Maximum efficiency prop for a WIG boat

Discussion in 'Props' started by Starman, Jun 24, 2010.

  1. Starman

    Starman Guest

    Hi, I'm working on the design of a WIG boat, a wing in ground effect craft, and am wondering about the possibility of using a water prop rather than an air prop.

    This is going to be a long range vehicle and so fuel efficiency is important.

    An air prop design for maximum efficiency will provide around an 80% efficiency.

    I would like to know what kind of efficiency can be expected from a water prop which is designed for maximum efficiency.

    I have read that surface piercing props operate with more efficiency that submerged props so that may be the way to go. Either way, there is no limit to the diameter and I can use a reduction gear to get the optimum rpm.

    So, with no limit to diameter of rpm, what kind of efficiency can you get?

    So, what do you think, which will win an efficiency contest, air drive or water drive?
     
  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

  3. Guest625101138

    Guest625101138 Previous Member

    The best you will get from a surface peircing water prop is 70%.

    High aspect submerged propellers of a similar shape to those used on aircraft will give in excess of 80% efficiency in the water but there is a large drag penalty in the supporting strut at higher speed. Also at higher speed a submerged prop will cavitate easily and this reduces efficiency.

    Surface piercing props avoid strut drag and the front face is aerated anyhow.

    So surface piecing props are only more efficient than submerged props in high speed applications but they will not match the efficiency of an air propeller.
     
  4. Starman

    Starman Guest

    Thank you both, it looks like the air prop will be the way to go for cruise efficiency, and it does have a benefit of being able to fly over sand bars and objects in the water without damage. Takeoff power will still be provided by a surface piercing water drive.
     
  5. apex1

    apex1 Guest

    I cannot see one single advantage for doing that? You would just add weight and complexity, imho.
    Can you enlighten us why?

    Regards
    Richard
     
  6. daiquiri
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    daiquiri Engineering and Design

    I agree with Richard. You will just add more weight, another failure mode, and will increase building and maintainance costs.
     
  7. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    By the way, the best grund effect benefits are obtained at heights below h/c=0.2-0.3, where c is the wing chord. So you need chord more than wingspan, if you want a true ground-effect machine. A high wingspan can be useful if you intend to fly frequently in OGE mode. In those moments it becomes an airplane, but a rather inefficient one. But then it will be a less efficient IGE machine, once it gets close to the ground again... So you need to clearly define your design goals, possibly expressed as a mathematical function to be optimized, lest you create a machine which is a good performer in just one task and a bad performer in all the others.
     
  8. Starman

    Starman Guest

    Yes, you are right.
     
  9. Starman

    Starman Guest

    WIG stability is a thorny situation that you may or may not be aware of, but almost all WIGs have crashed because of it, and this is my plan for solving it.
     
  10. daiquiri
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    daiquiri Engineering and Design

    May I express just two objections, if you don't mind.

    1) "if the front wing tries to lift up it will lift the rear up and won't flip over" - If you make the main wing fly in the chord-dominated height, the effect of height on Cl is so strong that it will be inherently stable in height. If the forward canard wing lifts up, it will very quickly loose lift (because of decrease in ground effect) and get back to the equilibrium height. Different is the case of pitch stability, which is a problem indeed. I am not sure which one you are talking about when referring to the rear-wing (or tail) height.

    2) "it's closer proximity to the water and wider chord prevent it from nosediving and digging in" - Maybe it is just a typo, but a closer proximity to the water is the opposite of safety. It means higher probability of hitting a wave crest, and is one of reasons why chords are kept as large as practically possible (while still keeping an eye on aerodynamic efficiency). More chord translates into a higher IGE flight level, which translates into a smaler probability of encountering a wave.

    Cheers!
     
  11. Starman

    Starman Guest

    Those are good points to consider, thanks :)
     
  12. kroberts
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    kroberts Senior Member

    Starman,

    I think you have some more studying to do if you don't mind my saying.

    First, your air propeller efficiency is extremely optimistic for a low speed WIG type craft. Other things than efficiency take center stage for a WIG, such as a lower tip speed to prevent noise near people, and to reduce erosion from water, sand and whatever else might fly up when you take off. In my experience, a surface prop as used in airboats, hovercraft and such (not including fans) tends to be closer to 60% all things considered. Marine folks who use air props learn to cut the noise down or they find a local noise ordinance far too restrictive for them to operate at all.

    Second thing, I have spent quite awhile messing with a hovercraft in the 60-70 mph range as measured by a GPS. I seriously doubt you want a water piercing prop when going 90 mph over a body of water that can contain sand bars, and from what I've read about WIGs (a moderate amount, starting with the entire contents of http://www.se-technology.com pages and most of their linked content) I think you've read up on them enough to be dangerous but not yet very effective. Worse yet, the way you're talking I doubt you have high speed water experience.

    I'm NOT trying to rain on your parade. I'm saying you should do a bit more research.

    One thing that alarms me is your power requirement. Your cruise engine is too large to be the sole engine for a properly designed WIG similar to your drawing. The take-off power is very large only when compared to the cruising power of the same vehicle. You've specified enough power to run a full aircraft that size way faster than your speed requirement. Most of the more experienced WIG designers choose a single engine in spite of your take-off discrepancy.

