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QWe are restoring a 1981, 58ft boat. There was a fair amount of water in the cabin bilge which was caused by a leak in the side hatch, now fixed.
While replacing the damaged floor I found rotten joists and that gravel had been used as ballast. A local boatyard said gravel is not good for ballast and I should use Class A engineering bricks. So I removed it a section at a time (weighing it first), cleaned the rust, painted the steel with bitumen and replaced the gravel with the equivalent weight in bricks.
The bricks are a fairly snug fit. There is room above to allow some airflow, but I am concerned I am overfilling the bilge. Should I be concerned that I have overfilled the bilge?
The boat seems to sit a little deeper than before and I am conscious that the gravel was damp when weighed. I have been considering removing a line of bricks at the back to create more space where the bilge water could collect.
ROB WAINWRIGHT, via email
ATONY REPLIES... Engineering bricks or steel off-cuts are perhaps the Rolls-Royce of ballast. Most narrowboats use concrete paving slabs. You did the correct thing as far as the longevity of the hull is concerned, even though it must have been massive amounts of work. You need actual engineering bricks because ordinary house bricks will absorb moisture like gravel, may flake over time and will weigh less.
Ideally the uxter plate (the flat plate above the swim and propeller with the cut out in it for the swim) should be about one to two inches below the waterline and the ballast placed so that the boat sits level or slightly stern down.
This attitude will alter as you empty and fill the water and fuel tanks, so make sure the uxter plate is still at least an inch below the water with an empty diesel tank and a full water tank. Altering the fore-aft trim changes the boat’s pivot point when turning. Normally ballast is placed before the floor goes down with steel offcuts often added to the uxter plate in the engine bay for trimming.
Under-ballasting allows the propeller to draw air from the surface which produces lots of foam but little thrust. Over-ballasting is not an issue for a private boat as long as the ‘through hull’ holes are safely above the waterline. You would need a greater depth of water, but the reduced headroom you need through bridges and tunnels might be an advantage. It is very important on narrowboats that the ‘straight-through’ exhaust outlet is sufficiently high above the waterline.
It is vital to leave an area at the back of the boat without ballast to allow water to collect; if the boat trims nose down, the water will flow the other way.
You also need to think about moisture draining from under the bricks and this is usually achieved by placing the ballast on something. If you don’t have much room above the ballast I would suggest plastic wall tile spacers or long scraps of house wiring insulation.
You should have more than enough air space for ventilation, especially if you leave the trap that is used to inspect the back of the bilge off when you are away from the boat in the winter.