WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

[dkwalker]

My boat is an 18 1/2 ft Manatee. I've been told it's heavy. The engine is a rebuilt Merc 115 o/b with about 25 hours on it now. I put most of that time on the engine varying the rpm as instructed<br />but without the benefit of a tachometer. Now that the tach is there, I read WOT is 4600 rpm. I realize 5500 is maximum.<br /><br />Am I ok running at 4600? Am I loading up my engine and asking for early failure. The mechanic says I could get more speed with a differant prop but (his assessment) because my boat is heavy, I <br />should keep it with the prop I have. He's asserting I'll be taking it easier on the engine by not running all the way up to 5500.<br /><br />Now... my use and environment. I am on the Columbia River, and conditions allowing, if I thought I wanted<br />to get somewhere, I could potentially run at WOT for an hour or more. Realistically, currently my <br />runs at WOT have been 15 - 30 minutes at a stretch. In the pursuit of economy, I have adopted 3500 as "no hurry" cruising speed, but 4600 really gets me there a lot quicker.<br /><br />I have no speedo but I'm guessing its something over 30mph at WOT. More would be nice, but I'm comfortable with what I'm getting. The engine sounds happy enough, not loaded up or wound out. It does sound like it could<br />run a long while "as is".<br /><br />I have no idea what prop is on there now. (diameter x pitch)

 

[KillerJello]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

im no expert with boats yet, but you should prolly run it at the motors rating.

 

[roscoe]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

When running your engine at WOT, if it is reaching but not exceeding the recommended rpm range, the current set up is acceptable. By setup, I mean engine performance, prop, engine height, etc.<br /><br />WOT rpm is used as a measure of how the whole package is working. If your engine is functioning properly, but not reaching recommended rpm, then something is putting too much stress or resistance on the engine. It could be a prop with too high a pitch.<br /><br />This is how you can zero in on the correct pitch prop to use.<br /><br />If your WOT rpm have been in recommended range, but now they have dropped 1000 rpm, you know to look somewhere other than the prop, perhaps an ignition failure, a carb problem, or 500 lbs of water in the bilge.<br /><br />2 stroke engines like to run at high rpm. You could run at WOT all day, or till you ran out of fuel and snacks. But best fuel enconomy is usually somewhere between 3000 and 4500 rpm. <br /><br />Maybe I should have said that 2 strokes run better when the resistance or load on the engine is limited to the point, that recommended rpm's can still be achieved at WOT. Any more load than that would "lug" the engine. Any less load than that could over rev the engine. <br /><br />The engine operates best when the load matches the engine. Your setup is how you match the load to the engine. On a car, you have a transmission that does this. <br />---------------------------------------<br />You don't have to run at WOT rpm all the time, just use it a a benchmark to verify the engine is performing properly. You can always throttle back to 3500 or 4000 rpm.

 

[dkwalker]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

It would not be correct to say I lost a thousand rpm. This set up is new to me, and in fact, I bought the boat then traded the 70hp it came with to upgrade to the rebuilt 115hp.<br /><br />What you've explained is exactly the thinking I"ve read on this site over and over... use 5500 as a benchmark and adjust my prop to attain that. It's exactly that thinking which led me to question the remarks my mechanic had made.<br />Your remarks about 3000-4000 being more economical is well in line with the 3500 I adopted as my cruise setting. Like the rest of us, if I can skip a trip to the fuel dock, I'd certainly rather.<br /><br />The Merc 115 is a 1978 model, so I am using traditional discussions for max RPM when citing 5500 as my target number.<br /><br />OK... let's extrapolate. I reprop so that my engine winds to 6000 with only myself aboard, but when I take on 2-3 adults and their food/gear/accessories, might I find that rpm drops to 5500? If this is the case, while I wouldn't open it up to 6000 normally, with that extra payload weighing it down, am I ok to go that same throttle position and run at the loaded up 5500?<br /><br />Thanks for all your help. Like many others, I have found this site to be a wealth of info and come here to read archived posts just for the grains of info and interesting stories that are found here.

 

[JasonJ]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

I had a similar situation with my Merc 125. I was barely touching 5000 rpms. I fiddled with trim angle, raised motor on transom 1 inch, reworked my controls(found out I was not getting full throttle opening), and switched from my 13x17 prop to my 14x17 prop (just as an experiment). Now I touch 6000 rpms when I am alone. The boat is plenty fast. While I have not confirmed it with a gps and my pitot speedo is not accurate, I can stay even with traffic along the bridge here, and it has a 55 mph speed limit. I am also thinking about the fact that I could either go up to a 19 inch pitch, or just leave it knowing I have the extra "gearing" for additional load. I rarely go about 3500 rpms because that 125 loves fuel, and I am usually not in a hurry, but I also thought that pitching up would get me there a bit quicker while cruising at 3500 rpms. I don't know. If a kickin' deal comes up for a 19 inch prop I'll get it. Otherwise, I'll probably stick with the current setup.<br /><br />Your mechanic is sort of right. The Mercs usually have a recommended rpm range of 4500 to 5500, but in my mind it should be set up to achieve the max rpms. Mercs just start producing real power after 4000 rpms, and if you are only getting 4600 rpms, you are missing out on the motors highest power output. You do not have to use it, but it is nice knowing you are getting every last bit of juice out of the motor. My 125 actually runs really well at 6000 rpms, but I do not go much above the 5500 rpms.

