Vibroacoustics 5

Reduced noise and increased comfort on board can also be obtained when refitting a boat, without needing overly invasive work

by Gian Piero Repetti

How many times have I climbed on board a boat and heard the owner say, disconsolately: “If only I had bought a sailing boat…”. 
As owners are always reminding us, their boats are their second home, and they expect them to be comfortable and quiet places where they can enjoy themselves without renouncing any of their home comforts.

Let us assume that every boat has its own charm, and that each owner chooses their boat because it meets their expectations and needs. So if they chose a motor boat, they did well – it was the right boat for them!

Our job sometimes involves using data and measurements to figure out what is keeping owners up at night and finding solutions that do not disturb the boat (or the owner!) but that improve the experience on the water, ensuring it meet expectations.

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Not a sticking plaster solution involving a certain type of insulation panel or make of motor, which does not get to the heart of the issue, but a carefully thought through, non-invasive refit. As promised in the last issue, we are now back on board, accelerometer and sound level meter in hand, going from cabin to cabin, into the saloon and onto the sundeck, the places where boat owners like to spend their time, and enjoy themselves.

Better shaking than shaken. The measurements are taken both in the port and on the move, and repeated at various speeds, or rather at different engine speeds, as the engine – always both a blessing and a curse – is the principal cause of sound issues. And here are the results: a table of numbers showing each source of noise (in dB) which, when combined in each room, whether the saloon or the owner’s cabin, create the total sound level (the noise we hear). Let’s study a real-life situation I faced.

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Look at the first column in the table, which refers to the noise in the saloon (Tab. 1). The highest value undoubtedly comes from the gear box, transmitted through the boat’s structures.

To reduce the total level of 66 dB (A), the noise experienced inside a car 20 years ago at 75 mph (!), we have to work predominantly on the gearbox, on the cardan joint and on the main engine. But how? The simplest way is to insulate the propulsion system – the engine and gearbox – from the structure to which it is connected, not forgetting that the softer a material is, the better the insulation it provides.

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In the system we’re looking at, the gearbox supports the thrust of the propeller, so it is placed on semi-rigid mounts that do not provide adequate insulation for the frequency components that stem from the meshing of the gears. The noise from the drive shaft can pass freely along the same route, or via the main engine mounts. If we remove the semi-rigid mounts from the gearbox and replace them with soft mounts, we have to add a thrust bearing and a shaft line coupling before the gearbox.

The Cardan joint, meanwhile, a common source of second-order vibration, could be removed by implementing a perfect constant-velocity system (an elastic coupling with an extension, for example). But don’t forget our overall aim – you can achieve anything on paper, but on board you have to compromise, and deal with the size and capacity limitations of a pre-existing, pre-built boat. We worked to reduce the sound transmitted through structures and through the air in various ways, bearing in mind how much each individual aspect affects the overall result (insulating the engine room, the exhaust pipe mounts, the underwater exhaust, the fans and so on).

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This is the finished result: a new series of measurements taken on board after the alterations to the system, with new parts, a shaft line coupling and thrust bearing and soft engine mounts all added (Tab. 2).

It’s like a new boat (and the owner will feel like a new person!) A theory-driven design based on measurements and products that offer excellent insulation, combined with the feasibility of the installation and the changes on board, means the owner can finally enjoy the peace and quiet and comfort they always dreamed of.

(Vibroacoustics 5 – – October 2020)