Newbie Question #2 - Tachometer

[utadventure]

I had the boat out on Pineview Reservoir yesterday with the family. I've been trying to determine if I have on the right propeller but needed some information for the prop recommendation sites. Here's what I recorded:

Tachometer: 2600 rpm
Speed (GPS): 26.4 mph
Drive: Mercruiser 120 - 1.94 Ratio (my assumption)
Propeller: SOLAS 15.25D/15P

I put this info into the program and it tells me that I have a negative slip (and that it's impossible). If I assume a .15 slip and have it calculate the rpms instead using all the other know factors, it tells me my rpm's are 4250, which is in line with the expected 4000-4500 rpm range at WOT.

Based on this, would you suspect that the tach in the dash is inacurate?? by that much?

Dave

 

[jevery]

Re: Newbie Question #2 - Tachometer

I?d like to take a stab at this, but I gotta know,

Tachometer: 2600 rpm
Speed (GPS): 26.4 mph
Drive: Mercruiser 120 - 1.94 Ratio (my assumption)
Propeller: SOLAS 15.25D/15P

Is this wide-open throttle or mid-throttle numbers?
Because the speed appears to be about right for an engine turning 2600 RPM at mid throttle.

If this is WOT, does the engine sound like it?s running at full throttle or not?

It this boat new to you or have you had it a while?

Based on the 26 MPH speed, I?d suspect that the tach is accurate and the engine?s not achieving target RPM.

 

[steelespike]

Re: Newbie Question #2 - Tachometer

First we need to know about the boat.For now I'll assume it is an 18 or 19 ft.
Then we'll need to know what "the Family" is?
I'll assume 500 lbs.18 ft with 500 lbs,15"prop a Healthy 120 I/O would turn around 5,000 rpm.If you were running wot(wide open throttle)Something is up could be as simple as a throttle adjustment.As suggested speed is about right to the tach rpm.You might start by checking the compression.

 

[utadventure]

Re: Newbie Question #2 - Tachometer

The boat is a 1968 Hydroswift 16'5" V-Hull fiberglass boat. The numbers given are wide open throttle with just my wife and I in the boat. (400 lbs)

We just purchased the boat about 3 weeks ago and have only had it out three times.

The confusion about he accuracy of the tach comes from the website that calculate the effectiveness of your setup. The calculators are saying that 2600 rpm shouldn't be able to produce 26.4 mph. I know that the 26.4 mph is accurate because I got it using a GPS. The motor seems to be running well. That's why I question whether or not the tachometer is accurate.

 

[jevery]

Re: Newbie Question #2 - Tachometer

That boat/motor combo should run around 40 MPH. You're not obtaining target WOT RPM's. Check the throttle linkage and make sure that full throttle at the control lever is moving the throttle lever on the engine through full range.

 

[utadventure]

Re: Newbie Question #2 - Tachometer

Thank Jevery - While out Saturday, I took the engine top cover off and tried to open the carb throttle once the boat topped out. No more throttle at the carb.

With my concerns about the tach, I checked the settings on the current tach and all was set correctly. The diagnostic tool was comparable in price to replacing the in dash unit so.... last night I replace the tach, and easily (and for just a short time) reved the engine past 4000 rpm. Now that I have a tach that I can rely on, I can put back on the lake and see how it does with a load on the prop. Then maybe I can figure out where I'm really at.

Thanks guys!!