What is world's biggest planning hull boat and how fast?

Discussion in 'Boat Design' started by Squidly-Diddly, Sep 16, 2020.

  1. gonzo
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    gonzo Senior Member

    How does it relate to the Lake Express?
     
  2. DogCavalry
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    DogCavalry I aim to misbehave.

    I was going to say...

    For reasons of the most basic physics, it is not even theoretically possible to have a pump be of comparable efficiency to a prop, under normal operating conditions.
     
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  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Jehardiman, how would you describe Destriero's hull form?
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Bild.jpg
    Nope DogC, here you’re guessing, and your guesses are wrong. It seems that the same misunderstandings about propulsion keep popping up every now and then. I feel there is a need for some clarification; can’t express in a short sentence, so sorry to hijack the thread for a while (to the moderator: please move to another place/thread if preferred!).

    To understand the basics of propulsion efficiencies with different technologies, you have to realize the combined influence of three major mechanisms. The following is valid for non-cavitating, non-ventilating propulsors. For cavitating/ventilating units, other limits apply.

    Consider the propulsor pumping mechanism as a “black box”, be it an oar, paddle or a pump (axial, centrifugal, injector or whatever….). The viewer, travelling with the black box will observe an incoming volume flow (Q) with the velocity Va, and the same flow ( for the ejector the inlet plus driving flow) exiting the device with the higher velocity Vj. For clarity, we leave the injector from the discussion for now.

    According to Newton, the pumping device is producing a force as follows:

    F = (mass flow) *( velocity change); or F = Q*rho*(Vj-Va), where rho is fluid density.

    This means that a required force at a given forward velocity (Va) can be produced, either by a large Q times a low velocity increase, or vice versa, or anything in between.


    Now here is the first major mechanism to consider:


    The efficiency here is a measure of the momentum exchange quality, ie how much momentum is wasted in order to produce a forward propulsion. We call It momentum efficiency (etam), and it can be written as:

    etam = (2*Va)/(Vj+Va);

    and also as (with some simplification):

    etam = 2/((Ct+1)^0,5+1); where Ct is thrust coefficient (= Thrust/(rho/2*Va^2*Ad)); where Ad = propeller disc area.

    Here you can see that the momentum efficiency is increasing with decreasing velocity difference, that is: the momentum efficiency improves when the force is produced by a large flow with a small velocity increase. It is also clear that the thrust loading is an important factor for efficiency. The inflow velocity Va is the ship forward velocity minus some average of the velocity of the boundary layers that arrive at the pump inlet, and here we find some huge misunderstandings regarding water-jet efficiency.

    The boundary layer has achieved a forward velocity from hull friction (mainly), i.e. we have spent power to increase its velocity from zero to some fraction of the hull velocity. Consequently, Va is lower than Vs (ship speed), and you can see that it will influence both the force production and the momentum efficiency. This fact has been neglected for a long time; I even found that in my 1967 edition of the PNA, the water-jet efficiency is not correctly defined! The obvious result is that the inlets for flush openings must be designed with a strong focus on the behaviour of the boundary layer flow, and that inlets are principally correct only for one speed. Looking at a number of “off-the-shelf” units it is obvious that this has not been understood, and the resulting efficiencies are thus suffering badly.

    When jets were applied to slender catamaran hulls, it was observed that a major share of the hull boundary layer was swallowed by the jet and this made all the difference to the efficiency of the propulsion.


    BUT………:

    There is a second major mechanism to consider:

    The pumping device is transforming the mechanical power into hydraulic power to the fluid, i.e. the product (pressure difference * volume flow), and this process involves various losses mainly in the form of friction and turbulence. The pump hydromechanical efficiency (etah) is a function of its operating point in terms of shaft speed, volume flow, pressure difference and physical size. This goes for all rotodynamic machines, from small high-pressure-low-volume radial pumps to big high-flow-low-pressure open propellers.

    The relation is written as:

    Ns=n*(Q)^0,5 /H^0,75; where H is pump head equals pressure/(rho*g); with rho=density and g=earth acceleration, all in consistent dimensions.

    In pumping circles it is called the “specific speed”, but is would be better to call it a “type number” since it relates to the physical configuration of the pumping device. The size influence is linked to the relative amount of boundary layer flow in the machine and the corresponding velocities, ergo a Reynolds number dependence. See attached diagram for typical values.

    This pump hydromechanical efficiency has a maximum for a configuration of a slightly diagonal flow impeller, falling off at both sides. The optimum is fairly flat, with a tendency to be pushed towards higher Ns-values with the ongoing technical evolution. Open propellers have a lower etah than a closed pump due to low pressure recovery and (generally) high tip vortex losses.

    The etah for the big water-jet pump units are about 90 to 93 %. Scaling down to “normal sizes” of inlet dia 230-350 mm, you will find realistic hydromechanical efficiencies around 80 to 85 %. The corresponding values for open propellers is seldom over about 70 % (but their etam is high at the design point).


