My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

[runninfarmer]

This is my first time posting on iboats, but I figured I'd share a project of mine. I recently acquired a partially restored 1988 19' Blue Fin with 2.3 OMC Cobra for $700. I like outboards though and didn't want to mess with the cobra, especially since their defunct now. I pulled the motor and all outdrive components and sold them for $500 quickly. Once I had them out I went searching for a bracket. I found right away the brackets are anywhere from $700 to $1500! Too much for my blood. This was a dilemma. After some deliberation and inspiration I designed one and decided to build one from scratch using 6061 angle aluminum and aluminum flat bar, with no welder. I bought a cheap $65 drill press from Harbor Freight and ordered the $450 worth of aluminum. That included all aluminum for the bracket, and plates/knee braces for transom reinforcment for the inside. Most commercial brackets don't come with that!

I'm about 30 hours into it and mostly done. Since the bracket is a paraellogram, it is fully adjustable, capable of adjusting to any transom angle. Will work with 25" and 20" shaft motors. I just have a couple more vertical members for the bracket to fabricate. I had to rebuild the transom, since it wasn't worth patching the old rotten one. Here are some pics:

 

[Teamster]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

No offense,............

But for some reason I don't see that "bracket" holding up to the torque of an outboard motor,.......

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

None taken, this is 1/2" 3x3 aluminum, bracket is not going to flex one bit. Transom would tear out before the bracket would fail. This is why there are extra 1/2" thick knee braces put in for transom. I actually borrowed the reinforcement idea from a poster on boatdesign.net.

It freely rotates right now because the vertical members aren't in yet to keep it from doing that. Stainless Marine, Moby Cool and Armstrong brackets are all made out of welded 1/4" plate aluminum. I basically extrapolated the jack plate concept, since those are made out of 1/2" aluminum or less.

Here is the link to the similar project by someone who used a 1/4" Stainless Marine bracket with the reinforcement. Seem to work OK for him Teamster:

http://www.boatdesign.net/forums/boat-design/transom-reinforcement-outboard-conversion-15706.html

 

[low277]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I am very interested in seeing how this turns out. I am considering a i/o to outboard conversion.

Keep working and posting!

 

[jbcurt00]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I am not an engineer, I've never attempted to build an OB setback bracket, nor attempted an I/O to OB conversion. Heck, I didn't even stay at a Holiday Inn Express last night, last week, or last month.

Although the fab work looks good, and the bracket looks like a heavy guage aluminum angle that should be sufficiently rigid, my concern is that long term, 2 thru bolts are sufficient per vertical angle to make the bracket function as a 'solid single unit':

And that 4 transom thru bolts are sufficient to transfer that energy thru the transom:

Increased leverage generated by the setback & weight of the motor look like it will work those 8 fasteners half (or more) to death...

A fully welded pod would act more like a single, complex shape unit. Although I suspect Magnumdeke will pretty much tell anyone that asks: "I over-built it' I'd much rather over then under build the primary point of attachment & primary point of weight/torque loading. Click the small tick mark next to his user name, it'll take you to his OB bracket setup. It's kind of spread out over a couple of pages, but starts about here:

Out board bracket swim platform plan, let me know if this shows upView attachment 153054View attachment 153054

Boat resto & boat safely. Layout of the parts & pieces look good, as does the fitment. But w/ several thousand pounds of boat to push, several hundred pounds of motor & gobs of torque & leverage, the bracket needs to be substantial.

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Thanks for the comments and concerns. I have had similar concerns, which is why I've been careful to fasten many points of contact. All the bolts actually weren't in the pics. I'll have 6 bolts per half of the bracket for a total of 12 1/2" SS bolts connecting it to the transom. I also have a couple cross members to connect the sides where the outboard will bolt on. I was planning on using 1/2" lock nuts for the bracket, but found they were extremely hard to tighten! So I'm going to roll with 1/2" lock washers. It is very much a research project, though I did also base the design on this commercial gil bracket:



I'm planning on throwing a Johnson 88 SPL (300 lbs) on it, so I won't get too aggressive with it. Once I get everything sealed and bolted, and the motor mounted, I will post more pics. My goal is just to investigate cheaper alternatives to a welded bracket. Aluminum welding is just too expensive to contract out, unless of course you can weld aluminum, in which you have a good advantage. In my opinion though, 1/2" SS bolts will be stronger than 1/4" aluminum welds, though I have nothing to really back that statement on my experience.

 

[Last Mohican]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Interesting concept. What keeps it from pivoting down? With only one bolt per joint on the external pieces that thing is going to pivot down when any weight is put on it. You should investigate 80/20 aluminum framing. Not sure it would work in this application but if you like building with aluminum with no welding required it is pretty versatile.

