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Thread: Hovercraft Design

  1. #17
    John2
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    Default Re: Hovercraft Design

    Hello to all,

    I need to thank John for helping me get back on since there were some technical log-in difficulties. Thanks. Curtis 5420 did you get my pm? I'm not sure if it went through when I was logged off.


    Quote Originally Posted by Ian Brooks View Post
    Plough-in is perhaps the most serious vice of hovercraft in general - it resulted in the SRN6 overturning in 1972 with the loss of two lives. It persists in light hovercraft today, but is largely conquered in large commercial craft.

    March 1972, SRN6-012, 5 souls were lost. Combination of wind and sea capsized vessel due to pilot error. Investigation noted that the pilot chose to run on a beam reach near shore and the surf from beam on caused leeward skirt to tuck under in the trough causing craft to list beyond angle of vanishing stability. Very tragic and very sad. I do not have the full specifics, but that’s the way Yung and Bliault published it in their book and no further explanation is given for their direct cause of death. I’m willing to bet that those souls were probably not restrained and may have perished from severe trauma or drowned from being unconscious from severe trauma.

    Quote Originally Posted by Ian Brooks View Post
    1 - the craft is operating at high speed
    2 - the bow pitches down just a little for whatever reason
    3 - the bow skirt contacts the water, and then lays on the surface
    4 - wetting drag on the skirt overcomes the skirt tension and the skirt begins to get pulled under the craft (called 'tuck-under')
    5 - as the skirt tucks-under, the center of pressure moves backwards making the situation worse
    6 - the process continues until the hull hard structure contacts the water
    7 - at this point the situation is critical. The hull could plane, in which case there will be an uncomfortable period of heavy deceleration, or it could sub-marine in which is very serious and can lead to structural failure, capsize and/or ejection of pilot & passengers etc.
    Ian, you have an excellent memory sir.
    I know it’s like hijacking my own thread, and I’m sure that this issue has been belabored in this forum to death, but I think we should expand a little on this and I am hopeful that the moderator will not find that I violated any anti-hijacking rules.

    Here is my understanding thus far. Lets first make some assumptions and these should be viewed as generalities. Let’s assume that all the required engineering is in place, longitudinal and traverse stability, skirt, etc etc. Further, let’s assume that all operational procedures are in place, emergency, pilot training etc etc. Finally, let’s assume that the worst conditions for plough in are in action, opposing wind and current directions, tuck under etc etc. We now only have to deal with action and reaction, which are always, equal and opposite for the engineering until design envelopes limits are exceeded, delayed and inconsistent for the operational and finally, unpredictable and non-responsive for detrimental conditions.

    It is obvious, and as it has already been pointed out above as a series of events, that a progression takes place which increase in complexity leading to the “point of no return”. The specific and principle reasons for these events can be defined and measured. The bottom line is that any action that puts the craft beyond their angle of vanishing stability, whether a high thrust line or excessive wetted drag, is an action which will cause the crafts inertia to seek neutrality on the opposite side of the stability curve, which at worst is going turtle. This progression can be measured in time and upon closer observation, it will be shown that there are “signs” which if left unheeded, eventually lead to the next progressive step. The signs follow a pattern and recognizing these patterns is of utmost importance if plough in is to be eliminated, avoided, or if imminent, reduced in force for minimal effort for recovery. It would be a disservice to attempt to provide pure clarity on this issue in such a small piece of wavelength, so simple points will be made.

    Test results by Yung and Bliault show that it takes up to 9 seconds to go from normal travel through a recovery of trim from a plough in and, the plough in occurs at 4.5 seconds. That is a lot of time and there are 5 stages that have “signs”. Finger makes contact with water, light, moderate, serious and unstable tuck under take place and then, plough in begins.

    That is an awful lot for a trained professional pilot or prudent private skipper which are both charged with souls to just arbitrarily miss and not recognize. The signs for the progression needs to be identified and I suspect that the most important will manifest at the gap between skirt and surface because that is where it all begins when disturbed, in other words, when the finger touches the surface.


    Quote Originally Posted by Ian Brooks View Post
    Up until step 6 things aren't too bad and can be recovered by a skilled pilot operating a well-designed craft. In a poorly designed craft, the fate of the occupants is sealed at step 3, they are swimming. In the best craft, this phenomenon is tamed and used as a sort of brake. My craft has vents, designed to deliberately promote a plough-in when I want to slow down.

    Excellent observation and this is the reason that I always have one ear to the ground. Books are nice, but I’ll share something I believe to be true. “Not even a perfectly engineered bilge pump designed by the book can outdo the resolve demonstrated by a terrified skipper with a bailing bucket. This may not apply to full flotation structures, but you get the meaning. I will concede that I cannot prove otherwise, but I suspect that much can be done to prevent going for a swim before step 3 as you have indicated above if we look for the signs that establish patterns for progression and then counteracting them expeditiously.

