Ballast is a high density material placed in the hull of a narrowboat or canal boat below the waterline to counteract the effects of weight above the waterline and keep the boat stable in the water.

Narrow boat ballast material ready to be laid. Bricks are one of many options available
Narrow boat ballast being laid in position for stability
narrowboat ballast neatly positioned ready for sub flooring

Ballast… Why Do We Need It?

It’s a Question of Design

The nature of our canal network is such that canals average from 3 to 5 feet in depth. That’s one of the restricting factors on how low our canal boats can sit in the water. The other factor is the design of the hull itself.

In order to maximise load capacity, a canal boat is like a long rectangular box. The restricted depth of water means no keel is fitted for stability & somehow there needs to be a clean flow of water along the hull to the propeller. Therefore at the back of the boat, underwater, the hull is shaped to a point and this section of the boat is known as the swim. The top plate of the swim, parallel to where we stand and steer, is known as the uxter plate. It is necessary for this uxter plate to be 1 to 2 inches underwater to prevent propeller ventilation. Ventilation is where the prop can draw air in around itself causing noise and poor steering. And therefore we need canal boat ballast to keep the boat stable in the water.

Stability and Safety

It is vital that any canal boat ballast is fitted with a focus on final stability and safety.

There are regulations enforced on Commercial BSS examinations concerning the height of deck drains, weed hatch and skin fittings above the waterline, while it is not a requirement on Private Craft BSS Examinations it is very much worth while considering and following these regulations.

There are stability and safety parameters determined by the Recreational Craft Directive so if you are at all unsure of any of these areas discuss them at length with your canal boat shell builder or BSS examiner. This section is intended to be a guide only, giving an overview of the canal boat ballast subject.

Where is the Ballast

Ballast is fitted into the shell directly onto the bottom plate. It is standard practice to lay the canal boat ballast on top of a non absorbent layer to separate it from the steel of the hull. If an air space can be incorporated between the canal boat ballast & the bottom plate, even better & this can be achieved by laying the canal boat ballast onto a non reactive spacer material such as large plastic tile spacers.

Treat the bottom plate with a thick layer of bilge paint or similar which further protects the steel from any condensation water or future leaks. It is worth it at this stage. Once the canal boat ballast is in, it will be a very long time, if at all, until the inside of the plate is exposed so treat it now & forget about it!

Types of Ballast

Different types of canal boat ballast are available, the most common of which are concrete pavers or engineering bricks. Any material such as this should be as dense as possible to prevent soaking up any moisture that may get into the bilges. For example, a standard house brick is designed to absorb water in order to ensure a good bond with a wet mortar. Some of these varieties can have a 20%-30% water absorption rating. A class A engineering brick is designed to be dense and strong and must have a rating of less than 4.5% water absorption by mass. This would be the obvious choice.

Other types of canal boat ballast include iron ingots & lead weights. Any metal canal boat ballast should be protected from contact with the steel of the base plate to avoid galvanic corrosion.

Also ensure that canal boat ballast is well secured from moving around, especially when laid in areas that will be inaccessible in the future. A clout into a bank or lock side could permanently affect the way the boat sits & handles if the canal boat ballast shifts around.

Think Ahead

Leave allowance for the positioning of heavy items such as waste tanks and iron stoves. A common rule of thumb is to ballast the boat two thirds to the bow, one third to the stern. A pivot point will be created relative to the ratio of ballast front and rear around which the boat will turn. Remember there is a heavy lump of iron in the boat at the stern in the form of the engine & its components. Batteries, calorifier, heating system and stern gear all add weight to this area.

Consider a heavier bottom plate. 12mm or 15mm plate will be heavier than 10mm and call for less canal boat ballast. If considering this, talk to your builder and use his experience.

Finally, accept there will be some fine tuning at a later date. Plan for access into the bilges so you can see whats going on in terms of moisture and consider fitting an automatic pump into the cabin bilge area, at the lowest point, just in case. A good place for this would be on the cabin side of the steel engine bay bulkhead as most boats will be trimmed with a slightly bow up attitude.

It is worth bearing in mind that ballast on a new canal boat will be minimal. Once you have ‘moved’ on to your new home, you may find you need to add to the ballast installed by the narrowboat builder. If you are purchasing a sailaway canal boat, you will generally need to source the ballast to install yourself.

Ballast… How Much Do You Need?

Get out the Measuring Tape and Calculator

So, on to working out how much ballast you need, and we start by working out the weight of hull we need under water. Very simply put we need to overcome the buoyancy of this underwater section of the hull and therefore need to work out how much additional weight we need to place in the bilges to sink the boat to a certain depth.

LxWxRequired Draft

Three dimensions are key. The length, the width, and how much of the hull you want underwater. Lets work an example, as it’s easier we’ll use metric units so we’ll convert a standard narrowboat’s dimensions of feet & inches into metres.

57’ x 6’10” x 22” draft equals 17.37m x 2.08m x .56m

The volume of this “box” is therefore:

17.37m x 2.08 x .56 equals 20.23m³

Allowing For The Shape of the Hull Underwater

As the hull is not a regular box, we multiply the above figure by what is known as a block co-efficient. The closer to a box shape the hull is, the closer this co-efficient is to 1. Lets use 0.85 to take into account the swims & irregular sections of the hull.

So, 20.23m³ x 0.85 equals 17.20m³

Displacement Volume Converted to Weight

Fresh water has a specific gravity of 1 which means 1 litre weighs 1 kg.
1m³ is equivalent to 1000l & 1000l weighs 1000kg or 1tonne.

Therefore 17.20m³ of volume (l) displaces 17.20 tonnes in fresh water.

Take off the Estimated Weight of the Finished Boat

We then need to account for the weight of the fully fitted boat. Decide whether you want to account for full water & fuel tanks & have a stab at guessing the weight of all the sundry items. It’s not rocket science & as you will find out, there will be a need to trim the canal boat ballast at the end of fitting to get your boat sitting at the right attitude anyway.

So let’s guess at 8 tonnes shell (FYI steel weighs 7850kg per m³ so if you really want to work out the weight of your particular shell you can), 1 tonne engine & fittings, 2 tonne woodwork, 1.5 tonnes liquid (fuel, water, waste, gas), 1.5 tonne everything else (food, furniture, equipment etc) equals circa 14 tonnes.

Subtract this approximate 14 tonnes from the displacement of 17.20 tonnes leaves 3.20 tonnes.

The Theoretical Answer

This 3.20 tonnes is the theoretical amount of canal boat ballast required to sit the fully loaded shell .56m or 22 inches into the water.

That’s the theory. In practice, perhaps it’s better to start with a couple of tonnes of engineering bricks on a pallet at the side of the boat, start loading them in & see what happens!


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