First of all, ULDB''s are not water ballasted. Volvo 60''s and Open Class 60''s are water ballasted but there is a very big diffference between the way a V60 or OC 60 is water ballasted and the water ballast systems used on trailerables.
To explain, there really are two types of water ballast; movable and what I will call fixed position water ballast. Moveable water ballast is the type of water ballast used by the Volvo round the world ocean racers and consists of a tank on either side of the boat and water is shifted from side to side every time the boat is tacked or jibed. This form of water ballast is typically used in conjunction with a fully ballasted boat. (I personally would love to see this is the type of ballasting system improved and incorporated in production boats.)
The second type has a tank (or tanks) in the bilge and uses water in conjunction with some small amount of higher density ballast. The issue with this type of water ballast is the same as with all forms of low-density ballast. If you compare water to lead, water is approximately one tenth the density of lead. That means you need ten times more volume of water to equal the weight of lead. This means that you will end up with some combination of either:
- The water being higher in the boat resulting in a higher center of gravity and less stability than the lead,
- More water ballast to overcome the higher center of gravity meaning a heavier boat (Remember weight, in and of itself, does nothing positive for a boat and does have a lot of negatives.),
- Appendages that are shaped to hold water rather than to be efficient as sailing foils,
- More dependence of form stability which means a less comfortable motion and a poorer ultimate stability,
- Less interior storage or no sump for bilge water to collect before soaking your clothes.
The bottom line a well-designed water ballasted boat will always be an inferior sailor when compared to a properly designed fin keelboat. As in all things in sailing there are trade-offs. In my book, even if water ballast reduces towing weight (which is questionable since the retractable bulb keel boats do not have to weight that much more than a dry water ballast boat), I really think its too much of a compromise in performance and safety for my taste. There are people who are perfectly comfortable with water ballast, but having been aboard a variety of boats from 20 to 41 feet that have been knocked down to close to 90 degrees, I see water ballast as too much of a risk for my taste.