Go Back  Offshoreonly.com > Technical > General Q & A
Please help with my setup. 16ft superboat with 12” JP w/120 Johnson. Only 40mph? >

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

Notices

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

Old 09-15-2020, 06:43 PM
  #1  
Registered
Thread Starter
 
Join Date: Apr 2010
Posts: 7
Likes: 0
Received 3 Likes on 1 Post
Default Please help with my setup. 16ft superboat with 12” 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 50’s low 60’s 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 12” 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 I’m only turning 5000k. With another mid range prop I had lying around the boat pushes 6200 rpm and does 39mph almost the same speed. I’m at a loss

experienced people please help!!!

thanks in advance

Last edited by PoshDesignNY; 09-15-2020 at 06:50 PM.
PoshDesignNY is offline  
Old 09-16-2020, 12:16 AM
  #2  
Platinum Member
Platinum Member
 
mcollinstn's Avatar
 
Join Date: Sep 2001
Location: tn
Posts: 5,663
Received 69 Likes on 44 Posts
Default

Was the previous motor a shortshaft? 15" ?
Was it also mounted on the same jackplate?
Give us some history.

mcollinstn is offline  
Old 09-16-2020, 12:39 AM
  #3  
Charter Member # 55
Charter Member
 
Griff's Avatar
 
Join Date: Oct 2000
Location: Omaha
Posts: 18,281
Received 684 Likes on 330 Posts
Default

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.
Griff is offline  
The following 2 users liked this post by Griff:
PA.WOODCHUCK (09-16-2020), TomZ (09-16-2020)
Old 09-16-2020, 07:36 AM
  #4  
Registered
Thread Starter
 
Join Date: Apr 2010
Posts: 7
Likes: 0
Received 3 Likes on 1 Post
Default

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 20” 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) I’m thinking the boat doesn’t 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 can’t lift the boats extra weight. I’m think’n

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
PoshDesignNY is offline  
The following 3 users liked this post by PoshDesignNY:
Griff (09-16-2020), Padraig (09-16-2020), TomZ (09-16-2020)
Old 09-16-2020, 11:32 AM
  #5  
Registered
 
Join Date: Oct 2010
Location: Cheboygan, MI
Posts: 1,369
Received 184 Likes on 115 Posts
Default

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?
ThisIsLivin is offline  
Old 09-16-2020, 12:53 PM
  #6  
Registered
Thread Starter
 
Join Date: Apr 2010
Posts: 7
Likes: 0
Received 3 Likes on 1 Post
Default

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 doesn’t 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 it’s 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?
PoshDesignNY is offline  
Old 09-16-2020, 03:59 PM
  #7  
Registered
 
Join Date: Jul 2007
Posts: 2,770
Received 432 Likes on 259 Posts
Default

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
speicher lane is offline  
Old 09-16-2020, 04:10 PM
  #8  
Registered User
Platinum Member
 
Join Date: Mar 2002
Location: Cardington Ohio
Posts: 3,202
Received 450 Likes on 194 Posts
Default

Move the battery to the back of the boat.

Should be faster than old motor.

Rakers are great props. I’ve 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 16’ boat liked.

Both of your motors are great motors.

My brother had an old 16’ 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 04:21 PM.
Twin O/B Sonic is offline  
The following 2 users liked this post by Twin O/B Sonic:
resurrected (09-17-2020), ThisIsLivin (09-16-2020)
Old 09-16-2020, 05:21 PM
  #9  
Registered
 
Join Date: Oct 2010
Location: Cheboygan, MI
Posts: 1,369
Received 184 Likes on 115 Posts
Default

Originally Posted by PoshDesignNY View Post
6000k with old setup, I def lost 1000k with the new. Trim for the new setup doesn’t 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 it’s 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.
ThisIsLivin is offline  
Old 09-16-2020, 05:28 PM
  #10  
Registered
Thread Starter
 
Join Date: Apr 2010
Posts: 7
Likes: 0
Received 3 Likes on 1 Post
Default

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 12” as well. I’m 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 ‘setup is everything’

I still feel like it maybe running on only 3 cyl, maybe a spark wire or coil. It’s 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 it’s 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, it’s 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 it’s 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 8-10”

Last edited by PoshDesignNY; 09-16-2020 at 05:37 PM.
PoshDesignNY is offline  

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.