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Author Topic:   Prop size for 70-hp Evinrude
fisher801 posted 03-04-2007 12:44 AM ET (US)   Profile for fisher801   Send Email to fisher801  
Hi all,
I would like to know what the best size propeller would fit a 1982 model 70-hp inline 3-cylinder Evinrude. It currently has a 13.25" x 17" prop on it, but I'm only getting 3500 rpm max. It seems as if the prop is a bit too big since I only see 1/8" clearance between the top of the blade to the bottom of the cavitation plate.
Another concern I have is that I'm getting only 30 mph max. This motor sits on a 15' Sport I recently purchased and from what I've read in other discussions I should be getting close to 40 mph with a 70-hp.
Thanks to all who respond.
fairdeal2u posted 03-04-2007 02:59 AM ET (US)     Profile for fairdeal2u  Send Email to fairdeal2u     
something is not right. you should easily get over 5K rpms.

I had that size prop on a montauk and with the 70 vro johnson 1987 model. top speed was about 38 mph.

With your lighter rig you should hit 50 mph.

SpinalTap posted 03-04-2007 08:29 AM ET (US)     Profile for SpinalTap  Send Email to SpinalTap     
I agree something is wrong with your engine only turning 3500rpm. I had a 60(pretty much the identical engine as the 70 tuned to 60hp) with a 13x19 turning over 5000rpms and if I remember right around 42ish on the speed. I don't know enough about props to know what and extra 1/4 inch in diameter and 2inchs less pitch would do to my rpms, but me thinks it would not go to 3500.
Hal Watkins posted 03-04-2007 10:58 AM ET (US)     Profile for Hal Watkins  Send Email to Hal Watkins     
I have a 60HP Evinrude VRO on my 15'SS. I run a 14x17 aluminum. Don't know wo RPM, but it will do 36 on GPS w/350#'s of passengers and 12 gal fuel. Hal of Waseca, MN
Tom W Clark posted 03-04-2007 12:13 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

What you report does not add up. I cannot see how a 1982 Evinrude 70 HP can achieve even 30 MPH at 3500 RPM with a 17" pitch prop. Crunching the numbers you would have to have negative propeller slip to do so.

You say there is only 1/8" of clearance between h blade tips and the AV plate yet you also say the prop has a diameter of only 13-1/4". I know you one can fit a 14" diameter prop on this motor, indeed Hal reports he is running one on his motor at present.

Can you verify the propeller make, material, part number and if it has been worked on?

Can you verify your tachometer settings on the back of the instrument?

Can you tell us how you are measuring your boat speed?

Can you verify the motor is running on all three cylinders?

A Sport 15 with a 70 HP Evinrude will not see 50 MPH but should be able to hit the low 40s with the right prop.

fisher801 posted 03-04-2007 09:26 PM ET (US)     Profile for fisher801  Send Email to fisher801     
Hi everybody
Thanks for responding and thanks for the RPS Z-leg replacements for my Montauk, Tom. They work great!
Anyway, there definitely is something wrong with this 70-hp Evinrude O/B. I'm measuring the speed with a handheld Garmin GPS and also have a working speedometer on the console. They both reflect fairly close readings so I know the speed readings I posted are accurate.
The tachometer(OEM Teleflex) readings are as I stated at around 3500 rpm at WOT. It sounds as if all the cylinders are firing, but I have yet to perform an actual test. There is one obvious symptom to note besides the RPM-- whenever I open the throttle from a standstill, the motor really lags in the first 2000 rpm range and then it accelerates to its current WOT rpm and speed. This should definitely be an indication of a specific problem.
The aluminum propeller currently on the 70-hp Evinrude is one that was on a 75-hp Evinrude E-tec that serves as the power for my Montauk. I've since upgraded to a 13.25 x 17" SS Stiletto prop for the 75-hp E-tec so I decided to use the existing aluminum prop for the afflicted motor. From what I've read in ealier posts, this size prop should be fine for a 70-hp Evinrude OB.
I need help in finding a solution. I don't want to spend a whole lot of money on an O/B that already has problems, but also knowing that many California lakes & reservoirs will probably ban many carbureted 2-strokes that do not fulfill the increasing stringent emissions laws, I might be forced to repower to either a 4-stroke or 2-stroke DFI eventually. For now, I would just like to get this O/B to run within its normal parameters.
Sal DiMercurio posted 03-04-2007 10:55 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Hold on partner.
Your problem is your tach.
I have found 9 out of 10 teleflex tachs don't work on omc engines.
Get yourself an omc or Bombardier tach & you will see the difference.
Borrow one if you must, just to see.
Just be sure to set the number on the back of the omc Bombardier tach to the number 6 I believe.
Tom W Clark posted 03-05-2007 12:22 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The good news is that if Sal is correct, the solution is as simple as finding an OMC tach.

The bad news is that if Sal Is correct, your true engine speed at 30 MPH is more like 4000 RPM which does not explain where the other 1000-1500 RPM, and 10 MPH or so went.

I think you should have the motor serviced. Something is very wrong with it and it is not the propeller.

where2 posted 03-05-2007 10:48 PM ET (US)     Profile for where2  Send Email to where2     
Have the carburetors service, before you melt down the top of one of the pistons. Those 3 cylinder engines can lope along on 2 cylinders, and if you're starving one of the cylinders for fuel at high speed, you'd experience quite a performance lag.

The 13-1/4"x17" should get you 38mph. My 1985 15' Sport with a 70Hp Johnson will run that prop up enough to bury the tach above 6k. I have a 13-1/4"x19" if I want to go above 40mph.

The Judge posted 03-07-2007 02:07 PM ET (US)     Profile for The Judge  Send Email to The Judge     
I would start with tach and carbs. That engine redlines at 5500 so remember that when propping it.

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