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Please help with my setup. 16ft superboat with 12Ē JP w/120 Johnson. Only 40mph?

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Please help with my setup. 16ft superboat with 12Ē JP w/120 Johnson. Only 40mph?

Old 09-15-2020, 07:43 PM
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Default Please help with my setup. 16ft superboat with 12Ē JP w/120 Johnson. Only 40mph?

Had the boat 10 years now but recently changed the motor. Previous motor was a 1.6ltr 110 v4 vro Johnson with raker prop. Boat did 50mph exactly as a center console then I converted to dual console(maybe added at the most 100lbs) swapped the old 1.6 for a 2.0 v4 Johnson 120hp from early nineties. Was expecting in the high 50ís low 60ís but instead topped out at 41. Motor has good compression and is in great mechanical condition.

Any ideas why a small motor with less displacement was able to push the boat 10mph faster than the newer 120?

is it running on all cylinders?
is the extra weight the issue?
is the shaft longer? Should I raise the motor?

the boat has a 12Ē manually adjustable jack plate.

aslo from what I researched the newer motor is 65lbs heavier. With the same raker prop that was on the older motor Iím only turning 5000k. With another mid range prop I had lying around the boat pushes 6200 rpm and does 39mph almost the same speed. Iím at a loss

experienced people please help!!!

thanks in advance

Last edited by PoshDesignNY; 09-15-2020 at 07:50 PM.
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Old 09-16-2020, 01:16 AM
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Was the previous motor a shortshaft? 15" ?
Was it also mounted on the same jackplate?
Give us some history.

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Old 09-16-2020, 01:39 AM
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Not that we don't want you here but you would probably get a lot more advice on https://www.screamandfly.com/forum.php. It is geared more toward smaller boats with OB's.
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Old 09-16-2020, 08:36 AM
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Originally Posted by mcollinstn View Post
Was the previous motor a shortshaft? 15" ?
Was it also mounted on the same jackplate?
Give us some history.
Both motors have 20Ē shafts and were mounted to the same jackplate in the same position. They were a direct swap from the 1988 110 to the 1992 120.

With all the additional weight. (from console conversion, bigger motor, larger battery) Iím thinking the boat doesnít lift as well increasing hull/shaft drag in the water, because the raker prop is a high range prop not much low or mid torque it prob canít lift the boats extra weight. Iím thinkín

thank you in advance

btw, this is also posted at screamandfly, just trying to get as much advice as possible. Infact OSO responded before SAF
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Old 09-16-2020, 12:32 PM
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What rpm's where you running with the old setup? adding 100lbs forward on a 16 foot boat is a lot. How does it react to trim old to new?
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Old 09-16-2020, 01:53 PM
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Originally Posted by ThisIsLivin View Post
What rpm's where you running with the old setup? adding 100lbs forward on a 16 foot boat is a lot. How does it react to trim old to new?
6000k with old setup, I def lost 1000k with the new. Trim for the new setup doesnít raise the bow as high, also because of the extra motor weight I moved the batt from aft to more than mid way forward to help balance. Should mention itís a heavy pig 1000 deep cycle, used a smaller 650in the old setup. You thinking weight is the major factor? Even with the newer motors extra hp and displacement?
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Old 09-16-2020, 04:59 PM
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a few extra hp is fighting the added weight (which you are probably underestimating the true number).... have you run a traditional prop with an appropriate pitch to start as a base line at the proper max rpm?

Raker props are a top end rpm on a light boat prop - you don't really have that anymore and your slip #'s are probably through the roof
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Old 09-16-2020, 05:10 PM
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Move the battery to the back of the boat.

Should be faster than old motor.

Rakers are great props. Iíve run tons of them on everything.


Check compression.

If your no longer carrying the bow at speed, you probably added more weight than you thought (as already stated) or moved it farther forward.
Neither of which would a 16í boat liked.

Both of your motors are great motors.

My brother had an old 16í OB yrs ago. It was heavy and slow.
Was trying to convince himself he was OK with 30 mph.
W/him at full throttle I got up and walked to the back of the boat.
It picked up 3 mph!

He was blown away.

He asked if I knew it would do that 😉

Last edited by Twin O/B Sonic; 09-16-2020 at 05:21 PM.
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Old 09-16-2020, 06:21 PM
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Originally Posted by PoshDesignNY View Post
6000k with old setup, I def lost 1000k with the new. Trim for the new setup doesnít raise the bow as high, also because of the extra motor weight I moved the batt from aft to more than mid way forward to help balance. Should mention itís a heavy pig 1000 deep cycle, used a smaller 650in the old setup. You thinking weight is the major factor? Even with the newer motors extra hp and displacement?
Get a smaller battery and move it to the back. Where is your tank? I had a 16' Larson Senza with a very modified Chrysler 115, great boat but the 18 gallon tank was in the front and really hurt top end. I just pulled 2 giant subs out of the back end of my 28' Velocity and it totally changed the ride. I now run more trim than I used to, I'm now trying to find ways to move weight to the rear.
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Old 09-16-2020, 06:28 PM
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Originally Posted by speicher lane View Post
a few extra hp is fighting the added weight (which you are probably underestimating the true number).... have you run a traditional prop with an appropriate pitch to start as a base line at the proper max rpm?

Raker props are a top end rpm on a light boat prop - you don't really have that anymore and your slip #'s are probably through the roof
in all honesty I think I added an extra 165lbs to the boat in total with everything included. It may be that the raker was at the cusp of what it could handle on the old setup To squeez 50mph and with the extra weight On the new it probably just past the point it could lift the boat and turn 6k. Should mention that the seating position moved forward about 12Ē as well. Iím hoping that if a get a smaller battery , move it aft again and tilt the motor up 2-3 inches I can get the extra rpms out of that raker.

Like it is said many time Ďsetup is everythingí

I still feel like it maybe running on only 3 cyl, maybe a spark wire or coil. Itís not running rough but that maybe because of that heavy ass flywheel.

think about it!

the new motor is a 2.0, the old a 1.6, 3cyl working on the 2.0 is only a 1.5, almost a 1.6 like the old motor.

did notice one day that 1 plug looked perfect and the other 3 looked normal. Might have to look back into that and track down the problem cyl. And FYI my old 110 had only about 100psi and the new motor has min 125psi across. One day I felt the boat jolt forward at speed like it was just let loose of itís restraints but that was only once.

when I read about heavier boats with similar hp going 50 I feel like I should be doing 60 especially with this superboat, itís so simple there is nothing to the boat

also with the raker on the new motor it is unusually slow to plane even with 2 people, the mid range prop I have now gives it an amazing hole shot about 3 seconds to 30mph. Another reason I feel itís not getting full power.

Originally Posted by ThisIsLivin View Post
Get a smaller battery move it to the back. Where is your tank? I had a 16' Larson Senza with a very modified Chrysler 115, great boat but the 18 gallon tank was in the front and really hurt top end. I just pulled 2 giant subs out of the back end of my 28' Velocity and it totally changed the ride. I now run morey trim than I used to, I'm now trying to find ways to move weight to the rear.
the old tank was the same capacity but was deck mounted under the cc, the new tank is custom aluminum bilge tank mounted rear of the old tank by at least 8-10Ē

Last edited by PoshDesignNY; 09-16-2020 at 06:37 PM.
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