Designing a Galvanized Iron Pontoon with Plastic Barrel Floats

Designing a floating pontoon involves understanding the principles of buoyancy and carefully calculating the required flotation to support the desired load and structure. This guide outlines the process for designing a galvanized iron pontoon measuring 1.2 m by 5 m, utilizing plastic barrels as flotation, while maintaining a 0.3 m freeboard and supporting a 600 kg load capacity.

Key Design Highlights

Buoyancy Calculation: The core principle is Archimedes' principle, where the buoyant force equals the weight of the displaced fluid. Understanding this is crucial for determining the required volume of the floats.
Plastic Barrel Utilization: Standard 55-gallon plastic drums are a common and cost-effective choice for DIY pontoons due to their inherent buoyancy when sealed and properly secured.
Freeboard and Load Capacity: Maintaining a sufficient freeboard (the height of the deck above the waterline) is essential for stability and safety, while the load capacity dictates the minimum required buoyancy.

Understanding the Science of Flotation

At the heart of any floating structure lies the concept of buoyancy. According to Archimedes' principle, an object submerged in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. For a pontoon to float, the total buoyant force generated by its pontoons must be greater than the total weight of the pontoon structure itself plus any intended load.

The density of the fluid is a critical factor. For freshwater, the density is approximately 1000 kg/m³ (or 62.43 lbs/cubic foot). Saltwater is slightly denser, providing slightly more buoyancy.

Calculating Buoyant Force

The buoyant force (\(F_B\)) can be calculated using the following formula:

\[ F_B = \rho \cdot V_{sub} \cdot g \]

Where:

\( \rho \) is the density of the fluid (e.g., water).
\( V_{sub} \) is the volume of the object submerged in the fluid.
\( g \) is the acceleration due to gravity (approximately 9.81 m/s²).

For a floating object, the buoyant force equals the total weight (\(W\)) of the object:

\[ F_B = W \]

This means that the weight of the pontoon and its load must be supported by the weight of the water displaced by the pontoons.

Designing the Pontoon Structure

The pontoon structure will be constructed using galvanized iron for durability and corrosion resistance, especially in marine environments. Galvanization involves coating the steel with a layer of zinc, which protects it from rust.

Pontoon Dimensions and Materials

The desired pontoon dimensions are 1.2 m in width and 5 m in length. A galvanized steel frame will form the main structure, to which the plastic barrels will be securely attached. The weight of this frame will need to be estimated and included in the total weight calculation.

Utilizing Plastic Barrels for Flotation

Plastic barrels, particularly 55-gallon (approximately 208-liter) drums, are commonly used for DIY floating structures. The provided barrel dimensions are a diameter of 58.1 cm (0.581 m) and a height of 98.5 cm (0.985 m). The volume of a single cylindrical barrel can be calculated using the formula for the volume of a cylinder:

\[ V_{barrel} = \pi \cdot r^2 \cdot h \]

Where:

\( \pi \) is approximately 3.14159.
\( r \) is the radius of the barrel (half of the diameter).
\( h \) is the height of the barrel.

Calculating the volume of one barrel:


  radius = 0.581 m / 2 = 0.2905 m
  volume_barrel = 3.14159 * (0.2905 m)^2 * 0.985 m
  volume_barrel ≈ 0.2605 m³

In freshwater, the maximum buoyant force provided by one fully submerged barrel is its volume multiplied by the density of water and the acceleration due to gravity:


  max_buoyant_force_per_barrel = 1000 kg/m³ * 0.2605 m³ * 9.81 m/s²
  max_buoyant_force_per_barrel ≈ 2555 N

To convert this force to a mass equivalent (the weight it can support), we divide by \(g\):


  max_weight_supported_per_barrel = 2555 N / 9.81 m/s²
  max_weight_supported_per_barrel ≈ 260.5 kg

This is the maximum weight a single barrel can support when fully submerged. However, for stability and to achieve the desired freeboard, the barrels will not be fully submerged.

Calculating Required Buoyancy and Number of Barrels

The total weight the pontoon needs to support includes the weight of the pontoon structure itself, the desired load capacity (600 kg), and the weight of the barrels. Let's assume the weight of the galvanized iron frame and decking is approximately 200 kg (this is an estimate and should be refined based on the actual design and materials used). The weight of a single plastic barrel is typically around 10-15 kg; let's use 12 kg for calculation.

Total weight to be supported by buoyancy = Weight of frame + Weight of barrels + Load capacity

We need to determine the number of barrels required to provide sufficient buoyancy while maintaining a 0.3 m freeboard. A freeboard of 0.3 m means the top of the pontoon deck is 0.3 m above the waterline. If the barrels are 0.985 m tall, a significant portion of each barrel will be submerged.

