How do propellers propell a boat.

Discussion in 'Props' started by tom kane, Mar 12, 2015.

  1. tom kane
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    tom kane Senior Member

    How do propellers propel a boat

    Add more infoA pitch only propeller, that is a propeller with a flat plate, no foil shape no skew,ect., ect.,
    blade, similar to the image works well running forward and reverse and using twin screws that can run in both rotations gives you forward and reverse and turning right and left and spin on the spot and no need for dragging inefficient rudders.
    The only thing a propeller needs to do is to move water forwards and backwards as efficiently as possible and with the desired velocity or volume to move the boat.
    Do not forget to add retractable (Pivotal) shaft drives.
     

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  2. daiquiri
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    daiquiri Engineering and Design

    I guess that what you have seen is a direct consequence of some discussions I've seen in forums about human-powered boats (HPB), where somebody came out with calculations which apparently demonstrated that R/C aircraft propellers could be used for HPB with the same efficiency they have in air, if both boat speed and the prop RPM were reduced to 1/15 of the speed and RPM of the prop in the air.

    Too bad that said calculations didn't account for the blade strength.
    At Vwater = 1/15 Vair and RPMwater = 1/15 RPMair one keeps the same Reynolds number between water and air case, hence the thrust and torque COEFFICIENTS do not change.
    However, the thrust FORCE and the TORQUE do change, and pretty much so. By adopting the above criteria, the thrust (and hence the blade-root bending moment) of the propeller increases by the factor of 3.72 when working in water, which is probably more than the safety coefficient used for the design of that prop. The end result - the R/C aircraft prop bends when mounted on a human-propelled boat.

    The moral of this story - let the R/C aircraft propellers propel the R/C aircraft, and make purpose-specific propellers for water craft. :)
     
  3. jehardiman
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    jehardiman Senior Member

    I do

    Hardiman's theory of propulsor design. Propeller, oar, paddlewheel, sail, keel, putt-putt boat...

    Everybody else, Lanchester, Prandtl Lerbs, Morgan, Kerwin,...just degenerate forms of the theory.

    Basic theory...there are one or more surfaces in the fluid causing an energy gradient and placed to take advantage of that energy gradient by flow continuity non-normal to the surface.

    That simple; no more, no less. Now the math models.....lol ;)
     
    Last edited: Mar 23, 2015
  4. jehardiman
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    jehardiman Senior Member

    Don, I agree.

    Lanchester and Prandtl laid the framework, but there were a lot of underlying assumptions in there (i.e. the Kutta–Joukowski condition). I think Tom is asking whether the theory is physically correct vice does it predict within acceptable error.

    No, modern theory is not physically correct...but it does a good job. I have found for lowly loaded and highly loaded blades, modern theory does not apply...but that was known. I always have my junior engineers READ Lerbs' 1952 SNAME paper "Moderately Loaded Propellers with a Finite Number of Blades and an Arbitrary Distribution of Circulation" where he specifically states it is only for moderately loaded propellers. I have developed several rules of thumb for the off loaded condition, including very low loaded and very high loaded propellers as you state. Those require very different analysis methods.

    As we NA's are fond of quoting and should remember....

    "As Leo has quoted on numerous occasions (and I believe he was re-quoting another): All models are wrong, but some are useful."
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That important quote by George E.P. Box is up there with another important one
    due to one of your better philosophers, viz.,
    "A man's got to know his limitations".
     
  6. DennisRB
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    DennisRB Senior Member

    For every action there is an equal an opposite reaction. If you push water out the back of the boat, the boat will move forward. Simple. Now how a prop does this seems a little more complicated.

    It has been said in this thread the blades act like wings (or sails, which are wings anyway). The standard theory is that the accelerated air on the top of the wing causes upward suction by decrease in pressure. Not that I disagree. But what about the more simple Newtonian explanation?

    I think most people would agree a helicopters blades are just wings spinning in a circle. much like a boat prop. Clearly there is a lot of air being pushed down to hold the helicopter in the air. Probably if you put the heli over a scale the downward force of the air would equal the weight of the heli? That would be an interesting test anyway.

    When I was a kid and built my first balsa airfoil, a flat bottom Clark Y, it seemed obvious to me how it worked. The air went almost straight passed the bottom. But the air coming over the top was smoothly directed downward. So just like the helicopter the wing is pushing air down to hold the wing up.

