I had previously restored the teak and holly sole to its original beauty throughout the whole boat, (Cruising World April 2017), but this had made no difference to the noise level from the machinery below. When both diesels and extractor blowers were running it was very noisy in the saloon, with a dull drumming you could almost touch. So I decided to do something about it. Britannia’s engine is the tried and trusted — but noisy, Perkins 4-236 85hp four cylinder diesel. There is also a Kubota three cylinder diesel generator, along with five electric pumps and two bilge AIR blowers, all under the floorboards — or to use the correct nautical term, the cabin sole.
I call it the equipment bay and it runs 12’ feet under the saloon floor. The space is 3’ feet wide at the floorboards and tapering 4½’ feet deep down to the bilge, where it is 15” inches wide. The seven removable floorboards give amazing access to all the equipment below, but the large space also acts as a massive boom-box.
There are certainly a lot of products which claim to significantly reduce noise from machinery, and some which are specifically aimed at boaters. The trouble with most of these is, they are also specifically aimed at your bank balance! I found prices for my boat size from $350 for simple 3/4” foam, to $800 plus for double thickness sound insulation sandwiches.
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. It would be practically impossible to eliminate this altogether, but I had effectively reduced 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 wanted to take a reading of the sound levels, to have a numerical comparison after the modifications were complete. I downloaded into my i-phone an app’ of a neat little decibel meter by DB Meter Pro, for the vast sum of $0.99 from 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, which registered 85 decibels. Then I started the generator as well, along with the twin extractor fans. The level went up to 93 decibels, which is roughly equivalent to a power lawn mower. When the fresh water pump was activated it added another few decibels. I don’t know how accurate these readings actually are, but it doesn’t really matter, because what I wanted was a comparison, between before and after the modifications.
FITTING THE FALSE FLOOR
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 1/2” inch plywood and cut it into 4” inch wide strips with my table saw. I also made 3/4” 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 3/4” 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 sheets1/2” 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 1/2” 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 3/4” 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 1/2” 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 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 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