I/O (Sterndrive) Conversion to Outboards

[Pmt133]

It was just a curiosity more than anything. I have seen the occasional transom with 2.25" core (3x 3/4 sheet core vs the normal 1.5 and layup). I wonder if it was just a selling/cost saving point and nothing more. I agree that it doesn't add anything. There are some out there for whatever reason though.

 

[redneck joe]

I've been a semi retired shiftless bass turd for about 5 years. 8 think next year is cutoff for me as grandkids are starting to pop.

Mamma has another five minimum. I need her to support my boat habit.

 

[Lou C]

Been retired 8 years but have worked part time throughout. Wife just retired. It's nice to have time, which is one thing money can't buy!

 

[tpenfield]

More progress . . .

I got the exterior panels cut an installed. Lots of measuring and odd angles . . . my head started to hurt.
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I'm letting the 'solar curing' do its thing.

 

[tpenfield]

I got to start thinking more about the internal structure. . . stringers, gussets, bracing . . . that sort of thing. Box structures are weak . . . a triangle is the strongest shape.

 

[redneck joe]

Isn't a circle the strongest?

 

[tpenfield]

Isn't a circle the strongest?

No, a circle is the roundest shape

That was pretty much day 1 in Engineering school . . .

"A triangle's resistance to deformation is fundamental to its structural stability, making it a common shape in trusses and frameworks. Engineers use this property to create strong, lightweight structures by ensuring applied forces result in only tension or compression along the triangle's sides."

I just don't want 'busy-up' the internal structure too much. Since the structure is about 3 feet tall, I'm also thinking about impact buckling . . .

 

[flashback]

Maybe a continuation of existing stringers thru the transom into the addition?

 

[redneck joe]

No, a circle is the roundest shape

That was pretty much day 1 in Engineering school . . .

"A triangle's resistance to deformation is fundamental to its structural stability, making it a common shape in trusses and frameworks. Engineers use this property to create strong, lightweight structures by ensuring applied forces result in tension or compression along the triangle's sides."

I just don't want 'busy-up' the internal structure too much. Since the structure is about 3 feet tall, I'm also thinking about impact buckling . . .

But if I'm putting a pipe thru a beam, round is best?

 

[tpenfield]

I've been looking at some twin engine brackets to see about the internal structure and hopefully get some ideas.

 

[Lou C]

Morning talk . . . While I wait for it to warm up outside.

Some folks (here and elsewhere) have expressed concern about sterndrive transoms not being as strong as outboard transoms. Probably true, but I like to work with facts & figures as to how much stronger. So, I did some web searching and did not come up with much.

So, I threw some numbers at it and came up with some approximations . . . This would be for 350 HP in a boat going 41 mph (i.e. 60 ft./second (YMMV) ) and having a sterndrive vs. a 30" shaft outboard. It looks like the torque on the transom is about 2/3 more with the outboard (a lot having to do with the shaft length).

View attachment 413030
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From this, my rule of thumb would be the need for the transom to be about 2X stronger.

Very interesting, so you can see besides the location of the lever arm being higher on the outboard transom, with the I/O, the front engine mounts & sheer weight of the cast iron engine in effect could counter act the tendency of the thrust of the prop to try to force the power package up. Whereas with the outboard, it is all concentrated at that upper section of the transom.
Yet you see big center consoles and all kinds of pleasure/sport boats with 3 or 4 very big OBs....

 

[tpenfield]

Very interesting, so you can see besides the location of the lever arm being higher on the outboard transom, with the I/O, the front engine mounts & sheer weight of the cast iron engine in effect could counter act the tendency of the thrust of the prop to try to force the power package up. Whereas with the outboard, it is all concentrated at that upper section of the transom.
Yet you see big center consoles and all kinds of pleasure/sport boats with 3 or 4 very big OBs....

Keep in mind that with an I/O engine the outdrive and transom plate are not rigidly connected to the engine. Perhaps the slightest upward movement at the rear mounts could be measured as the transom flexes, but the entire engine is not actually acting as a lever arm

 

[tpenfield]

But if I'm putting a pipe thru a beam, round is best?

Pretty much, the key thing is to stay within the center 1/3 of the beam's height. The smaller the 'web hole' the better.

 

[tpenfield]

I took my prop force model one step further and came up with some 'typical & maximum' numbers for the force exerted on the transom.

Since HP curves for engines are not prevalent these days, I came up with a typical power curve for a 350 HP engine (YMMV).
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This gave me a number of 4,700 lbs. 'maximum' force that I could expect from the engine in a scenario of coming up on plane.

I applied the 4,700 lbs. to my force diagram from before . . .

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Basically, it is like '3.5G' . . . which is good to know for the purpose of finalizing the structural details of the extension pod.

I think @Scott Danforth 's numbers of 2-5X the forces of an I/O are pretty good numbers.

 

[tpenfield]

Maybe a continuation of existing stringers thru the transom into the addition?

Essentially, the location of the 'stringers' within the extension pod line up with the stringers in the boat.
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Based on my force numbers, I am going to add some structure/stringers in line with the engine spacing and also add corresponding stringers in the boat.

 

[tpenfield]

Quick Update . . .

I moved the Extension Pod into the garage. This is the same space where I built the swim platform (about 6 years ago).
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Each day I only get a piece or 2 done . . . not a problem though as I can think through the next steps and hopefully avoid a disaster.

I got 2 sheets of untouched Coosa left and 3 partial sheets.

I am wondering about the transom thickness . . .
I could go with 1.5" of core and 0.25" of fiberglass each side, making the transom 2.00" (+/-) thick.

Alternatively, I could go with a 2.00" core and the 0.25" of glass each side, resulting in 2.5" thickness.

Not sure what thickness would be best (or typical) for large V8/V6 outboards.

 

[Pmt133]

I would lean towards more glass. Like said, you won't necessarily gain much with core but you will with a better layup to an extent. I imagine you get to a point where its more weight than its worth in added strength.

 

[Scott Danforth]

Make sure to use a stainless transom cap when you hang the motor. Would be glad to send you a sketch from work

 

[tpenfield]

Make sure to use a stainless transom cap when you hang the motor. Would be glad to send you a sketch from work

Yes, please and thank you . . .

I have some stainless steel "L" angle stock that has been sitting on a shelf waiting for a purpose. Wondering if that would fit the need? I assume the 'cap' is to distribute the load.

On a different topic . . . I know the outboard motor industry has come under some sales volume (downward) pressure in the past few years. The large HP (300+) engines are holding their own, due to the trends in that segment of the market. I'm just wondering what to expect price & market-wise in early 2026 when I'll be writing deposit checks on new outboards?

 

[tpenfield]

Thinking about next steps . . .

At some point (soon), I'll need to cut some access holes in the mating surfaces of the Ext-Pod. These holes (obviously) would be needed for installation and maintenance . . . and probably fabrication. I'll be making a matching set of holes in the transom and underside of the swim deck cavity.

My thoughts are smaller is better . . . and round is better than square.

There will be a stringer running in between the 2 smaller holes on each side, and crossing stringers above & below the large access holes . . .

The smaller holes need to be large enough for access to fastener (bolt) locations.
The large access holes will need to be big enough to fit my fat torso into the Ext.Pod to do the fastening, etc.

Sort of like this . . . bolt holes shown 1 side for illustration purpose.
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The transom modifications in the boat will match those additional stringer locations (dotted lines), so there is something substantial to bolt through.

Thoughts/comments welcome . . .