Prop QUestion for a Mercury Thunderbolt 800 80 HP

[jimmbo]

Your engine is a 72, looking at the wiring diagram,
http://162.144.28.33/lib/mercury/manuals/mercury_1965-1989_40-115.html#/514,
it shows the Tach wire going to the switchbox for its signal so... An alt driven tach might not work. The Faria Application sheet(http://www.marine-j.com/pdf/faria_tacho.pdf) mentions it(note D), and says it can be made to work by moving the wire from the switchbox to the rectifier

 

[krizm200]

Hi Jimmbo,

I had moved the wire from the switchbox to the rectifier. There were 2 yellow lead connections on the outside of the rectifier box, I connected the wire (brown as I remember) from the switchbox to one of these outside rectifer posts. I did not use the red post in the center of the rectifier. The tach itself is set on position 5 for 12 poles. I can check to be sure it is snapped in at the 5 setting.

So if I am getting above 37 mph on the GPS speedometer with about 4300 rpms on a 13x19 prop, maybe the tach is faulty? I'll check it again this weekend if the weather holds. I think I will try increasing slower to see if the rpms are increasing while the speed is, and listen to the engine if the rpms seem to be increasing but not shown on the tach.

Again, always thankful for your help.

 

[jimmbo]

To get 37mph, with that gearing and prop and 12% slip, the engine would need to be turning about 5425 rpm

 

[krizm200]

Thanks for doing the math. I was thinking about the pitch and distance, compared to mph, to find rpm at that speed. Your 5425 rpm at 37 mph makes sense, and points to a tach issue. I'll try to check it again this weekend. I want to see if the tach actually moves above 4200 or so.

 

[jimmbo]

One way to do it is to over trim it to the point where the prop loses grip, the rpms will go up

Can you check the gear ratio? With engine in forward, rotate the flywheel, two turns, the prop should NOT have made one complete rotation. I say to verify it as, there is a chance that in its existence, the original gearcase may have been replaced with one of a different ratio. If so, then all the number crunching, needs to be redone

 

[krizm200]

OK, I did a test on the crank shaft and the prop. 2 turns of the shaft rotates the prop exactly one full circle. So my gear ratio is 2:1, not the 2.3:1 as stated in my SELOC manual. The prop calculator is showing about 4670 rpms at 37 mph. Guess someone rebuilt the lower unit, or replaced it with a different one.

 

[krizm200]

Hi everyone,

So to finish up this thread, I found the gear ratio is 2:1, not the 2.3:1 as stated in my SELOC manual. I also took the boat out on Saturday, and did a test again of the throttle. My speedometer shows the boat maxing out it's speed at 37-38 mph on flat water and no head wind. I put the MPH into the prop calculator with a .15 slippage, and the rpms show about 4969, which seems to be in the ball park for the 4800 to 5400 rpm range. I am still using the 19 in pitch prop. The interesting thing is the tachometer will not increase above 4100 rpms. I increased the throttle slowly to WOT, and the engine is definitely increasing in rpms after 4100, and in speed as well. I may try re-seating the adjustment switch on the tach to be sure it is set for proper number of poles. Other than that, the engine and prop seem to be doing what they are meant to.

Thanks everyone for your comments and suggestions.