Mercury 115 HP Outboard

[eelaforge]

What is the stock size prop for my outboard? I have a 17 foot fiberglas bass boat 6 feet wide. Weighs approximately 1150 lbs. and typically has 2 or 3 people on it. It currently has a 22 pitch lazer 2 cupped prop on it and runs 4000 rpms and about 55-60 mph at wide open throttle. Upon light to moderate acceleration from a dead stop, the bow rises too high in my opinion and seems to take a long time to level or plane out. I feel the prop may have too much pitch. I appreciate any responses I can get to alleviate this problem. I'm going to put a hydrofoil on it to hopefully fix some of it.

 

[redfury]

Re: Mercury 115 HP Outboard

You are running well under max RPM for that motor...too much prop for sure. You'll carbon up that motor if you keep it running like that. I'd be willing to bet that a 19p might be pushing your luck a bit. Be nice if you could find someone willing to let you try out their prop for a test run to find the right one ( nice clean, debris free body of water to run on of course, to avoid damaging someone else's prop of course ).

 

[codertimt]

Re: Mercury 115 HP Outboard

What is the stock size prop for my outboard? I have a 17 foot fiberglas bass boat 6 feet wide. Weighs approximately 1150 lbs. and typically has 2 or 3 people on it. It currently has a 22 pitch lazer 2 cupped prop on it and runs 4000 rpms and about 55-60 mph at wide open throttle. Upon light to moderate acceleration from a dead stop, the bow rises too high in my opinion and seems to take a long time to level or plane out. I feel the prop may have too much pitch. I appreciate any responses I can get to alleviate this problem. I'm going to put a hydrofoil on it to hopefully fix some of it.

As you're likely to be told by a few experts here, there is no such thing as a stock size prop. It's all a matter of what is the proper prop for your outboard on your particular boat.

Now, before you go any further, IMHO you need to verify your tach reading and speed reading before trying to maximize your setup. I say this because...

1. 4000 rpms on that motor is only going to equal high 30s in speed.
2. Even at max rpms, unless you have a ultra efficient hull, you will not be approaching 60mph with a 115.

It's my guess that your tach reading is off(possibly not set correctly for the motor, check if there is a dial/switch on the back) and you speed reading is not from a gps and thus is off also. My dad has a 115 Merc with the same prop on a Ranger fish and ski and it runs just under 50mph at just over 5000rpms. Depending on your hull design, I would expect somewhere around the same for you...give or take a few hundred rpms and few mphs.

There will be many folks to also say avoid the hydrofoil...although it should indeed keep the bow down and help somewhat with planing. Smart tabs, although a bit more coin, will also accomplish the task...with some added benifits and without the negative associated with the hydrofoil. Do some searches on the board and you will find lots of information about both...

 

[eelaforge]

Re: Mercury 115 HP Outboard

You are running well under max RPM for that motor...too much prop for sure. You'll carbon up that motor if you keep it running like that. I'd be willing to bet that a 19p might be pushing your luck a bit. Be nice if you could find someone willing to let you try out their prop for a test run to find the right one ( nice clean, debris free body of water to run on of course, to avoid damaging someone else's prop of course ).

How many RPMs should I be running and what is the max RPMs I should be running? What do you mean by a 19 pitch might be pushing my luck? I haven't had this boat very long, but I have concerns about the performance.
Thanks.

 

[jimmbo]

Re: Mercury 115 HP Outboard

Which engine? I know you said 115, there are 3, 4, and 6 cylinder 115s and even then it helps to know what year it is

 

[redfury]

Re: Mercury 115 HP Outboard

How many RPMs should I be running and what is the max RPMs I should be running? What do you mean by a 19 pitch might be pushing my luck? I haven't had this boat very long, but I have concerns about the performance.
Thanks.

Need to know the engine year/number of cylinders to tell you that. What I meant by the 19p might be pushing your luck was addressed. There is no stock prop. 19p may be too much, it may be just right. We need to know hull type, load, max operating RPM of the motor in order to get into the right ball park. At this point, it's just guessing.

 

[eelaforge]

Re: Mercury 115 HP Outboard

Thanks to everyone for their input. The outboard is a 1988 Mercury 115 hp inline 6. The boat is a 1988 GlasStream bass boat which is fiberglas. The hull weighs approx. 750 lbs. The engine is between 350-375 lbs. There is usually 2-3 people and gear in it when underway, you could add about 550-600 lbs for that. On take-off, the bow will rise higher than I would expect it to, and takes a long time to level or plane out. It has power trim and is trimmed all the way down. When it finally does level, I trim it up. Another thing I noticed, is that it will turn right fairly easy underway, but you need to muscle it a bit to turn left. Anyway, I like the boat in general just somewhat aggravated with the issues. Thanks again...I look forward to resolving these minor problems. Oh, and by-the-way, I forgot to mention that the Tach reads 4000 RPMs at WOT.

 

[codertimt]

Re: Mercury 115 HP Outboard

So, how long is "a long time". 10 seconds? 1 minute?

I stand by my earlier comment, I think you need to verify the tach reading and/or speed via gps. Either one would probably be enough to let you know what's going on. I see one of two possibilities here...

1. The tach is off, you're actually turning more rpm and getting somewhat close the indicated speed and nothing is wrong except you're boat just has a crummy holeshot. Research lower pitched props, hydrofoils, smart tabs as options to help.
2. The tach is correct and you're only turning 4000 rpms...in which case the speed can't be anywhere near right. If this is the case, most likely the old 115 isn't completely healthy as those inline 6's were strong motors and although prossibly a bit overpropped, it should manage to turn the 22" prop alright on a 17' bass boat...

Given you're speedometer would have to be off by almost 20mph for the second scenario to be true, I would think(hope) that scenerio #1 is the case.

BTW, if you want to play with any online prop calculators, the gear ratio should be 2:1 and I think the WOT range is 5000-5500. I use the below site...

http://continuouswave.com/cgi-bin/propcalc.pl