Speed/efficiency at various kts - prop, surface drive, jet, etc.

Discussion in 'Propulsion' started by black_sails, May 10, 2016.

  1. black_sails
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    black_sails Junior Member

    Could someone share some rules of thumb for what 'the zone' is for different types of propulsion and it's efficiency at different speeds? Like i'm told surface drives are more efficient (i'm not sure how much more efficient) at higher speeds... i'm wondering whether jet drives are ever more efficient than props... and how wide of a speed range that a single prop selection can normally be good for.

    Ie - say I wanted to design a boat capable of 0-65 knots (thats wider than I would probably expect to design, it's just for purpose of discussion) - if I used a prop designed for 20 knots, how much efficiency loss is there by 40 knots... and vice versa? If I wanted to efficiently cruise at 9 knots (or whatever hull speed was) when not planing for a long range cruise, yet have immense reserve speed capacity throwing power at the problem when on plane, at what point would it make sense to start designing something like separate props for high and low speeds? (like two passagemaking submerged props or jetdrives and one big surface piercing screamer) I'm wondering how they do it on some of those CODAG systems/if they already do that and when "one prop" (or one speed of prop with twins) is no longer adequate.
     
  2. jehardiman
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    jehardiman Senior Member

  3. Barry
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    Barry Senior Member


    The following is an excerpt from Hamilton Jets information

    http://www.hamjet.co.nz/global/waterjet-overview

    Waterjet propulsion has many advantages over other forms of marine propulsion, such as stern drives, outboard motors, shafted propellers and surface drives


    HamiltonJet waterjets are designed to be optimized for vessel speeds from 25-50 knots, dependant on applications engineering variables including hull resistance and type, engine power rating and rpm, impeller rating, etc.

    High Efficiency
    •Propulsive coefficients as good or higher than the best propeller systems achievable at medium to high planing speeds
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Are you referring to the theoretical "open water" performance, or an actual real vessel i.e behind a hull?

    If theoretical, yes, I would agree. But a prop needs a hull...once the hull is in front..the open water characteristics change and shall never achieve the open water theoretical values - no brainer.

    In practice jets v props is always a mixed bag as you noted. But at higher Fn jets win hands down. The only caveat is a Swath. Props on swaths operate at near open water efficiencies and are more efficient than jets.
     
  5. jehardiman
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    jehardiman Senior Member

    I personally never believe ad notices.

    Notice the very carefully worded 1st paragraph. Basically it says "your mileage may vary".

    The second paragraph is patently a false flag. It doesn't say "efficiency" it says "propulsive coefficients". Note that it doesn't even say "propulsive coefficient" or more correctly quasi-propulsive coefficient ( eta d) which is the ratio of useful power obtained to input power. Realistically, from all data, it is similar to what is shown in RANCHI OTTO's figure: water jets have efficiencies ~10% less than open water wheels.

    While jet drive excel at specific applications, efficiency is not its forte. To quote PNA, 1988 ed, Vol II, Chapter 6, Section 10.2 : "Many claims have been made for jet propulsion, but these cannot be substantiated of grounds of efficiency". The text then goes on to explain why and shows actual trials data. Even allowing for improvements in inlet design, the efficiency of the overall unit is limited by the losses to the impeller, which is designed in the same manner as any propeller. So a properly designed prop, which does not have inlet and duct losses will always have a slightly higher efficiency...as all the engineering data shows.
     
  6. black_sails
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    black_sails Junior Member

    thank you jehardiman for that wonderfully specific response which i'll relink here because it's so good!

    [​IMG]

    Assuming that's accurate anyways but what even are those other propeller types i've never heard of? :) So the crossover point is right at 35 knots it seems? Is there any reason to expect it to ever be higher or lower than that on certain hullshapes and types?

    It would seem that a jet is best for specialized conditions where you don't care about efficiency as a top priority, a conventional propeller is 'best' for efficiency up to 35 knots and it's only above that that other prop designs are worth looking at. What are surface piercing drives considered to be?
     
  7. black_sails
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    black_sails Junior Member

    Would you have any references you suggest I read up on (including up to naval engineering level) to better understand how the hulls in front change characteristics either up or down and might change the order/conclusions implied by the efficiency graph I relinked?
     
  8. Barry
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    Barry Senior Member

    The text I supplied was written by Hamilton Jet and I am not arguing its validity.

