what causes a boat to porpoise

[roscoe]

Re: what causes a boat to porpoise

2 photos, one side view, one rear view, of the transom with the motor lowered down would answer a lot of questions for us. Also. a shot of the trim tabs would help.

 

[180shabah]

Re: what causes a boat to porpoise

2 photos, one side view, one rear view, of the transom with the motor lowered down would answer a lot of questions for us. Also. a shot of the trim tabs would help.

Prop info also...

 

[bates1]

Re: what causes a boat to porpoise

i have a 1988 bayliner cobra br had a 125 force on it changed it to a 115vro johnson. not matter what we do to the trim it wants to start jumping out of the water, about the same time its starting to plane out, no china walk just like a bucking bronco (HELP)

d river rat dats me;now what is the dates upon the motors.and what is the top H.P. on the boat.From my personal experience johnson is a much better engine.
history could be a route to take.They measured H.P. at the prop until around 1984 which the vro Johnson was very dominant in the late 70's thru early 80's.
if you have over powered the boat along with it's age and improper propeller pitch for your rig.
they went from measuring at the prop to the crank,which can make a huge difference in total output power on the vessel.

 

[mhyder63ford]

Re: what causes a boat to porpoise

its not that i cant get full throttle the boat starts jumping hard at about half throttle and im not brave enough to ride it out lol its like being on a wild bull ride will post oics of the back of the boat tonight. the motor i put on it is a 1984 115vro johnson it had a1988 125 force on it and it handled like a dream but you know the story they only put out about 88hp at the prop and i got a really good deal on the 115

 

[NSBCraig]

Re: what causes a boat to porpoise

no the only thing that i did to the boat was first take the 125 force of of it, then we put a 115vro johnson on it the shaft is the same and every thing we left the trim tabs on for the first time out tried the trim in all 4 holes, we couldnt get the boat to full throttle . so we went back home took the trim tabs off and back to the lake it didnt change any thing the boat just starts to plan out then all hell brakes lose never get past half throttle and we didnt have any problems with the force on their but it did have the stingray on it and the johnson does not

Your really not being clear at all if you want help you kind of have to help us help you.

 

[mhyder63ford]

Re: what causes a boat to porpoise

here are the pics you asked for

 

[Mark42]

Re: what causes a boat to porpoise

Not sure, but the side photo looks like the skeg hit the floor before going all the way down to the running position, right? I can't tell if the cavitation plate is even with the bottom of the hull or not.

 

[62 pipestone]

Re: what causes a boat to porpoise

its not that i cant get full throttle the boat starts jumping hard at about half throttle and im not brave enough to ride it out lol its like being on a wild bull ride

come on, that sounds fun

what is the weight diff from the new to the old motor.

also im with mark42. something doesnt look write with that side shot.

 

[180shabah]

Re: what causes a boat to porpoise

something doesnt look write with that side shot.

...too much bow down.....

 

[mhyder63ford]

Re: what causes a boat to porpoise

yes the skagg is just touching the floor but have the front of the trailer down as low as it will go ,but the motor is lined up with the closest hole to the transum also. the 125 also touch the floor in the same manner. this motor weighs about 65lbs more than the 125 from what ive got off the internet

 

[NSBCraig]

Re: what causes a boat to porpoise

Buy some smart tab blems from iboats. Someone added those cheese tabs and the fin on the old motor to fix the balance, luckily you have a better option now with smart tabs.

 

[JimS123]

Re: what causes a boat to porpoise

Buy some smart tab blems from iboats. Someone added those cheese tabs and the fin on the old motor to fix the balance, luckily you have a better option now with smart tabs.

The tabs in the pic are from the 1980's and are either Perko or Tempo (I forget). They are fixed in the down position (curved, not a straight chord)and are not adjustable. Their effect is quite pronounced. They were only available for a few years as I remember....probably didn't sell too well.

Those tabs should always raise the stern and/or lower the bow. If the boat is still porposing with THEM on, then it would be my guess that there surely is a hull problem. That being the case, IMHO, Smarttabs will not solve the problem, but make it worse at higher speds because they will retract.

I hope that we are truly talking about porposing here. Could it be that those tabs are forcing the stem in to the water causing bow steer?

 

[mhyder63ford]

Re: what causes a boat to porpoise

well we got it taken care of come to find out the motor had to raised up about 2 inches. thank you all for your help and input she rides like a caddy now. now we get to tear her all the way back down and replace the floors and patch the holes from the other motor. that will be my next project so im sure i will neede some more help from you guys, thank you all