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Merc Inline '81 and up 115s and older 150s

Vandalizer

Vandalizer

Well-known member
Hey everyone,

I'm not sure how many out there are still using the inline 6s, but I'm hoping we can come together to get a better idea of how these motors act. I currently have an '81 115 hanging on an '81 Enticer with no jackplate or setback and a 20p laser II SS prop. This gets me to a top end of around 53 when the motor is trimmed to pull the bow up. Assuming 2% slip at top end (which I hope is reasonable), I'm running right about at 5800 rpm WOT. I don't have a tach, so I had to figure this out with a calculator app. I'd like to try getting more top end out the boat, but I'm really wondering if the engine can push bigger props up to the same rotational speed.
I know that 85predictor is running basically the same setup with a 22p, and getting ~52mph. His engine is an '85 as opposed to my '81, but mine was just completely rebuilt.

Any suggestions?????
 
Yours is a true 115, his is a newer 115 that used to be the 150
84-87 are the adi rockers
81 has the adi, but is also still a 115
not sure exactly the cutoff if 82 or 83
 
So I would assume his with the higher power output and bigger prop would push his rig faster than mine- which it apparently doesn't.
What gives?
Basically, could I go to a 23p and gain a good deal of speed?
 
Actually pre84 115's are only 90 they are about 115 at the crank the newer ones would be the same as a 140 which was essentially a rebadged 150. That being said first you need a tach so you can truly setup your boat 2% is usually tough to achieve. Inline will spin if you set them up too.
 
Meant 90hp....not 115
My reply was a bit confusing
Kary said:
Yours is a true 115, his is a newer 115 that used to be the 150
84-87 are the adi rockers
81 has the adi, but is also still a 115
not sure exactly the cutoff if 82 or 83
Click to expand...
 
Alright... I'm really confused now.
If 85Predictor's rig is really a 150 and mine is 115, why is he only running just as fast (or slower) than me... especially with a bigger prop?!?!?!?
85Predictor, your input would be greatly appreciated!
 
you have a 90, he has a 115
How long does it take to get there?
Bottom condition?
Weight?
Setup?
RPM's accurate?
It could be any number of things, but basically if you are turning the recommended rpm's as he is with a close pitch prop, it will be nearly the same, until you get more rpms or hp. Even 300 rpm can make a difference. How much gear, anchors, etc.

I can put a 125 on my boat and do 68. A 150 will do the same if it is set up the same. With more power, the setup changes and thus your rpm and power. With more power I can trim higher and get more speed than with the 125. Your boat is the same situation. You may be runnin 100#s lighter. Elapsed time would be a better comparison. If you have a mile of lake to get to 55mph then you can trim until you get it. If you have less distance, then your power will show it at the finish line.
 
I have a really hard time believing that a predictor newer than my enticer (same exact hull) by four years can possibly be slower when it has an engine that is 25 percent more powerful (as you've both been saying) with a higher pitch prop, and running at 5800. Something doesn't make sense here! When I'm alone on the boat, I'm going 40 by the time the bow comes back down- about 3 seconds.
Also, what would a reasonable amount of slip be at the top end? I originally assumed 2%, but more slip would push my actual rpm's even higher.
What I'm really trying to get at is whether I can go up in pitch without significantly dropping my top end rpm. I appreciate the help- I just really don't want to drop money on a new prop only to find that I'm going the same speed as before, only with a lighter wallet and no hole shot.
 
sounds like something is wrong with the other boat if it cant go faster than you. Prop setup, driver, water logged foam, lots of variables...
 
One prop size in general will knock off about 200rpm's. Unless he's owned the boat since new you don't know if it spent it whole life in the water and is wet and heavy. Also your prop could be bigger than stamped or his could be smaller than stamped there are about 100 things that could cause his boat to slower or faster..
 
Thanks Blizz... you've been helpful.
Using 10% slip instead of 2%, it would seem that I'm running at around 6125 rpm at wot. Merc specs are 5800 for the top end, but others have also said that they've run these inlines consistently up to 6300.
If I went to a 23p instead of 20p, looks like I'd only gain about 4mph.
 
You run that inline that hard and you will burn it down or blow it up...
it is okay to do a couple 6000 bursts a day, but they are not designed to run 6300 all day.
If you want a motor that will last 10 years, run it at 5800-leave it.
Now I understand that we all like to run fast, but to run it at higher rpm's, it needs to be tweaked a bit. Rejetting, making sure all cylinders are even on compression, reeds okay.
What happens is a lot of guys get a motor and because it runs, they think it will race anything on the lake.

Do yourself a favor and check things out a little to keep that motor alive.
 
Kary said:
You run that inline that hard and you will burn it down or blow it up...
it is okay to do a couple 6000 bursts a day, but they are not designed to run 6300 all day.
If you want a motor that will last 10 years, run it at 5800-leave it.
Now I understand that we all like to run fast, but to run it at higher rpm's, it needs to be tweaked a bit. Rejetting, making sure all cylinders are even on compression, reeds okay.
What happens is a lot of guys get a motor and because it runs, they think it will race anything on the lake.

Do yourself a favor and check things out a little to keep that motor alive.
Click to expand...
Yay that means I can go fast next year!!!
 
Alright- I took the rig out today after changing from the 20" LaserII SS to the 12.5" x 23" ALUMINUM Black Max prop. I wasn't expecting too much, but it turned out to work pretty well. The black max had a slightly higher tendency to start blowing out in turns, and a bit less giddy-up out of the hole, but I did gain 3-4 mph up top. Lucky me, I also found out that she likes to start chine walking at about 52 or 53, but I think it was exaggerated by me being in the boat alone.
So, I'm thinking trying a Turning Point Hustler 23p might be worth a shot. Probably higher rake than the black max, plus the vent holes...
Any other suggestions would be great!
 
Stainless props are more efficient! I ran a 24" Turbo at 6300 and got 63 mph on my
Predictor. Also had a jack plate with 5" setback set to 1 1/2" below, LWP lower, moved tank back as far as it could go, battery moved to port side (balance) Put a 200 XRi on it and
couldn't drive it over high 70s. It was uncontrollable! Went back to the 115.
 
Perdictor speed

As Blizz and other people have said, setup, hull weight, rocker, hook etc. can have a huge effect on actual peed between two identical boats. I would get a tach and water pressure guage and experament with setup. How far is the propshaft centreline below the pad( narrow long flat section at the rear bottom of the boat? If you don't have a jackplate, try moving the motor up higher on the transome.If the top hole on the transome puts the propshaft about 3.5 " below the pad, that would be a good starting point I would think. If it's starting to chinewalk, you're getting the hull up on the pad and thats good!
 
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