Broadly speaking, it's done all the time in larger vessels.

Fintry was built with tanks for 50,000 pounds of seawater ballast. We' re
putting a bow thruster in the forward one, so we'll replace the weight there
with lead, but the aft pair (14,000 pounds each, p&s) allow us to get her up
to a draft of less than seven feet for sheltered waters or down to over
eight feet at sea.

Actually doing it in a boat that will go between planing and displacement,
is another thing. The hull forms are quite different and it might prove to
be a challenge. And, of course, you're talking about a lot of water -- you
might not like what it does to the interior in a 46 foot boat. You can see
Fintry's tanks between frames 2-6 and 41-45 at
http://www.mvfintry.com/details.htm -- the scale on all the drawings shows
frame spacing, which are 20" apart.

--
Jim Woodward
www.mvFintry.com

"AP" wrote in message
...
I want to build a cruiser with the following characteristics
46 foot with very deep V foreward. 25-28 degrees deadrise at transom
Overpowered.
And I want to have a " ballast tank."
The idea is :
If the sea is calm I will have the ballast tank empty and I will make her
plan using my extra horsepower.
If the sea is rough (or I meet heavy weather on the way) I will ballast my
tank, increase the displacement, will slow down the engines and I will be
sailing like a deep V boat, good for
rough weather.
Does that make sense??
Comments/suggestions are kindly invited.
Regards
AP