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Thread: Quiet Hovercraft Project

  1. #1
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    Default Quiet Hovercraft Project

    I’m proud to announce that after 40 years of noise research I have finally built a hovercraft which is radically quieter than any other of the over 150 craft I have measured worldwide; without compromising the craft’s performance.


    For the research work we have rebuilt my 15year old F25 craft, originally built as a junior racer/cruiser for my son Simon and affectionately known as ‘the Heap’. I’ve now renamed it the Quiet One – Q1. The craft is a conventional single fan, single engine, integrated craft with a 35 bhp engine.


    Measuring the craft static at full power at7 points in a 25 metre circle around the craft it averages 70dbA at 25 metres, which compares favourably with similar F35 craft which typically average 83dbA on the same test. This is the test specified by the WHF for Endurance Racing. The last time I adjudicated on some ER racing the quietest craft scored 79dba on this test. The reduction of 13dbA is dramatic – if you go to www.raceresults.info/noise/Qsigm1b.wav you will hear a before and after comparison, but you may need some decent speakers to get the full effect! Airbus and Boeing talk of an aircraft’s noisefootprint - the ground area around an aircraft that receives a certain noise level. Our reduction of 13dba represents a 95% reduction in footprint.



    The difference between a loud F1 racing craft (typically 96 dbA at 25 metres) and a racing F35 (typically 83dbA) is 13 dbA – Q1 is another 13dbA quieter than that racing F35.



    Uniquely the craft will comfortably meet the EEC directive for personal watercraft (jetskis). That calls for the maximum level of 72dbA (for a craft under 53bhp) at 25metres when a craft is driven by at full power. On this test the craft achieved 66dba when flying by from right to left of the meter and 68dba on left to right, - an average of 67dba.



    The craft is a specifically designed to cruise at low noise/ low speed in sensitive areas. It will maintain hump speed at 8 knots in calm conditions at just 2200rpm emitting a noise level of around 62dbA at 25 metres. Driving it is a new experience. It has all the amphibious performance etc of a normal craft but is simply quiet. International visitors who’ve seen it have commented that it’s drowned out by traffic noise on a nearby road. We’ve also had one bored test pilot have a lengthy normal mobile phone call whilst sitting in a tethered static craft with the engine on full power.



    Several HCGB members including myself have written articles over the last 40 years arguing that the key to quietness was the use of good silencers, low fan tip speeds and clean airflow but the challenge has always been how to apply these concepts. The advent of wider chord fans has now enabled lower tip speeds, and the work I did a few years ago with my NoiseCam system, which produced pictures of where specific noise sources were, has shown where the worst airflow obstructions are in front of and behind the fan. My test craft Q1 was rebuilt to take on board these lessons and then to research further various technical features of what did and didn’t create noise.



    I think this development opens a new era for cruising and other recreational hovercraft. Such hovercraft are already very environmentally friendly,because they cause minimal damage to the surface they are hovering over, and have the low fuel consumption and pollution of a 4 stroke engine. They now also have much quieter noise levels. I would like to see the HCGB, and in the longer term the WHF, introduce a certification scheme, that tests a craft and awards a Q (for Quiet) certificate to a craft which meets the EEC standards for personal watercraft. Although the EEC directive specifically excludes air cushion vehicles I think it would be a powerful lever with harbourmasters and authorities worldwide that we have voluntarily met that stringent test and can prove it.



    This development is less relevant for racing craft. The racing limit is 96dbA at 25 metres, and most racing craft are in the range 85-95dbA. Their use of high revving 2 stroke engines, high fan tip speeds, and general emphasis on peak performance make dramatic noise reduction on this scale challenging. However there are some noise reduction tweaks that may reduce racing levels a bit and offer performance benefits.



