Money not a problem inboard to outboard conversion for a 72 Starcraft SuperSport

[Kjmfisher]

I have a 72 Starcraft 18 foot or super sport that I really love and I want to convert it from an inboard to an outboard so I can get more fishing time out of it I just blew the hundred 20 horse outboard up in it the outdrive is just fine but I would like to switch it to a 90 horse outboard

 

[M2HB]

I'm certainly no expert in converting to an outboard. A modern 4 stroke outboard isn't cheap.

If your '72 is a Mercruiser, a late model, donor boat with a fairly new straight 4 I/O would be very reliable and much cheaper.

 

[Scott Danforth]

the cheapest and best way to convert is to sell what you have and buy what you want.

or find a 1992 or newer 3.0/alpha1-g2 and swap over everything

 

[211libwtfo]

Aluminum? Gut! Remove transom wood. Fill holes by tig welding replace wood Mount new hydrolic jack plate buy motor with controls. 90hp e-tec fill with gas repeat

 

[jbcurt00]

Not exactly ^^^

an I/O boat typically doesnt have knee braces to support an OB or OB pod/bracket

nor the support supplied by the splashwell w an SC outboard model

a 90hp is at the bottom of the ideal HP for an 18ft SC, 75% of a 120-140hp max, depending on year of the OB 18ft SS.

check your SS for chine cracks, and are the side panels below the gunwales still present and intact?

 

[No-AH]

That would be a huge undertaking, very expensive, painstaking conversion imo. Do-able...I would think, but I would be tempted to pursue a boat that has your requirements/needs instead of a conversion of that magnitude. Maybe use/sell it as a parts boat?

 

[Decker83]

What you are asking can be done. All the parts required can be fabricated out of sheet aluminum. The keyhole can be welded up as well..
The biggest problem would be the cost. If you have access to the required equipment and have the skill set to fabricate the parts it would be doable without a great cost. You will also need to be able to weld the keyhole without warping the transom
skin. If you have all of this you could do it for under $500.00.
If not then you should repair what you have or buy another boat with the outboard..
Just my 2 cents..

 

[Kjmfisher]

I put more on this boat then I money value it has more sentimental it was owned by my grandpa when I turn 16 he gave it to me I so money is not really that important

 

[Kjmfisher]

And according to my research a hundred and thirty horse is the max horsepower you can put on an 18 ft with a 6 ft wide transom the boat I got is a 72 it doesn't say anything about max horsepower rating because it was before they started putting Max worst power rating on to the identification plates

 

[GA_Boater]

You don't have a max rating because it's an I/O. Look at the same boat with outboard power.

 

[achris]

Also consider the weight change. I have seen these conversions. They don't work. You're pulling 250kg of I/O out and slapping about 130kg of OB on.... That makes the boat badly bow-heavy and dramatically prone to broaching...

The best option for you, if you intend keeping the boat, which it sounds like you are, is to find a late model 3 litre Mercruiser and put that in. It will bolt up to exactly where the current 120 is. Change everything, engine, drive and transom plates...

Years ago, I did this for one customer, he had an old 120, I put a brand new 3L in.... When he called back for the 20 hour service, he told me he had only one regret.... Not doing it years earlier.

My 2c worth.

Chris.............

 

[DLNorth]

Our 1970 SS has a HP rating of 125 hp.
After being in several boats with I/O's, when/if we upgrade, it will be to an I/O.

Dan

 

[crankbait cowboy]

I have a 18 ft ss, outboard built in 1973 and max hp is rated at 140

 

[M2HB]

I am having a low time, late model, Mercruiser I-4 put in my 1976, 18' SS. It was the logical choice. Wrecked boats provide cheap power plants for rebuilding old aluminum boats. It is certainly cheaper than a modern outboard. It is also cheaper to work on. Besides, with the sentimental value and the reliability, the modern Mercruiser is the only way to go. That is what makes it so nice, if your original boat is a Mercruiser. The swap from old to new is a piece of cake.

 

[bob johnson]

Also consider the weight change. I have seen these conversions. They don't work. You're pulling 250kg of I/O out and slapping about 130kg of OB on.... That makes the boat badly bow-heavy and dramatically prone to broaching...

The best option for you, if you intend keeping the boat, which it sounds like you are, is to find a late model 3 litre Mercruiser and put that in. It will bolt up to exactly where the current 120 is. Change everything, engine, drive and transom plates...

Years ago, I did this for one customer, he had an old 120, I put a brand new 3L in.... When he called back for the 20 hour service, he told me he had only one regret.... Not doing it years earlier.

