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Author Topic:   1986 Mercury 115-HP: Maximum RPM
joe horwath posted 05-28-2008 05:27 PM ET (US)   Profile for joe horwath   Send Email to joe horwath  
What is the recommended [range of maximum engine speed] for [a 1986 Mercury 115-HP]?

I [recently got] a 1986 Mercury 115 on an 18-foot [non-Whaler] center console on which I have done some extensive work. I estimate the weight to be in the neighborhood of 1,800 to 1,900-lbs. The motor had a 23-inch pitch stainless Stiletto propeller. I ran it a couple times before installing a tachometer. At WOT I was getting 4,400-RPM and 43-MPH. I assumed [this boat, motor, and propeller combination] was [lugging] the outboard so I traded it in on a 17-inch pitch [Mercury Trophy II]. Now I'm getting 5,500-RPM at WOT and 41-MPH, but [these results] sound like I'm on the other end [of the engine's recommended operating range]. I don't know a lot about this particular outboard when it comes to the manufacturer's recommended maximum [engine speed], but it sounds like I'm hitting the rev limiter (if it even has one). I have re-powered up from a 50-HP so it is hard for me to tell what is a good [acceleration from a standing start] as [the boat now] jumps up on plane so fast compared to the 50-HP no matter which propeller that I have to hold on to the steering wheel with a deathgrip. I like the performance of the 115 on the boat. I just don't want to wreck it running outside of [the maximum speed range] it was intended to run. Any suggestions on propeller type and pitch? I fish Lake Erie on a few times a year and thought that the four-blade would keep the bow a little higher when running in the slop. Thanks in advance!

jimh posted 05-28-2008 09:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Joe--You changed to a 17-inch pitch propeller from a 23-inch pitch propeller. That is quite a big change in pitch--minus five inches. Usually the rule of thumb with propellers is that you can expect the engine speed to increase about 450-RPM for each two-inch decrease in propeller pitch. If we apply that to your situation we'd expect about 1,125-RPM increase. You report an increase of 1,100-RPM--hey, that's right on the money with the rule of thumb.

The odd thing is that your boat speed dropped to 41-MPH from 43-MPH when you changed to the lower pitch propeller in spite of all the extra engine speed. Perhaps the optimum is somewhere in the middle range, between 17- and 23-inch pitch.

brisboats posted 05-28-2008 09:26 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Hmmm 23-17 and I get 6. How are you measuring rpm and speed?. Is this motor a 6 cylinder inline? What hull are you running the motor on?

Sounds like your tach is adjusted on the wrong pole and you are hitting the rev limiter with the 17 pitch prop.


jimh posted 05-28-2008 10:06 PM ET (US)     Profile for jimh  Send Email to jimh     
Yeah--bad math on my part--paying too much attention to the hockey game!

OK --six inch change in pitch implies 1,350-RPM increase compared to a reported 1,100-RPM. Sound like we are missing about 250-RPM, but still rather close for a rule of thumb.

Joe--I am sure that at 5,500-RPM you are under the limit for that engine, and, in fact, you are probably where you want to be if you reached that speed with a light load.

L H G posted 05-28-2008 10:28 PM ET (US)     Profile for L H G    
I have a pair of these 1985 model year engines on my 18' Boston Whaler. Last year of production was 1988.

The operating band is 5000-5500, but I have heard you can run them up to 5800, as the 140 HP version did.

Mercury claims each inch of pitch is worth 200 RPM, so what you are getting in going from 23" @4400 to 17" @ 5500 is just about right. Technically, you should get 5600 rpm, but the 4 bladed Trophy Plus probably is a little harder to turn than the 3 bladed Stiletto. Both are high efficiency props, so there is no other difference between them.

The Trophy is designed for elevated running, so your engine should be mounted in the middle set of holes out the five available. It might even tolerate the 4th set of holes. That will bring your speed up. Running it fully submerged adds effective pitch.

From the prop charts I have, 17" pitch is about right for your boat weight. The engine has 2.0 gearing. By lifting the engine up (if it's not already there), you may find you're in the 19" pitch range. Then you might want to consider the 19" Laser II (small hub version) or a 19" Trophy. If you're running 42 or 43 MPH you're doing quite well.

