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craigdskilling
10-12-2015, 07:30 PM
so I am running a 18 donzi and a 350mpi supercharged pushing 500hp. I am running 85 mph now with 27's and was wondering I have a set o 29/and 31 and can I try to change it up with 31/29 or31/27 has any one done this or is it to hard on the gears. And yes I am going to do the padded bottom over the winter.:boggled:

MOP
10-12-2015, 08:10 PM
When you say padded bottom, do you mean you will bring the bottom down enough so the entire gear case it clear of the water path leaving only the fin below below the bottom. I hate to continue to repeat myself but I can not understand why you do not raise the X up to 17- 1/2, Steve's 18 hit 79 with a bone stock Merc 300. You are pushing the same basic HP as Dom Tam had in his 18, had he raised his X he would have topped three digits. If you truly want the best out of your rig get hold of Steve Mars! You will never get the best with even a fraction of the gear housing below the the bottom, the stern lift kills HP $$$$!!!

osur866
10-12-2015, 08:28 PM
What's your RPM's at 85? What's the rev limiter set at and where did it make the most TQ and HP on the dyno?

MOP
10-12-2015, 08:45 PM
Until he gets his gear case clear of the water flow he will never reach the speeds he should, setup correctly he would be in the lower three digits. Your shortie has two advantages over the BH, trim and the case is fully below the bottom cancelling out stern lift. Having even the slightest bit of the lower half of the case below the bottom makes the boat run to flat giving it to much wetted surface to allow it to preform properly. Frankly if he raises his X he will blow your doors off!

craigdskilling
10-13-2015, 04:02 PM
I'm not looking to blow anyone away I have gone so far with my set up and I just want it to be the best that it can be so with that being said my motor is coming back out in a few days it wasn't running perfect so my builder wants to send the ecu back to whipple and do some changes and then we are going to re dyno it and set it up right. we might go more boost ad at that time I am going to take the boat back to my other guy to look at raising the x I talked to him and shouldn't be that bad but the hatch will have to be modified a bit do to my risers will see keep you posted thanks.i might have some props for sale soon.and I have another drive also.

yeller
10-13-2015, 06:47 PM
To answer your question...Yes. I have never tried it because I only have the one set of props, but I have read where people have had success in running two different pitches together. If you have the props, try every combo to find out what works best.

Here is an excerpt from a posting from Don Tamm on performaceboat.us back in 2006: (Gary Eisenberg was involved in the development of the BH)

"Excerpts from a conversation with Gary Eisenberg Sr. at Performance Marine in WI: DonCig

A Blackhawk case is 8” shorter that a Bravo 1 case. So if you raise the “X” Dimension 3” per Mercury’s recommendation, you have an outdrive that is protruding 11” less in the water than a Standard Bravo 1 outdrive, hence the large reduction in parasitic drag.

A Blackhawk drive has about 70/80 pounds of parasitic drag at 70 mph.

A Bravo 1 drive has about 300/400 pounds of parasitic drag at 70 mph.
Most boat riggers raise the transom holes (X Dim.) 2”-3” when installing a Blackhawk versus a
Bravo 1.

The bottom of the torpedo should be located ˝” above the bottom of the keel on my Donzi Classic 18 per Gary Eisenberg Sr.

Gary felt that the front prop pitch and the rear prop pitch should be different by 2”. "

craigdskilling
10-13-2015, 07:19 PM
Cool stuff I hear changing them up is hard on the gears but I will try.

MOP
10-13-2015, 09:23 PM
The 18 is quite a bit lighter then the 22' BH, I think that gives you a better safety margin on gear sets. Just for info there was a guy on Off Shore Only that was doing a special heat treating process that made the upper gears a lot stronger than originals.