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Catalina 250 Specific Forum

How Water Ballast works, and dangers if defective.

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Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Initially Posted - 08/08/2007 : 21:28:24

Following my "Water Ballast Leak" post, it came to me that some people are possibly unaware of the dangers a leaky WB can bring. I often read lengthy threads about the buoyancy of water in water trying to discredit WB boats telling me how little people understand the underlying concept. Below is an attempt to explain how the WB ballast works and possible inherit dangers if one suspects a WB leak. I made the illustrations myself which try to illustrate a cutout of a sailboat from behind with the water line.

Normaly filled water ballast without leaks.

Normaly filled water ballast without leaks and healed. The heal is counterbalanced by the WB weight above the water line on the opposite side.

Leaky water ballast tank floods boat. Water remains level with no WB weight above the water line to counterbalance. Ballast is non-functional and serves no benefit. Sailing with a leaky WB tank will eventually create this situation, it's just a question of time factored by the speed of the leak.

Eventually the wind will knock the boat over. The water inside the boat will simply keep level. Waves can crash into the cockpit then inside the boat furthering the flood and possibly sink the boat. Try leveling the boat while evacuating the water and lowering the sails.

CONCLUSION:
1. If you notice a substantial WB leak, you should NEVER sail back home. Lower the sails and motor back home instead.
2. Always make sure your WB valve is firmly shut since this limits the quantity of water the boat is going to flood to the quantity already contained in the WB tank.
3. With the boat leveled (motoring back) if you notice the water raising above the floor level and rising (and with your WB valve firmly shut) then you are taking on water from a source other than the WB tank and you are sinking.

Steve Blackburn, Calgary, AB
C250WB - 1999 - Hull 396

Edited by - Steve Blackburn on 08/08/2007 21:31:34

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 08/09/2007 : 04:58:27

<blockquote id="quote">quote:<hr height="1" noshade id="quote"> The heal is counterbalanced by the WB weight above the water line on the opposite side. <hr height="1" noshade id="quote"></blockquote id="quote">

You point out an interesting dynamic to water ballast.

To be sure we have clarity, "water line on opposite side" should be thought of as the interior to the boat water line not the lake level water line. My reason for suggesting this nit picking clarity is because there are those who would be tempted to think that water ballast works because of a relationship with the lake water level. The value of this clarity can be best understood if the drawings were slightly modified in the following way.

The ballast tank is actually slightly under the exterior water level with only the center line valve area having parity. It would be incorrect to think that this ballast below the water line does not offer righting arm until it starts rising above the lake water level. Simply put, the ballast is not in relation to the lake level, it is in relation to density dynamics within the hull.

You are spot on about your warning as any interior flooding would compromise the density dynamics at play.

Edited by - Arlyn Stewart on 08/09/2007 05:01:55

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 08/09/2007 : 12:50:10

Correct, ballast is ballast. But there is a direct relationship with the lake level because of the buoyancy differential. If the WB tank is at the same height as the lake then only a very slight ballast effect is being created due to the hull flotation (air). If the WB is higher than the lake then you get a reversed effect. The fact that the WB is slightly lower than the lake and has an effect proves this. Instead of a sailboat imagine a WB submarine, even when completely submerged it would roll in the upright position since the air on top is lighter than water. Healing with a WB boat is like trying to lift buckets of water above the water line.

However I was trying to explain the WB properties in a very simple manner. My first picture should show the lake water line slightly above the WB water line. In other words, the boat is still ballasted slightly when leveled. The ballast effect is accentuated the more heal you got.

My main purpose is to have people understand that having a WB leak is indeed a big issue and should not be taken lightly. Too many times do I hear the words "no problem" when problems arise and I could just see someone think they are safe sailing away with a WB leak on a constant heal until they get knocked down.

Thanks for pointing this out Arlyn.

Edited by - Steve Blackburn on 08/09/2007 13:14:22

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 08/09/2007 : 16:23:22

Steve, I don't think we are on the same page yet. Another way of stating it is water ballast works by displacing a heavier mass with a lighter one all of which is happening within the hull. The force that creates the displacement is countered by an equal resistance (righting force).

When the boat heels, air within the hull takes a place recently occupied by a denser materiel (happens to be water, could be sand). The denser material seeks the laws of physics and a righting force now exist (the dense material pushes back). If the tank were filled with water and swamped then the two mediums would be equal and no displacement of dissimilar densities would take place unless the water in the tank rose above the swamping water, so as you well pointed out, internal ballast righting forces in a swamped water ballast boat are lost.

Water ballast boats are however designed with a higher degree of hull form righting forces to assist internal ballast so don't panic too quick but do get the sails down.

Edited by - on

Steve Blackburn
Master Marine Consultant

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Canada
1091 Posts

Response Posted - 08/09/2007 : 16:33:35

Arlyn, I agree with you. Haven't thought of the air space now being created on a heal where once the water ballast was. So in fact 2 forces are being created, the weight of the water in the WB tank being pushed up (heavier mass) and air on the opposite side (lighter mass). So the wind tries to pull up the heavier mass and push down on the lighter mass. Thanks for that, I didn't even think of the lighter mass effect.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 08/09/2007 : 19:00:47

I think it helps to forget that it's water in the ballast tank. It could be wood... it could even be foam--neither would be as effective (due to their density), but either would have some affect. Whatever it is, the issue is where the center of gravity (essentially based on what's inside the hull and where) is relative to the center of boyancy (essentially based on the hull form). As the boat heels, the CG shifts to the high side, and the CB shifts toward the low side and becomes a righting force.

If the water is allowed to migrate inside the hull, which would also happen if the tank were only partially full, the CG shifts the wrong way (toward the low side) and the CB pushes it further over. Anyone who has stepped into a partially swamped dinghy knows the effect. So, correct--don't sail with a major tank leak, and never sail with a partially full tank.

Edited by - Dave Bristle on 08/09/2007 19:02:36

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