Go Back  Offshoreonly.com > Technical > Drives and Lower Units
Extension boxes and blackhawks >

Extension boxes and blackhawks

Notices

Extension boxes and blackhawks

Old 11-18-2018, 10:51 AM
  #1  
Registered
Thread Starter
 
Join Date: Jul 2017
Location: Harrington DE
Posts: 32
Received 0 Likes on 0 Posts
Default Extension boxes and blackhawks

I am doing basically a CT525 conversion on a Scarab P29. I have already pulled a bunch of weight out of the boat last year (with the original 5.7’s) and the boat ran too flat. Going to the aluminum aluminum engines it will be worse. I am considering Imco extension boxes. My question is what height should I go. I am thinking +3. My current x dimension is 16.5”. Thinking +1 for the 12” setback and going a little higher. I think the drive is a little low and forcing the bow down.
Hot Rod 29 is offline  
Old 11-18-2018, 11:56 AM
  #2  
BUP
Banned
iTrader: (9)
 
Join Date: Jul 2011
Location: Ft. Worth TX
Posts: 9,594
Likes: 0
Received 41 Likes on 35 Posts
Default

I can not recall Blackhawks on extension boxes and I am even currently working on boats that were set up with blackhawks.

On the GM crate dirt car engine CT 525 will need some internal parts changed out for the marine set up. If I am correct the same CT 525 GM engine here as you posted
BUP is offline  
Old 11-18-2018, 12:50 PM
  #3  
Registered
Thread Starter
 
Join Date: Jul 2017
Location: Harrington DE
Posts: 32
Received 0 Likes on 0 Posts
Default

Basically ct 525. I got 2 19329865 crate long blocks, Howard’s ASA cams, with the correct springs, new valves, refitted the pistons and ring gap, sheet metal intakes, and a few other things. Not worried about marinizing the engines I know what to do there. Just not real clear on drive height and my thinking on it. I know the blackhawks work good with a rocker in the bottom (my boat was already built for that) and world good with notches (the drive extension boxes take care of that).
Hot Rod 29 is offline  
Old 11-18-2018, 01:18 PM
  #4  
Charter Member # 55
Charter Member
 
Griff's Avatar
 
Join Date: Oct 2000
Location: Omaha/LOTO
Posts: 19,518
Received 1,783 Likes on 884 Posts
Default

FYI, when drives are deeper, they give more leverage and will lift the bow easier. A drive being deep does not force the bow down.
Griff is offline  
Old 11-18-2018, 01:30 PM
  #5  
VIP Member
VIP Member
 
Join Date: Nov 2002
Location: ankeny,ia.
Posts: 4,032
Received 217 Likes on 126 Posts
Default

Was this boat originally equipped with blackhawks?
JaayTeee is offline  
Old 11-18-2018, 01:32 PM
  #6  
Registered
Thread Starter
 
Join Date: Jul 2017
Location: Harrington DE
Posts: 32
Received 0 Likes on 0 Posts
Default

Normally I would agree, but blackhawks ride on top of the water. Just experience with outboards that propshadts even to 1” above the bottom can cause the lower unit body to push the bow down. Like a trim tab.
Hot Rod 29 is offline  
Old 11-18-2018, 01:34 PM
  #7  
Registered
Thread Starter
 
Join Date: Jul 2017
Location: Harrington DE
Posts: 32
Received 0 Likes on 0 Posts
Default

It’s a scarab p29. All p29’s came with blackhawks.
Hot Rod 29 is offline  
Old 11-18-2018, 01:35 PM
  #8  
BUP
Banned
iTrader: (9)
 
Join Date: Jul 2011
Location: Ft. Worth TX
Posts: 9,594
Likes: 0
Received 41 Likes on 35 Posts
Default

The P29 bottom and balance was designed for the blackhawks. They all were mounted WITHOUT extension boxes from the factory build. Larry Smith redesigned the P 29 bottoms for the blackhawks . I have worked on 2 different P29s in the past and another one 1 last year.
BUP is offline  
Old 11-18-2018, 02:03 PM
  #9  
Registered
Thread Starter
 
Join Date: Jul 2017
Location: Harrington DE
Posts: 32
Received 0 Likes on 0 Posts
Default

Originally Posted by BUP
The P29 bottom and balance was designed for the blackhawks. They all were mounted WITHOUT extension boxes from the factory build. Larry Smith redesigned the P 29 bottoms for the blackhawks . I have worked on 2 different P29s in the past and another one 1 last year.
But, will the hull work with boxes? My main issue was getting my balance back from loosing 600 lbs from the stern.
Hot Rod 29 is offline  
Old 11-18-2018, 02:42 PM
  #10  
VIP Member
VIP Member
 
Join Date: Nov 2002
Location: ankeny,ia.
Posts: 4,032
Received 217 Likes on 126 Posts
Default

If you don’t go +3, you’ll probably get the stern lift from the gearcases being too deep as you described earlier. Marc @ BAM did a ITS with a zieger extension box( to bring it back a foot) on a 20 Cig, maybe he’ll chime in

Last edited by JaayTeee; 11-18-2018 at 02:45 PM.
JaayTeee is offline  

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.