Buoyancy Required at Desired Freeboard

The submerged height of the barrels will be approximately the total height of the barrel minus the freeboard, assuming the barrels are positioned vertically and the deck is built directly on top of them. However, the freeboard is measured from the deck to the waterline, and the barrels provide the flotation below the deck. A more accurate approach is to determine the submerged volume required to support the total weight.

Total weight to be supported = Estimated frame/deck weight + Number of barrels * Weight per barrel + Load capacity

Let \(N\) be the number of barrels. The total weight is approximately \(200 \, \text{kg} + N \cdot 12 \, \text{kg} + 600 \, \text{kg}\).

This total weight must equal the buoyant force at the desired waterline. The desired waterline is such that the top of the deck is 0.3 m above the water. Assuming the deck sits just above the barrels, this implies a certain submerged depth for the barrels.

A more practical approach for design is to determine the total buoyant force needed and then figure out how many barrels, submerged to a certain level, can provide that force.

Total required buoyant force (in terms of mass) = Total weight to be supported.

To support a total mass of \(200 \, \text{kg} + N \cdot 12 \, \text{kg} + 600 \, \text{kg}\), the volume of water displaced must have this mass. Since the density of water is 1000 kg/m³, the required submerged volume (\(V_{required}\)) is:

\[ V_{required} = \frac{\text{Total mass}}{\rho_{water}} = \frac{200 \, \text{kg} + N \cdot 12 \, \text{kg} + 600 \, \text{kg}}{1000 \, \text{kg/m}^3} \]

This required volume will be the sum of the submerged volumes of all the barrels:

\[ V_{required} = N \cdot V_{submerged\_per\_barrel} \]

The submerged volume per barrel depends on the submerged height (\(h_{sub}\)). For a cylinder, \(V_{submerged\_per\_barrel} = \pi \cdot r^2 \cdot h_{sub}\).

However, we also have the freeboard constraint. A 0.3 m freeboard means that the top of the deck is 0.3 m above the water. The barrels are 0.985 m tall. If the barrels are placed vertically and the deck is directly on top, the maximum submerged height of the barrels to maintain some freeboard would be less than their full height. A common guideline for stability is to have a significant portion of the barrel submerged. Let's aim for the top of the barrels to be at or slightly below the waterline when fully loaded to achieve the 0.3m freeboard above the deck.

Let's consider the total buoyant capacity needed. We need to support at least the weight of the structure and the load. Let's estimate the weight of the frame and deck as 200 kg. So, we need to support approximately 200 kg + 600 kg = 800 kg, plus the weight of the barrels themselves.

Let's approach this by determining how many barrels are needed to provide significantly more buoyancy than required when partially submerged. If each barrel provides approximately 260.5 kg of maximum buoyancy when fully submerged, and we need to support around 800 kg (plus barrel weight), a few barrels will be necessary.

Consider submerging the barrels such that the waterline is, for example, 0.6 m up the barrel (leaving 0.985 m - 0.6 m = 0.385 m of the barrel above the water). The buoyant force per barrel at this submerged height would be:


  submerged_volume_per_barrel = 3.14159 * (0.2905 m)^2 * 0.6 m
  submerged_volume_per_barrel ≈ 0.1602 m³
  buoyant_force_per_barrel = 1000 kg/m³ * 0.1602 m³ * 9.81 m/s²
  buoyant_force_per_barrel ≈ 1571 N
  weight_supported_per_barrel = 1571 N / 9.81 m/s²
  weight_supported_per_barrel ≈ 160.2 kg

If each barrel can support approximately 160.2 kg at this submerged level, and we need to support around 800 kg (excluding barrel weight for now), we would need 800 kg / 160.2 kg/barrel ≈ 5 barrels. Let's use this as a starting point for the number of barrels, say 8 barrels to be safe and account for the weight of the barrels and frame.

With 8 barrels, the total weight of the barrels is 8 * 12 kg = 96 kg. The total weight to be supported is 200 kg (frame) + 96 kg (barrels) + 600 kg (load) = 896 kg.

To support 896 kg, the required submerged volume is 896 kg / 1000 kg/m³ = 0.896 m³.

With 8 barrels, the required submerged volume per barrel is 0.896 m³ / 8 barrels = 0.112 m³/barrel.

Now we can calculate the submerged height required per barrel to achieve this volume:

\[ h_{sub} = \frac{V_{submerged\_per\_barrel}}{\pi \cdot r^2} = \frac{0.112 \, \text{m}^3}{3.14159 \cdot (0.2905 \, \text{m})^2} \]


  h_sub = 0.112 m³ / (3.14159 * 0.08439 m²)
  h_sub = 0.112 m³ / 0.2651 m²
  h_sub ≈ 0.422 m

If the barrels are submerged by 0.422 m, there is 0.985 m - 0.422 m = 0.563 m of the barrel above the waterline. If the deck is mounted directly on top of the barrels, the freeboard would be approximately 0.563 m. This is more than the required 0.3 m, which provides a good safety margin.

