Water Ballast

There is a lot of misunderstanding and misinformation about water ballast boats. These boats have a reputation for being "tender".   How "tender" a boat is depends on many factors, such as hull and keel design, displacement, length, beam, and sail plan. In general, long, narrow boats with deep ballast will be more "stiff" than wider, shorter boats with a higher center of gravity. However, it is possible to design a boat with water ballast that has performance equivalent to similar sized boats.  

Here's a discussion about water ballast boats that might help set the record straight:

Technically Speaking—How Water Ballast Works  
By; Ramblin’ Rod Brandon
rodney.brandon@sympatico.ca
 

First Published in Clipper Snips the magazine of the Trailer/Sailors Association Fall 2007 Pg 10

I remember as a young child, my father posing the question, “What weighs more, a pound of feathers or a pound of lead?” Of course the answer is, that they both weigh exactly the same. This is a critical premise to understanding how water ballast works.  

Some people believe that because water has neutral buoyancy in water, that water ballast would have to be lifted above the waterline, to have any effect on righting moment. This isn’t true. To understand why, one needs to know how righting moment is generated. 

Every vessel has a centre of gravity (CG) and a centre of buoyancy (CB). The centre of gravity is the axis on which the boat will tend to roll when heeled. The centre of buoyancy is the relative volumetric centre of the water displaced by the vessel. 

As the boat heels, CG remains stationary, whereas CB shifts laterally. The lateral distance between CG and CB is proportional to the righting moment GZ. The greater this distance, the stiffer the boat will be. 

Dense ballast such as lead has the benefit of lowering CG which helps extend GZ. Since water ballast must be carried higher in the vessel, raising the CG, more weight in water ballast is required to create the same GZ as lead.

Note how in Illustration 4, the water in the ballast tank is not raised above the waterline outside of the vessel, and yet contributes significantly to righting moment. Without the water ballast, the CG would be much higher still, resulting in less GZ. This is why a water ballasted boat is so much more “tipsy” (technical term totally unrelated to beer or rum ballast), before the water ballast is loaded. 

Water ballast must be raised above the water line to be effective? Myth busted!  

Water Ballast Q & A

Water ballast has been used for many years in many types and sizes of boats. In addition to trailerables, ocean going maxi-racers use water ballast since it can be pumped to the windward side to reduce heeling.  These questions and answers only apply to the trailerable water ballast boats manufactured by Hunter, Catalina and Macgregor.   These vessels are safe when operated within design parameters. Comments are welcome at: george@kobernus.com.


 1.

Q: 

What is the biggest difference between a trailerable water ballast boat and a trailerable conventional fixed or swing keel boat? 

A:

The biggest difference is that the water ballast can be removed while the ballast in a conventional boat remains with the boat at all times. As a result, a smaller vehicle is needed to tow a water ballast boat and/or a larger water ballast boat can be towed by the same vehicle.  


 2.

Q: 

How do you get the water into the tank?

A: 

Water ballast boats are designed to be self-filling without overflowing the vent hole. If the water intake valve and vent hole are open when the boat is launched it will automatically fill. Once the tank is full and all vents and valves are sealed no further action is needed.  See these pictures for more information.


 3 .

Q:

Does the water "slosh" around in the tanks of a water ballast boat?

A:

If the water intake valve is opened completely, the tank will fill to the top, replacing the air in the ballast tank. There is no movement of water & air in the tank; as result there is no "sloshing". It is important to remember to close the valve and plug the vent hole after it is full.  


 4 .

Q: 

Can you sail the boat with the ballast tanks empty? 

A:

NO!  These boats should never be under sail with the tank empty. The McGregor is designed to operate much like a traditional powerboat with the tank empty under power but should never be sailed with an empty tank. Some water ballast models have lead ballast in addition to water ballast, but both types of ballast are needed to ensure boat stability under sail. There is one famous case of a overloaded McGregor 26 that capsized with an empty ballast tank; however, the boat was not under sail, the operator exceeded the designed passenger limit of the boat, and alcohol was involved. For more on this incident go to this link. For more on this subject go to this link.


 5.

Q: 

Can the water in the tank leak into the interior of the boat?

A:

With some designs it is possible to experience water spilling out of the tank vent hole if the hole is not plugged when heeling . Also, since the water tank top is usually the floor of the cabin, drilling holes in the floor is a bad idea (don't laugh, this has happened).


 6.

Q: 

Can the water in the tank "leak" out of the boat into the surrounding water?

A:

You can't really "leak" ballast. Water in the tank will always maintain the level of the surrounding water. There is a gasket on the valve that helps seal the tank when closed. However, I would guess that 99 percent of all hunter water ballast boats have "leaky" gaskets. A "leaking" ballast valve will not significantly effect the righting moment of your boat. Think about it - the ballast tank level will never drop below the surrounding water; even if the gasket leaks, the amount that leaks out out when you heel will just leak back in when you level off. The next time you pull the boat, keep the valve closed and see how much water leaks from the tank. If the amount of leakage concerns you, try cleaning the valve surface and the gasket. If still not satisfied, replace the gasket.


 7.

Q: 

How do I get the water out of the tank?

