To A New Paradigm With FPB

Exhaust Noise

We said we’d report on how the revised exhaust system was working out after some time living with the change. We’ve put 1200 miles on Wind Horse since making the modifications, but before telling you what we think, we need to review what has been done. We had a very quiet propulsion system before we started this fine tuning. Wind Horse under power is quieter than Beowulf was under sail. But still, after 2000 hours of engine time, you forget how quiet things are in an absolute sense (57 dB as measured by PassageMaker magazine) and start to concentrate on what you are used to in the present. Machinery noise has always been muted, but exhaust noise, coming from the outside into the saloon through our 3/4″ (19mm) thick windows, was annoying.
dry exhaust to aqualift muffler
We started out with a straight exhaust – no muffler or aqualift – to see if we could get away with it. Obviously with 57 dB as our cruise sound level we could, but there were still improvements to be made. Our exhaust is dry to the deck, and then an injection elbow is added to mix cooling water into the hot exhaust, the combination then running through hose to the stern. We added an aqualift (the black “barrel” to the left of the photo) to improve the mixing of gas and water, and provide noise reduction. Note the top of the exhaust pipe. This is hung from the deck with line connected to rubber mounts. This is to isolate noise and vibration between the exhaust pipe and the deck. The aqualift is connected with just the silicone bellows and a tie point – again to isolate noise and vibration.
aqua lift muffler
The injection elbow is inside the insulation just above the silicone bellows. You can see the small diameter corrugated black hose to the right, which has the raw salt water flow coming from the engine cooling pump. We then ran a 5-inch (125mm) hose from the bottom of the aqualift to the stern. This replaced one of 4 inches (100mm) to help offset the restriction of the aqualift. Towards the stern, along the exhaust hose, you will see a piece of 1/2″ (12mm) line for support. Once again this is to isolate exhaust noise from structure (these ropes will be later replaced with something more professional looking). Result? Significantly lower noise levels. The engines now have just a soft purr. The muscular exhaust noise is gone. While that deep-throated rumble would have excited us when we were teenagers (that’s why “cut outs” were invented for hot rods!) today we prefer a more sedate sound (except for rock and roll and blues – which we still like loud). We mentioned earlier that our exhaust gas temperature appeared to be lower during sea trials and this is indeed the case, indicating an overall improvement in efficiency. Maximum RPM at wide open throttle is also up. “Why not just go with the aqualift in the first place?” you might be asking. We did not want to have a conventional aqualift set up as these are at risk to back flooding if an engine is shut down under way. However, with our hanging aqualift, and its ability to drain automatically, this shouldn’t be a problem.

Posted by Steve Dashew  (May 19, 2007)




2 Responses to “Exhaust Noise”

  1. Christian Says:
    Hi, 57db? this is under power? and measured from where and with what? ( make / model of sound meter) I am in the middle of a new engine install on a Four Winns 268 (sorry a power boat but using WVO or biodiesel) and I want to see if I can match or better that 57 number. Fascinating! Your engine room insulation ? and I wonder why you didn’t use a hospital grade muffler after the aqualift. Christian

    [Reply]

    Steve Dashew Reply:

    Yes the 57dB is under power (see Bill Parlatorre’s article in PassageMaker – there is a link on the SetSail press page – for measurement details. You will find lots of references amoungst the posts for how we insulate the engine rooms. The exhaust system is wet, so a steel muffler would not last very long.

    [Reply]



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