It was first necessary to make support battens for the false floor panels to lay in, under the existing plywood sole. I bought a 24” inch by 48” sheet of ½” inch plywood and cut it into 4” inch wide strips with my table saw. I also made ¾” inch square battens out of hardwood. It was also necessary to reposition some pieces of equipment fastened to the sides of the floor beams, like wire hangers, water pipes and the big main engine filter, all had to be lowered below the beam level. The 4” inch wide plywood strips were then screwed to the underneath of the 2” inch wide sole bearers, to form a 1” inch lip on either side. I screwed the ¾” inch square battens to the sides of each aperture to support the ends of the false floors. This framework then supported the boards all round. I painted the bearers and new timber white.
I found some Medium Density Fiberboard (MDF) in 8’ foot by 4’ foot sheets ½” inch thick. I calculated I could cut all the boards out of two, 4’ foot by 8’ foot sheets to make the seven false floors. MDF is a heavy manufactured board, similar to particle board, but smooth on both sides, with a density of 44lbs per cubic foot and used to make stereo speaker boxes and other things where sound control is required. The sound deadening properties of this ½” inch thick board are actually better than the ¾” inch thick marine plywood sole, which is roughly 35lbs per cubic foot. It was also available in ¾” inch thickness, but would have been half as heavy again and more expensive. So I decided to compromise between weight, density, and price, for the ½” inch board.
One problem to be aware of with these types of manufactured boards is their susceptibility to deterioration in damp conditions. If there is a chance they might become wet it would be better to use marine plywood, but this is much more expensive.
The store assistant cut these heavy sheets to the sizes I needed with their vertical circular saw. This saved me having to manhandle them and enabled them to fit in my vehicle. I had them cut ½” inch smaller than the spaces between the individual beams to prevent them jamming when I needed to lift them out to gain access. A few boards still needed trimming to fit round obstructions which I could not reposition, but that was easy with my jig saw.
The simplest, time honored method to handle boards covering apertures is to cut a hole in the board big enough to get a couple of fingers through to lift it in and out. But these MDF boards were too big and heavy for that, and it would also have allowed a little bit more noise and heat to escape. I therefore drilled 3/8” inch holes in each board and threaded some 3/8” inch diameter rope through, knotting it on the underneath to form simple handles to easily lift the boards in and out.
The combined weight of all the fiberboards was 60Lbs but they are all positioned low down in the hull and a small price to pay for reducing the noise. When lying between the beams their weight also keeps them firmly in place. The floorboards now had a combined thickness of 1¼” inches with a density of about 80lbs per cubic foot.
There was yet one more thing I wanted to do to complete this project:
The section around the Perkins engine was particularly awkward because parts of the top of the engine were higher than the bottom of the beams. In fact the valve cover was only 1” below the actual floorboards. This was of course the principal source of all the noise, so it needed special attention anyway. I fitted battens all round the engine like all the other openings, then shaped pieces of fiberboard to fit round the engine. Next I cut pieces of foam and fiberboard to the size of the aperture and pressed the foam down over the engine with the fiberboard on top by actually standing on them. This indented the soft foam with an exact pattern of the high points of the engine, which I then cut out of the foam with a sharp blade. After managing to scoop out all the high areas I glued the foam to the fiberboard, which then fitted snugly under the floorboard. This gave an added layer of fiberboard over the motor, plus some foam insulation.
We could often feel heat permeating through the single thickness cabin sole when either of the diesel engines had been running a long time, especially on our own soles when walking barefoot. There must have been some sort of insulation glued to the underneath of the floorboards at one time, but this had disintegrated long ago. What was now left was a dirty layer of dry adhesive which had to be scrapped off by hand using Goof-Off paint stripper and a sharp 1½” inch chisel. I then painted the underside of the floorboards white.
I decided to add a layer of thermal insulation in the space between the boards, to try and reduce the heat coming through. I bought two 4’ foot by 8’ foot sheets of Rmax Thermasheath R6 foam insulation board These are 2” inch thick with aluminum foil on one face and an insulation rating of R6, which is the highest available for this thickness of foam. I cut them to the different sizes myself at the store using a sharp knife, to enable them to fit in my car. I then glued them to the underneath of each plywood floorboard using cheap construction adhesive by Liquid Nails which does not melt the foam.
The remainder of the floor now had the ¾” inch plywood floorboards, with 2” inches of foam glued underneath, then a ½” inch air gap, then the ½” inch MDF false floor. It was now certainly a compact floor.
After all this back-aching work I was naturally keen to take new readings on the decibel meter. With only the main engine running with the same revs’ this recorded 65dB, a reduction of 20 decibels! Adding the generator raised this to 70dB, 23 decibels less than before and now about equivalent to an electric sewing machine. This reduction may not sound much, (forgive the pun), but decibel ratings are logarithmic, so the noise reduction is very noticeable. Now we can comfortably listen to the TV or music at anchor, even with the genny’ running.
In addition to a considerable reduction in noise, there is now no perceptible heat coming through the floorboards, which helps to keep the living areas cooler. Heat is all carried outside by the engine room extractor fans, the noise from which is also much reduced.
Most projects I have undertaken on Britannia resulted in visible improvements, most notably when I renovated the actual teak and holly floorboards. But this noise and heat abatement project showed no outward improvements and the cabin looked exactly the same as before I started the job. It was only when the engines were running that the improvement was really appreciated.
This method of sound insulation would be very worthwhile for any boat, offering excellent noise reduction for minimal financial outlay. I actually used some spare pieces of MDF to double the wall thickness in the spaces where my two air conditioning units were installed, and this reduced the noise of the compressor and fan.