What is Narrowboat Ballast?

Ballast refers to the weight added to a boat to ensure stability and balance in the water. For narrowboats, this usually means distributing weight in a way that prevents tipping, ensures an even keel, and maintains the right draught (how deep the boat sits in the water).

Why is Ballast Important?

Proper ballasting is essential for:

• Stability: Prevents the boat from tipping over or listing to one side.
• Maneuverability: Ensures the boat responds well to steering.
• Comfort: Provides a smooth ride without excessive rocking.
• Safety: Helps avoid capsizing and other dangerous situations.

Additionally, ballast plays a critical role in preventing propeller ventilation, where air is drawn in around the propeller, causing noise and poor steering. To avoid this, as a guide the uxter plate at the back should be 1 to 2 inches underwater whilst underway.

Understanding Draught and the Uxter Plate

The draught of a canal boat is determined by the distance from the base plate to the uxter plate, the horizontal plate above the propeller. The design of the canal network, with depths of 3 to 5 feet, limits how deep a canal boat can sit in the water. The hull design, being a long rectangular box, also impacts how ballast is used to achieve stability.
Typically, a canal boat’s draught ranges between 18-32 inches. More modern builds tend to have a draught between 18-24 inches. Ensuring the correct draught is important for navigating various waterways and ensuring the boat’s overall performance and safety.

Types of Ballast Materials

Most canal boat ballast uses either concrete block pavers (also known as concrete commons) or paving slabs. These materials are readily available and easy to fit into the boat. You can often find good deals on seconds at builders’ merchants, as the aesthetics of the blocks or slabs are irrelevant for ballast purposes. This form of ballast is the most common due to its availability and ease of installation.
Steel Ballast: Extremely expensive and doesn’t gain any additional height inside the cabin because the steel bearers provide the boat’s structural strength and set a height under the subfloor. Steel ballast is very good for trim ballast due to its density and ability to fit into small spaces to trim the boat correctly.

Considerations for Canal Boat Ballast

• Water Absorption: Some ballast materials, such as standard house bricks, can absorb moisture, which is undesirable in a boat. Engineering bricks or concrete pavers, which are denser and have lower water absorption rates, are better suited for canal boats.
• Galvanic Corrosion: If you are using metal ballast like iron ingots or lead weights, ensure they are separated from the steel base plate to avoid galvanic corrosion. Using a protective non-reactive layer, such as roofing felt along with a substantial layer of paint protection, will help avoid steel corrosion.

Get Out the Measuring Tape and Calculator (Or Maybe Just Start Loading Bricks!)

Calculating the ballast for your narrowboat might sound like a job for an engineer with a PhD, but in reality, it’s a bit more of an art than a science. However, for those who enjoy getting into the nitty-gritty of numbers, here’s a light-hearted attempt to put some structure around the process. So, grab your calculator, measuring tape, and maybe even a cup of tea, and let’s dive in!

Step 1: Start with the Hull – L x W x Required Draught

First things first, you need to figure out how much of your boat’s hull should sit below the waterline. To do this, you’ll need to calculate the volume of the underwater section of the hull. This helps determine how much weight you need to add to sink the boat to your desired draught.

For simplicity’s sake, let’s work in metric units. Here’s an example using a standard narrowboat:

1. Boat Length: 57’ (convert to 17.37m)
2. Boat Width: 6’10” (convert to 2.08m)
3. Required Draught: 22” (convert to 0.56m)

Now, multiply these together to get the volume of the boat that should sit underwater:

Volume of Underwater Hull = 17.37m x 2.08m x 0.56m = 20.23m³

Step 2: Account for the Shape of the Hull

Since boats aren’t perfect boxes, we need to adjust our volume to account for the rounded swims and the tapering shape of the hull at the back. This is where we apply something called a block coefficient, which adjusts for the shape of the hull underwater. A block coefficient ranges from 0 to 1, with 1 representing a perfect box shape. For a narrowboat, let’s use a coefficient of 0.85 to account for these irregularities.