    This work was conducted over the past year in an intensive R&D program with Flying Fish (now the British HovercraftCompany). The findings in turn have now been incorporated into a redesigned British Hovercraft Co Marlin Q prototype, which is currently being tested. At present this test craft, though quiet, is not as exceptionally quiet as Q1. We are working through to identify the cause. It’s a fundamental of any scientific research that you must be able to repeat the results of any experiment before you can claim to have fully understood your theories so clearly we haven’t reached that stage yet! As a result I won’t fully publish the recipe for a quiet craft yet, I don’t want to lead others down the wrong path.



    However once we do have a clear recipe I will be keen to work with other HCGB members, including other manufacturers, who wish to produce their own quiet craft. My work with the British Hovercraft Co, as with other manufacturers and members in the past, has always been on a ‘Pro Bono’ basis so I will be publishing details of my research over the coming months. Probably as a series of HCGB magazine articles. It’s important to note that although I’ve already published for many years the general principles behind a quieter craft actually applying them to a specific design type is challenging and requires a lot of iterative development work. There is no ‘quick fix’- I’ve had to find solutions in at least 12 different areas. Tests show that fixing just one or two areas, such as a new silencer or different fan, may give some improvement but not a dramatic overall reduction,- you have to fix all the noise sources. Although I’ve now shown a way to quieten an integrated craft successfully quietening other design types, eg a twin engine or shaft drive craft, or one using an unducted propeller, will require more work. Right now I don’t know all the answers, but I do have technology to help, if members want my support.



    I’m afraid I don’t frequent Facebook because the level of ‘Trollism’ there isn’t helpful for my blood pressure but I’m happy to answer questions via the HCGB Bulletin Board or the private message system there. Whilst thinking of the dark side I’m sure there will be flat earthers who believe this is all untrue,that I’ve achieved the noise reduction using a volume control on the mic or something! I agree that would be the easy way but I could have done that in 40 secs, it wouldn’t have taken 40 years. Various independent folk from around the world have driven the Q1 test craft and I’m sure will vouch that it’s real.



    All hovercraft are an evolution of ones that have gone before. All integrated craft, ours included, are derived from Nigel Beales Simple Cyclone in the mid 70s. We’re using the rear drive fan approach I first used on Spirit of Snodland Mk3 for noise reasons in 1984. We’re using an A shaped splitter as used by Kip McCullom in the mid 80’s because it cuts down on lateral eddies. We’re using rear flow ducting as pioneered by Owen Ellis many years ago because it smooths airflow. I’m particularly grateful to Owen for the help and support he’s given the project. We’ve also included many quite subtle novel new features including one derived from Humpback Whales.



    Some folk will also argue that their craft has lower noise levels at cruise power. That’s absolutely true, Q1 for example is 8dba quieter at 2200rpm minimum cruise power than at 3600 full power. But for statutory purposes the WHF sets the limits at full power, as does the EU directive for jetskis. Full power at a fixed distance (25metres) is the only measurement thats repeatable thus allowing comparison between different craft and allowing a craft to be tested against a fixed standard. Cruise power is a very useful indicator of what it might be in typical use but different folk have different ideas of what rpm it is, revs may vary with load/weather/surface conditions and anyway theres always situations where someone will use full power, eg getting up a slipway or to get over hump.Some folk might also go flat out just for the hell of it.



    More info to come as the R&D program cranks on – in the meantime I’m happy to answer your questions on this HCGB Bulletin Board - I’m also starting some posts on the members only HCGB Cruising Talk Forum with more detailed information and I’ll be happy to try to answer some of your questions there.

    Keith Oakley
    Last edited by Keith Oakley; 11-Apr-15 at 01:45 PM.

  2. #2
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    Default Re: Quiet Hovercraft Project

    Hi Keith

    Nice to see all of your hard work is paying off!

    Where abouts are you gaining the most significant sound loss in your craft? Intake, fan, exhaust or is it a combination of them all and or diverting air in the plenum?

    Kevin Eastwood
    Last edited by Scuba Kev; 11-Apr-15 at 08:23 PM.

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    Default Re: Quiet Hovercraft Project

    Hi Kevin

    It's all of them, roughly 12 or more areas need attention. Assuming the craft is reasonably quiet in the first place if you fix just 1 or 2 unfortunately you get very little overall reduction.

