maybe changing from 4.3 liter cobra to 175 Suzuki outboard on a Starcraft Islander Islander.

[rolmops]

I am almost done pulling the transom (very sloddily installed 2 years ago) out of my 1989 Starcraft Islander and as usual I ask myself if I should take the big step and remove the I/O and change to an outboard. With the transom being replaced, now would be the time to do it. I'm eying a new 175 horse Suzuki which would sit nicely next to my Suzuki 10 horse trolling kicker. The idea is to get a 25 inch shaft one (the transom on my Islander is 44 inches high). Get a heavy duty outboard bracket, cut the transom 19 inches down and make that cut out wide enough for a main engine and kicker, while also creating a splash box in order to be able to raise the engines, but still have them close enough to the transom.
My cobra engine and outdrive weight about 700 pounds while the kicker is 128 pounds. the Suzuki is about 500 pounds, the center of gravity would move farther backward to the stern. I already moved the batteries to the front of the boat and this will hopefully offset this shift in balance. And the transom will be strengthened in the center with extra supports to the stringers. The new transom will be high end marine plywood 1.5 inches thick and have 25 plies. The two four by eight foot sheets were $185 each.
What holds me back right now is my worry that this change will create vibrations that may create hairline cracks in the 39 year old hull or in the stringers. This boat has never seen salt water and has always been trailered and there is no corrosion anywhere on the hull.
Do you have any thoughts or advice pro or con changing from cobra to outboard?

 

[roscoe]

A new 175 horse Suzuki will look much more beautiful, and be easier to hug.

 

[Lou C]

The outboard puts different force to the transom than the I/O does and the weight is distributed very differently. You can't just add an outboard bracket and replace the transom as it was designed for the I/O the transom itself must be strengthened, but you're planning on that I can see.
Don't forget much of the weight of the I/O is on the stringers for the front mounts and the rear weight is on the bellhousing and transom, but that's not levered weight.
Not saying it's not a good idea, as a long time I/O owner I know full well how much easier it is to deal with a OB boat, but don't be misled into thinking it's a simple conversion. See Ted's outboard conversion for his boat.....
Personally, if it were me, I would swap all my Cobra stuff for Merc, add closed cooling and a remote oil filter mount, making the engine less of a headache to winterize and service and boat on!
Although the Cobra I have certainly has given excellent service, can't complain one bit about it, too bad they are no longer in business.

 

[dingbat]

My cobra engine and outdrive weight about 700 pounds while the kicker is 128 pounds. the Suzuki is about 500 pounds, the center of gravity would move farther backward to the stern.

Loosing #200 off the stern would move CG towards the bow, if anything

 

[Chris1956]

Gee, you can likely get a 30" shaft on that 'zuki. That would save cutting down the transom 5 inches. You also want to keep following seas out. Do you plan to add a splashwell?

I looked at a boat years ago, i the 21 foot range. The transom was cut down to just a few inches above the waterline, and it had no splashwell. The cockpit floor was level with the wateline as a lot of self-bailing cockpits are. There was a panel kinda where the front of the splashwell would have been, to keep following seas out of the boat. The salesman remarked that this was the perfect style, because when you hit the throttle, your mother-in-law could easily be swept past the motor and overboard.

 

[rolmops]

Loosing #200 off the stern would move CG towards the bow, if anything

The 700 pounds is divided between the engine and the outdrive. The engine is 560 pounds and rests on the stringers. The outdrive rests on the transom and weights about 150 pounds. So I would shift the weight backward onto the transom when installing the outboard.

 

[rolmops]

Gee, you can likely get a 30" shaft on that 'zuki. That would save cutting down the transom 5 inches. You also want to keep following seas out. Do you plan to add a splashwell?

I looked at a boat years ago, i the 21 foot range. The transom was cut down to just a few inches above the waterline, and it had no splashwell. The cockpit floor was level with the wateline as a lot of self-bailing cockpits are. There was a panel kinda where the front of the splashwell would have been, to keep following seas out of the boat. The salesman remarked that this was the perfect style, because when you hit the throttle, your mother-in-law could easily be swept past the motor and overboard.

I have been looking for a 30 inch shaft. So far I have not found one. And yes I plan to have a splashwell, although a fairly small one. Part of the reason for the change is that I want to get rid of the big doghouse so I have more work space for fishing which is what this boat is all about. I will be talking to an engineer today and hopefully he can help me make a decision.

