Conversion from I/O to outboard on Islander.

[rolmops]

So I completely finished my 221 Islander project. It works like a dream . This 1989 rig with a new 4.3 engine and cobra outdrive in great condition is very nice indeed. The only problem is that it is all done. I like to putter, build and change things. So now I am thinking about changing it to an outboard driven setup. This will enable me to fish until the water freezes over without having to worry about winterizing.
I am toying with hanging a 150 or 200 horse engine on it.
Now there is the bracket possibility which to me is not so attractive. There also is the seadrive idea. Which seems a bit more attractive to me, but the they are very hard to find and most comments about them begin or end with negative opinions.
I am thinking of using a hydraulic jackplate with an extension piece to go between the jack plate and the transom. The extension piece would bring the outboard to about 12 inches behind the transom so the engine can be tilted. The outboard itself would be a 25 or 30 inch shaft so it can sit high up on the transom while with the help of the jackplate I would be able to bring it even higher up or down.
It would be more like a sea drive than a bracket because it was be closer to the the transom. But It would not be the monstrous weight of a seadrive.
Do you have any opinions about this? Is it doable or is this just another impractical idea
?

 

[rolmops]

This is what the bracket would look like

 

[rolmops]

 

[Chris1956]

The question is wether the transom will support that motor's power. I have heard, but never saw that the transoms of the i/o boats were intentionally made weaker than O/B models of that year. If that is the case those jackplates will damage the transom.

 

[mr 88]

My thoughts are , replace the wood in the transom, put knee braces in , 45* angle from new wood to stringers . I would find a similar hull with outboard hanging on it and look at the inside transom setup , including width of transom in case they use less width on the I/O setup .

 

[rolmops]

The sample pictures that I used are from a 1985 Boston Whaler 18 Outrage that was originally outfitted with a sea drive. The problem with the sea drive is the 30 inch setback from the transom which makes towing for longer road trips very hard on the transom. With only a setback of 12 inches and the transom cut out with a small splash box added. I would feel far more comfortable towing this outfit.

 

[jimmbo]

Few Fiberglass Boats use Knee Braces, They rely on the Glass around the Transom combined with the Motor Well on Outboard Models and Hull Cap on Inboard Models, combined with the Engine being bolted to the Stringers and to the Transom, which may or may not be full Width, to be the Transom's Bracing. Most I/O Boats have fairly Thick Transoms, as the all Modern Stern Drives are Clamped to the Transom.
As mentioned Replacing the Transom to eliminate the Hole in the transom cut for the Driveshaft and Exhaust.
Jackplates will also increase the Twisting Load on the Transom. With a 12 in Setback, the Engine can/should be mounted with the AV Plate being about 4 inches above the bottom of the Hull, even higher with some Higher Performance Props

 

[Lou C]

If you’re in freshwater even though I have at times cursed the work that I/Os require (salt water use and moorage) I would not get involved in a project like this. With an I/O the 2 options that would make maintenance easier to deal with are a remote oil filter mount & closed cooling. The main advantage of the OB is easier winterizing/maintenance. Adding these 2 features makes oil changes and winterizing easier and you still have the advantages of simple low cost GM marine inboard engine parts. OBs are great for moorage/use in salt water but due to design and materials used engine repair costs are very high. In freshwater I would stick with the I/O….
If you want a project when the time comes drop in a 5.7 Vortec with remote oil filter & closed cooling you’ll gain about 90 hp and a nearly equal amount of torque. The Cobra drive can handle 300 hp easily. With the V6 drive ratio that thing will jump on plane. Might need a 21” pitch prop though.

 

[mr 88]

When the OP wrote Islander my first thought was a Starcraft Islander , not the glass , Grady White ? . So knee braces ,as noted above , are not used .

 

[Pmt133]

My 84 was thinner in the transom for most of the width and was thicker around the keyhole. They used a 1-1/8 inch full width sheet with a 3/8 sheet around the drive. When I rebuilt it I did full width of 2 laminated 3/4 sheets....

 

[rolmops]

When the OP wrote Islander my first thought was a Starcraft Islander , not the glass , Grady White ? . So knee braces ,as noted above , are not used .

Yes it is a Starcraft Islander. In my opinion it is the best towable great lakes boat. The I/O to outboard conversion is probably going to happen on a 1983 22 foot Sylvan , which is a Starcraft in but name and location of the fuel tanks

 

[Texasmark]

If I recall correctly, back in the '60's I was in Austin, Tx. and Glastron was built there. Glastron has always been a top rated boat in my opinion. Anyway, on one occasion I was looking at the engine well of a good sized boat..maybe 20' and the transom wood didn't go all the way to the side of the boat. I do recall it being an I/O. I had a 60 era Starcraft I/O with a 110 HP Mercruiser I/O and a hole was cut in the transom for access to it and the outdrive was bolted to the transom. On my boat there was no fixed (rigid) connection between the engine, bolted to the hull stringers below, and the outdrive...just a sliding drive shaft and hoses.

