Converting IO to OB?

Just wondering if anyone here has converted an IO to a OB? I have a 175 OB and would like to mount it on my IO powered bowrider. I was figuring I could get a sheet of 2" marin plywood seal up the transom and bolt a manual jack plate to the transom...... Any thoughts. I know steering electrics etc would all have to be worked out as well.

Re: Converting IO to OB?

This isn't really an engine question as much as it is a boat restore/build question, JM.

I will move it there.

Re: Converting IO to OB?

This is one of those kinds - but depends on - - Some designors will design a hull for both I/O or outboard power so that they sit in the water about the same (balenced). Other designors will design for a specific type of power. Some will work well if the power is changed and some will not. I have read a few posts here over the years where the owner was happy with the change and a few who couldn't wait to dump what they ended up with. It is a weight distribution thing.

Re: Converting IO to OB?

the most i've seen are unhappy.

Re: Converting IO to OB?

If u can glass you can do it, the ob has to be mounted at the right height for performance.

Re: Converting IO to OB?

Do a search on this forum on this topic. Lots of posts covering the ins and outs of converting i/o to outboard. Also do a yahoo and google search. I found a few projects on the net from some very happy owners who converted.

Re: Converting IO to OB?

i almost did this on a 27 foot stamus. but deceieded to scrap the idea because the transoms on I/O arnt as tought or at the right position (sometimes) as OB. and its just a PITA. I would try and find something that is made for the application.

Re: Converting IO to OB?

I'd be willing to bet that almost all inboard transoms are tougher than almost all outboard transoms... they have to withstand a lot of force. I'll buy the part about them not being at the right height.

My understanding is that you have to align the cavitation plate on the outboard with the bottom of the hull. So the motor has to be at a certain height, and to mount it on the top of the transom you need either a certain transom height, a cut-out, or a stand-off bracket on the back of the transom.

For an IB/OB conversion, if it's a smaller boat you can use the cutout, for a larger one you'll need the bracket or something similar. You'll probably want to build a splash well if you don't use the bracket.

The important thing weight wise is that you don't want to alter the boat's center of gravity. So if you mount the motor farther back by going to an outboard, you'll want to use a lighter motor than the inboard was to maintain CG in the same place.

You can compensate by moving weight forward in other ways (batteries, fuel) but you'll alter the handling characteristics of the boat. Same problem if you add ballast forward, plus you're changing the overall boat weight.

On the bright side, using a lighter motor farther back it's possible to keep the same CG, lower the overall boat weight, and improve the boat's handling. Plus you may get a fuel savings. You'll probably pay for that in the cost of the outboard, though.

Erik

Re: Converting IO to OB?

I'd be willing to bet that almost all inboard transoms are tougher than almost all outboard transoms


how much u willing to bet...i'd think the oppisite...a io all the weight is on the stringers...

Re: Converting IO to OB?

rob711 said:

I'd be willing to bet that almost all inboard transoms are tougher than almost all outboard transoms


how much u willing to bet...i'd think the oppisite...a io all the weight is on the stringers...

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My thoughts exactly!!

An outboards transom takes much more stress than any IO

Re: Converting IO to OB?

Erikgreen, I think you would lose some $$ on that bet. With an outboard all the weight and thrust forces are transferred through the transom. Therefore it needs to be a sturdy and strong. There is also a moment acting on the transom from the weight of the motor, which is significant.

With an i/o, the engine is mounted to the stringers which carry a portion of the forces and all the weight of the engine. The transom still holds the drive assembly and needs to be designed to carry those forces, but in my opinion they are not as great as what one would experience with an outboard. Just my $0.02.

Have a great day,

Rob.

Re: Converting IO to OB?

He may be wrong on that point, but, He's definately right on the whole 'changing the center of gravity' deal. Move your motor outside the boat, and the center of gravity will most certainly change.

This could be fixed, by putting my mother-in-law in the bow, though.

Re: Converting IO to OB?

newbie4life said:

He may be wrong on that point, but, He's definately right on the whole 'changing the center of gravity' deal. Move your motor outside the boat, and the center of gravity will most certainly change.

This could be fixed, by putting my mother-in-law in the bow, though.

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WOW...

But in a lighter note (no pun intended to the mother in law) everyone is correct. The stringers take alot more of the thurst force than the transom. i am embarking on this quest (maybe) with a 24' scarab hull. and the transom is nothing like that of my old 21ft well craft OB.