My 2c worth.

Chris.............

not only do i dissagree, Id go so far as to say you are wrong when it comes to the Starcrafts..... Ive taken an 1971 I/O 21 ft Islander down to bare aluminum hull and stringers, and ive taken a 22 ft 1981 outboard Holiday.... down to the same spot... and Ive taken a 1983,18 ft outboard Super sport down to the same spot..... structurally once you get 2 feet away from the transom..thje boats are structurally the same...there is no ballast up front... there is no extra weight or less weight...the hulls are identical in the same length and model styles between I/O's and outboards EXCEPT the transom and splashwells and transom bracing......so once the hull is stripped you can brace up the I/O's transom to suit... you can weld, or rivet or bolt!!! I welded up the key hole in my I/O islander and built a offshore bracket... i had planned to fabricate some triangular corner pieces to share some stresses with the gunnels... maybe 24" triangles with 90 deghree bends all around to rivet to the transom and gunnels.....

 

[achris]

not only do i dissagree, Id go so far as to say you are wrong when it comes to the Starcrafts.....

Can't see what you are saying is wrong about what I said. A 4 cylinder I/O does weigh about 250kg, and an outboard does weigh about 130kg...So it can't be that... Removing over 100kg from the stern of a boat WILL make it 'bow-steer', seen that plenty of times, and doesn't make any difference, ali or 'glass. So it can't be that... And a boat that bow steers IS more prone to broaching, so it's not that....

The next paragraph is me expressing one of the options. Being an option can't be wrong, so it's not that.... And the last paragraph is an anecdotal account, and that's not wrong, unless you can talk to the now dead owner of the boat to find out if that's what he ACTUALLY said....

So yeah, can't see what you think is wrong.. Moreover sir, your statement, "Once you get 2 feet from the transom"... Sort of tells me that those rear 2 feet ARE different... They sort of have to be, being as one is stern drive powered, the other outboard... And tell me, exactly how many boats have you worked on? I would venture to postulate that is a few less than me....

Chris...........

 

[ab59]

:watermelon:----just sayin I think that we may be on the horns of a dilemma :joyous:. Hope everyone knows that's a joke to break the stress.

 

[bob johnson]

Can't see what you are saying is wrong about what I said. A 4 cylinder I/O does weigh about 250kg, and an outboard does weigh about 130kg...So it can't be that... Removing over 100kg from the stern of a boat WILL make it 'bow-steer', seen that plenty of times, and doesn't make any difference, ali or 'glass. So it can't be that... And a boat that bow steers IS more prone to broaching, so it's not that....

The next paragraph is me expressing one of the options. Being an option can't be wrong, so it's not that.... And the last paragraph is an anecdotal account, and that's not wrong, unless you can talk to the now dead owner of the boat to find out if that's what he ACTUALLY said....

So yeah, can't see what you think is wrong.. Moreover sir, your statement, "Once you get 2 feet from the transom"... Sort of tells me that those rear 2 feet ARE different... They sort of have to be, being as one is stern drive powered, the other outboard... And tell me, exactly how many boats have you worked on? I would venture to postulate that is a few less than me....

Chris...........

IM SAYING THAt the bow on an I/O is NOT any heavier, than a bow on a outboard version...... so removing the I/O motor and adding an outboard will NOT make it bow heavy!...the I/O bow is IDENTICAL to an outboard version bow!....i say that, because ive taken both apart, and seen for myself!............. and,,,,id think that a 250KG motor whos center of gravity is about 2 feet forward of the transom, would make a boat more BOW HEAVY, and than adding an outboard bracket and a 130KG outboard 2 feet AFT of the transom!!!!!!!!!! LEVERAGE!.........the outboard is cantilevered off the back of the transom........

bob

 

[ShoestringMariner]

I can see the value of having an outboard, but will it gain you that much more space? You will still need to put a splash well into it, will you not? Off the top of my head that only equals maybe a few extra inches of floor space?

If you do go through the build, and I do think it is entirely doable if you have the right equipment or people to do the TIG welding for you, would love to see the build photos.

subscribed

 

[bob johnson]

no splashwell needed..the transom is 40" high!!!!!!!!!(thats what it was on my 1972 Islander)it is as high as the gunnels!...so you do gain EXTRA standing floorspace... if there is a downside to the outboard bracket its the center of gravity getting moved so far back, and the work needed to get the motor height correct.... you should be able to mount an inch or more above normal because of the motor setback..if you had a jack plate , it would be easier, but if you want to be able to use all the movement of the jack plate, then youd be back to meticulously trying to find the motor mount height.