Summer projects: Adding more flotation to your trailer boat

Ideally, a boat that is completely swamped should float. But under boatbuilding standards introduced in 2006, all a boatbuilder needs to do is ensure one part of the boat remains afloat when it is swamped.

More often than not, this is the bow section of the boat, usually taking the form of a sealed air void that provides the flotation needed to keep the nose of the boat above the water. This is called basic flotation.

However, marine safety authorities recommend boat owners use boats that are rated as having level flotation. This is where the boat floats level with the water even while the boat’s occupants are still inside it.

How do boatmakers achieve this? It’s by adding more flotation materials inside the boat to counteract the weight of the boat itself.

Having a boat with level flotation has other obvious safety benefits compared with boats that have basic flotation.

The most important one is that the boat’s occupants have a chance to bail themselves out of trouble and refloat the boat.

It also allows boaters to reach and use any safety gear stowed in the boat, adding an extra layer of safety as well as making rescue easier.

The big difference between a boat that has basic flotation versus one that has level flotation is your chances of surviving a sinking.

A boat that floats level allows its occupants to climb back in and take shelter. The boat is also much more visible from the sea and air compared with trying to spot just the bow floating in the water. 

The boat’s occupants can also fire off distress flares and guide rescuers to their location.

There are two main types of flotation built into boats, and both will depend on what the boat is made from.

Fibreglass boats will often use air voids built into the structure to create pockets of positive buoyancy throughout the boat. However, you’re never going to know if these voids are completely airtight until your boat sinks. 

The other more widespread form of flotation is to use air-filled foam. This is mainly used for alloy boats where creating sealed air pockets is not feasible, but some fibreglass boats can also use a foam core sandwich that has an outer and inner fibreglass skin, but lightweight foam in between.

Fibreglass boats that use a foam core sandwich, such as those built by Boston Whaler, tend to have level flotation.

The best form of foam to use in a boat to add flotation is what is called closed-cell foam. That means the foam has a sealed outer layer that stops the foam from soaking up water.

A common and cheap type of foam is polystyrene. This works well and provides lots of buoyancy, although it dissolves when exposed to petrochemicals, meaning a fuel leak in your boat could potentially start to eat away at it.

A better type of foam to use is polyethylene or polyurethane foam. This is because it will not soak up water and it is resistant to petrochemicals.

However, polyurethane is susceptible to wear, so if it is considered as a solution for your boat it must be housed inside a durable outer layer.

You can get expanding polyurethane foam that can expand to fill voids. An easy solution is to spray it into plastic garbage bags that you can then seal to become watertight. However, this foam becomes brittle as it ages, losing effectiveness.

Polyethylene, the stuff that pool noodles are made from, is a better choice as it is more durable and longer lasting. 

The standard formula for adding any extra buoyancy to a boat is that half of what you add should be down at the stern of the boat.  This is because the boat will need to counteract the weight of the outboard engine to float at the waterline.

Of the rest of the buoyancy, 25 per cent should be placed in the bow, and another 25 per cent along the gunnels.  It should also be placed as high as possible so that the boat doesn’t try and invert once it is swamped.

Buoyancy added under the boat’s floor will tend to cause the boat to capsize, meaning anyone onboard will need to grab onto the boat’s keel once they’re in the water.

The easiest way to install foam buoyancy is to buy it in sheets that you can cut up to size. 

Major hardware stores all stock foam boards that are commonly used for insulating houses, and marine specialty stores sell boat-specific foams such as Microlen and Nealon.

The foam can easily be cut to size using a sharp knife or even a small handsaw.

If you’re glueing in the extra flotation such as when you’re adding it to a transom or under the gunnel, use an adhesive such as Sikaflex to ensure a lasting attachment.