1977 Evinrude 55hp motor height question

[Skeeterbait]

Recently put a Cook's CMC PT-130 power trim on my old rig. This set the engine back 6 inches. Its an Evinrude 55hp on a 16' true flat bottom aluminum boat. Cook's instructions say to mount the engine with the anti-vent plate 1-3 inches above the bottom. Originally it was at 3/4" below the bottom. I mounted two holes higher and that put me at 3/4" above the bottom. I gained top end speed probably simply because I can once again trim the motor (factory trim went out about 5 years ago and replacement not available). However I can now trim the engine to a point the prop vents and the motor over-revs, never could do that before, boat would porpoise first. And the hole shot now stinks. Trying to figure if I should lower the motor back to where it was. What is the generally excepted proper height of the anti-vent plate to the bottom on a true flat bottom boat?

 

[JB]

Re: 1977 Evinrude 55hp motor height question

Howdy, Skeeterbait.

Welcome to iboats.

I agree with the instructions, but only as a starting point. Every boat behaves differently in response to motor height.

A fast, flat bottom boat doesn't allow the wake to rise as quickly as most boats and doesn't squat on hole shot as much as most boats.

Experiment with the height, like bring her down a hole and try her out. You will find a "sweet spot" where she performs best.

Good luck.

 

[steelespike]

Re: 1977 Evinrude 55hp motor height question

Bring it down a notch.
Once you get it setup then we can see about a prop change.
There are props that will allow maximum height and speed.
If your prop is a little tired and has relatively thick blades
that seem to need sofistication you could do well wirth a new prop.
If you can tell us your prop size and wot rpm and speed we may be able to get you setup even better.

 

[Skeeterbait]

Re: 1977 Evinrude 55hp motor height question

I am thinking down a notch too. That will put the anti-vent plate even with the bottom of the boat. Currently running a 17 pitch SST. Am turning 5500 of 6000 max RPM at WOT and hole shot is sluggish, about 7 seconds. Will be testing a 15 pitch SST tomorrow afternoon.

 

[cyclops2]

Re: 1977 Evinrude 55hp motor height question

Caution

If you travel ALONE 99% of the time & get a fantastic hole shot & good top speed. You may not get on plane with 1 or 2 more people.
Party boat trip is a no do.
Or
You can set the height that gets the boat on plane, with the number of people you want to carry.
Count the # of holes up or down & mark it somehow.
Then go out alone & set the boat up for you alone & mark that hole somehow.

That is how I do boats.

 

[Skeeterbait]

Re: 1977 Evinrude 55hp motor height question

Well good results with the 15P prop. Full load, two large adults, full tanks, cut hole shot from 7 to 4 seconds. Also unexpectedly picked up 3 mph more at WOT. Guess my old (30 yr old) 17P prop was not as refined as a a prop recently manufactured. However If I can believe my old tach I am turning up 300-400 too many RPM now. Going to get an inductive tach test tool and go out one more time to test the accuracy of the tach in my dash. The engine does not "sound" like it is over reving.

 

[cyclops2]

Re: 1977 Evinrude 55hp motor height question

Skeeter

If the diameters & # of blades are the same. The better hole shot is the result of decreasing the pitch from 17" to 15 ".

That REALLY wakes up a loaded boat coming from a dead stop. Good choice for a full load of people !

Now you really want to watch out for WOT when you are alone. Serious long term overreving is a real possibility.

 

[Haysc]

Re: 1977 Evinrude 55hp motor height question

skeeter. thanks for the info i have the same motor.. maybe we could compare notes!

 

[cyclops2]

Re: 1977 Evinrude 55hp motor height question

Skeeter

If the 300 to 400 extra rpms is with just you in the boat. I would not change a thing. It will take a LOOONNNGGG time to damage the engine. If ever.

If that is with a full boat. You may want to adjust & depend on your freshly recalibrated tachometer.

I do not have a tachometer on the 9.9. But I did " calibrate " my hearing to the engine pitch, while being setup in a test tank.

 

[Skeeterbait]

Re: 1977 Evinrude 55hp motor height question

Sure, Haysc, any time. I have been round and round with this old motor since i bought it new in '78. Some times I have been happy with it and sometimes I have wanted to unbolt it over the deepest hole I could find and let it go to the bottom. :facepalm:

The 300-400 extra RPM is with full load and two adults, but not sure I can trust the tach. Will verify tach first and then decide what to do. Dropping it that one hole discussed previously might just bring the RPM back into align. Haven't done that yet.

 

[dazk14]

Re: 1977 Evinrude 55hp motor height question

What is your current WOT? At this point, I wouldn't drop the motor any lower.

What brand and model prop are you using. As steelespike mentioned previously, new props are a world apart from 30 year old prop technology.