Effect of I/O vs. OB on 25 Seafari CG

[FLexpat]

In the thread about the 25 for sale in CT, Connor was wondering about what happens to the CG of a 25 when you replace the I/O with a big outboard on the transom. Since I have modeled Seacraft hulls in 3D CAD software I decided to do a quick study on this. He was heading to the Bahamas for a bit and didn’t get a chance to make measurements before he left so I looked at a few photos and the spec drawings and roughed out a 3D model – it isn’t complete or exact but it works for this simple analysis and the Solidworks software does a good job of calculating the CG using weights and densities.

For the I/O engine I used a 5.7 model that I have and adjusted the weight to represent having iron manifolds. For the I/O drive I used a Bravo 1X model. Total I/O weight 974 lbs. which corresponds to a 5.7 Bravo. For the OB I used a DF300 Suzuki 30” at 627 lbs.
Both drives were set 2” up from the bottom and the OB was set on the transom.
I modeled the bare hull with hollow stringers but nothing else. Since I did not model the liner I increased the hull thickness to 0.38” so that it could roughly represent the mass of the hull with a liner. I modeled the cap as just fiberglass with no coring. The windshield framing is just solid aluminum to represent the frame with glass. I did not change the transom between the two versions since I figured about as much glass/core mass would go back in for a OB as comes out from the I/O.
Here is a side view of the I/O with the CG shown.

Here is a side view of the OB with the CG shown.

I was shocked that this indicates the CG with the OB is 0.2” AFT and 4.5” up from the I/O CG. I expected it to go up and forward a few inches. I know the actual location of the CG with this crude model is only generally representative but the difference between the CG locations should be a good indicator of the effect - even though I doubt it is accurate to better than an inch - if that.

I can make the model more accurate and add a liner if someone has dimensions and wants to use it to plan a rebuild.