Unstable after installing new battery bank

Where were the batteries before and how many and what weight?

This can happen with added weight low down. Compare with a sailing boat without her mast, the movement is faster.

You'll probably find that initial stability has decreased but that reserve stability is increased. Possibly due to a change in the water line. Initial stability is mostly governed by hull shape not ballast. The added ballast won't make much of a difference until the boat has heeled enough to use it as a righting moment.

To jhall767's excellent response I will only add that the waterline isn't the only thing to think about in initial stability. If the shelf you put the battery on is 20" below waterline and the battery is 16" tall then you loaded 1600 lb's 12" below waterline, and slightly to port. The center of mass of these batteries is undoubtedly above your boats initial center of buoyancy , which would detract from initial stability. As the boat heels what jhall discussed comes into effect and the center of buoyancy moves and you get a righting moment from those batteries that will help put you back where you want to be.

Did you replace these batteries (and they are a different type/weight than the ones replaced) or are they a new addition to the boat? 1600 pounds not in the lowest of the bilges can have a huge effect as you have noticed.

You changed the buoyancy factor of the center of the boat without changing it on the sides. Also the fact they're off center might play a role as you've created an imbalance. Altering any part of weight distribution can impact many other things. If anything the depth of it below waterline is a negative, not a positive. Vertical balance is important too.

Generally a boat with its mass distributed toward its perimeter will have a much higher resistance to changes in motion. It is therefore evident that it will be more dynamically stable. This can be intuitively thought of as the "gyroscope" effect.

Also, has this changed your waterline any?

The difference in this and the tank is I assume the tank is spread largely over the width of the vessel, not concentrated solely in the middle.

See, you put your AGM's to the outside of the engines.

Really think of standing with your feet together versus with your legs spread. Which gives you the best base?

J, have you ever heard of "free surface effect"?

This thread makes it clear that there are many misconceptions regarding stability and trim. It would serve the readership well for some to spend time with a good book on stability or a computer stability program and see exactly what happens when weight is added or removed from a given location.

Spreading the weight out to the sides will increase the moment of inertia and thus the roll period, making it slower to respond to dynamic forces (like stepping onto the boat) but it won't actually affect ultimate stability. If it lists 3 degrees when he stands on the rail with the weight in the center it will list 3 degrees when he stands on the rail with the weight distributed. Distributing the weight can make it more comfortable, however.

Exactly right on... however, the comfort or feel of the boat will be better with the longer roll period. This is something that is very important to naval architects and those that are sensitive to sea sickness. Basically you messed up you boat... should have hired a professional naval architect before installing things on the boat and unintentionally changing its dynamic response is the lesson here!

The preferred solution would have been to either split the battery bank up and put them equidistant from centerline as far outboard - port and stbd - as possible,, typically outboard of your engines if space / access allows or to place them in a single bank athwartship (across the beam).

Heavy weight, down low placed along the centerline would not be the first or second choice for the reason you found out, but the choices are typically complicated by space available in compact machinery rooms.

I suggest that any difference in susceptibility to roll due to the transverse location of added weights in a narrow beam vessel is so slight as to be of zero consideration. If anything, placing heavy items outboard exposes them to more rapid accelerations and will require heavier supporting structure.

As far as transverse stability is concerned, if the weights are equal, the transverse center of gravity will remain the same, and if the weight does not lower the CG then the roll rate will not change.

If the added weight is located below the initial vertical center of gravity, VCG will be lowered, GM will increase and produce a faster roll rate. That condition is probably what the OP describes as instability. The new condition is of course exactly the opposite of what the thread title claims, his boat is more stable.

It seems to me that the OP is describing initial stability.

He has essentially added an interior "keel" to what I assume is a round bilge narrow waterline without the added benefits of the dampening effects of a traditional external keel.

So I would still stand by my approach.

Considering the boat was at a dock and not suffering from compartmental flooding, that is a reasonable if superbly obvious conclusion.

He probably lowered the VCG, that is the sole reason the roll period changed.

Consider the statement made earlier in this thread "Compare with a sailing boat without her mast, the movement is faster." That sailboat still has the same keel as it did with the mast, removing the mast increases the GM and that is what shortens the roll period. If we use your theory, that sailboat would roll slower since the keel doesn't have to dampen the angular momentum of the mast.

I am not presenting an argument to you regarding GM, Roll period, sailboats/masts or whatever.

I will say it again, sloooowly this time - the OP will reduce his perception of a decrease in his initial (form) stability by NOT putting all the weight on centerline in a concentrated area, but by placing the weight symmetrically at the perimeter of the available machinery space on this particular MY.