blowing your ballast tank, DANGER, DANGER!!!!

[ruedi ross]

Many people have thought about emptying their ballast tank while in the water with compressed air. Here's an answer to how much air pressure it takes to empty the ballast tank on water ballast boats when they are still in the water. When you look through the vent hole when the tank is full and the fill valve is open you see the water about two inches below the vent opening. The pressure in the tank is equal to atmospheric pressure for a relative pressure of 0. A column of water 1 foot high exerts a relative pressure of .431 PSI, a 1 inch column exerts a pressure of .0326 PSI. Stick a tape measure in the vent hole to determine the difference between full and empty. This is the column height of the water in the tank. I measured my 240 and it has 16.5" of water in the ballast tank. 16.5" X .0326 = .5379 PSI, or 77.5 pounds per square foot. The next question is what is the surface area of the top of the ballast tank? I don't know this, but lets assume its 5' long by 2' wide = 10 sq feet or 1440 square inches. The top "clamshell" half of the tank is fiberglassed to the hull to form the ballast tank. That means that there is an upward force generated on the top half of the tank equalling 775 pounds. This is assuming 10 square feet of footprint for the tank. There 1s 1 assumption involved here, but I think it's pretty close, and may be conservative at that. I don't think I will try to blow out my ballast tank. I doubt that it would be covered under Hunter's warranty.Ross

 

[Sam Morgan]

Interesting

If you put the boat in the water with the fill valve open and the vent stopper installed, wouldn't the pressure be close to that required to empty the tank?Also, why would you ever want to empty the tank except when expecting freezing temperatures?

 

[ruedi ross]

sam

If you left the vent plug in and dunked the boat you are starting out with 0 pressure. The water would fill the tank compressing the air inside until pressure/water column equilibrium is reached. the pressure would be equal to the difference in water column heights, probably about 15-20% of the pressure required to empty the tank.

 

[Tom2909]

The weak link...

The pressure in the tank is going to be exterted in all directions on all parts evenly. Think of it as an inner tube with a weak spot. The weak spot is going to bulge out more and eventually break with increasing pressure.A given pressure (say 1 lb/sq inch or 144 lb/sqft) is going to be applied to all parts of the ballast tank. Which is the weak link: The hull, the lid, the seam? And how much force can it take, per sq. inch (or sq. foot)? There is a lot of force across the area, but it is also distributed and being supported across a large area.One other consideration: at rest the pressure in the tank from the weight of the water is the height of the water, 18 to 24 inches. Now lets say you are out sailing and you go down a very steep wave with your bow pointing down. Now the column of water that creates the pressure is oriented (more or less) vertically, and the pressure is much higher at the lower end (bow end) of the tank. Does the ballast tank seam split and water come rushing out? Of course, if I found myself in this situation, the last thing I would be thinking about is the water ballast tank .The dynamic forces are probably much greater when you are pounding through waves and wakes.For example, hold a water balloon in your hand, you stay dry. Throw it way up and catch it, you get wet.My point is, I wouldn't be overly concerned carefully applying LOW Pressure to blow ballast from a tank with the valve OPEN! ** DO SO AT YOUR OWN RISK **Fair winds,Tom

 

[Holdfast]

Blowing ballast

If you go to the Catilina 250 webb forum, you will see that using air pressure to emty the ballast tank is a standard practice among the Catilina 250 owners.