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Old 05-07-2016, 01:58 PM   #10
Nomad Willy
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City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 15,161
That's it.
The smaller they are the less noise ... but that's assuming all other things are equal and they never are.
Most of the noise comes from things like a bulkhead being like a drum. The engine shakes the hull and everything on the boat shakes. Many things on a boat will be excited (vibration wise) and shake the air and everything attached to it.

Low frequency vibrations from big and low rpm engines may be better at shaking big things like bulkheads. More noise. Noise is made up of vibration in the air consisting on intensity (as in amplitude) and frequeny or frequencies. The bigger the moving engine parts the bigger the amplitude. And the fewer the # of cylinders in the engine the lower the frequency. Think big 4 cyl engines. Worse yet a big 4cyl at idle. One could say low frequency noise is more pleasing and not so bothersome .. but have you been next to a boom box at a stoplight lately?

My engine makes a lot of noise but unlike a PU truck the engine is mostly sealed in it's compartment belowdecks and w some sound insulation it's not objectionable below 2500rpm, and we cruise at 2300. Also the engine is small (40hp) so noise wise life aboard Willy is rather pleasant .... for diesel power. The prechamber type of combustion chamber also helps.

North Western Washington State USA
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