    There are plenty of cases however where individual craft are very noisy for specific reasons - the most common for example is a poor exhaust silencer. Ricky Goosey's larger silencer a few years ago reduced many F3 craft from around 95dbA to around 87dbA

    Keith

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    Default Re: Quiet Hovercraft Project

    I seem to recall a few years ago when my storm with the polaris went into your noise-cam the most significant noise came from the intake on the carbs whether it was just air going in or maybe a vibration coming from the carb boots I don't know but definitely a noise hot spot..
    Would be good to see the noise-cam come out again sometime to do some testing of some of the newer craft and see what can be done to not only reduce noise while racing but to see what can be done to improve performance.
    Last edited by Scuba Kev; 11-Apr-15 at 08:23 PM.

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    Default Re: Quiet Hovercraft Project

    Intake noise on an engine is one of the significant sources. As the engine sucks in air it does so in pulses which create noise in a similar way to the way exhaust gas pulses come out of the engine. To get really low you need an air intake silencer (an airbox) but this often affects the breathing of an engine so many racers leave them off. One of the best airboxes is on the TZR250 F3 engine. I believe the TZR engine doesn't run well without them on so folk leave them on hence reducing intake noise, leaving just exhaust noise as a prominent source on that particular engine. Fix that, as Ricky did with his larger exhaust silencer, and you can get a useful reduction. But still not down into the very quiet levels of Q1, that seems to need many other things sorted.

    I'd like to get noisecam dusted off and back out on the road again as you suggest but in the last few years between maintaining ELS and providing livestream tv at race meetings I've been too busy. I need to work out my priorities! The final version of Noisecam (4D = 3D +time) proved capable of finding hithertoo unsuspected turbulence generated noise within the plenum which is exactly the area many racers (including Ricky again!) are exploring as a means of improving performance. That version has only been used in my garden however, its not field portable.

    Keith

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    Default Re: Quiet Hovercraft Project

    Well done Keith.


    Finally looking like hovercraft noise nuisance will be a thing of the past.


    I'm quite happy to use my TS3 for twin engine / twin fan noise reduction tests. It looks like I will be having to build another thrust fan frame & replace the thrust fan after the frame failed last week and chucked the belt into the fan. I'm not a happy chappie, but happy that I hadn't sold it to someone and this happened. I have found the causes of the frame failure so alterations to rectify them are under way.


    After a recent exhaust upgrade on the lift engine, made my craft more pleasurable to fly but it didn't lower the dB, only altered the pitch of the noise to a less irritating pitch.

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    Default Re: Quiet Hovercraft Project

    Thanks Alan, I appreciate your support.

    I'm glad you've kept the TS3, it is potentially a good starting point from which to develop a quiet twin engined craft and I'll be happy to work with you on that. I've sent you an email with some thoughts.

    Changing the lift silencer on your craft has knocked off some of the annoying high frequency engine harmonics but as you say it hasn't reduced the overall dbA level. That illustrates nicely my point that achieving a really quiet craft needs an assault on several sources, it's rare that theres only a single 'quick fix'.

    Keith

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    Default Re: Quiet Hovercraft Project

    Fantastic work and a good step in the right direction in our current ever changing world to reduce pollution, noise and fuel!
    It may be worth a mention that the air conditioning industry has for years been carring out extensive research in the same field, and as you are aware blade and air movement noise are also an issue. There research has developed some great change on the fan pitch and fan blade design but this is only any good using a fixed fan speed. The other large problem is these efficient fan sets are useless at high torques - they fail too easy! I did a few tests and research a few years ago on the designs then, but my intrest was for racing, this may be adapted better for the constant hum for cruising craft. One thought that I would love to try is for a centrifugal type fan set-up for lift engines on twin engined craft, they would provide a quieter, stronger and higher static lift, but be more susseptable to blockages from grass and debris, again, no good for racing craft. Altering full craft design is a way of adapting these methods but that is easier for larger craft, this won't be very practable on small craft. Food for thought...

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