Ballast in a catamaran

Speaking in terms of a catamaran vessel. If a non-compressed ballast chamber was added in the bottom of the each hull. For example, I create a 24" tall section, and flood the bottom 6". I expect the waterline to then be equal to that of a 18" tall section (plus the additional weight of the structure bellow the 18" mark). The extra 6" of structure would still have some affect on the actions of the vessel, Right? How would this affect the boat in a stationary position? Specifically in terms of stability and sea keeping? FYI, the initial design concept is that this would only be used for stationary use at sea, never under power.

 

Why would it need it?
How much more stability do you need when stationary?

 

To increase the resistance to rolling via higher moment of inertia?

 

sabahcat, It is for a small craft design that would be used for a specific purpose in a niche market. It doesn't fall under what I would consider typical multi-hull design.

That was my thought and part of the intent, along with lowering the waterline to the desired level when stationary. Yet allowing extra freeboard under power for the seakeeping side of things.

 

I had inquired several times about a landing craft being a cat and I thought of the same thing basically, water balast to stabilize the stern while loading and off loading. I just thought that a 24/27 foot cat would be rather light (which is the usual goal) and it would certainly see action from wakes on a lake for example. I simply based this seeing boats that were full of water, half sunk, as they don't move much.

I have moved on as I thought such a build would add difficulty that would yield a small return, it's probably easier to anchor and tie it off than filling and draining a lower compartment.....