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I have a 13 ft 1975 whaler with a late 80s 40 hp mariner 2 stroke. Motor has excellent compression and runs great. Need to replace prop but have no idea what rpm i am running because I have no tach. My gps is saying 27mph at wot. Comes on plain quickly about 12 mph. Should I stick with this set up when replacing or step up in prop. I would like more top speed performance if possible. Who wouldnt? what is the "typical" prop for this set up?
Read about props here at http://www.piranha.com
Not sure if the vibration type rpm guage will work on outboard, We use them for noise sound issues on off road bikes, I think JC whitney or radio shack as type,. or try ordeing a tach for dirt bikes that mounts and reads off spark plug wiring.
My 1966 20 hp has niether RPM or speed pitot tube. I guess GPS is accurate I will try that with my truck TomTom mao locator as a trip and speed tracker.
A 1975 Sport 13 with 40 HP should be capable of about 35 MPH not 27 MPH. What prop do you have now? Diameter, pitch, brand, style, number of blades, part number?
If you really want to dial in your set-up you need to know how fast your motor is running. A tach is an essential tool for propping a boat correctly. You can even use an inexpensive tiny tach:
