"Bucking" 25HP Motor at higher speeds
 

I don't have the boat anymore but maybe you can solve something that has been bothering me for a long time. Lifelong therapy on this forum!

In high school my brother and I were avid fishermen! And we couldn't afford a boat so shore fished all the time. One day our dad surprises us by pulling up to the garage with a 16' Aluminum boat with 25HP Evinrude on a trailer! Wow, we were excited!! He had to save for a very long time for the boat as we didn't have much money. He paid $1,500 I think. Keep in mind this was in over 30 years ago when you could buy a basic aluminum boat for $500 and a motor or $400, and get a good deal. This boat had a trailer, had front fishing deck built, was lined in astroturf....and (wait for it), had bucket cranberry color fishing "chairs" which were in a velvet type material. LOL! Not usual fishing chairs but these babies were right out of your living room type chairs! Built in arms, well rounded around our body. All that was missing was the foot rest! We'd get crazy, looks driving down the street with people taking photos! Hey, we were in high school, we didn't care! We had a boat!!!! Fun, happy memories to say the least with Dad. He has since passed and I wouldn't trade those memories of that pimped out boat for anything!

Okay to my post........ used 25HP Evinrude would "buck" at higher speeds. We could never figure out how to fix it. We tried mounting the thing at different levels because someone said it was from being mounted too low. But even when we raised it, it would still buck. It was fine at lower speeds, basically trolling speed, but as soon as you went any faster it was like a bareback bronc! Note: It was a "hollow hull". Hollow in the center of the entire length of the boat, solid left and right sides. Also, remembering back, the spot where the motor was mounted was scooped down about 4" inches. I'm wondering if the entire boat wasn't custom built and that scoop down made the motor entirely way too low no matter what trim height we set it at?

Thanks for figuring out what it could have been. Has been bothering me all these years because Dad felt bad he got taken and failed us boys. But in truth Dad, it was the greatest gift you ever gave us boys! The bucking made it even more of an adventure with great memories fishing with you! Thank you Dad!

Sounds like your trim needed to be set, not sure if that is what you meant when you said mounting it to low. A motor in that size class has about 5 different holes you can set your trim pin in. You may have to google it to know what I'm talking about as I'm not great with words.

Ya, that's what we tried. We set it at every setting we could try and still the same. I edited the post after you posted to say it was a hollow hull boat. That might be part of it. Tks

Sounds like you had a tunnel hull which is designed for shallow water use with a jet. The tunnel allows for a better water feed to the jet intake. They are not designed for use with a traditional prop driven outboard. They do however work with surface drive outboards as they provide enough water flow to pass over the prop in inches deep water where a regular outboard won't work.

Well that could be it. At the time I had not seen that style of hull where it was a tunnel in the middle. We put on a trolling motor which worked well. :sSig_cool2::sSig_cool2:

Might have been a cavitation issue as well, the water flow around the prop through the tunnel at higher RPM might have caused too many bubbles.

I should have corrected my post. Tunnel hulls are designed for use with a standard prop driven outboard but the motor must be raised considerably to the correct height. The prop has to be able to catch the water flowing out of the tunnel.

This.

Real simple to do just don't pull,the pin and fart around in deep water go to shore...pin falls out of your wet hands:sign0176:

Trim tabs would probably have solved the issue. The smart tabs(non electric) may well have worked well. You can YouTube them.
Porpoising can also be a result of improperly set trim of motor as mentioned in a previous post.

What you are experiencing is called porpoising.

It can be caused by hull design or trim settings too low or too high. Usually too high.
But it can also be caused by having the center of gravity too far aft. Or in layman's terms, too much weight too far back.

Without the boat to test, one can only guess at what the problem with a particular boat might be. In the end no one would gain from such guessing.

This is what i was thinking. I have an 8hp merc kicker on the back along with a 70hp yamaha. 16ft boat will get popoisingat speed unless my put the wife and kids forward.

Have not tried a whale tail and the trim does not fix it