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Converting a boat to I/O

Old 01-12-2020, 08:51 PM
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Default Converting a boat to I/O

Hi guys,

Iím working through my current restoration project (Bertram 20) and Iím at a point where this question is very relevant moving forward as well as for my next project.

My current B20 was converted to outboard years ago and Iím considering going back to I/O and my next project will likely be a true inboard that I want to convert to I/O.

Iíd like to hear from guys with real knowledge about the best way to go about converting a boat to I/O and anything that may not be obvious.

Iím considering:
-whatís best way to recreate the engine mounts as those were removed when my B20 was converted.
-how thick should a transom be for a Mercruiser Bravo drive to fit properly?
-if dual I/O, how to determine correct spacing between the drives when cutting holes in the transom?
-Best way to mate an engine to a Mercruiser drive that isnít a standard marine engine (sources for someone who can fabricate a flywheel/parts that can match up)

Iíd like to hear from folks who have real experience in similar projects and avoid the whole I/O vs outboard vs. inboard opinions if possible.
Old 01-13-2020, 12:34 PM
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The instruction manual that comes with the Mercury sterndrive package will answer most of those questions.
The situation is the same as it is for somebody building a boat, refer to the manual, layout the engine center lines as per the engine manufacture, layout and build the engine beds, layout and cut the transom hole... It's all in the manual.
Matching up a bell housing can be a little more difficult but there are kits to fit drivelines which opens up nearly unlimited possibilities.
Gerald
Old 01-13-2020, 08:54 PM
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Thanks Gerald,My main concerns are in regards to making a boat an IO that doesnít have any IO components currently.
And regarding checking with the engine manufacturer for info, thatís not an option as the motors Iím going to be using are high hp automotive engines that are rarely used in marine applications (please refrain from trying to dissuade me, this is what Iím doing. Just looking for help).

So my questions are based more in principal than in regards to a specific engine as I know nobody will have that specific knowledge. So basically, whatís the best method for adding IO engine mounts? Or do you have any good reading material to share?

The Bravo 1 manual tell me how thick the transom should be?

And can anyone recommend any resources or reading material on mating a non-standard engine to an outdrive?
Old 01-13-2020, 10:09 PM
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Here you go: http://www.mediafire.com/file/58e91m...anual.pdf/file
Old 01-14-2020, 05:42 AM
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Originally Posted by GaryDoug View Post
Thank you!
Old 01-14-2020, 06:21 AM
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Transom needs to be 2-1/4" thick and strong. You will need a Mercury jig to do the cutout as well as the entire drive and mounting hardware. Stringers will need to be in good condition. Pretty straight forward provided the transom cutout from the OB conversion was repaired properly.Not enough room on a 20' for twins. A donor boat with the entire drive and mount setup would be ideal. There are a ton of small parts that you don't want to be chasing as you go.
Old 01-14-2020, 07:57 AM
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Originally Posted by muskamoot View Post
Transom needs to be 2-1/4" thick and strong. You will need a Mercury jig to do the cutout as well as the entire drive and mounting hardware. Stringers will need to be in good condition. Pretty straight forward provided the transom cutout from the OB conversion was repaired properly.Not enough room on a 20' for twins. A donor boat with the entire drive and mount setup would be ideal. There are a ton of small parts that you don't want to be chasing as you go.
Yea twins will be for my next project. Was considering doing the IO on the B20 for practice. Probably just finish the B20 with the outboard and bracket that it has and just save the IO swap for the next boat with twins.

Next boat is gonna be twins running an automotive engine so just not much info out there. But getting the Bravo Drives mounted shouldnít be too hard.
Old 01-14-2020, 08:58 AM
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As above, transom needs to be 2 1/4" thick. two layers of 3/4 ply or high density composite plus glass is standard. The drive height is called the X dimension. You will need to do some research to find the recommended measurements but the info is available. Generally with a smaller, lighter, faster boat you can get away with at least 1/2" higher than Merc's official recommendation. Offshore only will be a good source but there are many others as well. Transom assembly should come with a template for the cut out but I am sure you can scrounge one off the net too. The jig is nice if you can beg or borrow one but not necessary if you are careful. There are common or convenient dimensions for stringer width. 28" comes to mind but it has been a long time. First step is to mount transom assembly. There is a jig to locate mounts from there but not very common to find one. Without the jig, lift the engine into the boat and set on the rear mounts. Make sure you use the proper isolation bushings or springs and torque to spec. The front of the engine will be supported with blocking and a jack or better yet an overhead winch. Use the correct alignment tool and set motor height. I would bump the height up 1/4" or so to allow for mount settling during final assembly. The actual mounts will be 6" x 1/2" aluminum angle stock about 10" long. Pre drill 4 1/2" holes for mount bolts on one web of each mount. With the engine in the desired location, position the aluminum mounts under the engine pads and mark the bolt hole location on the side of the stringers. Also mark the engine pad to aluminum angle bolt holes and label the pieces left and right. Pull the engine, drill your holes, install your mounts using plenty of caulk and reinstall the engine. It's all a bit of a pain in the neck but nothing hard or complicated about it. I would be surprised if there weren't numerous youtube videos showing the procedure.

