How do you figure out new CG? I/O to OB conversion

[the man]

So I did my searches, about 30 of them over the last few months. Alot of discussion regarding balance and how "you might get lucky". How can I try to take the luck, or bad luck out of it? it seems like there must be a way to figure out if I could get the CG close to the same place. For instance, the outboards would weigh less than the ancient out drives and engines coming out, possibly I could move batteries and tanks forward, etc. Lots of ways to move weight around.

Look at the attached picture- yes very crude but it illustrates something. There must be a formula, and/ or a way to figure this out? Taking X amount out that was placed in a certain location, adding Y amount to a different location, moving batteries, etc. Is this simple math or .... ?

Thanks

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

I've seen the build in red here on this forum, can't remember who did it and don't want to offend them... but dammit man! It just breaks all the rules of common sense (and probably several rules of physics too)... it really, really, really reminds me of a Jethro Bodine build from the Beverly Hillbillies.

If you're going to convert an I/O to OB do it right!!! They don't manufacture or sell boats with a prehensile tail with an OB on it for a reason, and it's a darn good one.

If you're going to install an OB on an I/O transom, convert the I/O transom to an OB transom. This means beefing up the transom with braces, cutting the transom down so the OB cavitation plate is in proper alignment with the bottom of the hull, and installing a splash well... it's not that hard to figure out.

Yes it's a lot of work, but it's the right way to do it... and if you don't want to do it right, sell that ol' turd box I/O and buy an OB hull.

 

[the man]

Re: How do you figure out new CG? I/O to OB conversion

They don't manufacture or sell boats with a prehensile tail with an OB on it for a reason, and it's a darn good one.

29z Outboard

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

29z Outboard

Yeah, ok... build it like that and see how it works for you.

Are there any others???

 

[the man]

Re: How do you figure out new CG? I/O to OB conversion

Yeah, ok... build it like that and see how it works for you.

yes, building it that way is what i am wanting to do! so that's why i am asking some questions before i start.

Are there any others???

Stabicraft. in the San Juans and Bellingham area these area highly regarded as safe and 'stable':



i am sure there are others. not hard to find. i am not saying mine will or would be safe and stable- i am trying to figure out if i can make it that way.

 

[Bamaman1]

Re: How do you figure out new CG? I/O to OB conversion

On an inboard outboard, most weight is put on the rear stringers and some weight on the transom. On an outboard, the weight's all on the transom.

You could remove the outdrive, transom plate and engine before fiberglassing the holes where the transom plate was. But you'd really need to do some diagonal bracing from the transom to the stringers, and that takes some knowledge of fiberglass.

In other words, the transom might need major fiberglass surgery to prepare to handle an external motor mount with a mid size outboard motor.

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

Well I gotta tell ya, those boats are very interesting... but as far as I can see the OB isn't mounted to a boom, it's mounted to a swim platform type structure.

I would be very interested to see the transom construction and below deck reinforcing for these boats, I'm sure there is some very serious engineering going on there.

Just an idea, why don't you buy one new and then take it apart when you get it home and show us how it works? I'd buy a $.50 ticket to see it!

 

[the man]

Re: How do you figure out new CG? I/O to OB conversion

Well I gotta tell ya, those boats are very interesting... but as far as I can see the OB isn't mounted to a boom, it's mounted to a swim platform type structure.!

that's what i want to install- a swim platform type bracket. that "drawing" was just outlines, and crude at that. it was meant to illustrate moving weight, not a bracket design

I would be very interested to see the transom construction and below deck reinforcing for these boats, I'm sure there is some very serious engineering going on there.!

i think that i could handle doing this reinforcing, but one thing at a time. I started this thread to try to figure out how to calculate the old and new balance point, and how this would affect performance. anyone know anything about this subject?

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

Well I'm not a math genius, so I would use the method that has worked for me all of my life... it's called the "Trial and Error" method, or if you want to get fancy you can call it "Research and Development".

I'd find the COG (balancing point) fore to aft as the boat is rigged now (I/O) and mark it on the boat, then build the boat the way I wanted it and add weight fore or aft (probably aft eh?!) until the COGs lined up again. Stuff can look real good drawn out on paper, but when you try to apply it in the real world you will more than likely have to make adaptations... I see this nearly every day on blueprints at work. Looked dang good on paper, but it didn't work worth a flip out in the real world.

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

...btw way, when you get ready for your build I have a couple of structural idea you may or may not be interested in.

 

[ondarvr]

Re: How do you figure out new CG? I/O to OB conversion

Don't stress over the CG, you can move it around after you get it built, this is what's normally done on new a design, they calculate (guess) at where it should be, build it, then move everything around to get it right.

A water tight offshore bracket like you want to use works well in a conversion and typically improves the ride.

Depending on how old your project hull is the conversion may be easy (relatively), these older designs used the same hull for I/O and OB, just a different transom, both had a flat transom though. Newer designs incorporated all sorts of angles and such which drastically complicate the project.

 

[sphelps]

Re: How do you figure out new CG? I/O to OB conversion

These are nice but pricey .. Prolly have to beef up the transom for it ..
fiberglass transom brackets

And figure on trim tabs ...

 

[the man]

Re: How do you figure out new CG? I/O to OB conversion

These are nice but pricey .. Prolly have to beef up the transom for it ..
fiberglass transom brackets

And figure on trim tabs ...

yes, those look like nice brackets. also was looking at Armstrong; aluminum. Trying to not to get ahead of myself though.

The boat already has trim tabs, of course i have no idea if they work or not, but they seem like a decent size(below). so that brings up another question: with the weight being farther back, does that mean bigger tabs are needed?

 

[jigngrub]

Re: How do you figure out new CG? I/O to OB conversion

Your current tabs should work fine if you bring the COG back to the original position on the boat.