Best Practices For Throttle and Trim management to not blow a motor

[lent0n]

I have a 1979 Evinrude 75hp, with some parts from two other Johnsons to give it power trim.
It has the old style wiring with a rectifier not regulator, but I have that wiring setup if I need it.

Presently I'm not having any issues going out and having a good time, I tend to cruise at 4k-4.5k RPM trimming the motor to ease up on the throttle which gives me about 40km/h-ish depending on weight, waves, etc.

I usually only have 2-3 people on the boat including myself so getting onto plane hasn't ever been a problem as I have a 15' foot trihull.

For getting onto plane there seems to be a few approaches, what I'm curious of, is what is best for the motor, especially if I have a larger group where it's going to be working its hardest.

How I currently go about it

Engine trimmed low but not the bottom, throttle up to get the RPMs at around 3k and trim motor up to relieve pressure in the combustion chamber and get on plane. From there I usually pull the throttle back as I'm trimming up to keep speed stable and bring RPMs down. I generally don't go past 5k unless the water is really calm in which case it might go as high as 5.5k rpm before I pull it down.

I've had some people tell me to go WOT before trimming but that feels like I'm generating a ton of pressure in the combustion chamber, and was the exact scenario that resulted in a previous motor blowing a hole out the combustion chamber years ago. (though who knows about its life before mine)

 

[saltchuckmatt]

Start with the motor all the way down, give the throttle everything she's got and then start trimming up until you hear the prop cavitate and put it slightly back down. That's how you test for the right prop, your boat should be loaded and trimmed all the way up slightly under cavitation and check out the RPMs. You need to be in The Sweet spot.

After that you can back down on the throttle.

 

[cyclops222]

Use 2 props. 1 for light load. Other for a crowd. Go to 4' of water. Jump in and change the prop. Havs spare nut and washer if needed. Been there many times.

 

[Mc Tool]

Well firstly ......you are not going to "blow a hole in the combustion chamber" by giving it to much throttle .....there must have been some other issue .
To get on the plane .... trim down and full throttle ( or very near ) and as the boat gets up on the plane trim up and reduce throttle . With a bit of practice you will be able to get the boat to pop up nicely and motor off on the plane without it lookin like a NASA launch .
Combustion chamber pressure is power basically and is not something you should be bothered about whilst out having a good time

 

[flyingscott]

prop that motor for 5800 rpm

 

[jimmbo]

The 3 cylinder 75hp is weaker in Holeshot than the 3 cylinder 70hp of the same displacement. That is due to the Port Timing to get the extra 5 hp.WOT range for it is 5200-5800, but that engine will run better and last longer if it propped to run as Close to 5800 as possible