1977 75hp Evinrude...need 500 more RPM

[BonairII]

Ran the boat with a new tach installed today. Got her up to 5500rpm and 35mph(fastest I've ever run with this motor). The RPMs are actually right in the middle of the correct operating range(5200-5800). From what I've read, these motors are happier at 6grand.

I'm running my spare prop, so I need to order a prop anyway. A 15 pitch prop should get me to 5700rpm.

Is chasing the extra 300rpm going to do anything for me? Am I going to lose speed by dropping to 15 pitch?

Boat- 1967 Glasspar Avalon Runabout 700 lbs
Engine- '77 75hp Evinrude 13 1/4 X 17P prop 220 lbs
2 people- 330 lbs
Gas, gear, 2 batts- 150 lbs
_______
Total- 1400 lbs

 

[BonairII]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

Just realized that dropping 2 inches in pitch should actually get me an additional 400rpm(not 200), so that should be perfect.

Found a deal($60) on a brand new BRP 13 1/2 X 15P prop, so I think I'm going to pull the trigger on it.

If I'm going UP a 1/4" in diameter.....and DOWN 2" in pitch....is my speed likely to go down or up, or stay the same?

 

[1080tommy1]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

I misread the post and this seems to be a good number for your prop. I am finalizing a new prop as well so hopefully you will be in range and happy. Good luck with her.

 

[eavega]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

From what I've read, these motors are happier at 6grand.

Sorry, where have you read that these motors are happier at 6K RPM? I run a 1976 70 HP Johnson with a 17p SS prop. Keeps my motor at 5400 RPM WOT which was in the acceptable range. I have avoided going to a 15p prop since that would put it about at the limit. I am always worried about running too close to a max range as I had a 72 50 HP evinrude that was supposedly rated to 6K RPM. I ran that at 6K RPM and blew a chunk of piston skirt through the crankcase. Since then I have been leery of running at max RPMs but wouldn't mind getting a little more out of my 70.

Rgds

 

[BonairII]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

Sorry, where have you read that these motors are happier at 6K RPM?

I've read it here somewhere. Can't remember where though. Hopefully, I'll get a few other opinions to either confirm or refute the 6k thing.

 

[steelespike]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

Calculator says with 10% slip a 17" gets about 32 Lower slip would help speed but I don't think enough for 35.Is speed by gps?
A 15 at 5900 10% slip would be about 31 mph.
At 6% slip 5500 17" gets 34.4 a 15 at 5900 6% gets 32.6
This assume about 400 rpm for a 2" pitch change

 

[BonairII]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

Speed was done by gps.

I ended up lower the trim one notch because I was ventilating in turns. I get 33mph now.

I'd rather not lose any speed by dropping down to 15" prop, but I'm assuming it's probably the best choice for me. If I have 4 total people on the boat, I'm probably going to need the extra 'grunt' that the 15P will give me.

 

[BonairII]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

Tried the 15 pitch prop today. Lake was flat.

I def could feel the extra oomph at holeshot. 30mph at WOT. Tach at 5800. (prev mph was 35mph on choppy water)

The boat setup was a little different. Transom trolling motor was replaced by a bow mount model, and battery was moved from stern to under the bow. Didn't get a chance to mess with trim settings, just left it at same trim(manual trim motor) as was used with 17p prop.

Since this was the first time I tested top speed on really flat water, makes me wonder if my 35mph top speed(17pitch prop) was because I was riding higher in the water because the lake was mildly choppy.

 

[steelespike]

Re: 1977 75hp Evinrude...need to 500 more RPM

Re: 1977 75hp Evinrude...need to 500 more RPM

A light chop can produce better speed than glass calm.
Your 15 should be easier to stay on plane at reduced throttle.Not have to constantly fiddle to maintain plane. Getting on plane easier should save a tiny bit of fuel.
Vertical height and trim can improve things
When you changed trim do to venting it might have been better to lower the motor so you might have been able to maintain up trim.
It does appear that if everything was accurate your slip with the 17 at 35 mph was 4%.
That was exceptional. your 15 is at 11% (high normal). Some of that may be from the changes you have made.Pretty sure that 4% was before you lowered the trim.
Its sure is hard to argue with 4% slip.Manual trim makes it difficult to run at ideal trim.Its normal to have to trim down for turns so people compromise do you tollerate some venting or sacrifice some speed.Generally if you compromise speed with trim you lose mileage.

 

[BonairII]

Re: 1977 75hp Evinrude...need 500 more RPM

Re: 1977 75hp Evinrude...need 500 more RPM

Currently my motor is at max height(using 4th hole on mounting bracket). When I trimmed up a hole(w/ 17p prop), I did get a bit of ventilating on turns.

I'm going to use the 17p for fishing and cruising around...and use the 15P for when I have additional passengers and/or we are pulling a tube etc.

Going to be pretty flat water this weekend, so I'm going to test the 17P on the calm water and see what speed I get.

Both props still keep me within proper operating RPM, so choosing one will just be a matter of what performance/speed I need on a particular outing.

 

[sutor623]

Re: 1977 75hp Evinrude...need 500 more RPM

What readings did you get on a calm day, with the 17P, assuming that you've even had a nice day. (All this East coast rain.....)