1985 Osprey 170

[Osprey17]

I have a 1985 Osprey 170.  I took off a 1985 Merc 90hp (Tower of Power) and installed a 1998 Evinrude 115 SPL (totally rebuilt).  The old Merc had a prop 13 1/4" x 17 pitch.  The new engine has the same type prop.  I had a tough time getting on plane.  Boat was not loaded.  It was a simple sea trial.  New tach was installed yesterday because I had some problems with it before.  Thought it was the issue perhaps.

[RickK]

So you're thinking the newer motor has the wrong prop?
A 115 should push that boat to 40mph at the rated RPM, around 5500 (if it doesn't have bottom paint).

[wingtime]

I have a 90 HP Ocean Pro on my 1987 170 and she gets up to 38-39 mph in a hurry.  So a 115 should do over 40 easy.  I think I have a 17" pitch prop on it.

[gran398]

Couple questions:

What type of boat did the 115  power before? Bass boat, flat's boat, party barge, half of a set of twins... different props than what you'll need.

What is the diameter and pitch? The diameter you'll measure. Stamped on the prop will be a number followed by a capital "P" for pitch. 17, 19P, etc.

Once she has the correct stainless prop (as mentioned) she'll fly out of the hole and easily break 40

If the prop is correct, you may have height issues. Too high, she won't bite out of the hole and will ventilate. Too low, she'll drag and kill your top end speed and cruise MPG.

Also we'll need an accurate tach reading at WOT.

[Osprey17]

The current prop is a 13 1/4" x 17.

I don't know the history of the engine.  I just know it was rebuilt.  The guy I bought it from is pretty reputable.  The new tach was installed and I hope to splash her today for a trial.  Thanks for the feed back.

[Georgie]

That boat with that engine should FLY!   Need to know what your max RPM's are to narrow down the issue effectively, but on a recently rebuilt engine, if you are not getting approx 5500 RPMs, then I'd suspect the issue may be related to timing linkage or timing advance setting if she otherwise starts and idles fine.  Is the engine wobbling like it's misfiring or not burning on 1 cylinder?  Have you checked spark to all cylinders?  Check compression yet?  Can you find an engine number on her anywhere?  Usually it is either on a round silver stamped plug about the size of a nickel on the top of the engine or on 2.5" X 1"  rectangular sticker/label on the motor mounts/mid housing.  If you're getting full RPM's then possible spun prop hub?  

[fitz73222]

Without any rpm/ gps speed data at full throttle, we're just guessing at root cause. The engine should swing a 17 pitch without any issues, also assuming you are running a 3 blade 17 pitch, not a 4 blade. The fact that the engine struggled to get on plane may point to some other issue that Ryan suggests.