 

[Lou C]

Look at Ted's conversion thread, he did a lot of these calculations....
regardless of what people say I do not like open transoms especially on smaller boats, if I ever did a conversion to OB on a formerly I/O boat I'd want a closed transom, nice scuppers for drains and a bracket.
The issue with OB conversions is that they shift weight back and up and the force on the transom is levered more so than on an I/O, because the pivot point of the OB is higher up than for an outdrive.
While I/O boats definitely have their issues, if you can re-do the rear of the boat to give better access for maintenance, the I/O actually has better balance due to the more forward & lower engine mounting. Yes, they can be a pain but that can be mitigated with smart design. I've been thinking of getting rid of the big rear bench seats in my boat and replacing with a pair of lightweight flip up seats. More room for fishing and easier maintenance.
If your engine & drive are in good shape you can just keep running them. Freshwater? I/O is fine there to me. It's here in the salt where those conversions make sense.

 

[dingbat]

The 700 pounds is divided between the engine and the outdrive. The engine is 560 pounds and rests on the stringers. The outdrive rests on the transom and weights about 150 pounds. So I would shift the weight backward onto the transom when installing the outboard.

Weight is weight..does not matter what structure it sits on

Weight X distance (CG of component to existing CG of boat)

(#560 x L) + (#150 x L) = I/O
#500 x L = outboard

Assuming the cg of the motor is back 6’ x 560 = 3,360

Assuming distance of cg between motor and out drive is 3’ = 6+3= 9’ x 150 = 1,350

Effective weight of 4,710 from boat cg

Assume distance from boat cg to cg out outboard is 8’ x 500 = 4,000 effective weight from cg of boat

 

[Chris1956]

You could also get a OB motor bracket (maybe a Jackplate) with enough clearance to allow the engine to tilt. Maybe that moves the weight back enough to make up for the loss of the 600# of the I/O engine block. You can then have a full size transom.

Side note, I traded in my 19' SeaRay 4.3 LX I/O for a 21 footer with a 150 Johnny. The cockpit space increased dramatically, much more than can be explained by the length increase. The doghouse took up lots of space. The one drawback for the OB boat was the loss of the gunwale to gunwale swim platform of the I/O boat. It makes it less easy to get in/out of the water.

 

[rolmops]

You could also get a OB motor bracket (maybe a Jackplate) with enough clearance to allow the engine to tilt. Maybe that moves the weight back enough to make up for the loss of the 600# of the I/O engine block. You can then have a full size transom.

Side note, I traded in my 19' SeaRay 4.3 LX I/O for a 21 footer with a 150 Johnny. The cockpit space increased dramatically, much more than can be explained by the length increase. The doghouse took up lots of space. The one drawback for the OB boat was the loss of the gunwale to gunwale swim platform of the I/O boat. It makes it less easy to get in/out of the water.

That was my original thought and I sort of retreated from that idea.
What I would really like is a bracket/jackplate with an hydraulic piston or electric actuator that raises the outboard vertically instead of angling away and up, but so far I have not found one that will raise the outboard high enough so the bottom of the skeg is above the bottom of the boat. Also it leaves me with the question why outboard manufacturers have not produced such a seemingly logical setup and I feel that there must be a reason why. Would it be a mechanical reason or an economical one?

 

[redneck joe]

Why do you want the skeg above the bottom of the boat?

 

[redneck joe]

And quation to those that know about the center of gravity. I assume islanders came with an outboard so why the concern? Assuming no big interior layout difference or if there were cant he just mimic that?

 

[redneck joe]

Yeah looking at that year the two ob versions seem exactly the same interior wise so not sure the center of gravity is going to be a big deal. Ted's conversion is way different and more extensive.

https://starcraftmarine.com/sites/default/files/catalogs/Starcraft.FishingBoats.1989_web.pdf

 

[rolmops]

Why do you want the skeg above the bottom of the boat?

So I can launch and load at the ramp without doing damage. And why vertical? It worries me to trailer travel with the thing sticking out backward and possibly cracking the transom up, while when vertical the center of gravity stays a bit farther forward and it has less back and forth energy.

 

[rolmops]

xxx

 

[redneck joe]

So I can launch and load at the ramp without doing damage. And why vertical? It worries me to trailer travel with the thing sticking out backward and possibly cracking the transom up, while when vertical the center of gravity stays a bit farther forward and it has less back and forth energy.

Look at other boats and tilter motors, my vote is above the trailer frame a couple three inches.

 

[dingbat]

So I can launch and load at the ramp without doing damage. And why vertical? It worries me to trailer travel with the thing sticking out backward and possibly cracking the transom up, while when vertical the center of gravity stays a bit farther forward and it has less back and forth energy.

Here is a data sheet for your motor. Looks like CG is pretty much identical to an outboard.

In the days of mass production, find it hard to a supplier wouldn’t design to power plants to be pretty much interchangeable.

With outboard in the up position, the center of gravity is directly over the transom. Weight is pushing down on the transom as it should, not fore and aft.