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Frozen Hoser
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118 Posts
Discussion Starter #1 (Edited)
For those that asked.

When I first installed a BH drive on my 25 Daytona with a stock 500efi, planing the boat off was very tough with any more than 2 adults and 1/2 tanks of fuel. Because the drive was originally designed to have the exhaust exit thru the drive infront of the props and my boat had straight thru hulls, I had an "issue" :).

The load of twin 31 pitch props would prevent the engine from reaching a decent section of its torque curve and it would lug along firewalled at 2500 rpm for what seemed like ever before forward speed would help unload the props allowing the engine to spin up over 3000 rpm and eventually plane off. I even twisted the splines off the vertical shaft once due to torque loading.

My solution was to fabricate and install an internal airation system. I had done some rubber tubes on another boat for a friend. Functional but fugly. I wanted something that looked more finnished. Once completed, it allowed the boat to plane off much easier with far less throttle, even with full tanks and 5 passengers. It lowered the dreaded BH idle speed 2mph and and softened the shifting as well.

Step one was to remove the rear cover and drill a bucket load of 1/2" holes thru the housing above the prop shafts.

Step 2 was modifying the rear cover. In order to seal the area above the holes in the main housing, I used high strenght epoxy putty to form a bridge in the under side of the rear cover to align with the recessed area in the main housing.

Step 3 was installing an aluminum tube in the rear cover tall enough to sit above the waterline when the boat is off plane. I welded it into a hole I cut in the cover just behind the epoxy bridge centering the opening above the holes in the main housing.
 

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Frozen Hoser
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118 Posts
Discussion Starter #2
I applied a thin bead of RTV silicone as a seal and allowed it to cure before installing the rear cover onto the drive.

Next up was fabricating a bracing system for the tube. I realized that any impact on the tube would put a fair amount of stress on the rear cover as it only attaches to the drive with two bolts.

I descided to use a piece of 1 1/2" x 1/8" stainless flat bar stock. I used a piece of leftover aluminum tube as a form to shape the center of the bar stock to the tube. I then fit the brace in place, drilled it and the tube and tapped the tube on 3 sides for a 1/4" bolt. Once fitted, I bent the bar stock where necessary to mate up with the drive shower cap. Cut to length, drilled and then polished, it bolts to the shower with 4 1/4" bolts and nuts.

Total cost was about 20 bucks for materials and 3 hrs worth of work.
Well worth it for the advantages it provided.
 

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Floatin dirty
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4,831 Posts
Great post...I think you mentioned that your planing time was cut in half also. I really wonder how this set up might work on a standard bravo drive as well. Some of us have modified bravo drives (TCM or IMCO) that are mounted with very high X dimensions so the drive is basically operating in a surfacing mode on cat/mod tunnel hulls even though it was never designed to work that way origianlly. At least for me, this creates a long time to plane out my deck boat. I would guess around 6-8 seconds depending on load and due to the fact that I do not go over 3500 rpms while coming out of the hole to save the drive from damage. I wonder if your set up would help on standard bravo also?
 

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Frozen Hoser
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118 Posts
Discussion Starter #4
Great post...I think you mentioned that your planing time was cut in half also. I really wonder how this set up might work on a standard bravo drive as well. Some of us have modified bravo drives (TCM or IMCO) that are mounted with very high X dimensions so the drive is basically operating in a surfacing mode on cat/mod tunnel hulls even though it was never designed to work that way origianlly. At least for me, this creates a long time to plane out my deck boat. I would guess around 6-8 seconds depending on load and due to the fact that I do not go over 3500 rpms while coming out of the hole to save the drive from damage. I wonder if your set up would help on standard bravo also?


I don't know if you could go thru the Bravo lower from above in the same manner due to case design. Would need to see a cut a way pic to see if anything is in the way of boring thru the case above the prop. In the BH its just dead space.

I now run an SCX/SC shortie deal and have a blower motor so planing isn't an issue at all. Just apply enough throttle to break the prop loose and go :)
 

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Frozen Hoser
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118 Posts
Discussion Starter #6
My BH started as an unused pre production "school drive" case that mercury had hanging around in a back room.
I bought two more parts drives from someone on OSO in 2002 and gutted them for shafts, shifter parts etc.
We put in the bravo X gears, all new bearings and seals and a hd vertical shaft ( after I snapped the original one )
 

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317 Posts
oh ok i just put the brovo x gears an a hd shaft in i havnt got the boat back yet i am only runin 560hp to so it should be ok as long as i am not beating in bad water
 
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