You will have to do your own research on engine connections but I would strongly urge you to use a power package that closely relates to one that has been available from the factory so you can source off the shelf parts. Trying to have custom one off couplers fabricated will be the death of your project. At least do some research on the old TRS drives. They used an external transmission and something like that may help with driveline connections.
Old 01-14-2020, 09:29 AM
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Not sure why anyone would put in an I/O setup, total bastardization of marine power-trains. Stick with the outboards!
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Old 01-14-2020, 12:40 PM
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May want to look into a jackshaft set up, even if its just a very short one, as you then do not have to make the engine and outdrive mate to each other. What are the engines if you don't mind sharing?
Old 01-14-2020, 02:13 PM
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Originally Posted by mpwitte View Post
As above, transom needs to be 2 1/4" thick. two layers of 3/4 ply or high density composite plus glass is standard. The drive height is called the X dimension. You will need to do some research to find the recommended measurements but the info is available. Generally with a smaller, lighter, faster boat you can get away with at least 1/2" higher than Merc's official recommendation. Offshore only will be a good source but there are many others as well. Transom assembly should come with a template for the cut out but I am sure you can scrounge one off the net too. The jig is nice if you can beg or borrow one but not necessary if you are careful. There are common or convenient dimensions for stringer width. 28" comes to mind but it has been a long time. First step is to mount transom assembly. There is a jig to locate mounts from there but not very common to find one. Without the jig, lift the engine into the boat and set on the rear mounts. Make sure you use the proper isolation bushings or springs and torque to spec. The front of the engine will be supported with blocking and a jack or better yet an overhead winch. Use the correct alignment tool and set motor height. I would bump the height up 1/4" or so to allow for mount settling during final assembly. The actual mounts will be 6" x 1/2" aluminum angle stock about 10" long. Pre drill 4 1/2" holes for mount bolts on one web of each mount. With the engine in the desired location, position the aluminum mounts under the engine pads and mark the bolt hole location on the side of the stringers. Also mark the engine pad to aluminum angle bolt holes and label the pieces left and right. Pull the engine, drill your holes, install your mounts using plenty of caulk and reinstall the engine. It's all a bit of a pain in the neck but nothing hard or complicated about it. I would be surprised if there weren't numerous youtube videos showing the procedure.

You will have to do your own research on engine connections but I would strongly urge you to use a power package that closely relates to one that has been available from the factory so you can source off the shelf parts. Trying to have custom one off couplers fabricated will be the death of your project. At least do some research on the old TRS drives. They used an external transmission and something like that may help with driveline connections.
Thank You! All your info is going in my notes. You just about nailed every question I have.

Originally Posted by kicker30 View Post
May want to look into a jackshaft set up, even if its just a very short one, as you then do not have to make the engine and outdrive mate to each other. What are the engines if you don't mind sharing?
Thatís not a bad idea either, Iíll consider that too.
Going to be running twin 2jz-gte Toyota motors for a few reasons but mainly because I have a lot of experience with them and they are abundant and incredibly reliable. Cooling will be using fresh water/antifreeze and a heat exchanger with the raw water dumped out post-turbo (will be replacing factory twins with a single turbo for each motor).

Hereís a gentleman running one already in his 25 eliminator mated to a Mercruiser drive. I just have yet to made the call yet to his shop to find out what coupling he used. https://www.offshoreonly.com/forums/...ator-25-a.html
Old 01-14-2020, 02:57 PM
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Originally Posted by marlinmike View Post
Not sure why anyone would put in an I/O setup, total bastardization of marine power-trains. Stick with the outboards!
Because I can is probably the best answer.

Many people today have no idea how good a boat can run with the power plant mounted low and near central rather than up high and leveraged well back off the transom. A traditional straight inboard may be best for simplicity and balance but comes with numerous drawbacks, most especially regarding fuel efficiency and speed. A jackshaft mounted I/O seems like a great compromise but they are finicky to set up and most can't compare to a traditional close coupled I/O in areas of NVH. I'm not looking to sidetrack the thread but the gentleman was asking for advise relative to repowering an ancient and already modified 20' hull not discussing having a modern 30' or 40' center console designed and built around twin or quad outboards.
Old 01-15-2020, 06:01 AM
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Turbos and gas engines in boats will be an experience.If nobody has the marinized parts like exhaust for that engine,that will be tricky too. Personally I'd forget the turbos and think larger traditional engines that have parts available for them.
Old Today, 07:17 AM
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If I were going to do a project like this (and I am not anti-I/O, have had a lot of experience with them) the only way I'd do it, is by using 100% Mercruiser parts and engines used by Merc for ease of parts availability. The only way. I'd never use engines not marinized before, or with hard to find parts to make it work. It is just not worth the frustration it will inevitably cause you.
What I'd do:
A pair of Bravo transom assemblies, Bravo 1 drives which are hell for stout and a pair of GM marine 6.2 liter gas engines fed by a simple Marine Edelbrock 4bbl, with Merc Thunderbolt 5 electronic ignition. Full closed cooling. This will work, you can find ALL the parts, no oddball custom fab work and will last a long time as long as it is maintained.
Now for advice with installation go over to the I/O forums at iboats and ask:
Scott Danforth
Achris
alldodge
Bondo

these are the posters who have the knowledge you are looking for.

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