    One thing you may not realize: If you are operating as a WIG over water with no aircraft rating, then you are required to obey marine laws. That means that on a no wake zone you are going 5 knots or minimum steerage, whichever is lower. No-wake zones on rivers go the entire width of the river, even if it's a mile or more wide. There are a whole lot of other laws too, that are not necessarily convenient to WIGs or hovercraft. I know about these things because of the hovercraft aspect, but WIGs and hovercraft are both surface effect vehicles and so fit into a category of sorts.

    Another thing I've noticed is that when surface effect vehicles are mentioned in a local waterway law, they are almost always mentioned in such a way as to restrict rather than to allow.

    You may have visions of cruising up the coast on your WIG, or up a river for an hour or three. From personal experience, going up a river at 60+ mph is intense. Canoes, logs, wreckage and swimmers seem to pop out of nowhere, and a duck or goose popping up in front of you can really mess up your day. Geese fly at about 50 mph or so, so you would be coming at them 40 mph faster than their top speed. I'm just guessing based on my hovercraft experiences, but that means you'll probably have about 5 seconds from the time they take off in a flock to the time you hit them. That's how long you have to change your course safely to avoid the birds, the terrain, the shore and whatever boaters or swimmers there might be. It's much better with a hovercraft since you're not going much faster than they are, but I've had some close calls even so.

    The water is not a road. What appears to be smooth and clear can turn on you in a moment. Things happen extremely quickly at even 60 mph, and 70 is noticeably worse. A piece of wire stretched across the river or sticking straight up can seriously mess your day up, because you just can't see it until it's far too late. A piece of rebar sticking up will wreck your machine before you even knew it wasn't a floating stick.

    Sorry for the soap box rant.
     
  13. apex1

    apex1 Guest

    Those were the basics of WIG.............;)

    and:

    ...that is not the case, the opposite is true. I would rethink the whole idea, including the Diesel as a main propulsion.

    And sure a surface prop adds a lot of resistance and tubulence when in flying mode. The second engine and tank, a lot of weight and complexity.

    You are on the wrong track, believe me.

    Regards
    Richard

    edited:

    thanks Ken!
    I typed a little slow.....
     
  14. kroberts
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    kroberts Senior Member

    Starman,

    Before you think there's a discrepancy between my "low speed WIG" comment and the water speed, for an aircraft prop 90 mph is not fast. For a vehicle operating within the space governed by maritime law, especially where there can be sandbars, it's extremely fast. You have everything from a half-buried tree to a fisherman and his dog trudging through in hip waders for an unbelievably long distance to get to his favorite fishing spot. You would think that sort of thing would be visible from a long way off, but when you're going that fast near a shore there's always something coming at you very quickly. Rivers never go in straight lines unless people got involved, and if people got involved it's a congested waterway with oppressive boating regulations.

    I'm not being antagonistic to you. I have had two hovercraft, both of which have more than enough power to go over 70 mph in real conditions. Both of which I have had going that fast, and don't intend to slow down much. I like speed. I just know that it's not nearly as cut and dried as most people seem to think it is. Most inland boaters think they have a speed demon if they can get 35 mph out of it. There are much faster boats out there in the same water, but you don't actually see a lot of people going truly fast on the water. Several people I know CAN go faster than 70 mph, but rarely do so.

    I suggest you get on the forum pointed at by the web site I referenced, there are a few really sharp wig guys there. There are also a whole lot of people who talk a lot of theory but have no practical experience. I suspect over half the enthusiasts on that site have never even seen a real WIG, even as a model. So be careful about that. Find out who actually builds them, read books THEY recommend and start building models. It's not nearly as well mapped as boat building and there are huge safety issues.

    It also wouldn't hurt for you to get some sort of fast water craft to fool around with so you know what the problems are with high speeds on shallow water.
     

  15. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    As Daiquiri has noted, the optimum Lift/Drag ratio in ground effect comes with airfoil proportions that differ from what you want for free flight, so you have to run a couple of additional turns in the design spiral there.

    The take-off power is more a question of creating low speed lift, than low speed thrust. This lift comes from an increased circulation around the wings, generated by properly directing the outflow from forward propulsion devices. If you look at the "Caspian Monster", the giant Russian Ekranoplan, the forward jets generate enough lift to let the vessel take off from the beach. The machine literally lifts from the ground, when the nose jets are speeding up, I have watched it and it certainly is impressive! A water propulsion clearly can not be of any assistance in this respect.

    The only reason to have water propulsion and steering would be to cheat the rules; this makes the vessel an airlift assisted boat, but you sacrify the amphibious qualities. Generally, dual mode propulsion will add weight and complexity as Apex already has noted, and any vessel that depends on dynamic lift is weight sensitive, so again back to the design spiral!

    Surface propellers generally have bigger diameter and lower rpm's than a corresponding submerged propeller, so your idea on direct drive SPP would have to be checked versus real propeller data, but as said above, its no good idea for a WIG!
     
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