Back to the Future

Discussion in 'Boat Design' started by Ad Hoc, Nov 3, 2020.

  1. BMcF
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    BMcF Senior Member

    We were flying along in one of the demonstrator/scale models we built when the control system had a malfunction that caused the craft to pitch up and literally "take off" from the water. It landed with a huge splash but quite stably..we likened it to a duck's landing. In pic below the foils are only supporting about 80% of the craft but we often flew with all three hulls clear of the water surface.
     

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  2. portacruise
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    portacruise Senior Member

    Indeed, the place to start would be with a foolproof collision avoidance system that works for quite prominent hazards as evidenced by HK Harbor and by cruise ship groundings, Plus U S Navy collisions like the USS John McCain, Etc.. Besides these hazards, hydrofoils would appear to be more sensitive to collisions with smaller objects because of their speed and altitude, compared to non foiling vessels. My guess would be that something along the lines of the Tesla automobile collision avoidance system might be enhanced so as to provide at least a modicum of improvement.

    Here's a link



    but this one is not related to motor powered hydrofoils or collisions.
     
    Last edited: Nov 15, 2020
  3. fastsailing
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    fastsailing Junior Member

    In terms of fuel cost for passenger mile, all the mentioned marine vessels will lose against a proper sized aircraft with a wide margin, assuming route distances are at least several hundred nautical miles. The ratio of weight of passengers and weight of the transport device is much worse for large marine vessels than aircraft, especially if aircraft passengers only need a seat, and marine vessels offer an own cabin with showers and sleeping arrangements due to longer travel time.
    If the requirement involves carrying cars or cargo in addition to people it won't be the same result.
    If the merit is cost of ticket for passengers instead of cost of fuel, it depends on length of route and profit margins for operator and either port or airport as well. The result could be anything, with large variations for all transport methods possible due to monopoly or just lack of competition.
     
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  4. jehardiman
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    jehardiman Senior Member

    I think you should go re-check the modern Gabrielli von Kármán diagram. The most efficient aircraft is only on par with high performance marine craft as far as fuel consumption per tonne/km. While it is true that the time required to move a passenger from airport to airport may be much less, the fuel expenditure will not be, leading to the comment that only people and berries should transported by plane.
     
  5. fastsailing
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    fastsailing Junior Member

    Check the difference between a passenger and tonne.
    In former case, aircraft is superior, and in latter case, a large diplacement vessel with low froude number.
     
  6. jehardiman
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    jehardiman Senior Member

    I understand the difference, but I think you are overstating the case. Lets take the case of two contemporary vehicles with similar P/WgV values (i.e. the same amount of energy is used to transport 1 tonne (vehicle and passengers) 100 km):
    ............................ W (kg) .... V (m/sec)..... Pass .................. Kg/pass
    Boeing 929......... 115,000 ...... 23 .............. 250 to 350 .......... 328 to 460
    Boeing 747-200... 377,800 ..... 259 ............ 366/440/550 ..... 1032/858/686 (the three passenger values are normal configuration/ certified lower deck max/ certified total max)

    So while the aircraft gets there 10 times faster (maybe...ever taxied at Atlanta?), it still spent more fuel per passenger.
     
  7. kach22i
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    kach22i Architect

    Good information, but to cover the more unconventional means and methods.

    1. WIG or Wing In Ground Effect, will avoid submerged obstacles and may have greater efficiency over conventional aircraft because of proximity to ground/sea.

    2. Hovercraft, can be 2-4 times more efficient with fuel (Diesel and Gas models - non turbine) over that of boats if speed in fair weather is your only metric. One of course has to add in skirt maintenance (and staff to do it), an added cost not to be ignored. Then again, port facilities can be as simple as an empty beach.

    How does the average fuel consumption of a hovercraft compare to that of a car and a boat?
    DiscoverHover: Hovercraft FAQ https://www.discoverhover.org/abouthovercraft/faq.htm
    Wind and weather can really mess up your math with any craft, including displacement vessels (boats).
     
  8. kach22i
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    kach22i Architect

    Yep, which is why the world still uses barges.
     
  9. fastsailing
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    fastsailing Junior Member

    I don't think this:
    same amount of energy is used to transport 1 tonne (vehicle and passengers) 100 km
    would be correct.
    Overall efficiency (engine and propulsion) is far better for turbofan at high altitude and cruise speed (above 50%) than for any propulsion system in a marine vessel.
    Take a look at: Figure 11.8 from source: 11.5 Trends in thermal and propulsive efficiency http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node84.html
    Also 747 is an old design, not having the best fuel consumption for passenger mile available for an aircraft, (around 3 liters of fuel/passenger/100 km).
    For short distances aircraft performance obviously degrades, even when using turboprops, and without them, climbing to altitude takes a lot of fuel making thu numbers much worse for short routes.
    In optimum conditions aircraft still has less drag/ overall weight than hydrofoil or cat at high froude number. This reverses when comparing to a large ship at low Fr, but that is meaningless if only transporting people, because such ships are very heavy (typically over 50 million kg) for the task, which is not a problem for heavy cargo transport.
     
    Last edited: Nov 23, 2020
  10. fastsailing
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    fastsailing Junior Member

    1) not so, because ground effect only reduces induced drag, which is a small fraction for an aircraft in cruise conditions.
    Wing used in WIGs are also typically of short span, canceling the benefit from ground effect, and have special less efficient aerofoils for stability reasons.
    Most importantly WIGs have much worse combined engine and propulsion efficiency than modern large aircraft at cruise conditions. Could even be worse than marine vessels.
    I thought the metric was (amount of fuel) / (number of passengers) / (distance travelled)
    thus speed has no direct effect on result, only indirect by changing amount of fuel needed.

    If you have any numbers for hovercraft that supports it for said metric, please post it.
    But I doubt they exist.
    Hovercraft uses propeller using air as propulsive media, and mass flow is typically too small for good propulsive efficiency, worse than marine propellers and worse than propaircraft have at cruise condition. That is already significant, and there is also power needed for air cushion.
     

  11. kach22i
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    kach22i Architect

    One way to get around most of that silly spray and inefficient air propulsion systems is to partially lift the hull out of the water, flexible bow and stern skirt with narrow side catamarans sponsons, utilizing water jets or propellers.

    Such a craft is known as a SES or Surface Effect Ship and is known to be more efficient than a hovercraft though not fully amphibious, and will probably still require conventional docking facilities (more overhead costs).

    What Are Surface Effect Ships?
    What Are Surface Effect Ships? https://www.marineinsight.com/types-of-ships/what-are-surface-effect-ships/
    [​IMG]
    [​IMG]
    In general, time is the greatest factor in determining cost efficiency and putting it into context.

    When time is important - fly.

    When time is not important go slow.

    Where the oddities of hydrofoils, SES and hovercraft come into play is in those narrow set of circumstances where their unusual features become assets and not liabilities.

    If your route becomes ice covered, drought stricken, flooded or otherwise compromised with unusable docking a hovercraft just may be the answer.

    If your biggest worry on your route is a floating coconut or two then a high speed catamaran or hydrofoil may be of great use.
     
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