Based on these calculations, using 8 plastic barrels provides sufficient buoyancy to support the estimated structure weight and the 600 kg load while maintaining a freeboard greater than 0.3 m.

Refining the Number of Barrels and Placement

Given the pontoon dimensions of 1.2 m x 5 m, placing 8 barrels strategically is important for stability. Two rows of four barrels lengthwise would fit within the 1.2 m width (2 * 0.581 m = 1.162 m, which is less than 1.2 m). Along the 5 m length, four barrels with some spacing would be appropriate.

Floating dock constructed with blue plastic barrels.

A floating dock built using plastic barrels as flotation.

Construction Considerations

The galvanized iron frame should be robust enough to hold the barrels securely and support the deck and load. The barrels should be sealed tightly to prevent water intrusion. Strapping or cradles can be used to attach the barrels to the frame.

A deck material, such as treated lumber or composite decking, will be added on top of the frame. The weight of the decking must also be accounted for in the total weight calculation.

Structural Integrity of the Frame

The design of the galvanized iron frame is crucial for distributing the load evenly across the barrels and providing a stable platform. Welding or robust bolting can be used to assemble the frame components.

Securing the Barrels

The plastic barrels need to be firmly attached to the frame to prevent them from shifting or detaching. Methods include using metal straps, custom-built cradles, or marine-grade ropes.

Load Capacity and Safety

The calculated load capacity of 600 kg is in addition to the weight of the pontoon structure and the barrels. It is crucial not to exceed this capacity to maintain adequate freeboard and stability. Distributing the load evenly on the pontoon is also important to prevent tilting.

Capacity plates, while often required for manufactured boats, serve as a good reminder of the design limits. For a DIY pontoon, clearly indicating the maximum load capacity is a good safety practice.

Understanding Stability

While buoyancy keeps the pontoon afloat, stability prevents it from capsizing. The width of the pontoon and the placement of the barrels contribute to stability. A wider pontoon is generally more stable. The center of gravity of the loaded pontoon should be kept as low as possible.

This video explains the relationship between buoyancy and weight in boating, a fundamental principle for pontoon design.

Material Considerations

Selecting appropriate materials is vital for the longevity and performance of the pontoon.

Galvanized Iron

Galvanized steel is chosen for its resistance to corrosion, particularly in freshwater or saltwater environments. Hot-dip galvanization provides a thick, protective zinc coating.

Example of a galvanized steel structure used in marine applications.

Plastic Barrels

High-density polyethylene (HDPE) plastic barrels are durable and resistant to UV rays and chemicals. Ensuring they are air-tight is critical for maintaining buoyancy.

Decking Options

Pressure-treated lumber is a common and cost-effective decking material. Composite decking offers greater durability and less maintenance but at a higher cost.

Summary of Design Parameters and Calculations

Parameter	Value	Notes
Pontoon Length	5 m
Pontoon Width	1.2 m
Barrel Diameter	0.581 m
Barrel Height	0.985 m
Desired Freeboard	0.3 m	Height from deck to waterline
Load Capacity	600 kg	Additional weight the pontoon must support
Estimated Frame/Deck Weight	200 kg	Approximation, requires detailed design
Estimated Barrel Weight	12 kg per barrel
Calculated Volume per Barrel	≈ 0.2605 m³	When fully submerged
Calculated Max Weight per Barrel (Freshwater)	≈ 260.5 kg	When fully submerged
Recommended Number of Barrels	8	Based on calculations for required buoyancy and freeboard
Required Submerged Height per Barrel (with 8 barrels)	≈ 0.422 m	To support total estimated weight
Calculated Freeboard (with 8 barrels)	≈ 0.563 m	Assuming deck is just above barrels

Frequently Asked Questions

How accurate are these calculations for a real-world pontoon?

These calculations provide a strong theoretical foundation. Real-world factors like the exact density of the water, the precise weight of the structure, and the distribution of the load will affect the actual freeboard and capacity. It's always recommended to build in a safety margin and test the pontoon's buoyancy with incremental loads.

Can I use fewer barrels if I don't need the full 600 kg capacity?

Yes, the number of barrels directly influences the total buoyancy and thus the load capacity and freeboard. If you require less load capacity, you could potentially use fewer barrels, but you must recalculate to ensure sufficient buoyancy and stability.

How should I secure the plastic barrels to the galvanized frame?

Common methods include using heavy-duty straps, custom metal cradles that fit the barrels snugly, or bolting through reinforced sections of the barrels and frame. The goal is to prevent any movement of the barrels relative to the frame.

What maintenance is required for a galvanized iron pontoon with plastic barrels?

Regular inspection of the galvanized frame for any signs of corrosion or damage is important. The barrel seals should be checked periodically to ensure they remain watertight. The connections between the barrels and the frame should also be inspected for integrity.