Porpoising...

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vahunter

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I have a 1548mv with a 25hp Johnson. It runs about 27-28mph with just me. Battery in front and battery in very back with gas tank. With a passenger in back with me the boat porpoises on flat water only above 3/4 throttle, but not terrible just uncomfortable. It never porpoised before I moved the rear battery from the front to the rear. I moved the battery because it sat too low in the bow when I was bass fishing up there. I tried moving passengers up front but didnt seem to help much.

Cavitation plate is roughly 1-1/4below bottom. I have the motor pinned in pretty good causing roughly 1-1/4 to front of cav plate and roughly 2 to rear of cav plate. I'm not interested in a fin/whaletail or preferably no smart tabs. And I'd rather not shift any weight. It planes out super fast with or without a passenger. The RPMs 'sound' to be a little low but no tach. I'm considering adding an adjustable jack plate.

What are your thoughts and opinions? would raising the motor, 'leveling' the cav plate some more, adding a jackplate and/or change prop be beneficial? What should I do first?
 
Get a fatter front passenger. :p

Yeah, I agree with Ahab that the weight redistribution is the best way to fix the problem.
 
I'd start by raising the motor until the cavitation plate is at least level with the bottom of the boat. After that, play with the pin in different holes. That'll be the least expensive way to fix it. Move it up until it cavitates, and drop it down one hole.

A CMC PT-35 jackplate would get you a few more MPH and solve the porpoising, but that's not a cheap fix.

Weight distribution would work. If nothing else, drop your trim pin down a hole. You'll lose a little speed, put it should get your boat to "dig" a little more and kill the bounce.
 
vahunter said:
FishinsMyLife said:
Move it up until it cavitates, and drop it down one hole.

When you say 'move it up' do you mean height or tilt via the pin? And how do you determine when it cavitates?
i beleive he means tilt..... thats what i would mean..
 
My 1654 with a 40hp yamaha did the same thing. I tried all of the mentioned fixes, and like you didn't like all the forward weight when fishing. Thus I was continually moving things back and forth. I finally installed a Stingray hydrofoil and the problem was solved.

I lost about 3 mph off my top speed, but the boat was inherently stable at any speed regardless of load.

I agree with the others who may tell you that the effects of the hydrofoil are just masking some other issue (in this case too much weight too far back. But, it definitely fixed my problems.
 
vahunter said:
FishinsMyLife said:
Move it up until it cavitates, and drop it down one hole.

When you say 'move it up' do you mean height or tilt via the pin? And how do you determine when it cavitates?

Yeah, I meant move the pin up. Cavitating is when the motor sounds like it's over-revving but it's not "catching" in the water. It's hard to explain, but you'll know when it happens.
 
FishinsMyLife said:
vahunter said:
FishinsMyLife said:
Move it up until it cavitates, and drop it down one hole.

When you say 'move it up' do you mean height or tilt via the pin? And how do you determine when it cavitates?

Yeah, I meant move the pin up. Cavitating is when the motor sounds like it's over-revving but it's not "catching" in the water. It's hard to explain, but you'll know when it happens.


I USED A BAD CHOICE OF WORDS.. CAVITATION IS CAUSED FROM THE WATER PASSING OVER YOUR TRANSOM AND CREATING TINY AIR BUBBLES AROUND YOUR PROP MAKING IT NOT GRIP THE WATER WICH MAKES IT EASIER TO SPIN FASTER AND OVER REV SO TO SPEAK...THE CAV PLATE BEING EVEN WITH YOUR HULL KEEPS THE BUBBLES FROM GETTING SUCKED THROUGH THE PROP...
 

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