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I'm going to use shorter vertical members in the parallelogram, which should keep it from pivoting down. They're not shown in the pictures yet. The four bolts make it convenient to fit the bracket to any boat transom with any angle. Since it's a paralleogram, the bracket automatically forms the same angle as the boat transom for the motor. Once you fit the bracket, you can add in the vertical braces to make it stationary.

 

[Teamster]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I think it's starting to make sense to me,.......

 

[magnumdeke]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

It is interesting for sure, I toyed with a similar idea on paper myself, just couldn't get past the lack of a displacement pod to help counteract the set back center of gravity change, seems like a porpise issue may arise on a short boat.???

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I was thinking the opposite, since the center of gravity shift will coincide with where the majority of your passengers are. I think it should help with having extra weight in the back of the boat, since the "lever arm" is lengthened. If you think of the outboard as a pivot point, if it sits farther back, identical weight in the same position should keep the bow from rising as much. You have a 19' lever vs a 21' lever, with weight being put on identical positions, with the pivot weight being shifted back 2'. Here's a pic to describe what I'm talking about:



The drawing isn't to scale, but it gives you an idea of what I mean. Also, putting on an offshore bracket is supposed to increase propeller efficiency, since it runs in less turbulent water and for a couple reasons. This is better explained in this link which is all about offshore brackets:
Classic Whaler: Boston Whaler: Reference: Engine Brackets

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

I finally finished the bracket and sealed everything up. Got the 88 SPL Johnson mounted too. Check out my pics:

 

[GT1000000]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Very interesting concept...would love to come aboard and see how this conversion works...Thanks for sharing!

 

[magnumdeke]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Ya, I want to see pics and video of it in operation, got plans for a smaller boat and would love to experiment with something similar!

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Definitely will get a video of it in water. First, I have the daunting task of rebuilding the floor and consoles. I also have to awaken the 88 SPL from its hibernation. Shes been asleep for many years!

 

[Nickypoo]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

If you loosened all the bolts to finger tight, would it still keep the motor up? If not, this isn't safe as it is now. Friction under the bolts isn't going to hold that in the intended geometry for long underway.

You could easily make an addition to make it safer and more robust. Although welding would be the far better option.

And yes, this will alter the center of gravity a bit.

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Yes, it geometrically holds shape with loose bolts. If the two vertical members weren't put in place, it would be free to rotate. They re a different length than the angles on the ends. The bottom bolts of the vertical members are at different positions on the bottom supports versus the top supports, thus preventing rotation. The first pics don't show those in place, but the last pics do.

There was some "settling" when the bracket took on the full load, but it actually helped since the motor is a short shaft, it will be positioned better. Now that I have the shape I could have a welder put some welds for safety, but sort of defeats the purpose for people like me without the resources for welding. Though the safety welds on this setup would prolly be inexpensive.

 

[Nickypoo]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Have you considered something like this? You mentioned that you would like something adjustable. The white and green pieces have holes drilled in them, and you could raise or lower the drive by adjusting the bolt holes on those two pieces. Use two bolts in the middle spaced apart attaching the white and green pieces so they don't collapse. This orientation would also make it overall more rigid. Just a suggestion.

 

[m casey stock]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

Why do all that? I understand when the setback is also used as a swim platform, It actually looks ok, but just as a conversion I don't see the real benefit ratio being that great to the added stress on the transom from the extra leverage. Seems like since you rebuilt the transom anyway, why not just build it to directly accommodate the motor of choice? I know that there are performance benefits, handling and power transfer, but I also know that everything has to be balanced just so or some real problems could occur. I read this article,How Transom Brackets for Outboard Motors Improve Performance, so I understand what you are trying to do and I really do honestly wish you the best and that it works properly. I can appreciate all the hard work that went into it as well, but, maybe I'm just a traditionalist or something like that LOL but I just haven't gotten used to the way it looks all stuck out there by itself. I, like some others, would love to see a video of all this in action.

 

[runninfarmer]

Re: My Custom Offshore Bracket: I/O Conversion for less than $500, No Welder Required

It depends on what you want out of a conversion type project. I didn't want a motor house or splashwell wasting boat space. I also don't want to mess with a defunct inboard setup. Outboards are a little simpler to work on. Brackets are nice because they preserve the top half of the transom. I was thinking it'd be wrong to cut into it and butcher it.

I do have concerns with performance, but I'm personally not trying to push the envelope for handling and speed. If worst comes to worst it will sit in the water like a bass boat. In terms of weight I estimate I reduced the back stern weight by 150 lbs or so. Wasn't quite sure what the total weight of the cobra package was. If anything it has a sweet mechanical look!