    A wing can still work enough to fly with the flow detached on the top (full stall). The air passing the bottom will still be directed downward, but it will need higher AOA, and create a lot of drag. In this situation the lift will be poor, but still there. After all, in stall, the wing has lost half of the downward deflected air, with a lot of added drag. There is no "suction" from the top, yet it still works, but rather poorly. Even a clark Y will still fly upside down and produce lift (poorly), even though the suction is now downward.
     
  7. DennisRB
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    DennisRB Senior Member

    Holy, crap. (I just done some searching on how an airfoil works upside down, as according to the textbook theories I read it seems impossible)

    My childish intuition and refusal to totally accept the "longer path of air around the top" theory is correct after all!

    Well according to NASA, my theory is correct! Its technically called "flow turning". The foil, flat plate, hand out of a window, boat prop work by changing the direction of a flow. The "longer path" theory is bogus, and so is the other Newtonion "skipping stone" theory (which only includes flow deflection from the bottom of the wing). When you include flow deflection from both sides of the wing like I did when I was a kid you get the correct theory.

    http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html

    http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

    So there you have it. A prop works due to Newtonian reactions based on flow turning.
     
  8. tom kane
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    tom kane Senior Member

    It seems no one has bothered to design and fit to a boat a fixed pitch propeller that works equally well in forward and reverse even though many people complain about bad reverse performance. Forward speed is king. That is probably because aircraft do not fly backwards and do not need that so the Marine industries just follow.

    Recreation and work boats would be more pleasant to maneuver with good reverse.
    Fixed S/P boats would benefit and Pocket and tunnel propulsion.
     
  9. jehardiman
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    jehardiman Senior Member

    Sure they have, just not how you think. Look at all the Azipod or Voith-Schneider tugs and ferries. Even the old outboard motors or modern trolling motors that spin 360 degrees. They do just as well moving the vessel astern as ahead.

    Now, yes, for a classic horizontal shaft inboard machinery rotation reversing hull protected wheel, you really can't make the performance the same astern as ahead. This is because the of the effect the hull has on the inflow and outflow ( i.e. the surfaces are not in the right places, see Hardiman's theory above). Ducted/Tunnel props or thruster types (think White Gill) can be made to have equivalent inflow wakes, but will always have a lower maximum efficiency than a similar open wheel due to the inlet/outlet and strut interference/losses.

    As long as you are turning the biggest wheel you can, often the answer is to just throw horsepower at the problem. My experience is you generally will spend 90% of the time and money to get the last 10% of the answer, so you need to know when it is time to stop worrying about it and just take the answer you have.
     
  10. tom kane
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    tom kane Senior Member

    How do propellers propel a boat

    I do not think many people would want to go very fast in reverse so there is a compromise.
    Props in pods and tunnels are not pleasant in recreational boats and are much like water jets in that you need to run near full throttle to get anywhere and just a nice cruising speed using each RPM to advantage is not there.You need to be able to adjust where the props run in the water as no way can you change the flow of water at speed efficiently.

    Props in pods/pockets using basic turbo fan theory is interesting
     

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  11. daiquiri
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    daiquiri Engineering and Design

    And they couldn't either, even if they wanted to. Boat transoms are not made for going fast in reverse, and some would be outright dangerous in that condition.
    But as you said, there is pretty seldom a need for going fast astern either, unless we are talking about special vessel types - so it all fits together.
     
  12. Joakim
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    Joakim Senior Member

    Sailboats are quite fast on reverse. My previous boat was a 28 feet 3 tonne C/R-boat from 1999 with Yanmar 1GM10 and saildrive. It had a top speed of 6.4 knots on forward and close to 6 knots on reverse. The only problem on reverse at that speed was tiller steering which was quite unbalanced on reverse. Also bigger waves hitting the transom would have been a problem. No benefit from this, but I tested it a few times.

    I guess the same would apply to double ended displacement motorboats, but their typical propeller installation is not that efficient on reverse.
     
  13. daiquiri
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    daiquiri Engineering and Design

    On the other hand, I have seen low-freeboard planing-type boats with waves climbing dangerously high up the transom while going in full reverse. It clearly depends on the general hull shape and layout.
     
  14. Joakim
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    Joakim Senior Member

    Yes you can sink many OB boats on full reverse, probably also some stern drive ones. The worst ones even by slowing down too fast from plane.

    Even wide transom sailboats have their transoms clearly in air and don't typically make a big waves on reverse and bury their transoms.
     

  15. DCockey
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    DCockey Senior Member

    I am not aware of any reason that a prop in a tunnel or a prop on a pod does not work well at slower speeds.
     
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