View Full Version : Verifying tach speed
Any techs out there know how a new tachometer is typically set up and verifyed when rigged? Just put a new 150 Opti on my current boat and also had a new merc tach installed as the other one went with the old motor. The tach never worked from the get go and the mechanic who rigged it just said to turn the adjustment screw on the back of the tach till it was set right. I did that, but it sure doesn't seem right. The motor is set down on the transom as far as it will go,I'm running a high 5 prop, and it says I'm pulling 5900 RPM at WOT. Although my speed is only 46 MPH. I'm getting no blowout or major prop slippage and I'm having a hard time believing that the tach is reading correctly, especially when others here with the same boat are getting 5 MPH faster speeds than I am. There doesn't appear to be any other screw setting that works at all and I'm not sure what to do next. Can anyone poiont me in the right direction?
SVaerst
05-21-2001, 09:00 AM
hello.
Sounds alot like you need to play with props. I am in the process of finding a different prop to maximize performance and with enough research, here is what I have found:
A 4 or 5 bladed prop will in many cases reduce the top end speed that your boat can have. It will help tremendously in handling performance of your rig. I am on a hunch why the top end falls off when I say this, but with more fins in the water, there will essentially be more water resistance...hence the lower top end speed.
With a 3 blade prop and some 4 blade models, you can maximize speed and keep the handling qualities you want as well. By reducing a fin on the prop, there is less resistance, thus the increase in speed.
From what I have read on the threads here, I would hunch that you may want to try some 3 and 4 blade props. I could not tell you where to begin on which brands to try, but if you check around on the threads for props, you may get an idea as to what to try. If anything, hopefully this gives you a start.
Have fun with it..
Stev
Jim Carroll
05-21-2001, 10:12 AM
Your Opti has a rev limiter and would not let the motor turn at those RPM's for any length of time. Your Tach needs to be calibrated. JC
I have already come to that conclusion I guess. It's how to do that that I'm confused about. Anyone know?
Phil T.
05-21-2001, 02:15 PM
Your dealer should be able to hook up your engine's management computer to a laptop and read the rpm's directly. There also exsist mechanical tachs that you set in the dimple at the end of the crankshaft. One of those could be used to calibrate your gauge at an idle. Farm implement dealers or even a farmer relative should have one of those (I do).
I put a new tach on my outboard and did not believe it was reading correctly.So I bought a Quartz tiny tach and hooked it up to the engine also.I believe the Quartz tachs are suppose to be pretty accurate.I compared the two readings and found the regular tach was reading 160 rpms to slow and adjusted from there .
Old Fishermanr
05-23-2001, 03:37 PM
Why not have your dealer check your tach against his/hers shop tach and set your's accordingly. You could then proceed to prop your boat.
I took my boat into the dealer cause the tach was factory tach which was on the correct pole setting was reading 5400 rpm's with a 15" prop (other people with this setup [F80/1775 pro v] were showing 5800 with 17"). The tach was off by 800 rpm's. It was quicker and easier to have the dealer do it than to dick with it myself (tach is adjusted, and I have a 17" prop). You need accurate rpm/speed before playing with props.
After toying with it some more I am now going to just bring it in to the dealer and let them play with it.Wanting top performance, it's aggravating to trust anything at this point....