So, I am guessing that Glastron built up the thickness of their transom to side connections with woven roving fiberglass and had the wooden part of the transom bolted to the transom with either outboard mounting bolts or outdrive bolts. Seems like a straight forward method to ensure a strong connection between the sections.

So, If you were hanging an outboard on a bracket 12" behind the transom, I surely would think that knee braces bolted to the hull stringers would be in order.

Something to consider: When you are riding out high waves or getting on plane, or coming off, with normal OB or outdrive power plants, bolted to the transom it puts the engine height in the area of the slip stream caused by the hull moving through the water. For an engine on a jackplate, especially a 12" setback, unless you have a 25" skeg length (thereabouts) on your OB, you are going to dunk your engine every time you do what I mentioned above....don't remember the origin of the pictures but I have seen pictures of what I said and the engines were Sea Drive OMC engines set on their Sea Drive mounting bracket out behind the transom.....which had them sitting back from the transom pretty far. Possibly that design had a good sealing of the power head hood to the mid section where the designers knew of the dunking process and made the engine seals adequate to keep the water out.

 

[rolmops]

If I recall correctly, back in the '60's I was in Austin, Tx. and Glastron was built there. Glastron has always been a top rated boat in my opinion. Anyway, on one occasion I was looking at the engine well of a good sized boat..maybe 20' and the transom wood didn't go all the way to the side of the boat. I do recall it being an I/O. I had a 60 era Starcraft I/O with a 110 HP Mercruiser I/O and a hole was cut in the transom for access to it and the outdrive was bolted to the transom. On my boat there was no fixed (rigid) connection between the engine, bolted to the hull stringers below, and the outdrive...just a sliding drive shaft and hoses.

So, I am guessing that Glastron built up the thickness of their transom to side connections with woven roving fiberglass and had the wooden part of the transom bolted to the transom with either outboard mounting bolts or outdrive bolts. Seems like a straight forward method to ensure a strong connection between the sections.

So, If you were hanging an outboard on a bracket 12" behind the transom, I surely would think that knee braces bolted to the hull stringers would be in order.

Something to consider: When you are riding out high waves or getting on plane, or coming off, with normal OB or outdrive power plants, bolted to the transom it puts the engine height in the area of the slip stream caused by the hull moving through the water. For an engine on a jackplate, especially a 12" setback, unless you have a 25" skeg length (thereabouts) on your OB, you are going to dunk your engine every time you do what I mentioned above....don't remember the origin of the pictures but I have seen pictures of what I said and the engines were Sea Drive OMC engines set on their Sea Drive mounting bracket out behind the transom.....which had them sitting back from the transom pretty far. Possibly that design had a good sealing of the power head hood to the mid section where the designers knew of the dunking process and made the engine seals adequate to keep the water out.

I did not even think about that problem. Thanks for making me aware of it.
My plan is to either have a 25 inch or a 30 inch shaft, but after what you said I will move to the 30 inch shaft length.

 

[Texasmark]

I did not even think about that problem. Thanks for making me aware of it.
My plan is to either have a 25 inch or a 30 inch shaft, but after what you said I will move to the 30 inch shaft length.

Its a guessing game. You want the correct AV plate height at/near the surface when running but you want all the height you can get otherwise. A 30 may work and if so great but running a straight edge back from the hull bottom, may find the AV plate several inches below the line.....which you don't want if you are expecting any kind of speed out of your rig.

 

[huggyb1972]

I did a Spectrum aluminum 1950 that was a 4cyl OMC Cobra I/O a few years ago. I constructed a aluminum bracket that added about 300lbs of buoyancy it was only a floation aid while the boat was below plane.

I added a 92 115hp Mercury 2 stroke to it.

The things that I learned right away.
-Every foot back the engine goes it has to go up about a inchn but that all depends on a bunch of factors
- Just because the power head is out of the water below plane doesn't mean it's safe from water ingesting during transition to plane, or vice versa. (Long story short I ended up adding 5" to the motor to get the power head higher because I was having some of stalling issues).

The boat was better in every way after the refit. You can add bracing to tie the transom to the stringers for added support. That boat made a trip to SC from Indiana in that configuration with no issues. I kinda wish I wouldn't have sold it now. Good luck with your project.