 

[akriverrat]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

another reason to get rpms in the high end of the recomended range is boat weight. mine never seems to get lighter. it is allways x plus a little or a lot. if it is x plus a lot the engine could end up lugging. equals holes burned in pistons.

 

[dkwalker]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

As is often said, thanks for sticking with me on this thread.<br /><br />I agree that it's producing best power after 4000. There is a noticeable feel and sound differance after just about there. At 3500 it's planing "OK" but not really sounding like its ready to jump. After 4000, every bit of extra throttle really translates to response.<br /><br />The point made about lugging equals holes in pistons is exactly the dynamic I am concerned about.<br /><br />One remark made that I don't understand... going from 13" to 14" with same pitch and you gained RPM? I would think the opposite would be true. And what was the remark about desiring a 19"? That must be referancing pitch?<br /><br />I'll be down to the Marina later, and whether I go out in the weather or not, I'll tip the engine up and get those prop numbers so I know just what I have. I can tell you, the prop has a veteran look about it and I can certainly justify something something new and smooth.<br />NOTE: I haven't noticed a vibe like it was out of balance or otherwise tweaked, I am new enough it might not register unless it was way out of wack.<br /><br />THanks crew.... I"ll be back with the info and pursuit of this topic. You all have told me what I all ready suspected. I'll be checking $$$ and maybe I"ll even look at SS props.<br />dw

 

[kenneths]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

DKwalker, you almost answered your own question....Yes, loaded it will pull down to maybe 5500...maybe 5400. That IS the case you want. RPM is a result of the prop and load, BUT, if your throttle plates are fully open at lower than 5500, you will have less economy, and more wear. 3500RPM would be a good cruise IF you can turn 5500 WOT. If all you get at WOT is 4600, the plates are almost completely open at 4000.....no fuel economy at 3500 in this case.<br /> May I also add your Mechanic is full of CRAP if he thinks 4600 is OK.<br /> Simply put, prop down to 5700-6000 r's by yourself, and you will not be making this mechanic money as soon as he would apparently like you to...<br /> Post your prop size if you would......200 RPM (generally) per 1" pitch change...

 

[JasonJ]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

I happened to have the 14x17 laying around and decided to see what it would do. When I put it on, the rpms stayed the same, but the bow was plowing. I trimmed the motor out one hole (I don't have power trim), which returned the attitude of the boat back to what it should be, but it also gave me extra rpms. The pitch is the same, but that extra inch must have given enough bite to affect the running attitude. It gets on plane better, and holeshot was improved a little bit as well. I did not expect any rpm increase, I was expecting a decrease. Thats just how it goes when you experiment with the setup. A little tweak here and there makes a huge difference.<br /><br />Yes, I was refering to pitch when I said 19. I have a 17 inch pitch right now. I probably have an optimal setup now, but I would like to try a 19 inch prop to see what happens.

 

[cobra 3.0]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

If you want your rpms to go up, you have to pitch down to a 15 ". A 19" will slow your rpms down further.

 

[Maximerc]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

Just a thought, have you tried a plastic prop?, I have run the compuprops and felt that they unloaded the engine..they are physically light. I dont have any numbers cause the old Johnson did not have a tach. The price is pretty good on them too. there is another company that makes one with removable blades and you can change blades (pich) at the dock. would be pretty handy if you ran empty.. then with 6 people aboard.

 

[dkwalker]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

OK.... I am back. I tipped the engine up and went round n round with the prop. The only number I could find was "19". I infer that was pitch. With my tape measure I decided it's a 12". Using "go / no go" philosophy, it's definitely not a 13".<br /><br />So. It seems I have a 12 x 19. <br /><br />You guys talk about hole shot. OH YEAH. Just to try it out, I hammered it from virtual idle and it got right with it. It veritably jumped out of it's own skin.<br />As I've said, I have no real complaint about performance, but I just wanna see more RPM on that tach... and I am only thinking about the longevity of my powerplant.<br /><br />1"=200 rpm? For easy math, lets shoot for any even thousand. 5". So a 12 x 15? Sounds flat by comparison. Fudge and go 12 x 16?<br /><br />And what if I go the other way, grabbing a 13"?That I can see going much flatter, but then it would seem like a speed prop. <br /><br />HHMMMMMMMMMMMMMMMM............. remarks?

 

[walleyehed]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

I'm seeing 2 different trends here.........you say you only get 4600 r's, and yet have a "hell-of-a-holeshot". In most cases, you sacrifice one for the other to some extent. This being said, I believe I would check the tach for accuracy. If the tach checks out, drop pitch until you get "in the ball-park", or start raising the engine one hole at a time until it ventilates, then drop back one.<br /> Other than what I've mentioned here, I can't say I could be of anymore help, so good luck, and keep posting results!

