Turbofan

Discussion in 'Boat Design' started by dskira, Feb 18, 2013.

  1. dskira

    dskira Previous Member

    I am wondering if a large SAWTH can gain by being propelled with two R&R turbofan, like the new airliners engines.
    I see no more shafting, easier to maintain the engine, been on deck level.
    The consumption can be high, but perhaps with a derating the engine?
    I know the combine air/salt/water is not the best for gas turbine but that can be resolved since some warship use the gas-turbine to power the gen-set and as a high speed propulsion also. Like some yacht.
    But the mode I am thinking si more simpler, get rid of shaft and propellers. Just plain R&R prop-fan on deck, that it's. This engine with fans are describe as very efficient, more than a gas-turbine by itself.
    The goal will be not for speed, but for simplicity of design.
    Note: I know nothing about gas turbine, I just put two an two together, and see if it can come to four, or go .........to five!
     
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  2. Number4

    Number4 Previous Member

    Turbofans are used on aircraft that fly above 25,000 feet and above 300 mph. They are efficient up there because of the thin cold air.
    Turboprops are used on aircraft that fly below 25,000 feet and below 300 mph. Props like thick air and cannot approach the speed of sound.
    A diesel engine is a more efficient power producer than a gas turbine, but weighs much more for the same power rating.
    A large slow turning prop is more efficient than a small fast spinning prop.
     
  3. BMcF
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    BMcF Senior Member

    Rolls Royce, GE, Pratt Whitney, Solar, and so on all make, or have made in the past, "marinized" version of their turbine engines to provide propulsion power. A number of SWATH vessels have been built that used gas turbines for their propulsion power.

    In almost every case, the power is transmitted via mechanical means via shafts and gearboxes.
     
  4. dskira

    dskira Previous Member

    Your last point is exactly what I was wondering. If its possible to bypass the mechanicals components, and using the turbine directly as a reactor, like on an airplane. As for reverse, the gas-turbine are mounted on a rotating system like the huge crane are, and they can turn around with a help of a hydraulic motor.
    As for the generator, a simple diesel engine also on deck, if possible using the same fuel as the turbine. The Arrow can do that I think.
    The SWATH have a huge platform, The space on deck will be not a problem, and I don't think the weight will be either.
    I am thinking of a large vessel, with small prop-fan.
     
  5. BMcF
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    BMcF Senior Member

    The net thrust produced by airscrews or a high-bypass turbofan is much less than what the same power can produce using water as the operating medium and turning propellors or impellors in that medium.
     
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  6. WestVanHan
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    WestVanHan Not a Senior Member

  7. Number4

    Number4 Previous Member

    The hydrofoil "Westfoil", built in Westport, Washington by a group including at least one ex-Boeing Marine Systems engineer (Robert Gornstein). She is now in Honolulu Hawaii, owned by Pacific Marine. These pictures were taken there in December 1997. She is powered by two diesels that run two cut down C-130 propellers, mounted above the cabin.

    http://www.foils.org/gallery/world.htm

    I think the main problem with an air prop is that it would generate a hurricane force wind and would be trying to suck everything not tied down into itself. It's a big problem at airports called FOD, Foreign Object Damage. It would also be extremely loud.
     

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  8. Red Dwarf
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    Red Dwarf Senior Member

    I looked into the concept of using a airplane propeller because it simplifies the drivetrain and packaging plus offers other benefits.

    The problems is it will be too noisy and obviously requires some safety protection to prevent ingesting items (takes FOD to a whole new level). Another issue, related to the fact that water is more dense than air, is the prop has to be 28 times the diameter of a water prop for the same thrust.
     
  9. daiquiri
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    daiquiri Engineering and Design

    The efficiency and the noise are the main problems I would see with that solution, Daniel.

    Turbomachinery (be it propellers, turbojets, turbofan or turboprops) all share the same basic physics of propulsion. Which is the one you know from the propeller theory - the propulsive efficiency increase as the velocity difference between the turbomachine's inflow and outflow decrease.

    An infinitely big prop disc which accelerates air or water by just an infinitesimal amount could give theoretically a 100% efficiency. But we cannot have infinitely long blades, so we have to compromise... again.
    The compromise consists in determining the prop diameter from various constructive, operational and physical constraints, and then designing the blades and gear ratios in such way to get the maximum efficiency under these conditions. At the end you will get a prop optimized for that operative condition, but it will not be the optimum prop for the whole wide range of other conditions.

