The engine in my 1977 Down East 45 schooner, Britannia, is a tried and trusted — but noisy — Perkins 4-236, an 85-horsepower four-­cylinder diesel. 

I call the space the equipment bay. It runs 12 feet under the saloon floorboards and is 3 feet wide at the sole level, then tapers to just 15 inches at the bottom of the 41⁄2-foot-deep bilge. Seven removable floorboards give amazing access to all the equipment below, but the large space also acts as a massive boombox.

There are a number of products that claim to significantly reduce noise from machinery, and some are specifically designed for boats. The trouble with most of these is they are also specifically aimed at your bank balance! 

In simple terms, the object of sound insulation is to absorb noise at its 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 similar diesel on a previous boat simply by installing a false floor beneath the cabin sole. 

Before I started work on Britannia, I wanted to take a reading of the sound levels to have a numerical comparison after the modifications were complete. I downloaded a neat iPhone app, a decibel meter by Decibel Meter Pro, for the vast sum of 99 cents, from iTunes. It was very easy to use, and I took readings at head height in the center of the saloon. 

Fitting the False Floor

To get started, it was first necessary to make support battens for the false floor panels to lie in, under the existing plywood sole. I bought a 24-by-48-inch 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. 

I screwed the ¾-inch square battens to the sides of each aperture to support the ends of the false floors. I painted the beams and all the new timbers white.

The sound-deadening properties of a ½-inch-thick sheet are actually better than the ¾-inch-thick marine plywood sole, which is roughly 35 pounds per cubic foot. (The MDF sheets were also available in ¾-inch thickness but would have been heavier and more expensive. In the end, I decided to compromise between weight, density and price, and go for the thinner stock.)

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.

The weight of the new fiberboards was 60 pounds, but it’s all positioned low in the hull, and it was a small price to pay for reducing the noise. When lying between the beams, their weight also keeps them firmly in place. The sole and subfloor now has a combined thickness of 1¼ inches, with a density of about 80 pounds per cubic foot.

Beat the Heat

To complete the project, there was one more thing I wanted to do. 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 bought two 4-by-8-foot sheets of Rmax Thermasheath R6 foam-board insulation from Lowe’s for $21.98 each. These are 2 inches 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 sizes I needed at the store using a sharp knife, which helped me fit them in my car. 

The section of floor around the Perkins engine was particularly awkward because parts of the top of the engine were higher than the bottom of the floor beams. In fact, the valve cover was only an inch below the sole. This was, of course, the principal source of all the noise, so it needed special attention anyway.

I fitted battens all around the engine as I had in all the other openings, then shaped pieces of fiberboard to fit around the engine as well.

The remainder of the floor now had the ¾-inch plywood sole pieces, 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 backbreaking work, I was naturally keen to take new readings on the decibel meter. With only the main engine running at the same revolutions per minute as before, my iPhone app meter read 65, a reduction of 20 db! 

In addition to a considerable reduction in noise, there is now no perceptible heat coming through the floorboards, which helps to keep the living area cooler. Heat is carried outside by the engine-room extractor fans, and the noise from them is much reduced too.

Most projects I have undertaken on Britannia resulted in visible improvements, most notably when I renovated the teak-and-holly sole. 

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 as well.

There are, of course, no labor charges factored into the cost of the job, which took me four days to complete, but messing about on boats is supposed to be fun.