    BUT…….!

    Now we run into the third major mechanism to understand:

    All flows in channels and around profiles obey the physical laws of Bernoulli. Any velocity change in the flow direction will be reflected in a change in pressure. When the inlet flow meets the rotating blade leading edge, there is a pressure reduction. The critical zone (for pumping performance) is normally the inlet tip, where the relative velocity is the highest. With increasing speed you first get an expansion of free gas, liberating free air bubbles, followed by vapour cavities evolving at higher speeds (lower static pressures).

    Again, we face a trade-off, where pumping devices of different specific speed show different cavitation behaviour. For the open propeller, increasing forward speed, keeping rotational speed constant, results in an increase of the tip relative velocity, that is an increasing negative influence on efficiency from cavitation. In lower speeds, the tip speed (and operating depth/pressure) is the limiting factor.

    The water-jet, with its fluid-guiding enclosure is completely different. With increasing speed (keeping shaft speed and flow constant), the nominal inlet opening needed will be reduced. This makes for a diffusing flow, where the incoming velocity is reduced and alas, there is a pressure increase that reduces the cavitation risk. At low speeds, there is no pressure recovery to suppress the leading edge cavitation, with the result that the water-jet has its operational cavitation limit at low forward speed combined with high power input. Again, the tip peripheral speed is the limiting factor.

    So, there you are; the achievable efficiency can be written as:

    Etap = etam * etah;

    where the influences of hull boundary layer, unit size, unit specific speed, hull speed, cavitational behaviour aso are wheighed in, but neglecting the obvious mechanical losses in transmissions. For open propellers, the losses from shaft and bearing brackets must be accounted for, both in terms of direct losses and due to the negative influence of the wakes on the propeller's etah.

    In addition to that, an additional balance to be found is the power/rpm matching between propulsor and driving machine. For big units with gas turbines, water-jet power characteristics follows the same trend as the turbine’s (power is proportional to n^3, with small influence from forward speed), easing the matching process. When everything is accounted for, I think the propulsion efficiency for the Destriero jets would be close to, or even slightly over 80 %.

    Edit: the original diagram comes from C.E. Brennan; "Hydrodynamics of Pumps" and I added the typical impeller sections to show the connection shape-specific speed.
     
    Last edited: Jan 28, 2021
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  5. Ad Hoc
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    Ad Hoc Naval Architect

    Bodo,
    In the paper by Rolf Svensson, KMW - Waterjet Conf. - RINA 1994, they show slight fluctuations - with some measuring imperfections - but with values in the 0.73 - 0.78 range.
    upload_2021-1-28_9-20-32.png

    I still recall this conference, as it was the first i co-authored at and presented!
     
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  6. jehardiman
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    jehardiman Senior Member

    I haven't seen a complete lines plan, but from what I have seen she is a shallow V planing or semi-planing monohull, possibly (from the image of the model) with aeration steps. Built by Fincantieri in 1991 and designed by Blount, the mould seems to be similar to Giles 1989 patent with a hard chine carried aft in lieu of a round bilge (note that the LCS FREEDOM class also has this hard chine). Given the contemporary time and using patent US5080032A to do the heavy lifting, it wouldn't have been a stretch...just the funding. Funny also that Blount worked on the LCS FREEDOM Class which was the subject of a patent infringement case based on US5080032A where " Lockheed Martin represents that the hull design is based on Destriero, the Jupiter ferry, and “the work done by Donald L. Blount & Associates and Fincantieri.”" The real takeaway though from the patent and suit is the manipulation of the pressure at the inlet.

    US5080032A - Monohull fast sealift or semi-planing monohull ship - Google Patents https://patents.google.com/patent/US5080032A/en

    I really like the post (and there are many concepts in there that I intend to address in my thesis if I ever write it) , but to drag this back on topic
    In one way yes, in one way no. The truth is buried in this chart from High Speed Craft by Keuning & Ligtelijn; note that this is etaO (i.e. EHP/SHP) for installed systems which becomes important.

    Destriero and US5080032A notwithstanding, the conventional propeller is most efficient up the mid 30 knots. Additionally, if you need less that 5,000-6,000 SHP, etaO can be significantly higher, so while that single screw 100,000 SHP LSD may have a etaO of 0.73 turning a huge B 5-95, a small launch could achieve 0.90 with a B 2-30, and specialist HPV's propellers even better. So why don't LSDs use B 2-30's? What limits propellers is bending moment to material strength and cavitation losses, so while that CVN is going significantly faster than the 100,000 SHP LSD, it's 250,000 SHP is spread over 4 props. This is necessary to prevent the props from failing. As we move through the props and speed, cavitation losses and the methods to handle cavitation change. From transcavitating to super-cavitating to surface piercing, the EHP, installed SHP, and propellers keep getting smaller and smaller until we get to ~120 knot Unlimited Hydroplanes mostly flying with a 3000 SHP GT and an 11 inch 2-bladed chopper prop. While I think the water jet curve above represents real, but un-optimized installs, jets suffer the same effects as propellers as specific speed increases. Only by good manipulation of the pressure throughout the unit can a water jet hope to approach an open water propeller etaO. However, the water jet impeller has two advantages over an open water prop, first by manipulation of the inlet it can increase the pressure at the disk to prevent cavitation onset, and second the tube can support the blade tips reducing the blade bending. The first means I can have faster flow into the impeller before the onset of cavitation drags down efficiency, the second means I can add more power and/or have a larger disk. In all this means that while etaO may be slightly lower at speeds above ~40 knots I can throw more SHP at it and therefore have a large EHP and therefore a larger vessel.

