# center of gravity changes with conversion?

Discussion in 'Boat Design' started by modunlavy, Aug 21, 2017.

1. Joined: Jan 2003
Posts: 20
Likes: 0, Points: 1, Legacy Rep: 10
Location: Mobile, Alabama

### modunlavyJunior Member

I have a 32.5' Cary Blackfin that I am converting from inboards to outboards, which obviously changes the center gravity. The boat currently has the main fuel tank in the back of the boat, so the plan is to move it forward to the approximate location of where the engines came out.

Based on some initial measurements and calcuations, the ORIGINAL CG of the boat dry (no fuel or water) measures about 139" from the transom. Full of fuel and water, it measures about 128" from the transom.

I have a few different options, but it looks like with the outboards the NEW CG of the boat dry (no fuel or water) calculates to about 119 - 123" from the transom. Full of fuel and water, it calculates to 123 to 127" from the transom.

Does anybody see any issues with this or have any suggestions that I may need to address in the design part of this project?

Thanks for any help!

2. Joined: Sep 2011
Posts: 6,876
Likes: 518, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Hull shapes should be known to determine if there are any problems, and how to solve them. You should also consider the change in cog height.

3. Joined: Jan 2003
Posts: 20
Likes: 0, Points: 1, Legacy Rep: 10
Location: Mobile, Alabama

### modunlavyJunior Member

TANSL,
Hull is a planning hull with 24 degrees deadrise. Here is a line drawing of the transom - 9'9" overall beam, 8'1" beam at the waterline

4. Joined: Sep 2011
Posts: 6,876
Likes: 518, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

I mean the complete shapes of the boat, not just a section by the transom.
If you wish, I can do a preliminary study, without going into great detail, of how the boat would float with the new weight distribution. But for this it is necessary to have between 5 and 10 cross-sections and the longitudinal profile. I can read the information in AutoCAD, which I think is the one you have.

Last edited: Aug 21, 2017
5. Joined: Oct 2010
Posts: 10,401
Likes: 1,029, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

It is not just the change in the longitudinal centre of gravity, but the lightening of the boat that will have the boat floating higher, that needs to be considered. How are you arriving at the lengthways position of the COG measurements ? It is likely 10 feet in a 32 foot boat is going to give problems, but 11 foot might be be good as gold, so you need accuracy. And if the boat has been considerably lightened as a result, it might have different stability characteristics at rest, and underway. But I'm guessing the weight difference won't be huge, in the context of the total.

6. Joined: Jan 2003
Posts: 20
Likes: 0, Points: 1, Legacy Rep: 10
Location: Mobile, Alabama

### modunlavyJunior Member

TANSL, I am still taking measurements and trying to get the boat drawn in autocad (I only know 2d), so it may take me a several more days or so to get the boat measured and drawn, but I will get it done.

Mr. Efficiency, the weight change will be just under 10% lighter (10,500 lbs to 9,600 lbs). I am thinking off adding a generator (350 lbs) and maybe even a seakeeper gyro stabilizer (550 lbs), so if I do both I will be right back at the original weight.

I am calculating the lengthwise CG measurements with known weights and locations of each. The only thing I have estimated is the CG of the bare hull (used published weight), but I have the ability to hang it and weigh it with two straps with scales, so it can be determined with accuracy, just have to get set up to do it, hopefully next week.

Here is a pic of the hull (the blue one..lol):

7. Joined: Oct 2010
Posts: 10,401
Likes: 1,029, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Does the boat have tabs, and if so, is there much need for them ?

8. Joined: Nov 2012
Posts: 811
Likes: 62, Points: 28, Legacy Rep: 41
Location: Delta BC

### JSLSenior Member

Two years ago I assisted with a similar conversion: 26' Bertram with twin 6 cyl. inboards/shafts to twin V6 outboards.
To maintain integrity (and space) the owner fitted an Armstrong bracket for the 2 x 300 hp Mercury Verados. After a 'before/after' weights and centers calculation it was determined that the changes in weight, LCG, etc was going to be minimal so a thorough hydrostatic analysis was almost redundant. The owner is very happy and manages a commute at good speed (30 knot cruise) and comfort.
The attached photo was taken on 'launching day' so the owner was still playing around with trim, steering, etc.

#### Attached Files:

• ###### Bertram 26 -repower at 30 knots DSCN3773.pdf
File size:
206.1 KB
Views:
303
9. Joined: Oct 2010
Posts: 10,401
Likes: 1,029, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

If it is possible to roll the back back along the trailer, supporting the back of the trailer first, when she starts to tip, is the COG or very near it. A bit difficult with a big boat though. Obviously you have the winch cable/strap still connected so it doesn't get away from you. It doesn't work to put fuel tanks too far forward, as they empty, it will behave more like a stern heavy boat.

10. Joined: Jan 2003
Posts: 20
Likes: 0, Points: 1, Legacy Rep: 10
Location: Mobile, Alabama

### modunlavyJunior Member

The boat had trim tabs, but I never got to run the boat so I don't know how much they were needed. I got it in "non running" condition. I do plan on putting trim tabs back on it.

11. Joined: Jan 2003
Posts: 20
Likes: 0, Points: 1, Legacy Rep: 10
Location: Mobile, Alabama

### modunlavyJunior Member

Very nice, always loved those 26' Bertrams and almost bought one a few years back to do the same exact thing, but decided in the end I wanted something a little bigger.

12. Joined: Aug 2017
Posts: 1,252
Likes: 298, Points: 83

### BlueknarrSenior Member

Don't forget you are also raising the CG and adding a lot of moments of inertia to the transom which probably was not originally designed to withstand.
Since you are considering the effects of the CG changes, you probably already knew to beef up the transom.

Paul

13. Joined: Sep 2011
Posts: 6,876
Likes: 518, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

You are probably right, but I do not quite understand your reasoning.

14. Joined: Feb 2015
Posts: 62
Likes: 4, Points: 8, Legacy Rep: 14
Location: Athens, Greece

### vkstratisNaval Architect

Excellent boat! You can also check quickPlaning tool (www.boatdesignlab.com) to estimate the effect (running trim and speed) for various COG locations.

15. Joined: Aug 2017
Posts: 1,252
Likes: 298, Points: 83

### BlueknarrSenior Member

I'm just a glass repair man not an engineer, my terminology may be faulty.

I have repaired many transoms after owners hung too large (heavy) of an outboard on them.

The transom on an inboard motorboat might not be designed to withstand the full force of pulpution or several hundred pounds hanging off it and bouncing around

Just my experience of people making major modifications to their boat without proper engineering

Paul

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.