    Perhaps Baekmo is around



    I will digress here a bit, from an engineering viewpoint, the term efficiency seems to be extremely abstract/undefined but in this case we would have to define what efficiency we are looking for. While the OP has asked for a broad answer for efficiencies for a yacht from 0 mph to 65 mph, without suggesting a size of yacht/boat and the speed at which he wants the "best" efficiency between drives. If he is trying to make a boat do 65 miles per hour, I would expect that the boat in his mind is 35 feet, fixed weight

    So when I refer to efficiency, I am assuming that it is the amount of fuel burned per hour to make this 35 foot craft run at 50 mph) [from which a person could calculate MPG at 50 mph]

    So with my definition of efficiency, I would wonder if a three stage axial flow jet with 3 thrust enhancing stators could be more efficient than a propeller.

    The propeller with one stage, higher pressure on the thrust side, lower pressure on the forward side, perhaps more cavitation due to pitch angle, more bleed off from high pressure differential, etc,
    might be in fact less efficient than a jet for this set of parameters. Additionally, any prop drive, surface piercing left out, will have additional drag due to struts, shafts, rudders

    Of course all drives might be more efficient at different speeds, with other parameters, but for the sake of further discussion on this thread, this might be a good discussion point.
     

  9. Ad Hoc
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    Ad Hoc Naval Architect

    b_s

    You firstly need to understand the interpretation of the data below. The data below is in terms of the PC or propulsive efficiency. This is not open water prop efficiency. The two are total different. An open water prop efficiency is as the name suggests. Take a prop with a shaft and test it with water flowing into it and out of it. It is tested with no hull in front of it at all…thus as the name suggest it is in “open water”. This means one is testing the prop and obtaining data based upon the maximum or best efficiency for that prop in open water – the data closely matches the theoretical maximums in ideal conditions – rather like your graph above. It is how we design boats with hulls…we need to know what is the efficiency of the prop at a given rpm and speed of advance (water into the prop) and the power being delivered.

    When the prop is installed and we go on sea trails we measure the speed at max rpm. What occurs?...well the speed does not match the “theoretical” open water efficiencies. The simple reason is that a prop has a hull in front of it, this alters the flow of water into the prop. It speeds up or slows down or a combination of both and varies circumferentially. What this does is change the efficiency of the open water characteristics because in the open water, there is no hull in front and thus the water is clean flow and uniform. This is no longer the case.

    So what we, as naval architects, like to measure is the propulsive efficiency, PC. This takes into account many factors such as: appendage, hull, prop efficiencies as well as the shaft and relative rotative efficiencies into one simple formula.

    PC = EHP/SHP

    EHP = Power to propel or drive the vessel when naked. Resistance of the vessel naked hull means no appendages, just the hull alone. The Power being R.V (R = resistance , V = speed).

    SHP = Shaft Horse Power, or the power delivered at the prop.

    In simple language is it a measure of the overall efficiency of the system. Power in power out. So whether a prop or waterjet the speed one gets on trails is influenced by many factors. Thus once a tank test is completed and the EHP data is obtained we can quickly estimate the speed of the boat given size engines. Since we now know EHP all we now need is the SHP. Thus SHP = EHP/PC. The PC for a prop boat is different to that of a waterjet boat. As a very quick rule of thumb, you can use a range of PC for a prop boat as 0.5-0.55 and a waterjet boat from 0.6-0.65. Thus it is quick to see a waterjet boat has a better overall efficiency in the system. In other words, despite props have a very high open water efficiencies, in the 0.6-0.7 range, when placed behind a hull, all the changes to the flow of the water into the prop, the inclined shaft as no longer horizontal as in the open water test and several other factors, the overall efficiency drops.

    But as always there are caveats. The efficiency of a waterjet boat changes when the LD (Length-displacement ratio) starts to get low. The heavier the boat is for its length affects the waterjet more than a prop. Also slow speed, again starts to be much less efficient. So another rule of thumb. Below 25 knots (note I have not used Fn) props are generally preferred and below LD ratios of around 5.5 waterjets tend to loss their efficient characteristics.

    But like most rules and caveats it does not mean it is not feasible or advisable. Since we have used waterjets on a 50m catamaran running at only 22knots; the LD ratio was high, which helped. Also on low LD ratio hulls, like those windfarm catamarans waterjets are still used because their top speed is circa 30knots. So nothing is cast in stone…you select the best for you and your boat, regardless what sales data suggest. Horse for courses.

    So below are curves of PC v speed of KaMeWa waterjets early data with a few spots and much later with many more from trials which confirms the early trends. KaMeWa is now Rolls Royce. You can see PC's reaching almost 0.80, which is very high indeed.

    KMW-efficiency curves-1.jpg KMW-efficiency curves-2.jpg

    Also a more generic PC v speed of different propulsion types.

    Generic-efficiency curves-3.jpg

    Does this helps?
     
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