no towing power

[fishingfriend]

115 in line 6 mercury 1978 runs fine with no one on tube or skiis
as soon as you try to pull a tube with someone it has no pulling power
but when turning sharp and creating slack in the rope it revs up no problem
and then once again when rope is taught it bogs down no power at all,
any advice would be greatly appreciated,thank you.

 

[emckelvy]

Re: no towing power

Sounds like you need a prop with less pitch to allow the engine to rev up into its powerband.

What rpm does the motor run at full throttle with only the driver in the boat?

What length/brand/style of boat is the motor on?

What's the pitch of the current prop? If it's a standard Merc prop you'll find a series of numbers stamped into the main body of the prop, ending in a 2-digit # such as "19" "17" etc.

With an aftermarket prop you may have to remove the prop nut (and thrust hub if not built-into the prop) to look for a number stamped in the prop hub.

Let us know what you've got and we can come back with some recommendations that'll get you out there enjoying your watersports (and likely burning a lot less fuel to boot!).

HTH.......ed

 

[fishingfriend]

Re: no towing power

i have a 16 ft sunray (bow rider ) with a 17" p prop
at full throttle it revs at 4100 rpms and 30 mph
used to have a 16 ft sidewinder with exact same motor and prop, it was
running at 40 mph and had no problem pulling a skier. not sure if its the
boat or motor,

 

[emckelvy]

Re: no towing power

Either you have a very heavy hull, incorrect tach or poorly performing motor.

The WOT recommended rpm range for your motor is around 5300-5500 rpm. If the rpm is accurate, you're way out of the motor's power range and that's why you've no power for pulling watertoys.

A 115 should pull a 17" pitch prop pretty well, with lotsa pep. Definitely more than 30mph. And frankly only 40 on a 16' Sidewinder is kinda slow, too. That's a light, fast ski boat hull. I used to have a 16' SS and if I recall was getting 40mph out of a 100hp Six. And closer to 50 (very scary, too!) with a '71 1350.

Maybe it's time to do a compression check and tuneup on the ol' gal. She may be a bit on the underperforming side due to state of tune or internal problems.

Here's a good place to start:

http://forums.iboats.com/showthread.php?t=168855

http://forums.iboats.com/showthread.php?t=158076

BTW don't lug the motor in its current state of tune/prop. You're gonna kill it. Detonation and other destructive internal issues occur when you lug an I-6 like this. Or most other outboards, but the Inlines are particularly sensitive to the issue.

So, you're gonna need at minimum a 15" pitch prop, if the rpm readings are accurate. Maybe even a 13" to get up into the rpm band. If you're in good with a local dlr or prop shop, they might let you try a prop on an exchange basis.

Otherwise, you can probably find a deal for a used prop on your local www.craigslist.org classified, or at eBay motors.

One last thought, someone could've played mix 'n match with lower units and installed one with a different gear ratio.

You can check this out by first pulling all spark plugs, then the engine 'tophat'. With the ign off, and the lower unit shifted to Fwd gear, turn the flywheel 2 revolutions. The prop should turn exactly 1 revolution. If the prop turns more than 1 rev, you have a 'overdrive' gearset which will 'skew' the prop pitch recommendations by at least 2" of pitch.

In other words, all things being equal you'd have to go at least 2" less pitch for any given prop recommendation, given a 2:1 gear ratio (which is what your motor should have).

Not that it's that likely to have happened, but it can't hurt to check.

HTH & G'luck.........ed

 

[madgadget]

Re: no towing power

Sounds like a very ill engine to me.

For reference. I have a I6 140hp on a 16ft V hull and it will pull 48mph with 4 people totalling around 900lb of people on board, and 90litres of fuel. (GPS reading too).

Towing anything is no problem, I don't even notice the difference pulling a ring or kneeboard.

I use a 21 inch prop too and just get about 5300rpm. (although may go back to a 19 and try that soon)

Your engine is obviously a little bit less power but you have issues if your 115 can only get to the RPM you state.

Compression check and inspect all plugs immediately if I were you.

 

[Jeep Man]

Re: no towing power

Experiment with another prop with less pitch. If you are now over-revving, you will know that you are running the correct one. If it is the correct pitch, then you likely have a spun prop.