Hi all,

How easy is it to convert a boat from an OMC Cobra stern drive to a
Mercruiser (i.e. can it be done and not have it leak). The local boat yard
says it can't be done because all of the holes in the transom are in the
wrong place and it will eventually start to leak. Of course they also told
me it would cost me ~$500 to winterize the boat... The current set-up is an
'86 Cobra 260 (350 Chevy) and want to convert to one of Merc's 6-cyl diesel
units. It is in an 1986 230 Chris Craft speed boat. The boat is used
primarily for low speed cruising so the (low RPM) diesel is not a problem we
are looking for dependability over time rather then performance.

thanks,
mark

rock_doctor wrote:

Hi all,

How easy is it to convert a boat from an OMC Cobra stern drive to a
Mercruiser (i.e. can it be done and not have it leak). The local boat yard
says it can't be done because all of the holes in the transom are in the
wrong place and it will eventually start to leak. Of course they also told
me it would cost me ~$500 to winterize the boat... The current set-up is an
'86 Cobra 260 (350 Chevy) and want to convert to one of Merc's 6-cyl diesel
units. It is in an 1986 230 Chris Craft speed boat. The boat is used
primarily for low speed cruising so the (low RPM) diesel is not a problem we
are looking for dependability over time rather then performance.



When OMC first rolled out the Cobra drive, one of the selling points was
that the transom cutout was identical with Mercruiser, thereby
facilitating an easy conversion in the hopes that OMC might take a
larger bite out of Mercruiser's dominant market share.

As far as I know, they are the same. Of course you can probably find out
for sure by looking up parts and doing a comparison. Or asking an
impartial mechanic that you trust.


Dave

"Dave Hall" wrote in message
...
When OMC first rolled out the Cobra drive, one of the selling points was
that the transom cutout was identical with Mercruiser, thereby
facilitating an easy conversion in the hopes that OMC might take a
larger bite out of Mercruiser's dominant market share.

As far as I know, they are the same. Of course you can probably find out
for sure by looking up parts and doing a comparison. Or asking an
impartial mechanic that you trust.

Unfortunately we are 110 miles from water so finding an impartial
mechanic/yard is difficult. But am very happy to hear that the holes are
the same or at least similar. Thanks

mark

On Mon, 10 Nov 2003 12:22:05 +0000, rock_doctor wrote:

Hi all,

How easy is it to convert a boat from an OMC Cobra stern drive to a
Mercruiser (i.e. can it be done and not have it leak). The local boat yard
says it can't be done because all of the holes in the transom are in the
wrong place and it will eventually start to leak.

Worst comes to worst, you could pull the old drive, completely "seal" the
transom using plywood and fg, then cut out and drill new holes.
"Mice-nuts" compared to the cost of a diesel, and you could repair any rot
in the transom at the same time.

Alternatively, look at sealing the transom and using a V-drive, so you
wouldn't have that pesky outdrive to screw up your reliability.

Lloyd Sumpter
"Far Cove" Catalina 36

"Lloyd Sumpter" wrote in message
news
Alternatively, look at sealing the transom and using a V-drive, so you
wouldn't have that pesky outdrive to screw up your reliability.

I actually considered this, the fuel tank sits in the center of the boat.
So I would have to relocate the tank and reinforce the stringers. It could
be done... A nice 4-71 Detroit and a borg warner velvet drive (they still
make these?) or since I am doing it, how about a steel frame, 3-blade prop
and a 454. Viola a "franken-air-speed boat"... :-)

I figure worst comes to worst just yank the 5.7 out and install a marinized
6.2 in the same spot. I was concerned the gear set in the drive would not
last very long. When shifting, the engine will drop a bank of cylinders to
reduce wear on the gear sets. This would not be possible with the
mechanical injection on the diesel. We will see, probably just go with a
Mercruiser 6-cyl diesel. It is a nice engine (Cummins 4.2L*) and should
last a long time. Thanks for the post.

mark

* http://www.cmdmarine.com/recreationalsterndrives.html

rock_doctor wrote:

"Lloyd Sumpter" wrote in message
news
Alternatively, look at sealing the transom and using a V-drive, so you
wouldn't have that pesky outdrive to screw up your reliability.

I actually considered this, the fuel tank sits in the center of the boat.
So I would have to relocate the tank and reinforce the stringers. It could
be done... A nice 4-71 Detroit and a borg warner velvet drive (they still
make these?) or since I am doing it, how about a steel frame, 3-blade prop
and a 454. Viola a "franken-air-speed boat"... :-)

I figure worst comes to worst just yank the 5.7 out and install a marinized
6.2 in the same spot. I was concerned the gear set in the drive would not
last very long.

The difference in torque between the GM 5.7 and the 6.2 is not all that
great, so that shouldn't be a problem.


When shifting, the engine will drop a bank of cylinders to
reduce wear on the gear sets.

This is the norm for dog-clutch drives. You would need to reproduce this
action for any new engine you would consider, or hard shifting will
result, and increased wear and stretch of the shift cable.


This would not be possible with the
mechanical injection on the diesel.

Possible yes, practical? probably not.



We will see, probably just go with a
Mercruiser 6-cyl diesel. It is a nice engine (Cummins 4.2L*) and should
last a long time. Thanks for the post.

Be careful when bolting up diesel engines to outdrives. Since diesel
engines have higher low RPM torque, they can stress an outdrive more
than a gas engine. I would opt for a Mercruiser Bravo drive, as it is
built to handle higher torque and has a cone clutch, thereby eliminating
the need for the ignition interrupter circuit to shift smoothly.

I would not run anything bigger than a 4 cyl diesel on a Merc Alpha
drive.


Dave

"Dave Hall" wrote in message
...
I would not run anything bigger than a 4 cyl diesel on a Merc Alpha
drive.

Actually Merc makes a production stern drive with a 4 and 6 cyl Cummins
diesels ranging up to 300hp. So a mercrusier diesel IO would be a over the
counter item. Thanks for you post it was appreciated.

mark