    Mono-hull or not? While I generally don't care about power yachts (What is the first thing you do when you buy a yacht?...offer it for sale at a higher price), there are several small combatants that fit this requirement. The only real issue with most is range.

    Check out the usual suspects, most builders are happy to share what they have on the ways.

    I'd prefer a Norwegian Skjold-class corvette which is a SES, see Leo Lazauskas paper.
    Wayback Machine https://web.archive.org/web/20080719002649/http://www.cyberiad.net/library/pdf/giam260a0.pdf
     

    Attached Files:

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  7. baeckmo
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    baeckmo Hydrodynamics

    Sic! Two additional observations regarding the KaMeWa diagram:
    A/ The hull is stepped (?), which should make for less BL flow in the jet inlet region. The jet suffers but the hull resistance is reduced; "what you loose on the swings, you gain on the carousel".
    B/ The remarkably good performance in the 30 to 40 -knot region indicates that the jet layout is optimized for a slightly lower speed range than pure top speed. Again it is important to see to the full operating range, even in a "racer".
     
    Last edited: Jan 28, 2021
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Pretty hard to get around
    Momentum p=mv
    KE=½mv²

    Thrust is momentum transfer, but fuel burned is KE.
     
    Last edited: Jan 28, 2021
  9. DCockey
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    DCockey Senior Member

    Fuel was a large fraction of Destriero's displacement when starting the trans-Atlantic run so the displacement at the end of the run was considerably reduced from the displacement at the start of of the run. I recall reading or hearing Blount talk about optimizing the design for the shortest time for the trans-Atlantic run, not for top speed.
     
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  10. DogCavalry
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    DogCavalry I aim to misbehave.

    I'm surprised to learn that boundary layer effects on propulsive efficiency were still miscalculated in 1967. On the air side this was well understood in the 1920's, with dirigibles.

    Squidly Diddly asks great questions!
     
    Last edited: Jan 29, 2021
  11. baeckmo
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    baeckmo Hydrodynamics

    You're forgetting that there is a velocity involved; Thrust is momentum change per time unit, or "impulse". And KE is the work done by moving a force from A to B without considering the time it took; again the time factor, which must be included for a relevant comparison.
     
  12. gonzo
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    gonzo Senior Member

    KE is not fuel burned. It is the energy on the fuel multiplied by the efficiency of the conversion of energy.
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    What the hell are you two talking about? Baekmo, that was nonsense.

    No, Gonzo, KE is not fuel burned. It is kinetic energy. Work done in accelerating water rearward so that the reaction force described under Newton's 3rd law (momentum) can propel your boat. So your power requirements scale with kinetic energy, but your thrust scales with momentum.
     
  14. baeckmo
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    baeckmo Hydrodynamics

    Acceleration of fluid mass is a continous process during the transport job, which is why you must consider the time scale. Take a look at the dimensions, for a start. Momentum is mass times velocity, that is kg*m/s. Thrust is a force, that is mass times acceleration, that is kg*m/s^2. Momentum does not propel any boat, but momentum change per time unit does.

    As for the kinetic energy, its dimension is mass times velocity squared, that is kg*m^2/s^2. The power developed is force times velocity, that is kg*m/s^2*m/s, or in cleaned form: kg*m^2/s^3. The total energy spent to do the job is power * time, that is force * velocity * time (aka Ws or kWh).

    You may very well calculate a transport efficiency from the kinetic energy, but then you must refer to the actual work done, which is the transport of a floating object a defined distance "S", while it exerts a resistance "R" to the movement. The job done is then S*R, which may be compared to the total amount of fuel energy spent, to get a figure for transport efficiency. If you include the engine process you go from a flow of chemical energy to hydraulic power. As the lady said, "you got to keep your tongue in the right mouth" here....
     
    Last edited: Jan 29, 2021

  15. DogCavalry
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    DogCavalry I aim to misbehave.

    Right. Ad Hoc emailed me to say that Baeckmo is top drawer in his field, and we are arguing definitions, not content, and talking past each other. So I'm climbing down from DevCon4. I'll try to learn what I can, from your particular expertise, Baeckmo. No doubt your knowledge of pumps is encyclopedic. I took vector mechanics in 1983, not pumps.

    J
     
    Last edited: Jan 29, 2021
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