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Author Topic:   Propeller Revisited: 1977 Johnson 70-HP on NAUSET
jtwhaler posted 07-14-2004 04:28 PM ET (US)   Profile for jtwhaler   Send Email to jtwhaler  
There are so many of you with performance experience that I am going to try for some advice. My 1966 Nauset has a 1977 70-HP Johnson just overhauled, old original tachometer, and a 13-3/4 x 17 propeller. With two people, 12 gallons of gas under console, and two batteries, the tachometer said 6000 at WOT; the book says 5500. The GPS said 25-MPH.

New tach, new speedometer, but add 500-lbs. to load: 4500-RM at WOT. 24-MPH, and slow getting on plane with the load.

Back to 13 1/4 x 15 prop. Same lake, same load within 100 lbs, still 4500 at WOT and 24-MPH.

No electric trim so in second hole out, where it was in the beginning, and anti-ventilation plate slightly above bottom of boat. Engine sounds good but definitely not at rated horsepower or rated crankcase speed at WOT. Can't miss a good two stroke maxing out.

Help. What do you think about the 4500 with two different pitch propellers? Sure would appreciate some advice.

Peter posted 07-14-2004 05:19 PM ET (US)     Profile for Peter  Send Email to Peter     
Only explanation I can come up with is a spun hub on the 17P. Is the tachometer set to the correct pole setting for your engine?
Joe Kriz posted 07-14-2004 06:06 PM ET (US)     Profile for Joe Kriz  Send Email to Joe Kriz     

I agree with Peter about the pole setting. I believe it should be set on 6.

Is the new Tach an OMC?

Is the old Tach and OMC?
I know you said it was original but I do not know who put it on or what brand it is.

Various brands of TACHS ARE NOT EQUAL!!!!!

You have the correct props. Either one should work. The 15 pitch will give you a better hole shot and a little more top RPM. The 17 pitch willl give you a little less top RPM but more top end speed and of course a little slower hole shot than the 15.

I would suggest you do these test with no one else on board.
No load at all except I would have FULL fuel tanks.
With the 15 pitch prop, no load, full tanks, your Tach should read around 5200 to 5400/5500 RPM.

Let us know what else you find out.

jtwhaler posted 07-14-2004 06:57 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Thanks Peter and Joe. The old tach was the original OMC and the new one is a Teleflex Sportsman 7K. I purchased new for this engine the two props and hub kit from Boater's World, a Turning Point brand. Don't think the hub is slipping. I will run unloaded next time the Grandkids are away, and see what happens. I have not in years run with a loaded boat, and in fact have not had a Boston Whaler boat since 1966, so memories fade as to what I USED to do. I did run a Homelite and did get 30-MPH or more. I could have two in the boat and pull (young) skiiers. Thanks again.
John T.
jimh posted 07-14-2004 09:41 PM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "Propellers revisited."]

knothead posted 07-15-2004 05:13 PM ET (US)     Profile for knothead  Send Email to knothead     

Perhaps one of the cylinders is not producing power.

I have a 90 hp Johnson and on one occasion I had a fouled plug and the engine would only turn up to 4400 rpms at WOT. It sounded fine at idle, but would not turn up to 5500 at WOT.

You may want to make sure that all cylinders are producing power.


jtwhaler posted 07-15-2004 11:45 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Thanks for the cylindar adviced knothead, I'll let this Kansas weather cool down a bit and run a compression test. I was sure the shop did a good powerhead rebuild but one never really knows for sure.

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