I have been adding red Stabil to my fuel and find it really helps acceleration performance and the fuel burns cleaner.

Also, be sure to replace the water impeller every 2 years or so. You need to keep the water getting all the way up to the top cylinder passages for longevity of the engine.

You should also learn how to adject the low speed jets yourself, so you can keep it idling and accelerating smoothly.

If yours is in good shape, and looks good cosmetically (I wax mine periodically) hold on to it. Good ones keep getting more valuable as demand for them is strong in certain segments of boating. Real classics!

Tom W Clark posted 05-29-2008 12:12 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The Stiletto Advantage I (which I am assuming is the Stiletto model you have) is a very different propeller than the Mercury Trophy Plus.

Rules of thumb about pitch change and RPM change hold true within a given line of propeller but will vary if comparing completely different propellers, so do not put too much faith in a 200 RPM per inch of pitch or 250 RPM per inch of pitch rule when comparing the Advantage I to the Trophy Plus.

If you are using a rule-of-thumb, mine is a 150-200 RPM per inch of pitch change for the intermediate size outboards.

I know the Trophy Plus, in spite of it four blades, tends to be less aggressive than other propellers. A 16" Mercury Vengeance, for example, will allow less RPM than the 17" Trophy Plus. Weird, but true. This is also one of the reasons why you are only seeing 41 MPH with this prop.

Likewise, the Stiletto Advantage Series tends to act like a propeller with two more inches of pitch. Generally I recommend dropping two inches of pitch when switching form an OE three blade like the Mercury Laser II or OMC SST. I would expect the 23" Stiletto to perform very similarly to a 25" Mercury Laser II.

Now for this particular boat, I would not recommend the Trophy Plus, but who knows? If the 17" is not enough, try a 19" Trophy Plus and see how it performs.

I think a much better choice would be a 13-1/4" x 17" Stiletto Advantage I, a 13" x 18" Mercury Vengeance or a 13-1/4" x 19" Mercury Laser II. The Advantage I and the Laser II will work better at higher motor mounting positions.

Tohsgib posted 05-29-2008 11:47 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
I agree with Tom. 5500 is redline and a 4 blade prop is gonna be much slower than a 3 blade on certain hulls, especially lighter hulls.
joe horwath posted 05-29-2008 04:55 PM ET (US)     Profile for joe horwath  Send Email to joe horwath     
Thanks to all, I'm glad to hear that 5500 rpm's is o.k. @ WOT. Not really worried about top end speed as I don't intend to run it wide open for any length of time likes to slurp the gas when its opned up. Truely amazing what an extra 65 ponies does for the handling and the ride, sorta puts it into a planing hull with the 115 as opposed to the semi-displacement way it was running with the 50. I put the 23p stiletto because i got a good deal, bought it brand new for 125.00 and then traded w/40.00 for the trophy II after I installed the tach and saw the rpm's were so low at WOT. I like the way the trophy runs on the boat so it will stay. Took me 6 years to find the right one on eBay so I doubt I'll part with it, and I'm a stickler for reg. maint. as new ones are way out of my justifiable price range. Thanks again.
cooper1958nc posted 05-29-2008 09:24 PM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     
There's no rev limiter on this outboard.

Losing top speed when reducing pitch is not that uncommon. Plus fuel economy suffers.

If you can accelerate OK with the 23, I would return to that. You will get quieter and more economical cruise.

Perry posted 05-29-2008 10:50 PM ET (US)     Profile for Perry  Send Email to Perry     
Let me get this straight, the 23" pitch prop was getting Joe 4400 RPM at WOT. The motor's operating range is 5000 to 5500 RPM. LHG says a 17" pitch prop should be what he needs. Tom Clark recommends a 17" pitch Stiletto Advantage I.

Now this Cooper fellow says he would return to the 23" prop if it accelerates OK.

Cooper, why in the world would you run a 23" pitch prop when doesn't even get close to the recommended RPM at WOT?

Joe, I would take the propeller advice from Tom or Larry.

joe horwath posted 05-30-2008 06:44 PM ET (US)     Profile for joe horwath  Send Email to joe horwath     
Going to stay with the 17" trophy and resist the urge to run at WOT for entended periods just because I don't feel comfortable with running an older motor at the top end of the rpm range. Really like how the boat handles with the trophy. thanx guys
cooper1958nc posted 06-03-2008 04:26 AM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     
We've debated this issue quite a bit. What many say is "overpropping" is often advantageous.

Piston engines are more efficient operating at a high load factor (unless they are detonating). High load factor means lower RPM and higher throttle (manifold pressure).

For midrange economy, keep the RPM down.

The parade of horribles that is supposed to happen when you run at a high load factor, well....I am waiting to hear. Most blown pistons are from overheating at high RPM.

If acceleration is acceptable, and ultimate top speed is irrelevant, you will do much better with more pitch, especially at optimum cruise in the mid 20's. Quieter, more economical, perhaps less engine wear.

Modern automotive design uses very high gearning to keep engine speed to a minimum. My little experiments show that, for example, shifting from 5th to 6th gear at 60 mph increases mileage from 24 to 29 mpg. (Steady state readings from onboard computer.) RPM drops only a few hundred, from about 2100 to 1900. Of course the ultimate top speed is higher in 5th, or maybe 4th.

The difference is due to less engine friction which increases as the square ofthe speed.

There is a current post on this forum, where an owner reports a 23" prop gives 43 mph at 4600 RMP using 23 gph, and a smaller prop gives 42 mph at higher rpm, 26 gph.

And he is told to use the smaller prop??? Perhaps we need a nautical "mythbuster" to get rid of the fallacy that you must prop for redline RPM.

2manyboats posted 06-03-2008 10:42 AM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
dumb example, cars run on land , boats run in water
Tohsgib posted 06-03-2008 10:51 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Coopper I totally disagree with you but that is the benefit of internet discusions. Your best mpg will be achieved when the engine is under the least load. With 2 strokes they have a "sweet spot" and that might occur at 4200 or it might be at 3300. For example Tom Clark's Whaler gets the best mileage at higher rpms and higher speed.
cooper1958nc posted 06-03-2008 12:45 PM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     
Internal combustion piston engines are internal combustion piston engines.

On the "least load" issue, well, I guess I don't understand the point.

Here's the analysis:

It takes a certain amount of hp to drive the car or boat at a given speed. To make that hp the engine burns a certain amout of fuel/air mix. Power can be produced with a relatively thin charge at a high rate of pumping, or a denser charge at a lower rate of pumping. Charge density is governed by the throttle. Pumping rate is governed by the RPM. For engines that can vary RPM and throttle independently, it is well known that fuel consumption is roughly related to throttle (indexed as "manifold pressure") times RPM.

Now which is more efficient, high RPM low throttle or high throttle low RPM?

The answer for piston engines is: the lowest RPM that will work, meaning you decrease RPM and increase throttle until (a) you are at full throttle; or (b) you get undesirable operation, such as detonation or vibration. This is because internal friction increases as the square of the RPM, and it is significant. This is the idea behind an "overdrive".

So you carry the biggest prop that allows you to get on a plane, IF you only want economical mid-range operation, and IF it does not cause detonation.

Certainly such a big pitch prop will not allow highest total horsepower, but for most people, who cares?

So, absent detonation, go low RPM high throttle, if you can.

Sorry if that does not jive with existing local nautical mythology. Expand your sources beyond manufacturers or dealers, to engineering texts on IC engines and you will agree.

cooper1958nc posted 06-03-2008 12:48 PM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     

The issue of certain boats getting better mpg at certain speeds is more related to the hull than the engine. Planing boats reduce wetted area with increasing speed, and this may account for it.

Except at really wierd very slow or very fast RPM or throttle settings, engine efficiency (as specific fuel consumption) is amost constant with throttle setting, absent detonation of course.

