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.

[Bushwacker]

Couldn't open your attachments, but your results are consistent with what Carl Moesly said when I asked him how the CG's compared between the I/O and OB 20' Seafari models. . . he said the CG on the I/O was further forward than on the OB, although I got the feeling that it was a lot more than 0.2". Of course this would be comparing the 140 hp Mercruiser vs. and I-6 Merc outboard of maybe 260-270 lbs.

[bilgerat]

The COG on My 25 with twin 150 mercs set back 3 ft on big heavy aluminum brackets must be wayyyyy back, the 300lbs of led in the bow helps some but she will still launch off a wave like flipper doing a tail dance!! still rides and preforms better than My old 23 Formula with the IO.

[Bigshrimpin]

Flex - Bigfluke mentioned that his 25 performed best when lcg was 7.9ft from the transom heel. Any idea what size ballast and how far forward you'd have to mount it to achieve lcg 7.9ft from the heel?

[FLexpat]

Originally Posted by Bigshrimpin

Flex - Bigfluke mentioned that his 25 performed best when lcg was 7.9ft from the transom heel. Any idea what size ballast and how far forward you'd have to mount it to achieve lcg 7.9ft from the heel?

Since this model does not have a liner or all the heavy cabinetry and other weight - like seating - scattered (mostly forward) around the boat it will over-indicate how much weight is needed. That said, the primitive model took 400lbs in the anchor locker to get the CG to 7.9' fwd of the heel. Based on that I'm guessing 250-300 lbs in the anchor locker - more if the weight comes aft. Sounds to me like the best option is to convert to a '25 Savage' with the motor under a CC and a jackshaft to a drive - I think that is pretty much what BigFluke did (I'm assuming the 7.9' is without the sponsons he added). An easier alternative might be to have 100' of heavy chain anchor rode and a fresh water tank low and forward. The problem with putting weight in the anchor locker is that the CG gets moved higher so it does not damp out rolls as well.

Bilgerat - PM me your bracket weight/dimensions, motor weights and ballast location so I can stuff that in if you want it for another comparison.

[flyingfrizzle]

Mine will have two 395lb 2.5 mercury 200 hanging off the back on a bracket with minimum set back of about 22" to get the motors to tilt enough for road transport. The bracket will be likely at least 165lbs due to the large floatation chamber. The lay up is a little heavy so may end up more around 185lbs. I haven't tried to weigh it yet but may try to do so once I get the swim platform bond down to the other half. I know Im asking for trouble and do have some tanks that I am going to cut down and add under the cabin floor and in the nose section for water ballast. Mine has the larger head room with more drop down than the earlier models so there may have to be some give there to fit the tanks. Going to be hard to come up with 400 plus pounds with out lead. I do have some ballast bar lead that I use in my race car that I can move around to play with the cg but not enough space to contain all the weight. Plus I don't want to make it a pig either. It is already a heavy hull to start with. The large tank mid ship will be shortened and moved far forward as possible but not much help there as I will still need the capacity for the twin efis. There is a guy with twin 150s with an yellow hull in VA and if I remember he said he had 30 or 32 inches of setback on his and clamed it to ride fairly good but his fairly good may be different than mine or others. I have even thought about adding bladder tanks under the bedding section for additional fuel or water to help but then you loose you storage that is already limited. There will be tons of storage aft where the old io sat but adding fish and ice there will defiantly be a negative effect.

[FishStretcher]

A pint is a pound the world around. 40-50 gallons of water forward of the *missing* forward bulkhead would do the job and you can pump it out on the way to a boat ramp to haul out. If you put in a low pickup and had a top vent, you could blow the ballast tank in an emergency for 400 lb of flotation.

Put it way up here:

[FishStretcher]

Also, I ran numbers on a flume/surge tank and think that something useful could be placed in the cockpit step down. When I return to civilization I can post what I have. I put some calculations up on boat design dot net perhaps 18 months ago.

[Bigshrimpin]

That spot looks like a nice area for lead along the keel and or a custom "sandbox". Mitch - thanks for the PM . . . will be in touch.

[flyingfrizzle]

I need to see just how much room is down there, the older hulls like the one you show FS is much bigger than mine. My cabin area floor steps down lower than the early year 25's to give you more head room which takes away from the space below. I think in 75 they changed it and done away with the space for Moesly's original space for the ballast system that was never installed. I could always raise it back up if needed tho. Seems to be the best place for weight. Down by the keel to keep it lower to help the roll. I really need to get back working on it. I was supposed to have it going this summer and now here it is summer. Oh well, maybe fall....