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Sub-floor, floorboards and ballast

I'm in the process of fitting out a 4 year old 58ft narrowboat. I bought the shell, engine etc. and it has the original ballast and 18mm ply sub-floor which is not in too bad a condition but will need some treatment. I'm in the process of putting down 25mm Celotex between wood batterns (on metal struts) about 55cm apart so quite close together and therefore quite firm.
My intention is to have hardwood T&G floorboards, do you think I need the sub-floor and spend time and money on it as it's already there or better just to remove it for the floorboards. Does the sub-floor make that much of a difference?
As I'm not sure about the ballast and how the boat sits in the water (it's currently on hard standing), I'm just a little concerned as I may need to get to the ballast, once it's in the water, to move things accordingly? The sub-floor will make this much more difficult.

Asked by: Martin Parker  | 11.24am, Tuesday 17 January


WW says:

Personally, I would use the sub-floor, as otherwise the T&G boards might warp slightly more if they are not supported underneath- however, any movement will probably be minimal. I should add that most boatbuilders would add a subfloor (and this also provides a good degree of insulation). However, as your bearers are quite close (which are nominal 600mm/2ft centres) then you might well get away with the flooring; however it might make bulkheads, etc, less stable if attached to the floor. In the end, it will come down to how well supported you feel the floor is
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On the other point of access, I would strongly advise that you make easy access to the cabin bilge- both for trimming ballast, and for being able to inspect the bilge regularly (to ensure no build-up of water from pipe leaks or condensation). Sometimes laying the floor in panels, or even adding hatches (especially right aft) are useful.
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You should also consider ventilation of the cabin bilge, which will help reduce decay in the future, of the flooring and the steel shell. If you have air inlets forward, and assuming that air can circulate in the bilge (so ensuring that each section between bearers is not airtight from its neighbout- you need to plan for good air circulation) you need some air inlets aft as well (such as in an access hatch). You might also consider drilling a series of holes into the floor right into the cabin bilge behind and underneath the fridge, so that the latent heat from the 'fridge helps draw air up from the bilge- this will help with circulation as well as make your fridge more efficient.
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It might be useful to fit an automatic bilge pump into the aft cabin bilge- with an running indicator light on a panel (probably by the main engine panel). This will both enable you to identify any leaks and also, if the worse happens (such as a split water tank) will enable you to empty the water out quickly. The only requirement is, like for ventilation, that every section of bilge can drain into the next- check the lumber holes are fitted into each bearer to allow water to drain aft (boats tend to trim stern-down).
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In summary- use a subfloor if you can, but if you are happy with the stability without, then you can get away with it. Add as much access as you can, and ensure a hatch aft at the very least.

Mark Langley  | 11.35AM, Tuesday 17 January


Readers say:

Thanks for the quick, and very useful, reply. Hatches might be the way to go but it does give me more to go on.

Martin Parker  | 11.47AM, Tuesday 17 January

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