Converting inboard to a outboard

[bksah2]

Hi All,,,I am a new guy here on the fourm..I justed picked up a 1976 18' bowrider SS.. It has a 120hp merc init ..The motor and drive are shot .. So heres my question I would like to convert it to a outboard with out useing a ext bracket ..I think if redo the trasom wood and add some sort of splash well i should be ok..When i look at all the rehabs on here the trasom look like it the same weather it a inboard or outboard minus the splash well So CAN THIS BE DONE thanks ps i plan on useing a 140hp

 

[Bamaman1]

Re: Converting inboard to a outboard

You'd probably need to look at an outboard Starcraft to see how the stern's structured. You might need to put some bracing from the bottom stringers to the transom.

I like the idea of an aluminum extended motor bracket, as it makes the boat ride like it's 3' longer.

 

[jbcurt00]

Re: Converting inboard to a outboard

It is somewhat more involved the just covering up the I/O hole & removing the motor mount stringers. OB versions usually have at least 1 knee brace that will need to be added:

from Ezmobee's 18' SS resto. I thought that some came w/ 3, but I couldn't find a pix quickly for reference...

That's just 1 of many you can find here:
Starcraft Rebuilds and Restorations

I'm not 100% but I think the transom angle for an I/O is different then it is for an OB....

Can it be done? Sure......... A donor boat w/ a viable engine & outdrive may be simpler, though............

 

[jasoutside]

Re: Converting inboard to a outboard

Can it be done? Sure!

But I'm with JB there, the path is much easier (and cheaper) to an IO replacement.

Cheers!

 

[magnumdeke]

Re: Converting inboard to a outboard

The transom angle should be fine, mine worked out, should be 13 degrees i believe.

 

[bksah2]

Re: Converting inboard to a outboard

I would love to keep the 120hp but as i said above the motor is toast and the outdrive is toast ,,I have a 140 evinrude in my shed i can use
I looked at several rebuilds on here but have not seen it done
I see i would have to add a keel brace and splash well I just done know if thats enough to carry the load

 

[JD in Milwaukee]

Re: Converting inboard to a outboard

There is a thread on here where the owner took out an I/O and hung a 140 Suzi 4 stroke. Granted, it was on a bracket. But the results were spectacular. Personally, I would go the bracket route. The interior space you will pick up is substantial.

I would love to keep the 120hp but as i said above the motor is toast and the outdrive is toast ,,I have a 140 evinrude in my shed i can use
I looked at several rebuilds on here but have not seen it done
I see i would have to add a keel brace and splash well I just done know if thats enough to carry the load

 

[jbcurt00]

Re: Converting inboard to a outboard

There in that list of SC rebuilds I posted, or in the SC owners group forum. Magnum added a pod to the back of his, but his (and other threads) can give you an idea of what's involved in closing up the I/O hole & beefing up the structure at the transom.

 

[CaptainKickback]

Re: Converting inboard to a outboard

I'm just getting to the transom on my rebuild project. I'm converting twin I/Os to twin OBs, but I am getting a bracket from Armstrong (I want to engine room space for genie, etc.).

I have just made my filler pieces for the stern drive holes and starting to glass them. My transom is sound, but in an abundance of caution I'm going to do the following:

1) More FG inside, several layers of 1.5 oz CSM and 1708.

2) Extra FG outside: CSM, 1708, CSM

3) Knees inside from transom to hull

Just make sure your transom is strong. Lots of forces working to rip it off. And don't forget to allow space for the OB to tilt forward.

Sea ya...

 

[jasoutside]

Re: Converting inboard to a outboard

I have a 140 evinrude in my shed i can use

Well now, that's a horse of a different color!

Yah, if your fabrication skilz are up for the challenge go for it

 

[bob johnson]

Re: Converting inboard to a outboard

my guess is that the angle of the transom on an I/O is not right for an outboard!!! I am having serious doubts as to wether that transom on my I/O is anywhere near 13 degrees!!!..looks like maybe 5 degrees at best. you would have to add a motor mount...

because for one..the top opf the transom is way to high!!! its like 40", as a matter of fact i just looked at the literature i just got for my islander, and it says the transom is indeed 40" high!!....maybe the SS is lower???

you will change the posture of the boat for sure hanging the motor way off the transom on a gill bracket...might have to forward weight that boat...like ballast up front...somethings that is static..( not like a fuel tank!!)

I would do the same project...and might still if my 6 cylinder mercruise cant come back to life... there would be soo much more room inside


EDIT!!!!!!

I just looked at some pictures of my islander, and the transom is angled way more than I thought...it might be 13 degrees after all!!!

sorry I guestioned anybody!! ha ha
bob

 

[jasoutside]

Re: Converting inboard to a outboard

Hey Bob, heads up, your transom is probably somewhere between 12 and 14 degrees.

And I'm pretty sure he's talking about cutting the transom down.

