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