    What craft do you have? Pictures of the vents or skirt? What must be done to plough in? Why would a need to plough in exist? Would it not be better to fishtail or do a 180 and make the velocity degradation tolerable?


    Quote Originally Posted by Ian Brooks View Post
    There are two main ways to do this - either the skirt is designed to prevent tuck-under and to move the center of pressure forward at 2) (SRN series bulbous bow skirts), or the skirt is compartmented such that the reduced hovergap leakage at step 2 causes the front compartment pressure to rise and oppose the pitch-down moment. This is how my craft does it - together with a hull designed to plane in the event of hull-water contact.

    This needs to be explored further. Have any pictures of the skirt? Are those compartments by design or did you incorporate the change yourself?

    Quote Originally Posted by Ian Brooks View Post
    Other considerations of skirt design would be water-skirt interactions. In theory, the wave-making drag is greatest at a Froude no. of about 0.56 when the bow sits on the peak and the stern sits in the trough of the wave, but in practise the worst drag often occurs at Fr=0.4, when the bow & stern sit on peaks, the mid-ship sits over the trough.

    Correct. That is exactly how I calculated for worst loads, simply supported, but, I have the craft upside down with full lift thrust pointing up. I wanted to make sure a capsize didn’t turn into one of those Star Trek moments where Scotty says “she’s breaking up Captain”.


    So, any suggestions yet on iteration 1,2 or 3 for further development? Comments? Increase LOA?

    Thanks
    J

  2. #18
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    Default Re: Hovercraft Design

    Does any one know the address of the Plain English campaign HQ??
    What is a velocity degradation for a start?

    Kip

  3. #19
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    Default Re: Hovercraft Design

    Quote Originally Posted by KipMac View Post
    Does any one know the address of the Plain English campaign HQ??
    What is a velocity degradation for a start?

    Kip
    Dont worry Kip. it was once explained to me as: get drunk, fall down, velocity degradation happens when you hit the ground!!

    John2 and Ian keep it going although I think you both need to get out there and get some more practical experience. EG some people on here have carried out capsize experiments in real life conditions.

    BTW Did Scotty ever say "She's breaking up Captain" I think it may have been one of the later engineers.

  4. #20
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    Default Re: Hovercraft Design

    To plough in at 76mph would destroy the plaining surface and the people in the craft. To drive an open cockpit craft at speeds above 50mph is not for the faint of heart. The conditions have to be spot on. Flat water into a 6 mph wind perfect. We do not get blessed with that sort of weather often here. I would be thinking along the lines of shedding as much needless weight from the craft. Toss the gull wing doors and decrease that skirt pressure. A compartment skirt would be a must. Still it looks good. Just never seen that many good looking hovercraft that work very well. Interesting thread i wish you all the best.

  5. #21
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    Default Re: Hovercraft Design

    From an Industrial Design aspect I like the twin thruster option. However I have no idea where your lift fan is. I also think that getting in and out of the craft would be helped with some flat decks.

    Here is a pretty hovercraft in my opinion, and it seems to be working for those Russian guys.

    http://www.seatech.ru/eng/ships/hove...ater/kater.htm

    The photo ID says: Kater


    http://www.seatech.ru/eng/ships/hove...ater/kater.htm



  6. #22
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    Default Re: Hovercraft Design

    The engine position, fan arrangement and skirt look very similar to another Russian craft I've seen.12-10-07_1536.jpg12-10-07_1535.jpg
    Last edited by Philip; 18-May-10 at 10:43 PM.

  7. #23
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    Default Re: Hovercraft Design

    Quote Originally Posted by Philip View Post
    The engine poition, fan arrangement and skirt look very similar to another Russian craft I've seen.
    Yes the Mars 600 looks to have been it's daddy.

    AK-Invest I think was the Mars maker.

    I have a long tread of nothing but photos and links in which it appears once in a while:
    http://www.hoverclubofamerica.org/fo...showtopic=1801

  8. #24
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    Default Re: Hovercraft Design

    John2

    With all due respect I think you are making it WAY more complicated than it's really needs to be. The formulas needed to design a hovercraft have mostly already been figured out. There is much in free information and books to purchase with these details already established. It sounds a bit like you are trying to reinvent the wheel. A couple of questions for you:

    Have you ever owned or operated a hovercraft?
    Have you ever seen a small or recreational hovercraft in operation?

    Someone had mentioned before the hull bottom design is critical. Flat is not an option. Again, no offense, it looks like you have some good ideas and I want to encourage that but hovercraft for the most part are amazingly simple machines with very similar operating characteristics despite their sizes, shapes and configurations. Design your lift system first and build the asthetics around that. I would recommend listening more to the people here who operate, build and work on hovers all the time. Their experience is invaluable and could save you lots of headaches..

    Nate

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