Do I need a new tach?
 

My tach is a auto meter that has a separate knob to recall/erase. At wot I'm getting 79mph at 5300 rpm but when I turn the recall knob it says 5800. I have the rev limiter set at 6000 and when you turn the key on the tach goes to 6000 and back to zero. Turn the knob and it recalls 6000. So I would think it was operating right. If I run 4500 and check the recall it says 4500. So do you think I'm running 5800 wot or 5300. I don't know where the reading comes from the position of the tach needle or the ignition box. I don't really want to buy another tach just to test it.

Can't really help on if you need a new tach but at 79mph GPS what size prop and gear ratio do you have? That might give some Insight. Like for example if you have a 26p b1 and you're spinning to 5300 with a 1.5 ratio you're at 10 percent slip which idk for a cat if that's good bad or bad. But at 5800 you're at 17 percent. Does that sound more realistic? I don't know either way but that will give you maybe some insight as to wether or not the tach is right. Also does it sound like 5800 rpm? I do believe the recall would come from the tachometer itself not the box.

I was told it's a xr outdrive but I don't know how to tell what the gearing is. I'm running a 24x15.25 hydromotive 4 blade cleaver. I want to say the po said it was 1:36. I don't know what you mean by slip bit sounds important. I'm not new to boats but I'm new to performance boats and working on them. Thanks

Here's a quick run down on prop slip and a calculator

http://www.mercuryracing.com/propell...calculator.php

Based on a quick calculation I'm thinking you're spinning the motor to 5300, that would be 10.4 percent slip and I did a quick search on cats and they claim 8-10 percent is optimum slip. If the previous owner had the boat dialed in then you are more then likely spinning it to 5300, now at 5800 you'd be at 18 percent slip which is realitively high and I think you'd really tell the difference in the way the motor sounds if you're spinning it up to 5800. If you had access to a known working tach you can temporarily hook it up and do a quick comparison.

Or possibly you could set the limiter at say 5200 or 5,000 do a run and see if you can get the boat to hit the limiter and see what speed you're getting out of it. If you are at say 75 mph at 5000 then you're probably spinning it it 5300 at 79. (These are just rough numbers) I apologize if i didn't explain it in an easy to understand way.

I bought the boat a few months ago and it spun a rod bearing about my fourth trip out. I installed a new motor and have spent all. Summer trying to find the right prop. Thanks for the info and I'll try setting the rev limiter down like you said.