Tohsgib posted 06-03-2008 01:17 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
If your theory was true then why do manufacturers have different gear ratios? We should all run a 1:1 ratio and sort it out with the prop? Also why would they state WOT range 5-6k but you think 4400 is better? Did you help in designing these engines or were you on the corporate debate team?
2manyboats posted 06-03-2008 04:23 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
Even dumber, True engines are engines and I am sure the would behave the same on a bench. But connect one to a multi geared transmission and put it in a car , the other connect to a single geared transmission and put it on your boat and the are completely different.
So expand your sources beyond engineering texts on IC engines and go out in your boat and you will understand.
brisboats posted 06-03-2008 06:18 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Mercury inline 6's hate to be lugged. They are a very tweaked motor that is already prone to detonation. Hence the reason the later 84-88 115hp is essentially the 140hp motor with an updated ignition and low dome pistons. Running it with a large pitch prop that cannot put the engine within the peak operating range would in my opinion be unwise. Cooper's therory doesn't jive with those that have been running and racing mercury inlines for decades. What Coop proposes is when driving uphill shift into the highest gear possible that will still allow headway. Just cannot wrap my brain around that.


jimh posted 06-03-2008 08:51 PM ET (US)     Profile for jimh  Send Email to jimh     
The recommended practice actually is more like this: select a propeller that allows the engine to reach the top end of the recommended engine speed range when the boat is lightly loaded. Using this advice, the propeller will not be too much load for the boat in adverse conditions, such as when the boat is heavily loaded, is facing head seas and head winds, and is operating in adverse conditions for the engine (such as high temperature air, high humidity air, and very warm water).
Tohsgib posted 06-03-2008 11:07 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Your point Jim?
brisboats posted 06-04-2008 08:43 AM ET (US)     Profile for brisboats  Send Email to brisboats     
Maybe on paper Coop's therory works but in practice it does not. Take a two cyle string line trimmer out for a low cost example and experiment. Remove the factory installed guard and line cutter. Then run a larger than recommended stringline at a greater diameter and length than the guard originally would allow. While running this trimmer now one cannot reach peak operarating rpms but will however cut a larger swath at a quicker speed causing some to proclaim it is more efficient. Run said trimmer for 20 hours (if it will last that long) tear down the engine and report back.

What you will find is a a carbon coked spark arrestor and most likely a scored cylinder wall. While the trimmer may have cut more area in the 20 hours of operation than a trimmer run within specification. The longevity of the engine has suffered. It cannot reach peak operating rpm and the engine struggles swinging the larger and heavier string. In the shorter thinner grass (on plane) it works well and is certainly faster but in the thicker deeper grass the engine bogs and struggles under load (coming onto plane or run wot).

Manufacturers recommend rpm operating ranges, propellor sizes, string line diameter and lengths for some very good reasons. They have talented engineers smarter than me and even Coop that get paid to make these recommendations based on the projected uses of the engine at a wide range of rpms and load. My advice is listen to them and those with real time experience running your engine and propellor combination.


brisboats posted 06-04-2008 08:45 AM ET (US)     Profile for brisboats  Send Email to brisboats     
Sorry I forgot to add, I think that is Jim's point.


jimh posted 06-04-2008 09:18 AM ET (US)     Profile for jimh  Send Email to jimh     
The conditions I mention (above) in regard to propeller selection permit a propeller to be chosen so that in no normal operating situation will the engine's maximum speed be exceeded. From this point we work downward in engine speed as more load is added. It may very well occur that a propeller selected in this manner will produce a lower engine speed at maximum throttle when the boat is heavily loaded, trying to make way upwind or against large head seas, or in any condition where the load on the engine is increased.

If we select the propeller for the lightest load conditions so that the engine operates at the very lowest speed of the maximum range recommended, then we can be certain that when the load on the engine increases (due to added weight aboard or sea conditions) the engine will not be able to reach the minimum speed recommended. This means the engine will be operating outside of the manufacturer's recommended conditions.

That's my point.

Tohsgib posted 06-04-2008 12:06 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Got it Jim...when reading it it almost seemed like you had something else to say but did not.
cooper1958nc posted 06-05-2008 11:10 AM ET (US)     Profile for cooper1958nc  Send Email to cooper1958nc     
Remember that manufacturers have vested interests in certain things, and they may not be the same things you are interested in.

