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PROPELLER GUARDS keep people and animals in the water from being gashed by propellers. While propeller guards have trade-offs, and boats can still hit cetaceans, propeller guards are a safety item which can help performance: "concentric ring vanes (looks like a bullseye target from the rear)... are said to improve handling at slower speeds, increase thrust, and slightly increase top speeds in some situations."


Many articles about propeller guards are at


Simple guards are widely sold for $100 to $500.


US Coast Guard proposals on propeller guards are reviewed at:


Jet boats use a water pump in the hull, with no propeller below water, reducing dangers of cutting large animals, and reducing noise underwater. Dangers remain for animals smaller than the water intake; they can be sucked through the pump.



ENGINE NOISE varies, based on engine size, propeller design, gears, whether exhaust (and noise) are discharged in air or, more often, underwater.


Basics has broad research on underwater noise.

The European Union has a Technical Group on reducing ocean noise


Ireland has a detailed survey of noise sources in the Atlantic west of Ireland 2015-2016. Passing ships were recognizable (p.62) on underwater hydrophones for 3-4 hours, at 108-112 dB re 1μPa. Three seismic vessels were present during monitoring, up to 150 km away, with sounds up to 145 dB re 1μPa2.s (SEL).



Ireland at the same time did aerial and bird surveys.


Vancouver’s Port has


Specific Boat Engines' Sound, Measured Underwater

A Finnish study (Seppänen and Nieminen 2004) compared noise from several boats underwater in a lake in Finland, in the summer of 2002.


A 100 HP outboard motor with exhaust underwater was louder underwater than a larger boat with diesel inboard engines.


The diesel boats did not put exhaust below water.

Jet boats use a water pump in the hull, with no propeller below water, and had less noise underwater than other boats tested.

Vessels on Lake

Sound Pressure Level in Decibels, Relative to

1 Micro-Pascal of Pressure, at 50-100 meters

Outboard Yamaha 100 HP, 4-Stroke


Slow Steel Boat, Diesel


Outboard Yamaha 15 HP


Outboard Johnson 55 HP


Outboard Johnson 20 HP


Fast Jetboat, Diesel



Specific Boat Engines' Sound, Measured in Air


Boating, a powerboat magazine, measured noise of a Canadian electric boat engine in 2012 in air, in a prototype boat,

Ski Nautique boat with LTS Marine engine, 206-hp maximum power, 79dB-A on the boat, or silent at idle

"Typically, our idle speed test numbers fall in the 60s [dBA, on non-electric boats], the same level as a casual conversation. As the boat climbs on plane, the engine noise revs up into the 70s. Then it climbs into the 80s at cruising speeds and into the 90s as it nears wide-open throttle; some noisy boats hit the 100s."


A consultant for an oyster company measured boat noises in air, in 2011, in California (p.36). (p.36 of pdf file)

Oyster skiff Powered by 20 HP, 4-Cycle Engine at 50 feet: 62 dBA Lmax, 58 dBA Leq

Oyster Skiff Powered by 40 HP, 4-Cycle Engine at 50 feet: 63 dBA Lmax, 60 dBA Leq


National Park Service overview


A widely-quoted 1995 study submitted to the New Jersey state police by the testing company Noise Unlimited, compared noise in air from several boats, apparently in 1995.

The Personal Watercraft Industry Association says personal watercraft became 70% quieter after 1998, and presumably some other engines are also quieter.

Type of Boat

dBA at 50 feet

Classic Inboard - 1930 Wooden ChrisCraft, Inboard Motor, Exhaust Outlet Above the Water Line 


Single Outboard Engine - Police Patrol Boat with single 175 hp Johnson Outboard Motor 


Twin Outboard Engine Police Patrol Boat with two 155 hp Evinrude Outboard Motors 


Personal Watercraft No. 1 - Kawasaki JetSki 750 cubic centimeter engine


Personal Watercraft No. 2 - Kawasaki 900 cubic centimeter 3 cylinder 100 hp, High Performance Model 


Inboard/Outboard with Exhaust Below Water Line - 350 cubic inch (ci) Chevrolet Engine with Outboard Drive


Inboard/Outboard with Exhaust Above Water Line - Wellcraft Nova 352 ci Engine with Outboard Drive


Racing Boat No. 1 - Runabout, 283 ci Chevrolet Engine Exhaust Below Water Line

105 at 77 mph

Racing Boat No. 2 - Hydroplane, 305 ci Chevrolet Engine 

109 at 107 mph

* A-Scale Sound level (dBA) measurements at a distance of 50 feet.

A-Scale approximates the sensitivity of the human ear and is used to note the intensity or annoyance level of sounds. Data from NUI Report No. 8077.1, New Jersey State Police-Marine Division. Nov. 1, 1995.


Overall Patterns of Noise


Boat speed is the strongest cause of noise, not boat size. Houghton et al. 2015 measured sound received by whales (with meters attached by suction cups), and characterized boats present (laser measurement).


Proposal for Quiet Zones, where there are many marine mammals and few noises.

Maps on right show where quiet zones could be established without much resistance, because there is not much noise there now. (Williams, Erbe, Ashe, Clark, 2015)


Reducing noise and whale fatalities in the St. Lawrence Estuary. Voluntary efforts by ships to avoid whale feeding areas and reduce speeds from 13 knots to 10 knots achieved a 36% or more reduction in the odds of whale deaths caused by ships, between 2013 and 2015.


Overall Effect of Boats in Orca Habitat.

US study (Veirs and Veirs 2007) measured sound in Orca habitat off Washington state from April 2004 to November 2005.

The quietest noise level (average for 30 minutes, 100-1,500 hz) was 95 dB, relative to 1 micro-pascal of pressure. They considered this "background" noise.

The loudest half hour averaged 130 dB. The difference of 35 dB means it was 56 times as loud.


"Large vessels like tankers, container carriers, ocean liners, fishing vessels, and tugs (hereafter "ships") pass in the shipping channels on the order of a kilometer from the hydrophones and create broad peaks in amplitude, typically lasting 15-60 minutes and having maximum amplitude ~20 dB above ambient" sound (10 times as loud as background, bold added).

"Each ship usually adds 20-25 dB [10-18 times as loud]...; some add nearly 30 dB [32 times] and a few add only 10 dB [3 times]"


"The relatively quiet periods between ships are usually less than 30 minutes, but sometimes last a few hours, particularly during the early morning hours."


"Small vessels like recreational power boats, whale watch boats, and sail boats under power (hereafter "boats") also add ~20 dB ... boats pass faster and closer to the hydrophones, typically creating a spike in amplitude that lasts a few minutes." (also 10 times as loud as background)


"boats usually add 15-20 dB. [6 to 10 times as loud as background]"


The graph of Sound Pressure Levels (SPL) shows that on summer days, noise is almost never under 110 dB [6 times background], and 60% of the time it is under 121 dB, so 40% of the summer days the sound is over 121 dB, 20 times background levels. In winter, 40% of the time sound is over 117 dB, 13 times background.

                                  SPL means "Sound Pressure Level" or Loudness

SPL means "Sound Pressure Level" or Loudness 


Other Links on Noise


A US boat builders' group summarizes the history of US noise limits and proposals in air, without considering underwater life.,_Status,_etc.doc


Erbe 2002 Noise levels  of orca-watching boats.


1,000 narwhals died trapped under ice in 2008, because they did not leave their summer area before it froze over. Researchers believe the reason they did not leave was a seismic survey with air guns on their migration route. The seismic survey had "four arrays of up to 37 airguns with a combined maximum volume of 4200 inch3 and a pressure of 2000 psi."


Scholarly articles on dolphins, whales and sounds