 

[dkwalker]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

hmmm. that's not very helpful.<br />I have looked in my Boater;s World catalog and they do not list a 12" prop, but a 12 3/4". And it doesn't go down to 19" but 21".<br />Maybe I don't know what I'm talking about for an awesome hole shot. It stands on its back end and proceeds to top speed in real hurry. There is real acceleration, well experienced by the seat of my pants.<br /><br />It could be the tach isn't calibrated right. The mech supplied it and commented it could be off, but his supposition was that it would read way off, not just a few percentage points.<br />I guess I have to go back to him, and have him bring a field tach out so we can wind er up and see what;s really going on.<br /><br />anyone else got two cents worth?

 

[JasonJ]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

Have you tweaked the motor position at all? You may get your rpms without prop change by tweaking the setup. What is the running attitude of the boat at wot? Is bow high or bow low? It should be running on about the back 1/3rd of the hull. The motor may also need to be physically mounted higher. The cavitation plate should be even or a bit above the bottom of the boat. How much gear is in the boat? You probably have a 13x19 prop. I am not 100 percent sure how a prop is properly measured, but I would think measuring from the exact center to the outermost edge of a blade and multiplying by 2 would give the diameter, but I could be on crack too....

 

[dkwalker]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

more detail? gear on board... basic fishing tackle, two rods and net, 4 pfd.s, a small tool sack, fish finder, 225 ft of anchor line, anchor bouy, anchor and chain rhode, binoculars, thermos of coffee, assorted snacks, spare clothes and 180 lbs of ME.<br /><br />power tilt n trim... little or no differance as motor is raised. It does relax a bit at 'mid throttle if I tweak the trim a little, but not much change beyond that initial nudge.<br />Boat does plane on the step, above about 3k - 3500.<br /><br />The previous post questioning my tach may be valid. The fact that he diagnosed my symptoms? if all seems normal, maybe the tach is wack!<br /><br />I will pursue this and post my progress. Thx to the crew.

 

[walleyehed]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

Jason, I don't think you're on crack........I question the 12" diameter here, although Merc had some in this size, I too, feel it could be a 13X19, and BTW, center of shaft to outside point X 2 is correct.<br /> Dkwalker, usually the diameter increases, with a decrease in pitch. (common on factory props)<br /> Make sure you are looking at the proper application...# of splines, hub size, and what-not.<br /> I don't recall for sure but I think Merc had some of the higher HP engines that still used the smaller gearcase, and swung a smaller diameter prop. One way to tell would be to measure from the center of the propshaft, straight-up to the cavitation plate...this would be the measurement that would determine the max diameter prop you could handle........all this being said, you never know what was installed by the last guy........

 

[walleyehed]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

Dkwalker, I just looked up props for your engine, and man, there is a load available!<br /> I would suspect the prop you have has been trimmed down, as the higher pitches, as I said, are smaller diameter...It was most likely a 12.75X19 at one time. I think I see what might be happening now. I think you have good hole shot because of the lack of diameter that should be there. The high RPM is a result of that, but I don't think your fwd speed is nearly as fast as you may think.<br /> You need to measure that prop from centerline to outside edge carefully, and determine the exact diameter...if it's really 12", you need to go to a 13X19, or 13.25X 17.<br /> I've got a 12X17 I've run on my Jonny 115, and it WILL set you back on blast off, the RPM goes thru the roof, and I think the sound of extra RPM makes it "feel" like you're hauling A*S, but this prop had alot slower speed than a 13.25X17"......close to 8MPH if I'm not on crack.............

 

[Boomyal]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

Hey DK:<br />Take your prop down to the prop repair place under the St John's Bridge.(name escapes me right now) They will be able to tell you the diameter and pitch. Once you know that info you may be able to find other props to try b4 you buy.

 

[Boatist]

Re: WOT RPM- 2nd guessing my mechanic + 12x19 prop. ????next???

DKWalker<br />If you trust your tach then run your boat with the lightest load you will ever have. If that RPM is 4600 then you would be better off with a lower pitch prop. You could go down to a 3 blade 13 inch dia by 15 pitch and hit about 5400 RPMs. This would give you a better hole shot and would not load the engine as much. However You likely will not gain any speed and will burn more fuel. A 13 dia by 17 should put you near 5000 RPMs so you would be on the low end of your normal RPM range. With this prop you would be faster than the 15 pitch and use less fuel and still be near your low end of your normal range. Stainless steel props I belive will gain you about 2 MPH with thinner blades and a better bite. The RPMS will stay about the same for the same size prop but will get a little better fuel mileage.<br /><br /> http://www.boat-props.com/mercury_rpms.html <br /><br /> http://www.boat-props.com/propellers/s/Mercury/Outboard/1978/115/R