    Same is valid for other turbomachines (a general category to which propellers belong to). Talking about aircraft, a turbofan is efficient for a certain range of speeds, but for lower speeds you have to either increase the fan or to switch to a turboprop. Slow down a bit more and you'll be much more efficient with a simple reciprocating engine driving a prop, because a turboengine spins too fast and requires a transmission with a big reduction ratio. A boat, as fast as it can be, is a snail - when compared to an aircraft. So, your application would be at the lowest end of this speed range.

    Then there's a matter of fluid used for the propulsion. Sea water is 840 times denser than air, so in order to get the same thrust from two geometrically similar props, one working in water and one in air, the air prop needs theoretically around 30 times bigger diameter.

    An air prop with big diameter and with high loading (because you never can have all the diameter you would like to have) will have blade tips working in the transonic regime, hence producing high aerodynamic noise. Think about the noise produced by a hovercraft or a turboprop airplane.

    These are some of the most evident factors which have made power transmission through water preferred over the transmission through the air.

    Cheers
     
  10. dskira

    dskira Previous Member

    Your last sentence make me remember found memories.
    In the mid sixties with my fiancée and our Mini Morris, hop we went from Main Land England to the Isle of Wight. The transport over sea was with a large hovercraft with turbines and large propeller on top of the roof.
    Several time they have to slow down for safety measure due to bad sea condition. And yes the noise was amazingly high. But I loved every moment of it. The vision of the hovercraft coming from the sea to the tarmac was amazing. The Mini was lost inside the huge garage.
    I thought the design of the hovercraft was outstanding, and I think also today it was quite a cool thing.
    But back to the prop-fan and SWATH
    Your very precise and knowledgeable informations put me back to hearth, thank you so much.
     
  11. BMcF
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    BMcF Senior Member

    The first thing Navatek did was remove the airscrews. That craft eventually became an operational passenger ferry (for however brief a period) with a completely different lifting foil configuration replacing the stuff that was on it when they acquired it from the Rust Bros...and it was propelled by a custom-engineered water prop system. I got a chance to go through the vessel just prior to the beginning of sea trials after all the mods.

    Having spent more time than I would consider "enjoyable" involved in the test and trials of the LCACs when they were being built, I and certainly attest first-hand to the incredible noise associated with those air screws and gas turbines..
     
  12. FranklinRatliff

    FranklinRatliff Previous Member

    Correct. There are reasons why the big English Channel hovercraft are equipped with air props not turbofans.
     
  13. FranklinRatliff

    FranklinRatliff Previous Member

    The formula for calculating propeller thrust for a given speed is:

    T = 550 np (HP)/V

    where

    T = Thrust in lbs

    np = Propeller efficiency as a percentage (decimal)

    HP = Horsepower

    V = Velocity in feet per second

    Assuming 500 horsepower and 85% (.85) propeller efficiency, thrust would be 1,593 lbs at 100 mph, 796 lbs at 200 mph, 531 lbs at 300 mph, and 398 lbs at 400 mph.

    Assuming 1,500 horsepower and 85% propeller efficiency, thrust would be 4,781 lbs at 100 mph, 2,390 lbs at 200 mph, 1,593 lbs at 300 mph, 1,195 lbs at 400 mph, and 956 lbs at 500 mph.
     
  14. Yellowjacket
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    Yellowjacket Senior Member

    What you have to look at is called "propulsive efficiency". That is, the velocity of accelerated flow, relative to the velocity of the craft you are driving determines the efficiency. If you accelerate a small mass to a high velocity and you aren't traveling at a high velocity, the efficiency sucks. Propulsive effiency is equal to 2/(1+(Vi/Vo) where Vi is the speed of the flow and Vo is the speed of the vehicle. In simple terms, the propulsive efficiency is 1 if the velocity of the jet is equal to the velocity of the vehicle. The faster the air that you move relative to the velocity of the vehicle, the poorer the propulisive efficiency becomes.

    That is why a big fan is more efficient than a smaller one. The bigger fan is accelerating a larger mass but at a lower velocity, where the small one is accelerating a smaller mass to a higher velocity.

    Bottom line is that any airscrew or fan is far less efficient than a prop in the water in terms of proplusive efficiency at low speeds. The reason is that if you are only going 30 mph (44 ft per second) and you are accelerating the flow of the fan to a much higher velocity (say over 400 ft per second), like in a turbofan, the the propulsive efficiency isn't going to be very good.

    A prop is far better than fan, but because air is very light, you need to move a lot of it and accelerate it to a pretty high speed to generate any useful thrust.

    If you are on the water and aren't going very fast, you need to think about moving water if you want to decent propulsive efficiency.
     
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  15. FranklinRatliff

    FranklinRatliff Previous Member

    Hololistic Design

    Propulsive efficiency is not the be all and end all. There are other factors that need to be taken into account and weighed against each other, such as what you're trying to do with the vehicle.
     
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