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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. |
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 |
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). |
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.
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Was this boat originally equipped with blackhawks? |
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.
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It’s a scarab p29. All p29’s came with blackhawks. |
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.
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Originally Posted by BUP
(Post 4659791)
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.
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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 |
I would set it at the height that will make the drive bullet sit on top of the water coming off the bottom at speed.
my Daytona is set up this way at +2 inches using a -3 IMCO shortie and an IMCO neutral ext. box. At speed the water coming under the boat goes under the gear case bullet. I even had to lower the water pick up to just in front of the skag. I also cut the bell off the back of my prop. This made a noticeable drop in the stern. and I agree that the closer the gear case gets to the surface the more it wants to surface causing a bow down attitude. |
Page 21 post 206 (adjustable extension box pics)
https://www.offshoreonly.com/forums/...iesels-21.html Guessing you know that the stelling boxes are adjustable up and down so the perfect drive height can be acheived, Also they are nla. No reason imco boxes couldnt be modified to accept the rear hangar for a driveline and no reason the boxes couldnt be mounted on custom plates/short boxes that would allow for the boxes to be adjusted up or down a few inches The extra weight of those parts would help balance, plus the mercury driveline still uses the rubber coupler Unlike the stellings so no extra abuse on the drive. More money to go custom and adjustable on the front end of this build rather than nasa level math for a one shot Hope its right x dimension and with no spacers have to recut and rebuild an inch in either direction later |
With the prop shaft parallel to the bottom of the boat my bullet/gear case is in the water. About 1-1.5”
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I can’t find any used boxes. IMCO and ITS from mercury Racing is it. I think I need 2” maybe 2.5” to be correct. I hope 3” will work so I don’t have to redrill. |
I have done a lot of research on the blackhawks amd they run best when bullet is out of the water so You want to go higher as your now even whig the bottom problem whit this is that You have less leverage to Carry the bow but the black hawk is not designed to Carry the bow the hull need to have natural bow lift build in so might check if your hull has decent rocker in the bottom |
Originally Posted by bor
(Post 4659919)
I have done a lot of research on the blackhawks amd they run best when bullet is out of the water so You want to go higher as your now even whig the bottom problem whit this is that You have less leverage to Carry the bow but the black hawk is not designed to Carry the bow the hull need to have natural bow lift build in so might check if your hull has decent rocker in the bottom |
2 Attachment(s)
Here is a drawing I made when I figured the X on my 20 Cigarette. Unfortunately I have not run it yet. But yes I think the gearcase needs to be above the bottom.
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I did my blackhawk on my 20 with an ITS, it sits back8 inches which is approximatley 5/8s of an inch lower, i set my X at 17 3/4 and it rides like a dream.. nice and flat at 86 mph so far.https://cimg3.ibsrv.net/gimg/www.off...8b6bbc1b0d.jpg
https://cimg5.ibsrv.net/gimg/www.off...6df315dd4c.jpg |
but totally different app - not an apples to applies comparison here. Lite weight narrow short single vs far from this 29 twins and full cabin. The props are important for blackhawks as well.
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the question was about a box and the difference in the drive height and I answered it. every 12 inches you take a drive back lowers your X dim by an inch as well as changes the COG. of course props make a diff, i have multiple sets of 31s, 29's and sold my sets of 27s. the set on now was done by cutting edge and hit the 86 on its first run with minimal trim and less than 5k rpms, at 5800 it will be 100 plus.. I nailed the X dim perfectly.
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I understand my statement was basically maybe not to stray away from the stock set up. The 29 did pretty well with the stock set up back in 1996 and blackhawks. Some boats like set backs and some boats do not.
With twins the RD might be not worth it depending on the wallet. Like I said I have not ever seen a 29 with set backs. I have driven these boats before as well. Might be best to ask Larry Smith before anything. Just saying. |
Originally Posted by BUP
(Post 4660005)
I understand my statement was basically maybe not to stray away from the stock set up. The 29 did pretty well with the stock set up back in 1996 and blackhawks. Some boats like set backs and some boats do not.
With twins the RD might be not worth it depending on the wallet. Like I said I have not ever seen a 29 with set backs. I have driven these boats before as well. Might be best to ask Larry Smith before anything. Just saying. |
Interested to see where you end up and how it turns out. Love Blackhawk boats!
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Gonna go of the res again on this based off some info on this thread so far.
Your x is 16.5, from bams drawing i gather crank to propshaft centerline is 13.375 16.5 - 13.375 puts propshaft @ 3.125 above the bottom. Diameter of the gear case is 5 inches approx. 3.125 - 2.5 puts the bottom of the gearcase @ 0.625 above the bottom of the boat. Trim the drives parrallel or to whatever setting the boat likes, ( guessing some positive trim ). with a string taped to the boat bottom pull it past the gearcase and tie it off behind the boat so the string is approx 0.625 below the gearcase bottom, or touching the rearmost edge of the gearcase (right where the first prop hub begins). From there measure back 12 inches and measure how far up the string line went. Its at least a starting point to place the propshaft height relation to the water 12 inches back as it is now. Tape that string to the skeg so it is just touching or below the rearmost edge of the gearcase and crank the trailer tongue untill the string line is level, does that look like the boats angle of attack, bow too high, crank it down, right now with the changes you said it was too flat. If you adjust the tongue and like the angle of attack, then go the 1 inch up for every inch back with the string line, wheres the string in relation to the gearcase ? With a couple straps and a chainfall the drive and extension box can be slid up and down the transom in conjunction with the string line Was me id go to the extremes like mentioned above towards finding the x especially when there are no inexpensive adjustments to had such as spacers, but i tend to go off the rails. Awesome project, your going after the set up, Look up ben perfected here on oso, different boat, 30 scarab, similar world as you, surfacing bravos, bottom work, and chased down the perfect balance, boat runs over a hundred, no detail too small or overlooked. |
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It’s a little hard to pull a string line as the bottom has a rocker in it. The factory height has the bullet about 5/8 above the bottom. I’m guessing the bullet is skimming the water and somewhat acting like trim tabs and the boat runs really flat. Jacking the bow up with the drives on the extension boxes I’m guessing 2” higher will get the bow where I want it before the drives start pushing the stern up (bullet starts touching the string). I opted for the standard boxes. They look better and would have to fill the transom and redrill anyways. Now that I have all the parts including the exhaust I can start on it. My exhaust I have will allow me to put the motors 2” closer together. Is it worth the effort? |
Have you also gutted the cabin or taken some weight out?
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No weight reduction in the cabin. No gear or trash in the cabin. Lots of weight reduction in the engine area. Single light battery Single alternator no engine water pumps exhaust changes all aluminum LS 6.2 engines (The old EFI system was 85 lbs. !) light flywheels engine hatch weight reduction i lost 40 lbs. lol i will explore cabin weight reduction, but will not gut it. Will glass/epoxy foam to replace plywood cushion bottoms, This boat has never spent the night outside. That and I covered it with plastic, and run a dehumidifier until no more water came out. It’s dry. I am blueprinting the bottom. The extension boxes odes I am installing to try and get the balance back but not sure it it’s enough. Hoping that raising the drives will help. |
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