A: 

It is a very bad idea to remove water from the tank while the boat is in the water as this affects the righting moment of the boat. See question # 4. When the boat is pulled from the water and the intake valve opened, the water will drain out. Some owners have rigged up a pump to remove the water but this is a bad idea as this converts the boat into a very large canoe! The McGregor is an exception to this rule as it can be emptied while operating under power, just as any small powerboat can. In addition, installing an access hole to install a pump could violate the integrity of the tank.


 8 .

Q: 

How much water does the ballast tank hold?

A: 

The water amount varies depending on the boat manufacturer and model. Water weighs about 8.3 lbs per gallon. The Hunter 260 ballast tank contains about 238 gallons or about 2000lbs of ballast. Some boats such as the McGregor series have both water and conventional ballast.


 9.

Q: 

Are there baffles in the tank?

A: 

No, there is no need for baffles since the tank is designed to be full at all times and this could increase the time required to fill or drain the tank.


 10.

Q: 

Does the ballast have to be raised up above the waterline to effect the righting moment of the boat?

A: 

No, this myth is debunked in the above article by Rod Brandon.


11 .

Q: 

Will a water ballast boat capsize easier than a conventional ballast boat?

A: 

Compared to what boat? Any boat can capsize under the right conditions. Trailerable water ballast boats are Class C vessels only certified for inland and coastal waters and are no more liable to capsize than similar trailerable conventional ballasted boats. Many water ballast boats have successfully weathered severe weather and have completed long open water voyages. One Hunter 260 owner recently completed a 5800 mile Great Loop trip, and more than one H260 has done the Miami-Nassau run, but this is not a blue water boat by any stretch of the imagination. 


12 .

Q:

I've heard that water ballast boats are "tender". 

A:

It is possible to partially quantify how "tender" a boat is using conventional engineering data. Go to this link to look at the data of similar boats. The Hunter 260 has a displacement of 5000lbs and carries 2000lbs of water ballast. The two foot shorter Dana 24 has a displacement of over 8000 lbs with 3200 lbs of conventional ballast. In this case the fact that the Dana is less tender than the H260 has more to do with the displacement and design of the keel than the type of ballast. With water ballast boats you'll notice it feels more like a powerboat when sitting at the dock or motoring with the sails down; but it's rock solid once it heels more than 5 degrees. 

 

After all, the H260 is still a 26ft boat with a displacement of 5000 + lbs and 320 sq. ft of sail. This gives it a SAD of 18.8. The hunter has a significant roach in the main so it is a good light wind sailor but it can become easily overpowered when the wind pipes up and will round up if you are not paying attention. As a result, you can't just lock-up the mainsheet and throw away the key like you can with larger boats. Tending the main and reefing the sails early are the keys to safe, comfortable sailing.


13 .

Q:

Will a water ballast boat heel more than a conventional keeled boat?

A:

No. Because the center of gravity of a water ballast boat is higher than a conventional ballast boat, the initial righting moment provided by water ballast is not as pronounced for about the first 5 degrees of heel, then the weight of the ballast begins to take effect. Beyond that initial point, there is no significant difference between the "tenderness" of water ballast and conventional boats given similar size, ballast weight and displacement. The ballast does not have to raise above the water line before it begins to resist heeling. See this diagram.


14. Q: Where is the water ballast. Is mechanism complicated or easy to break?
A:  Think of the water ballast as a long shallow flat keel attached to the bottom of the boat. It is a little more complicated than a lead fin keel that just sits there, but more complicated than a heavy swing keel that has to be lowered and raised. 

15 .

Q:

 Does the water have to be replaced periodically? 

A:

 There is no need to replace the water under normal conditions. Some people assert that the water gets "stale" and smells. This is should not be a problem if you keep the vent hole plugged. Few if any organisms can survive for long in a sealed tank without light and depleted oxygen. In tropical climates where the boat remains in the water all year, some owners have been known to treat the ballast with bleach, water bed solution or other chemicals. Despite this, in virtually all situations discharging chemicals into open water is against the law and heavy fines are possible.


16 .

Q:

What should be done to prepare a water ballast boat for winter. Is there a possibility that the water will freeze?

A:

Yes, the water can freeze and the tank can be damaged. Some people suggest putting anti-freeze or water softener in the tank. You'd need a lot of chemical, and this could result in a large fine for polluting the surrounding water. The whole idea of a trailerable boat is that it is easily retrieved. Pulling the boat and draining the tank is the best option when temperatures drop below freezing. Level the boat and ensure the ballast drain valve is open. This will insure all the water will drain from the tank.


17. Q: I'm confused. My friend has a water ballast boat and he says he has a keel that sticks down in the water about 6 feet. Is that where the water ballast is?
A:  No. Most water ballast boats typically have a centerboard that weigh less than 100 lbs. The purpose of a centerboard is to resist sideways motion of the boat when sailing. See this picture.

18.

Q:

does a water ballast boat point as high as a similar boat with a conventional or swing keel?"

A:

Again, you have to ask "Compared to What boat?" The great majority of the sailing manners of a boat depend on factors other than the ballast. Design of the Keel/centerboard, rigging, sail plan,  length, width, and displacement result in a whole greater than its parts.  Many observers agree that some water ballast boats don't point as high as some conventional keeled boats; however, others believe that gaff-rigged boats also don't point as well as some conventional keeled boats or water ballast boats for that matter.