each time the tone would reset or be off, but it would come on after a couple minutes. I guess I’m lucky it went off before getting to far away from the boat ramp. I’m pretty sure a steady tone indicates running hot, or over heating.

My plan is to drop the lower unit and replace the impeller one day after work this week, and then try to get back out for a test run.

[mdtritn21]

If your going to replace the impeller you would be better off to buy a whole water pump kit and replace everything because it probably needs it.

[Walkabout7781]

"I have a four blade prop on a spare older 1989 2.4 Mercury 200 engine, that I plan on installing and then see how it performs at WOT, and compare the two props."

I'm not 100% sure about the 2.4 L 200 having the same LU as my 150 XR-4 has the smaller lower unit. If it does, the prop will fit your newer '98 ob, but it isn't likely to work very well. There is a chance that the LU has been changed on the '89 engine, of course, or maybe the '89 engines didn't come with the small LU.

I'd sure measure the diameter of the prop hub before I bothered to try it on the water, at least. We tried my favorite XR-4 prop on my OFB's 175 and it didn't work well at all. But we were going a fair distance away and wanted a spare prop, so it got nominated by default. Threw a blade once (dodging a sleepy duck) on the XR-4 and found out it's a good idea to carry a spare prop!

PS: Be careful when catching strippers with the wife!

[Axhammer]

I agree on the kit, my local marine store usually has a kit in stock for about $50. I think I replaced the impeller/kit and thermostats in 2019, and I never got the boat out all through 2020. I have heard some folks replace the impeller every winter, maybe I should start doing that?

how often should the thermostats be replaced?

lol…on the stripper, is that what the pole is for on the casting deck?

[Axhammer]

I got the boat out yesterday. I had less than 1/4 tank of gas. I replaced the impeller & housing with a kit from Budget Boats in Chesapeake VA, for $42. I was getting 20 PSI on the gauge, much better than the 10-12 I am used to. When I got out into Tulls bay, where there is enough room to run at WOT safely, the wind was blowing pretty good 10+ with a 1’ chop at least, not good conditions for running at WOT, but I did it anyway.

My tachometer & mini tach were within 100 RPM, so that was good. After opening the throttle to WOT, and trimming the engine up for maximum MPH, the bow lift going into the wind was more that normal, but I stayed in it…the boat was on the pad, and all over the place, chine walking. It was hard to read the mini tach, but I stayed in it, and I listened to the engine RPM’s climb, sounding like I am used to. About when I felt like it was unsafe to go faster, I had 63.5 on my GPS, and 5600 RPM on the tachometer, so I pulled the throttle back, and made a U turn, and headed for the boat ramp. I think I have it fixed!

I'm pretty sure in better conditions with full fuel, I could get close to 68 MPH at about 5800-6000 RPM.

Looking at my Tempest A45 25P propeller closer, I can see it needs to be worked. I’m going to see if my 4 blade propeller fits my engine. If it does I’m going to have my Tempest prop worked. Should I find someone local to rework my prop, or should I send it to Mark C.?

Thanks for the help everyone, BB forums comes through to help me again…this place is awesome!

[outsidesmoke]

How old is the gas?

[Axhammer]

Fresh gas, I’ve filled the tank a couple times in the last month with non ethanol 89 octane gasoline. The engine runs & preforms well, my tachometer was reading low, but it is fixed now.