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Mercury 115 hp top end

Discussion in 'Boats and Motors' started by Gfunk, Jun 24, 2007.

  1. I switched from a 1996 75hp Mercury to a 1999 115hp Mercury, both 2 strokes. I kept the same prop, a 21". With the 75, I could get my boat up to about 30mph, with the 115, I only get it to 34 mph. My tach says I'm turning about 4800 rpm. If I go by the book, I'm supposed to drop to a 19" prop to get the rpm's up to 5200. My question is this, shouldn't I be getting alot better performance out of the bigger motor in the first place? It's got a great hole shot, but seems to putz out on the top end, leading me to think I should go up in prop size, but the tach says otherwise. I was looking for something in the 50mph range, not just 4mph difference. The 115 seems to run nice and smooth, in fact it's quieter than the 75 was, so I don't think there's anything wrong with it. I just don't get it...
    Any advice is greatly appreciated, I may try to check the fuel pump, filter, fuel ball, etc. tomorrow, but it all looks good visually. Looks almost new just opening the cowl..??????? Help!
  2. first find out what the max rpm should be at w.o.t. that will tell if a prop ghange is in order...dont expect 50mph though...thats asking alot from a 115 imo...good luck!!!

  3. That's how I came up with going to the 19". 5250 WOT-the 4800 I'm getting = 450rpm too slow. The internet resources I've found say that you gain 200rpm's for every 1" drop in pitch means I drop to a 19" to get in the proper range....
  4. I do 49mph (GPS) in a 17'8" Skeeter. I'm running a Yamaha 115 2-stroke. I realize I'm probably going to get more speed in my boat given the hull shape, but not sure how much of an effect that might have and how your weight (maybe lower than mine -- I'm not sure) would offset that. Anyhow, I'd doubt 15mph should be the difference. My 17'6" aluminum Tracker ran 29-30mph (GPS) with a 50hp...
  5. keep us posted...would like to know after prop change!!!
  6. I was reading last night and came across recommendations for mounting height of the motor. The website said that the anti-cavitation plate was supposed to be 1 to 1 1/2" above the bottom of the boat. Mine appears to be below it by about 1 1/2". I don't know how much this would change things, but I may find myself an engine hoist sometime this weekend.....
    Anyone ever try this or know whether it's true or not?
  7. You can move the engine up by having someone steady the engine, put a block under the skag, and crank the trailer up after pulling the bolts.

    It will help to raise the motor, if it blows out on a turn, you went too high. They do that a lot on flats boats down in Florida, usually by installing a jackplate, but raising yours by a hole or two should make a difference.

    That may bring up your RPM also....worth a try.

    I had a Tracker 18' deep 'V' with a merc 115 and got about 44 MPH at WOT ( wth the wind at your back, going down stream).

    I beleive it had a 19" SS prop.

    Not sure what size boat you have, but mid-40's is about tops for the 115 HP, unless you're in a small, light boat.

    Good Luck.
  8. Champion188 Elite

    Champion188 Elite Smallie slayer

    What kind of boat? Does the 115 have power tilt/trim? In most situations the cavitation plate should be even with the bottom of the hull or slightly above.
    I would think something is wrong with the set-up if you only gained 4mph with 40 more hp.How is the holeshot? Is it plowing water on plane?
  9. The dealer raised the motor to the 4th hole down on the mount. This leaves the cavitation plate about 1" or maybe a bit more above the bottom. It was in the top hole to start with. He also put a 22" 4 blade stainless cupped prop on it. I took it out and scared the crap out of myself in it. It planed quickly and took off, I trimmed it all of the way out and the rpm's topped out @ 5000. Not sure of the speed, (left my GPS at home) but it was a lot faster, BUT it was also completely out of control, the back end walked around like a thirsty cow trying to find water, scary... so anyway, off with the pretty stainless prop and back on with the aluminum 21"er. I'm trying it out tomorrow. If all goes well, the rpm's will be up and the boat will creep it's way into the 40's and still be under control. If that doesn't work, I plan on lowering it by 1 hole and trying again. Then , if I'm still not satisfied, I'll drop to a 19" prop and call it quits with whatever that gets me.
    What I don't understand is this. My old motor was exactly the same as this one, just a 75hp, and it was mounted in the 2nd hole down. Why would the marina put it in any other hole? don't know, but I have an inkling that's where this one will end up before I'm done.

    To answer the question about the boat, it's a 20 Lund Alaskan, 75" beam more or less, 20'-4" long and dry weight of about 800 pounds, but loaded like know, it's probably about 1500 pounds....

    Thanks for the feedback guys, keep the suggestions coming....
  10. So, what happened? :)
  11. Sorry for not posting sooner, my company does a lot of industrial maintenance and installations and this is the busiest time of year for us.....
    However, I did find time to take it out last Sunday morning. I didn't lower the motor yet, just changed the prop back to the original 21" that came with it. It ran fine, just not up to speed. RPM's topped out around 4900 and speed was 34mph, again. The big news was that I was able to actually control it. Today, I'm buying the 19"prop and hopefully trying it out later this afternoon. I'm going to pick up some new plugs and a fuel line. The motor seems to run fine, just want to rule it out as a possibility. I'll post the results of this trip if I can get out today...
  12. EDD


    I don't know about alum boats , but unless you have a chopper prop to run higher on the boat , most fiberglass motors run best 2" to 4" below the pad on the boat ( a 20 ft lund I think may be heavier than you think , mid 40 's might be you best ) If the water tail at wot is near or higher than the motor your losing some of your motors hp , lower the motor with less trim . my opinion