 

[bksah2]

Re: Converting inboard to a outboard

Yes the height of the trasom would be 20/21" the same as a standard outboard I am not trying to be a pain in the a** about this it just seams to me that the inboards and the outboard transom look identical except for the splash well and keel bracket on the outboards Thanks in advance for all your replys and PLEASE keep them comeing
OK heres kinda of what i am talking about check out the pics in this post

http://forums.iboats.com/showthread.php?t=491394&page=2

 

[bob johnson]

Re: Converting inboard to a outboard

Yes the height of the trasom would be 20/21" the same as a standard outboard I am not trying to be a pain in the a** about this it just seams to me that the inboards and the outboard transom look identical except for the splash well and keel bracket on the outboards Thanks in advance for all your replys and PLEASE keep them comeing
OK heres kinda of what i am talking about check out the pics in this post

http://forums.iboats.com/showthread.php?t=491394&page=2

the transoms should be a lot different..on an inboard..they is very little potential energy....the weight is inside being supported by the hull floor...on an outboard it is cantilevered off the back...and has a moment arm. the transom on an outboard designed boat should be much stiffer...i'd install some brackets that tied the top half of the transom to the stringers in the floor or a cross brace that is attached to the stringers....

i took the splash well out of my 180SS man it is afixed all over the place!!! from the looks of it the islander I am going to get has no where near the amount of structural support....the weight is low and there is no moment arm of force to try and pry the transom up and down....

you would need a lot more structural support on your conversion just to keep the transom sound for trailering on the road!!!

i think more damage comes from trailering and running on bumpy roads that the effect of the HP force on the transom while under power in the water!!

good luck

bob

 

[jasoutside]

Re: Converting inboard to a outboard

the transoms should be a lot different..

Actually, not that different.

For a Mercruiser, transom must be 2" - 2 1/8", OB is typically at 1 1/2" (my SN is 2"). Outside of that we are talking knee braces and splashwell like BK mentioned.

Not that it woulld be easy to make the conversion, but as for the transoms, IO an OBs are very similar on our ol Starcraft boats

 

[Georgesalmon]

Re: Converting inboard to a outboard

For what's its worth. I have worked at manufacturers that used the same hull mold for both I/O's and outboards. We built and cut the hull differently and of course used a different deck mold with the splashwell built in for the outboard in fiberglass boats. One of them was an aluminum builder that made a boat called the "navigator" it was 20', same deal, both the I/O and O/B used the same hull. MirroCraft was the builder. The transom angle was the same. The hull was also structurally the same. However, (big HOWEVER) The splashwell on the outboard design was such that it added a lot of added structural support to the transom.

 

[bksah2]

Re: Converting inboard to a outboard

And that is my point the only differance that i can see in the inboard vs. outboards is the hight of the trasom and the keel brace and the splashwell..The splashwell on the outboard design is whats gives it alot of added structural support to the transom..They call it a splashwell but what it should be called is the upper transom support bracket because without it the outboard would rip the transom off and that is with all boats not just the starcrafts

 

[bob johnson]

Re: Converting inboard to a outboard

I dont understand how anyone can with any kind of physics knowledge....couldnt see that the needs of an outboard mounted transom differ a lot to those of an inboard.

hanging an outboard off a gill bracket that is bolted to the transom of a stern drive boat.......creates ALOT more amount of leverage and force than the inboard motor did!!!!

heck there hardly any forces of torque on the transon of an inboard boat from the motors weight....the weight of the motor is completely supported by the hull floor.

on an outboard it is totally supported by the transom!!!

I think the splashwell on the ouboard models is mainly for transmom support....if we look at most other manufaturers, they dont have those huge splashwells... its because they arent really for splashwells....it is starcrafts version of transom support, as much as it is for water deflection

the other makers have knee braces in several spots... and cornder braces that are large to tie the transom into the gunnels



bob

 

[jasoutside]

Re: Converting inboard to a outboard

the needs of an outboard mounted transom differ a lot to those of an inboard.

Yup, the needs of support/structure are certainly different with IO vs. OB. That's where the splashwell and knee braces come in for an OB (like what has been mentioned here on this thread).

If a fella wanted to run with an OB on a bracket then the knee braces (and additional structure) are even more important.

 

[InMotion]

Re: Converting inboard to a outboard

Hey Bk... ok been following along and thought I'd like to chime in some here

From what I understand, the bottom brace's main purpose is to prevent the motor from pressing into the boat at the bottom of the transom. Secondary purpose is to add strenght to the upper part of the transom from pulling out. In my Jupiter, the bottom brace is super strong at the hull and tightly riveted to the hull. The upper part of the brace had three bolts on either side that went through the wooded part of the transom.... but as most had experienced from rotted wooded parts of their transoms, the flanges off of the transom brace had cracked or the bolt holes had pulled through... see picture here...sorta...

I believe that all the elements of the transom combine to give it strength --- the U channel that the wooden transom board sits int, the laminated transom board itself, the transom brace for preventing the bottom of the motor from pushing in and the splashwell. I think the splash well tied the gunwales and transom together --- but man that transom was super strong once I bolted it to the metal of the boat! The splashwell to me just makes it a system --- sorta locks the gun wales and transom together if you know what I mean!

I added a SS U channel across the top of the transom just for added strength --- and even with the motor on I cannot make the thing flex at all!!!

I am really interested to see this project! I too had thought about it! Keep in mind that the motor really is bolted to the transom rather than sitting on it! The real strength of the transom to keep from bending outwards is the transom board itself. Most but not all of the forces when you accelerate are on put on the lower transom brace inwardly and some of course pulling at the top outwardly where the top motor bolts are.

I think your bottom brace will be a tricky part --- in construction, strength and attachement to the hull and metal part of boats transom. If you can nail that and develop a splashwell... you'll be a long way towards something strong and safe!!

All the best,

Jim