Adjusted Volume = 20.23m³ x 0.85 = 17.20m³

Step 3: Convert Displacement Volume to Weight

Freshwater has a specific gravity of 1, meaning 1 litre of water weighs 1 kilogram. Since 1 cubic metre (m³) of water is equivalent to 1,000 litres, 1m³ displaces 1 tonne of weight.

So, our boat with an underwater volume of 17.20m³ displaces 17.20 tonnes in freshwater. This is the weight of the water your boat needs to displace to float at the desired draught.

Step 4: Estimate the Weight of the Finished Boat

Now, let’s move on to what your fully fitted boat will weigh. This includes everything from the steel shell to the engine, furniture, fuel, water, and all those knick-knacks you’ll bring onboard.
Let’s take a rough guess at some of the main components:

• Boat Shell: 8 tonnes (steel weighs around 7,850kg per cubic metre)
• Engine and Fittings: 1 tonne
• Woodwork and Fit-out: 2 tonnes
• Liquids (fuel, water, waste): 1.5 tonnes
• Everything Else (furniture, food, equipment): 1.5 tonnes

In total, this gives us an estimated weight of around 14 tonnes for the fully fitted boat.

Step 5: Subtract the Boat Weight from the Displacement

Now, subtract the weight of your boat from the displacement volume we calculated earlier.

Displacement Volume = 17.20 tonnes
Estimated Boat Weight = 14 tonnes

Remaining Ballast Required = 17.20 tonnes – 14 tonnes = 3.20 tonnes

The Theoretical Answer

The theoretical answer is that you need 3.20 tonnes of ballast to get your narrowboat sitting 22 inches (0.56m) into the water. Sounds simple enough, right?

Hybrid boats, whether they are parallel or serial hybrid systems, come with significantly more weight compared to typical canal boats. A serial hybrid boat, if sensibly spec’d, will include a diesel generator (potentially 250kgs), a large number of batteries (potentially 1000kgs), and an electric motor. A parallel hybrid system includes a standard diesel engine modified with potentially two electric motors and will also have a significant battery bank weighing up to 1000kgs. Both types generally feature much larger inverters, which also add to the weight. Depending on where these components are positioned, ballast needs to be removed or positioned to compensate for their substantial weight. This can mean a significant reduction in the amount of stern and potential bow ballast required, so careful planning and calculation are necessary to achieve the correct balance.

When positioning the ballast in your narrowboat, it is essential to achieve the correct trim, which means the boat should have a slight lean from front to back. The bow (front) should be higher than the stern (back), with the stern sitting lower in the water. This setup ensures optimal handling and manoeuvrability.

To achieve this, more overall weight is required towards the back of the boat. However, it’s important to consider the weight distribution of major components:

• Engine at the Back: Often requires careful ballast distribution to offset the heavy engine.
• Water Tank at the Front: Counteract this by leaving a relative amount of ballast out at the front.
• Interior Layout: If the interior layout is lopsided, such as a corridor running along one side, you may need extra ballast on the opposite side to balance the weight. Similarly, allowance for the positioning of heavy items such as waste tanks, iron stoves, and batteries. As you fit out the boat, remember that additional ballast might be required depending on the weight of furniture and equipment you install.
• Also, consider the thickness of the bottom plate of your boat. A heavier bottom plate than the standard 10mm will reduce the amount of ballast required, but this should be discussed with your builder to assess feasibility. However, it’s important to note that opting for a thicker base plate, rather than traditional ballast, won’t increase the internal headroom of the boat. The construction still requires steel bearers, which dictate the height underneath the subfloor, meaning there is no significant gain in overall space inside the cabin.

Ballasting a canal boat is a very manual, tiring, and labour-intensive job. On average, it will take a professional a day to ballast a narrowboat and potentially two days for a wide beam. If you have not done this before, do not have access to the right equipment, or cannot position the ballast in a way that makes the job easier, this task could easily take two or three times as long.