    If the book say's 5250 rpm you want at least that by yourself or you will not be happy , try the 19p
  13. Well,
    I lowered the motor back to where the original was putting the anti-cavitation plate about 1" below the boat. I changed the fuel line and bulb, rebuilt the fuel pump, replaced the fuel filter and plugs, and guess what, no change, 4800-5000 rpm's and 32 mph. I just don't get it, everytime I see one of these motors on another boat, I stop and ask what kind of speed they are getting, and all of them say at least 40 mph. This motor seems to run fine, just no power on the top end. I have an appointment at another marine in a week and a half to let them diagnose it, I'm pretty much at the end of what I know.... may be the carbs need rebuilt? Never tried it....
  14. UFM82

    UFM82 The one others want to be

    OK, let's nail down some basics. The hull weight of a new 20' Lund Alaskan is 1,275lbs dry. That means the hull- no batteries, no fuel, no seats, no carpet, no tackle boxes, no people, etc. Add a 500lb engine, fuel, two batteries (I'd guess) tackle, life jackets, a cooler of pop, etc AND you and we're probably talking 2,500 lbs all told. Now, let's go to a speed calculator and see what we get. www. If we plug in our numbers, assuming 14% slip ( true speed divided by theoretical speed), our RPM which you said was 5,000, our lower unit gear ratio of 2.07:1 and our pitch which you stated was a 21", we get a predicted speed of 41.8 mph. You said you saw 34 mph? So we know there's some speed there to be gained.

    First off, you said the engine runs fine. So stop wasting money throwing parts at it. All you're doing is replacing good parts. Unless it's running bad, don't mess with it. If you are able to get to the proper RPM range which you said you did with the SS prop (or at least close) you know the engine is fine. You also ran a 22" prop at 5,000 which tells me the 21 you have should be just about OK once we get your set-up right.

    Second, put the engine BACK up where you had it when the anti-ventilation plate was above the keel. It should be 1" or more ABOVE the keel. Any lower and it just increases your drag and that kills speed. Put the engine back up- the plate should be riding on TOP of the water when you are up on plane.

    Once you have the engine up where it should be, try the 21 at WOT and trim up until it cavitates. Then trim back down and see where you are. If everything is working correctly and RPM's are 5,000, you should be in the 40 mph range. I can see a top end on this boat of maybe 44 if all is perfect. An SS prop in the 21" range like a LaserII would probably work well on the boat as the SS would be a little more efficient than aluminum. The LaserII will also give you a lot of bow lift and that means you can trim up and get more of the boat out of the water, thus reducing drag.

    The loose feeling you got when running the 4-blade prop was most likely due to two things- the bow had some lift which you've never gotten before from an aluminum prop and that reduced the wetted surface of the hull. This got more of the boat up out of the water and got your speed up. Secondly I'd bet you were trimmed too high. This boat and engine combo will NOT throw a rooster tail like a bass boat will. It's not a shallow water set-up and the prop should stay fully submerged. If you were throwing a tail you were probably slipping like crazy too. The bottom of your hull isn't a high-speed hull either. My ProLine has a hull like a bathtub-smooth and flat. When I hit top end and trim out a hair too much, it squiggles around too as prop torque tries to push the boat sideways. It's a wild feeling.

    Anyway, raise the engine and work the trim and see what WOT you can get. I see no reason why you shouldn't see the 40's. There was a boater here years ago who claimed to be running in the 60's with a 115 and despite my arguments he insisted it was true. However, his GPS was in fact incorrect and he wound up getting some true assistance fromn some people that knew what they were doing. His engine came from the dealer with the anti-ventilation plate below the keel and rasing that alone gained him speed. He also changed from his aluminum prop to an SS prop and that helped. I believe mid-40's was his final top speed and that was faster than his boat had EVER run. It's just a matter of setting things up properly.

    There are a few sites on the web that deal with set-up and they can be very beneficial. Check 'em out and follow the guidelines. It's worth a shot.

  15. I've got a 1996 Mariner 115 on a 18' Smokercraft aluminum deep V. The motor runs like a champ and I usually get around 43 mph (GPS). I'm running a 21" (slightly dinged up) prop and can get the 5280 at WOT. I went to a 19" Michigan Match prop and it went well over 5280 so I don't use it (will sell if anyone wants to buy...comes with hub). Your boat is about 2-3' longer and heavier, I doubt you'll get the same speed I get. I'm not an expert but don't think you'll find a magic prop that'll give you over 40mph. FYI, my cav plate sits at the bottom of the keel and I usually have about 3-4 feet of the boat wetted at top end. Good luck in your search.
  16. EDD


    I was not talking about the anti-cavitation plate on my post ( that's about 8" above the prop shaft) I talked about the prop shaft being around 2 - 4" below the pad working on fiberglass boats.
    sorry if I was not clear - That would be alot of drag
  17. UFM
    I tried running the boat at the higher motor setting and trimmed it out until it started losing water about a week ago with pretty much the same results, no rpm's above 5000 and no speed above 32, using the 21" prop. I agree with you though, I see no reason why this isn't working, motor runs fine, I checked the throttle arm at WOT in the driveway, and it's open all the way, so the throttle cable is fine, uggggg!
  18. UFM,
    i used the calculator on your post and came up with a 37% prop slip, any idea if this could be the problem?
  19. Just a quick are you measuring speed? GPS, paddle wheel sensor or pressure probe?

    37% slip is the question...that would mean an extraordinarily bad prop design.
  20. GPS,
    but I don't really think I'm slipping that much, I'm losing power somewhere and that would account for the low RPM's on the 21" prop, I think that motor ought to be able to push that prop to 5800-5900 rpm's, it's just not for some reason. I checked the spark tonight with a spark tester, all of them were working @ 1000 and 2000 rpm's, and measured between 2.75 and 3.00 on the scale on my tester. Don't know if that's good or not, but they were all pretty close.