Re: Converting IO to OB?

newbie4life said:

He may be wrong on that point, but, He's definately right on the whole 'changing the center of gravity' deal. Move your motor outside the boat, and the center of gravity will most certainly change.

This could be fixed, by putting my mother-in-law in the bow, though.

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I'd beat you the center of gravity wouldn't move much if at all. The outdrive for the I/o would hang in the exact same location as the outboard and weighs 2/3 as much as the outboard. Given that a 200Hp outboard weighs roughly 1/2 what the I/O would weigh I'll beat it's a toss up.

Re: Converting IO to OB?

In both cases the stringers carry the load. However with an outboard, the thrust and weight is transferred to the hull and stringers through the transom. I would be more concerned with the cg with an outboard than with an i/o. With an outboard, all the weight is on the transom which would make the cg closer to the stern, whereas with an i/o it would not be as far back. My engine cl is about 24 inches forward of the transom, and it weighs about 600 lbs, the leg and transom shield probably weigh another 200 lbs. Given the outboard may be lighter for the same hp, I doubt it would be 50% lighter, especially a 4 cycle. If you did switch to an outboard, you could move the cg closer to the bow by installing the fuel tank further up. This would only benefit when there is lots of fuel in the tank. I would investigate a little more regarding how such a change would affect the handling and safety before I proceeded.

Have a great day,

Rob.

Re: Converting IO to OB?

From what I read when researching this transom conversion is that moving the motor farther rearward improves boat handling and control. Some people put the motor brackest on that extend the outboard 18" or more from the transom just for that purpose. This is common on off shore boats with dual outboards pushing 200+ hp each.

My thoughts were to reinforce the existing transom with another layer or two of 3/4" ply, and then two or 4 large knee braces from mid height of the transom to about 20" in on the stringers. Then hang one of those motor brackets on the back with the outboard. Think of all the space you will gain in the rear of the boat with that big dog house gone!

Re: Converting IO to OB?

imported_Robj said:

Given the outboard may be lighter for the same hp, I doubt it would be 50% lighter, especially a 4 cycle.

Rob.

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Not 50% but your getting awfully close if you compare the 2 stroke with with the Bravo option.

225HP Optimax (2 stroke)weighs 497 lbs.
200HP Verado (4 stroke)weighs 510 lbs
225 HP Yamaha (4 stroke)weighs 583 lbs.

A Mercruiser 4.3L with an Alpha drive weighs in at 865 lbs and with a Bravo Drive 910 lbs.

Re: Converting IO to OB?

dingbat said:

Not 50% but your getting awfully close if you compare the 2 stroke with with the Bravo option.

225HP Optimax (2 stroke)weighs 497 lbs.
200HP Verado (4 stroke)weighs 510 lbs
225 HP Yamaha (4 stroke)weighs 583 lbs.

A Mercruiser 4.3L with an Alpha drive weighs in at 865 lbs and with a Bravo Drive 910 lbs.

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HOLY SH*T

Re: Converting IO to OB?

Another advantage of the big brackets is they are hollow and give additional floatation...





These are listed advantages from continouswave.com :

The propeller runs in cleaner (i.e., less-turbulent or less aerated) water that exists behind the boat.

The engine can be raised (sometimes several inches) which reduces the hydrodynamic drag by having less of the engine lower unit submerged in the water.

Bow lift (in static trim) may be gained from the changed weight distribution aft. When underway this in turn may allow the engine trim to become more vertical, resulting in the propeller thrust becoming more aligned with the forward motion of the boat.

The engine thrust will be applied with a longer moment arm to the boat's center of lateral resistance and the boat's center of gravity, producing greater leverage of the engine thrust on both the boat's course and the boat's trim.

Operation in reverse will be improved because of the increased distance between the propeller and the hull.

Radically reduced immersion of the propeller may permit use of more efficient propeller designs, further increasing efficiency.

Underwater drag of the lower unit may be reduced when its trim becomes more nearly vertical because of improved flow of water around the "bullet" of the gearcase.

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BTW, these babies aint cheap. Some people make their own out of plywood and many layers of glass.

Re: Converting IO to OB?

Mark42 said:

BTW, these babies aint cheap. Some people make their own out of plywood and many layers of glass.

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that the bracket you are refering too? i am thinking of just taking my time and making a new bracket or catch tank that sits off my transom just to say hey i did it Iboats and give my review, it is scraps i will scrap the hull and by another out board hull. i am 21 so i have all the time in the world. but so far i ahvnt seen a review on someone who actually did it. it just get discouraged.