Narrowboat Ballast (We Explain What One Is)

Ever had that annoying feeling where something doesn’t feel quite right? Well in some instances you may very well be right. A boat which is not sitting level can be really irritating. We as boat owners can become obsessed with ‘listing’. Fortunately, there is a solution. Narrowboat ballast. In this article we are going to look at what it is used for and give a couple of suggestions.

Why Does a Narrowboat Need Ballast?

Water sits level. Always. It’s a fact of physics. However, boats are movable and pitch around something called the centre of gravity. It sounds technical, but it really isn’t. The centre of gravity is basically a given point on the boat where the combined weight of all the things inside  acts.

Still too complex?

OK what we are essentially saying is if all the weight is at the back, the boat will tend to sit down towards the back. If the majority of the weight is concentrated to the right then the boat will list to the right… and so on.

This can actually be rather irritating. A constant lean is not what you want. To counteract this, you are going to need to ‘trim’ your boat. Trimming is the act of adding weight in order to ensure that it sits level. It’s that simple.

You will tend to find that boats that are under laden will tend to list much more easily than those that are correctly weighted. And to achieve this we can use ballast to make the boat sit more correctly.

What to Use as Narrowboat Ballast?

Well, this might be slightly more complex than you first think. The reason? Well what you are trying to achieve by trimming your boat is consistency. Therefore, you will want something that weighs the same amount all of the time.

How can something change its weight?

Well one thing we have in abundance on around a boat is water. Depending on the material you use for ballast you may find that it can change in weight significantly when saturated. Want an example?

You might think that house bricks would be ideal to use as narrowboat ballast as they are geometrically shaped and relatively cheap. You’d be wrong. House bricks are actually designed to hold a certain amount of water to increase bonding with the cement. If one house brick weighs around 4kg, by the time it is soaked in water it will in fact weigh closer to 6kg! Obviously, you can see the implications.

Likewise, with sandbags. Sand can hold a great deal of water (hence why they use it in flood defences) and once a bag of sand is wet, it will tend to never dry out. Again, achieving any measure of consistency is practically impossible.

The best ballast to use is something that is uniform in weight and non-porous. Metal ingots seem to be the most reliable solution. Lead is extremely dense, heavy and non-reactive. It’s also relatively cheap. Iron ingots can also be used.

How to Work Out How Much Ballast You Need for a Narrow Boat?

If you are buying a new boat or fitting out a boat with a lot of different equipment then you may be at a loss as to how much ballast you will need to get the boat to sit correctly, well don’t worry here is a simple guide.

It can get quite technical; however, we are going to try and simplify it as much as we can. A boat is held afloat in the water by buoyancy. Essentially this means that its weight is supported by the amount of water it displaces. In simple terms if the shape of the boats hull displaces 10 tonnes of water and the boat weighs equal to or less than 10 tonnes it will float.

All makes sense so far? Good.

If we want the boat (or part of the hull) to sit correctly in the water than we will need to add weight in order displace more water. This weight is known as ballast.

First, we need to thing about how much of the hull we ideally want under the water. And we need a starting point.

To work out how much displacement is required we do the following: –

We measure the length of the hull.

We measure the width of the hull. (Also known as the beam)

We measure how deep the hull ideally needs to be in the water to sit correctly. (Also known as the desired draught)

Let’s use a really easy example.

The hull is 20 metres long, 2 metres wide and we need a desired draught of 0.5 metres. Let’s plug this into our formula….

20 x 2 x 0.5 = 20 cubic metres.

But remember that a hull is not a perfect rectangle, so we’ll assume that the bow and stern shape results in 10% less volume than we originally thought. So, the amount of displacement required is 18 cubic metres allowing for this reduction.

Now here’s a handy fact, water weighs exactly 1 tonne for every cubic metre.  Therefore, we will need to displace 18 cubic metres of water, or to put it another way 18 tonnes of water, in order to get our boat to sit correctly.

We now have a little more maths to do. We need to work out how much the boat already weighs. This will be the weight of the hull plus everything that is going to be on board, including water, fuel, fittings, the engine, everything! In this instance let’s say we have worked it out and it comes to 14 tonnes.

Remember the first part of our equation? We said that to sit correctly the boat needed to displace 18 tonnes of water to sit correctly? Well, so far it only displaces 14 tonnes… So we need to add ballast.

How much? Well it’s super simple. Simply subtract the existing weight of the boat from the total amount of displacement you need: –

18 tonnes – 14 tonnes = 4 tonnes.

So the amount of ballast we need is 4 tonnes!

In summary

A boat that is too light will not sit correctly. And it pays to know how much you need to load onboard in order to make it nice and level. Once you know the amount of ballast you need you should choose your ballast wisely. Anything which will hold water, move or change in weight over time should be avoided, it would be a shame to do all of those maths only to find you still had a slight lean in the water.

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