Ship noise reduction techniques - why surface ship can’t be made as quiet as submarine?.

[T. A. Gardner]

On ship's machinery... I know, for a fact, that the USN does test for and have standards for electrical motor noise. This mostly comes from the bearings on one. I've seen at least one case of a rebuilt motor failing noise testing repeatedly for excess bearing noise until--

Digressing a moment. I was in shop 10A, Ship's Superintendent's office at SIMA San Diego at the time. The motor went on one of the ships I had the "book" on (the list of all outstanding work in the shops ashore to that ship). 51B Inside Electric and 92A a test shop that did the testing on electric motors, had this motor repeatedly fail. This was a big motor, something in the 20 to 40 HP range.

--anyway, until I showed up for one of the tests because I had to get the job moving out of the shop and back on the ship. The problem was the rotor was wound in this motor and not just a solid mass. As soon as I saw that, I suggested they dynamically balance the rotor, something they hadn't done. This solved the problem. The rewind of the rotor had left it slightly out of balance and that vibration was enough to cause noise in the bearings.

 

[Nicknick]

The sounddsmping of mashinery noises on ships should be easier as in submarines. Ships tend to have more space (especially catermerans (like SWATH ships) which allow better damping. A highly dampend coupling to the hull with the mashinery in the open air would keep most of the noise out of the water and in the air.

So, waterjets can potentially reduce the noise compared to a screw by a factor of 10 to 100. The under water noise of the mashinery could be reduced as well. The open question is, how much does the SWTH design help in reducing the noise of the interaction with the water surface.

 

[Scott Kenny]

The sounddsmping of mashinery noises on ships should be easier as in submarines. Ships tend to have more space (especially catermerans (like SWATH ships) which allow better damping. A highly dampend coupling to the hull with the mashinery in the open air would keep most of the noise out of the water and in the air.

So, waterjets can potentially reduce the noise compared to a screw by a factor of 10 to 100. The under water noise of the mashinery could be reduced as well. The open question is, how much does the SWTH design help in reducing the noise of the interaction with the water surface.

Catamarans have LESS space down in the hulls than submarines.

 

[Nicknick]

But more space in.the open air. It wouldn't make sense to put the engines and pumps under the waterline.

 

[Yellow Palace]

On ship's machinery... I know, for a fact, that the USN does test for and have standards for electrical motor noise. This mostly comes from the bearings on one.

Naturally – anybody who worries about acoustic signature will do. You don't have noise standards for something that isn't noisy.

Incidentally, that includes some commercial applications. Seismic survey ships handle towed acoustic sensors that are plenty sensitive to self-noise, and in some cases even carry out RAS to avoid the difficulty of bringing in and paying back out their arrays.

Ships tend to have more space (especially catermerans (like SWATH ships) which allow better damping

Multihulls, and especially SWATH vessels, are notorious for a lack of usable hull volume. They don't put machinery in the superstructure because they can, they do it because they have to.

Placing machinery above the waterline is not innovative, the Royal Navy did that with the Type 23 propulsion diesel generators in the 1980s.

 

[Nicknick]

Never said this would be innovative, outboarders are around since a long time.... In the GDR, many older river barges were motorized by "Z-Antrieb" which looked a bit like outboarders, but with the engines stationary under a hood at the rear of the ship.

 

[tychosis]

The literature I have to hand points out that below cavitation inception speed, the dominant acoustic signature is discrete LF tones related to machinery (prime movers, gearboxes etc.), which are independent of the propulsor choice. Above cavitation inception speed, the ship is going to be making an almighty racket whatever you do, and also probably can't hear anything anyway.

Interesting. While that's largely accurate for submarine noise, it isn't entirely accurate for surface ships.

Circumferential wake variations from props passing through the turbulent wake a skimmer generates results in modulated broadband at the blade rate (and is the major reason you can get a blade rate/PSR count well below cavitation inception.)

 

[Yellow Palace]

Circumferential wake variations from props passing through the turbulent wake a skimmer generates results in modulated broadband at the blade rate (and is the major reason you can get a blade rate/PSR count well below cavitation inception.)

Surely that's a function of screws, rather than surface ships – AIUI similar interactions occur between the fin wake and the screw on submarines not fitted with pumpjets.

 

[tychosis]

Surely that's a function of screws, rather than surface ships – AIUI similar interactions occur between the fin wake and the screw on submarines not fitted with pumpjets.

I'm not entirely certain what you mean... yeah, it's absolutely something we don't want and why we try to avoid any turbulent or uneven flow at the screw. These small variances in thrust are transmitted through the shaft to the hull and ultimately into the water.

But any propeller moving through a non-uniform inflow will behave this way, and a non-uniform inflow is largely unavoidable on a surface vessel.