Recommendations of manufacturers are not always done by genius engineers. Sometimes they are sales related.

The recommendation for redline RPM at FT is based on wanting to make sure the engine is good for all the hp advertised. For many people that is not very important.

I have zero documentation and even less theoretical basis for concluding that a 2 stroke spec'ed to run 5000-5500 will be unhappy at 4500 FT. True it will produce slightly less hp at FT, but the mid range operation will be more economical, quieter, and possibly with more longevity.

Full throttle is full throttle, and BMEP (brake mean effective pressure, a measure of cylinder pressures post combustion, and an index of detonation likelihood) is not different at 4500 than 5500, not much anyway. Even throttle timed engines (old carbed 2 strokes) are not much different in timing at 4500 than 5500. Modern engines without throttle timing could care less.

The anecdotes are full of pontoon an houseboats with 2 stroke OB engines that turn, despite low pitch props, maybe 3500 tops. These engines run for years without problems.

As fuel prices continue to rise, I predict manufacturers will re-prioritize and find a way to reduce mid range RPM in favor of greater economy of operation. Maybe two speed lower units, maybe just carrying a little more prop and sacrificing top speed.

In the aviation world we have seen sacred cows die right and left as fuel prices became more important. One sacred cow was called "don't run over square." That was the aviation equivalent of the sacred cow we are talking about in this forum: you must keep engine RPM "above" the equivalent manifold pressure (RPM in hundreds, MP in inches). That was shown to have no basis, despite it being universally acceped for many years (based on the same arguments I hear here.) The second one was, "don't lean beyond peak EGT." That is also the norm today in marine engines, although we can't directly control the fuel mixture. Then it was demonstrated that engines ran fine on the lean side of peak EGT, saving a bundle of money in gas. This was pure heresy, and was thought to lead to immediate engine destruction. Many, many people would remark that the manufacturers dictated rich operation, and after all, an engine was more valuable than fuel. However, as it turned out the manufacturers were just repeating past wives' tales, and no one wanted to rock the boat. So, when certain private individuals demonstrated the reality, they all eventually caved in.

Anyway, run your engines any way you want. As this forum seems to dislike theoretical discussions, I am over and out.

Perry posted 06-05-2008 12:46 PM ET (US)     Profile for Perry  Send Email to Perry     
As this forum seems to dislike theoretical discussions, I am over and out.


prj posted 06-05-2008 02:23 PM ET (US)     Profile for prj  Send Email to prj     
I've never understood the animosity that cooper's theoretical engineer-based questions and comments have created.

I'll miss his/her contributions were they to cease.

Tohsgib posted 06-05-2008 02:36 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Ok Coop...I do not dislike these discussions but you have nothing backing you up. Tell us you have been running your overpropped 200hp? for 1800 hours and she runs like new. Tell us you ran the recomended range and got 2mpg and then over propped it and now get 2.5mpg at the same speed....just tell us something of fact please.
brisboats posted 06-06-2008 11:02 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Nothing wrong with thinking outside the box as long as someone else is paying for the cardboard. Running the 23 Pitch at 4400 rpms could have cost Joe his expensive outboard.


Tom W Clark posted 06-07-2008 10:23 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Internal combustion piston engines are internal combustion piston engines.

Oh brother.

If that were true there would be no reason for competing engine technologies. A two stroke would be the same as a four stoke There would be no advantage of direct injection over electronic fuel injection or even carburetors. Inline four, six, eight twelve cylinders, V-4, V-6 V-8, radial. Who cares, they are all the same internal combustion piston engines, right?

It is rather absurd to postulate that an outboard manufacturer's recommendation about the WOT RPM range can be ignored. It is very much in the outboard manufacturer's interest to see the outboard consumer with a motor that performs well and lasts a long time.

It is illogical to think that a manufacturer would advise the consumer to operate their product so that it consumed more fuel, produced more noise and wears out more rapidly when all the consumer has to do is prop the motor to reduce RPM.

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