If you have a forklift truck, it can help hold the bricks outside a side hatch, allowing you to lift them straight into the boat. Alternatively, some people create slides at side hatches to slide the bricks or slabs down into the boat.

Stability and safety are critical when fitting ballast. The Recreational Craft Directive outlines parameters for canal boat stability, and while not all aspects are legally required for private boats, it’s worth discussing stability and safety with your canal boat shell builder or Boat Safety Scheme (BSS) examiner.

Additionally, consider the placement of deck drains, weed hatches, and skin fittings above the waterline, as these are important for maintaining safety.

Step 1: Protecting the Base Plate

Before you begin ballasting, it is crucial to protect the steel base plate of your narrowboat. This base plate must be painted to prevent rust and corrosion. Your boat might already have a paint scheme in place; if so, continue with the same type. If the base plate is just primed, determine whether it uses a single-pack or two-pack primer.

• Single-Pack Paints: Often bitumen-based, these are widely available at DIY stores. Brands like Blackjack are common choices.
• Two-Pack Paints: These offer a more durable finish and are available at specialist chandleries. Options for two-pack blacking are varied and provide excellent long-term protection.

Research thoroughly to select the appropriate paint for your base plate to ensure it is adequately protected against water and dampness.

Step 2: Laying a Damp-Proof Membrane

Once the base plate is protected, lay down a damp-proof membrane to prevent moisture from rising into the ballast material. Most canal boat builders use roofing felt due to its waterproof properties. Cut the felt to the width of each bay, typically just short of 600mm (24″), and lay it down across the base plate.

Alternative Methods: Some people use tile dividers to create a gap between the ballast and the base plate, though this is less common.

Step 3: Positioning the Ballast

With the membrane in place, you can now position the ballast. Follow these guidelines to ensure optimal balance and stability:

Leave Channels on Each Side: Keep a channel on both the port and starboard outer sides. This is crucial because there is a 45-degree cutout in the bearers to allow water to flow from the front to the back. If you have a leak, water will run down these channels to the back of the boat.

When placing the ballast inside each bay, it should be compacted tightly like a jigsaw puzzle. This ensures that if the boat takes a knock in a lock or has a collision, the ballast remains secure and doesn’t shift, maintaining the boat’s balance and stability.

Create an Inspection Hatch: When you install the subfloor, include an inspection hatch at the back of the boat. This will allow you to remove any accumulated water easily with a wet and dry vacuum cleaner if necessary.

Step 4: Installing Insulation

Once the ballast is in place, you can decide whether to add insulation on top. The bottom of the boat is typically the coldest part, and insulating this area can improve comfort.

PIR Insulation: Using 20mm PIR insulation on top of the ballast is a good option. This material helps to mitigate the cold floor often associated with narrowboats.

Step 5: Fixing the Subfloor

To complete the ballasting process, fix the subfloor to the bearers. Ensure there is a batten on top of each bearer that you can attach the subfloor to.

Mechanical Fixing and Adhesive: Best practice involves using both mechanical fixings and adhesive to secure the battens to the bearers, providing a sturdy and reliable base for the subfloor.

By following these steps, you can ensure that your narrowboat is properly ballasted, insulated, and protected against moisture, creating a stable and comfortable living environment on the water.

Ballasting your narrowboat is somewhat of a black art; it can be very challenging to calculate the exact amount and optimal positioning needed. If you are fortunate enough to be fitting out your boat while it is in the water, you have the advantage of being able to adjust the ballast during the build, ensuring it is correctly positioned.

However, if you find yourself in the more common situation of fitting out your boat while it is out of the water, this poses a significant challenge. Once the boat is in the water, it’s practically impossible to access the ballast to make adjustments without dismantling parts of the interior. Therefore, it’s wiser to under-ballast the boat rather than over-ballast it. Adding ballast is much easier than removing it. Consider placing additional ballast under plinths in kitchens, under beds, in voids, and plumbing spaces as needed after the boat is in the water. This approach allows for more manageable adjustments to achieve the perfect balance and stability for your boat.