I/O (Sterndrive) Conversion to Outboards

Scott06 · Dec 30, 2025

tpenfield said:
My retirement job (sort of) . . .

Nice definitely not a lot of guys doing that kind of work anymore.

I did the interior of my 65 GTO but I was just putting covers on that were made by Legendary interiors, so I wasn't making them...

tpenfield · Dec 30, 2025

Scott06 said:
Nice definitely not a lot of guys doing that kind of work anymore.

I did the interior of my 65 GTO but I was just putting covers on that were made by Legendary interiors, so I wasn't making them...

Yes, apparently I do good quality work and don't charge enough, so they keep coming back.

harringtondav · Dec 31, 2025

tpenfield said:
Holiday Update . . .

I hope everyone is having an enjoyable Holiday season. We hosted Christmas and the kids/grandkids have been staying with us for the week.

Fiberglassing has been on hold while the family is here to avoid the possibility of fumes in the house. Meanwhile . . .

1) I have been continuing to clean-up the outdrives of the 'old' motors, using the non-caustic paint remover. It works pretty well.

2) I also have looked into the engine choice and the Dometic Seastar Optimus stuff, which is all the high-tech electronic controls the the boat will have. As I gather information it looks like the Suzuki motors may be the easiest to integrate. I was leaning towards Mercury, but I keep running into potential gotchas on the compatibility of Mercury controls with the latest & greatest Optimus components. It seems to me that Mercury is more geared to the OEM market with their JPO, and not as easy to deal with in the aftermarket. ( let me know if I have the wrong impression )

If I went with Yamaha, it would be all the Helm Master EX stuff. Pricing and convenience will tell the story.

One nice thing I found was that Optimus has a Bill-of-Material generator on their website, which has been helpful in understanding the components needed and the cost.

3) I'll be interested to see how the final 2025 figures on the boat market look. It seems that things have slowed down for boat sales in general, and I'll be interested to see how the outboard figures look. (hoping to get a good deal on the engines, which ever brand I choose).

4) My project schedule is sliding a bit, given the winter temperatures. I was originally hoping to be sending the boat off for re-power in the Feb/March time frame, but that may be more like April.

Anyway, I'll be back at it doing the fiberglassing after New Years.

Wishing everyone a happy & safe 2026.

Your design drawings calculated several important factors, CG, forces etc based on a specific O/B package. Do your alternatives above fit within your design?

tpenfield · Dec 31, 2025

harringtondav said:
Your design drawings calculated several important factors, CG, forces etc based on a specific O/B package. Do your alternatives above fit within your design?

I'm not sure what you mean exactly, but the answer is yes

tpenfield · Thursday at 1:40 PM

harringtondav said:
Your design drawings calculated several important factors, CG, forces etc based on a specific O/B package. Do your alternatives above fit within your design?

Oh, I think I know what you are asking . . .

I used the higher weight of the OB's under consideration and also the thrust of a 350 HP OB, whereas I plan on going with 300's.

In terms of strength calculations, I have been using 25,000 psi tensile strength for the fiberglass. Each of the 3 stringers can take a 30K lb load, based on the amount (thickness) of fiberglass used.

The compressive loads have been more guesswork, but they are probably substantial enough.

tpenfield · Thursday at 1:46 PM

Also, I just came across a piece of information about the Mercury OB option . . .
It looks like with electric steering and DTS, then JPO can be added on a re-power and the 'personality profile' can be created for an individual boat. This appears to be a recent change, but certainly a welcome change.

Any Mercury Gurus feel free to verify/comment.

tpenfield · 2026-01-06T11:11:29-0700

I am getting close on the internal structure of the extension pod. One tricky aspect of the build will be glassing in the internal areas of the transom pieces. I laid them in place, just to get a sense of the access and reach that I will need. My plan has been to access the insides via the splash-well openings. I do need longer arms, but I'll have to make do.

Here are a bunch of photos . . .