I had previously restored the teak and holly sole to its original beauty throughout the whole boat, but this had made no difference to the noise level from the machinery below. When both diesels and extractor blowers were running it was quite noisy in the saloon, with a dull drumming from the engine you could almost touch.

Britannia’s engine is the tried and trusted, but noisy, Perkins 4-236 85hp four cylinder diesel. In the same area is also a Kubota three cylinder diesel generator, along with five electric pumps and two bilge air blowers, all under the same floorboards - or to use the correct nautical term the cabin sole. I call it the equipment bay and it runs all the way under the saloon floor where it is 4 1/2’ feet deep, and acts as a massive boom box. So I decided to do something about it.

There are a lot of products that claim to significantly reduce noise from machinery, and some that are specifically aimed at boat owners. The trouble with many of these is they are also specifically aimed at boat owners wallets! I found noise reduction material from $350 for simple 3/4” inch foam, to $1600 plus, for double thickness sound insulation sandwiches.

This is a neat decibel meter for an I-phone which gave relative reading. In simple terms, the object is to absorb the sound waves from the noise source and thereby minimize what filters into the interior of the boat. I had managed to effectively reduce the engine noise from a Perkins 4-236 on a previous boat, simply by installing a false floor beneath the cabin sole. This is quite an easy and inexpensive do-it-yourself way to achieve a significant sound reduction.

Before I started I decided to take a reading of the sound levels, to have a numerical comparison after the modifications were complete. I downloaded an app’ into my i-phone of a neat little decibel meter by DB Meter Pro, for the vast sum of $0.99 from the iTunes app store. It is very easy to use and I took readings at head height in the center of the saloon. The first was with the main engine running at cruising revs that registered 85 decibels. Then I started the generator as well, along with the twin extractor fans. The level went up to 93 decibels and when the fresh water pump was activated it added another few decibels. I don't know how accurate these readings actually are, but what I wanted was a comparison between before and after the modifications that I planned

                                                                                     FITTING THE FALSE FLOOR

It was first necessary to make supports for the false floor panels to lay in under the existing plywood sole. I bought a 24” inch by 48” sheet of 1/2” inch plywood and cut it into 4” inch wide strips with my table saw, and also made 3/4” inch square battens out of hardwood. It was also necessary to reposition some pieces of equipment that were above the level of  the floor beams, like wire hangers, water pipes and the big main engine filter. All had to be lowered below the floor beams 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 battens to the sides of each aperture to support the ends of the false floors. This framework then supported the boards all round and I painted the bearers and new timbers white.
 
Sound deadening the complete floor made a big difference in the saloon. I found some Medium Density Fiberboard (MDF), and I calculated I could cut all the false floorboards 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. It has a density of 44 lbs per cubic foot and is used to make stereo speaker boxes and other things where sound control is required. The sound deadening properties of this 1/2” inch thick board are actually better than the 3/4” inch thick marine plywood sole that is roughly 35 lbs per cubic foot. One problem with these types of manufactured boards however is their susceptibility to deterioration in damp conditions, and if there is a chance they might become wet it would be better to use marine plywood. The store assistant cut these heavy sheets to the sizes I needed with their vertical circular saw that enabled them to fit in my vehicle. I had them cut 1/2” 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 that 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. Instead I drilled two 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 60 lbs but they are all 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 4” inches with a density of about 80 lbs per cubic foot. There was yet one more thing I wanted to do to complete this project

                                                                                        HEAT 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.

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 that 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 had painted the underside of the floorboards white when I restored the cabin sole, so I now glued the insulation to the underneath of each  floorboard using construction adhesive by Liquid Nails that does not melt the foam

The engine is enclosed with a board, then the floorboard.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” inch 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 a piece of fiberboard to the size of the whole aperture and pressed 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, that 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, that then fitted snugly under the floorboard. This gave two layers of fiberboard over the motor, plus foam insulation.

The whole floor now had the 3/4” inch plywood floorboards, with 2” inches of foam glued to them, then a 1/2” inch air gap, then the 1/2” 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 65 dB, a reduction of 20 decibels! Adding the generator raised this to 70 dB, 23 decibels less than before and now about equivalent to an electric sewing machine. This reduction may not sound much, (excuse 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 that 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, 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 cost. I actually used some spare pieces of MDF to double the wall thickness in the spaces where my air conditioning units were installed, that reduced the noise of the compressor and fan.

 

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This is the equipment -bay in which all the boats machiners is housed.