CONTINUOUSWAVE

Hello. I am running a 1977 Montauk 17 with a 1977 Mercury 115-HP [six-cylinder in-line] Tower of Power. The tachometer came up to 6100. The propeller is a 13 -1/4 x 17 Solas aluminum. My GPS speed was 42 to 43 MPH.

Is the tach reading correctly?

I put the [above] data into a calculator and it say about 5500.

I am not sure about prop slippage. if [the engine is] turning 6100 I think that is entirely too high.

I read those 115 Mercury engines were rated at 5,300-RPM. Let me know your thoughts. Thanks.

Just wanted to add the 5,500 I got from the calculator was putting in a 3-percent prop slip. I was just reading it is not out of the ordinary to have a 10-percent or more slip. That puts me at 6000 plus. Just wanted to get some thoughts on it. Thanks.

What is the gear ratio of the engine?

There is really no way to measure SLIP with sufficient accuracy to use that as a basis for assessing the accuracy of a tachometer.

A tachometer whose accuracy is suspect must be tested by connecting a second tachometer of known accuracy to the engine and comparing readings between the two.

Thanks Jim. I ordered a digital photo infrared tach reader from Amazon. You place a piece of reflective tape on the flywheel and point the infrared beam at it. The reviews claim it will read dead on. Will try it next weekend and report back. For 17.99 we shall see.

Gear ratio is 2:1

For this data:

PITCH = 17
RATIO = 2
MPH = 42
SLIP = 7

The calculated engine speed is

RPM = 5611

Whaler672: what did the new tachometer read this past weekend?

Hi Dutchman. I haven’t been on the boat yet. Bad weather and commitments. Will post after we check it. Thanks

https://www.amazon.com/HDE-Professional ... op?ie=UTF8

Made it down to the boat. Put a piece of reflective tape on the flywheel and the optical tach reader would not read. If I covered it up with my hand around it shading it, I could get it to read at idle but would not want to put my hand that close to the flywheel at 4 or 5000 RPM's. Might work if tried when it was darker out. Might be better to just buy another tachometer.

Something is off.

Is this boat new to you? If not, what was past performance that has you questioning this latest run?

Is the hull heavy? Weight?

Do you go out running light or loaded with gear, people?

Is the engine tuned? Carbs clean and synched?

Typically, a 1997 Montauk, lightly loaded with a classic 115 with the appropriate prop, mounted 2 or 3 holes up would be able to attain 49+.

LHG, a respected member who runs many Mercury engines in his fleet reports 50+ running a 22" Laser II prop.
See his comments down a bit in this archived thread:
http://continuouswave.com/ubb/Forum4/HTML/007546.html

[The MONTAUK 17 boat under discussion in this thread] is a 1977. The [Mercury 115-HP in-line six-cylinder two-stroke-power-cycle] engine is a 1977.

The boat is not new to us. We used to run a 1986 Johnson 110-HP on it. The Mercury is new on the boat. It is mounted two-holes-up. [Boat speed is] 42 to 43 on the GPS receiver, but the tachometer is showing over 6,000-RPM. Once we figure out if that's the true RPM, we can go to a 19-pitch stainless propeller. The boat is usually loaded heavily and has a 28-gallon fuel tank under the seat.

PhilT--what year is that 115? What pitch and propeller are you turning on yours?

The boat image in Phil's post is not Phil's boat. Based on the Illinois registration and the 25-year-old Cadillac sedan towing it, the boat must be Larry's.

Unless your goal is to get the maximum possible boat speed under the calmest sea state with the lightest possible load, choosing a LASER 22-pitch will probably not be the best all-round propeller. Your intention to try a 19-pitch is probably a good choice.

Thank for the data on the propeller, RPM, boat speed, and slip.

We bought a new tachometer and the engine speed indication came up to 5,600-RPM with a boat speed of 42 to 43 on a GPS receiver using a stainless 13-1/4 x 17 slightly cupped propeller.

The engine ran great all summer until the exhaust manifold started leaking. I found another one on eBay. But I am going to mount a 1986 Johnson 110-HP engine for next spring.

Again, Thanks.