Surface propeller and low speed displacement boat

Discussion in 'Surface Drives' started by Alberto78, Dec 12, 2008.

  1. Alberto78
    Joined: Dec 2008
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    Location: italy

    Alberto78 New Member

    Hi to all
    I am new in the forum and my name is Alberto from Italy.

    I have a big question for You and I hope could You help me.

    Is it possible to design a surface propeller with an high diameter and very low propeller turning in order to fit a 3 kw high efficiency solar boat?

    Which is the parameters to consider in order to define if there is any convenience to use a surface propeller instead a traditional one.

    The average speed is 8-10 knots and maximum speed can reach 20 knots.

    What Do you think about? Any experience regarding will be appreciate.
    Could indicate me a reference for very low speed surface propeller design?

    Kindest regards from Italy
    Alberto
     
    1 person likes this.
  2. Guest625101138

    Guest625101138 Previous Member

    No. An unconstrained open water prop for 3kW at 20kts will achieve around 90% efficiency while a surface prop will achieve a best efficiency around 70%.

    The trade-off between added drag for appendages on a conventional prop and the reduced efficiency of a surface prop. Usually somewhere around 40kts the surface propeller gains an advantage. There are other factors such as draft constraints that might favour a surface propeller but you have indicated a large diameter anyhow.

    Attached photo shows a surface prop I experimented with to understand operating conditions. The aim was to achieve good efficiency at relatively low speed - I determined it cannot get good efficiency for a variety of reasons. However I did not optimise the shape based on what I learnt but I doubt anyone can get better than 70% efficiency.

    You will need a very special boat to achieve 20kts with 3kW with on-board solar collectors. The solar panels will need to be specially built and I expect they will be expensive. If you have some in mind I would like to know the source and price.

    Rick W
     

    Attached Files:

  3. brunello
    Joined: Jul 2008
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    brunello Junior Member

    Efficient low speed semi submerged props

    In 2005 I experimented, with Genoa and Naples universities, three different designs for five-bladed large diameter semi submerged propellers.

    The research was a comparative one, aimed at finding out if an already optimised conventional fully submerged twin screw installation, on a displacement vessel, could be beaten.

    We choose twin vs twin, counter rotating, in order to avoid the drawbacks normally associated with single partially submerged props. A single large diameter surface prop, is normally difficult to tune and make efficient, mostly because of the high side forces it develops.

    All trials were conducted on the scale model of a 73 mts lwl fully displacement ship, with a bulbous bow and dry transom stern. The model had already been tested with a conventional submerged twin screw set up, in self propulsion mode, and all data were available. The actual ship had been built after the tank testing and tank data were found to be reliable when scaled up.

    We asked Renato Sonny Levi, an undisputed master, to design the three 'challenger' surface props and the Naples university tank to modify the ship model to take the new drive system.

    During the entire work, we were continuously recommended by many experts (famous propeller designers and builders, lecturers from the university, ship designers etc.) to drop such a stupid research and to avoid an useless loss of money and time. According to the ‘experts’ it would never work, since surface drives are ok at high speed only. All usual arguments about appendage drag etc. etc. Slow speed, let alone displacement vessels, with a 'surface' drive? "Creazy".

    Sonny designed radical props, even if there is no knowledge at all into this area of surface prop design. We got all the props models nc machined from billet, tested them first in the Genoa cavitation tunnel and then in isolation in the Naples tank, using a Z-drive style pod. Already some results were quite interesting, since these props, due to peculiar geometrical factors and slow turning, showed some surprising results, departing from ‘conventional’ surface prop behaviour. It is still to be understood if there was a scale factor contribution.

    The ship model was towed naked (no appendages) and then all final trial were run in self propulsion mode with each set of props, at different speeds, different displacements and all the like. Matching resistance vs power absorption curves, it was clear that the props needed tuning for top performance (pitch adjustment), which in fact was not done for this first set of trials.

    Regardless of the not perfect prop to hull match, a consistent certified increase in performance was achieved, reaching a +13% compared to the ‘traditional’ optimised screw propulsion. A lot of work needs to be done, but we feel the result was spectacular – and a lot of sceptical ‘experts’ became our sponsors.

    Full pictures and videos are available, with all tank results.

    The project led in fact to a worldwide patent, hold by Flexitab (my own company) which has recently been entered in an European funded research program. We have now developed an in house code for a full CFD analysis on a surface propeller, matching the available physical test data we have, with very good results.

    So I feel that your project should be fully feasible and I am convinced that you can beat, with a properly designed slow turning large diameter partially submerged (surface) multi bladed propeller, in a twin counter rotating set up, any submerged prop, at any speed.

    Conversely, I am not surprised that a single, large diameter, twin blade prop, with not optimised blade design, in ‘surface’ mode, should not work properly and produce poor performance.

    Brunello.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Brunello
    I am interested in the videos and pictures if you can provide links.

    What efficiency did you achieve?

    I can get 85 to 87% efficiency for lightly loaded boat props. I would be interested to learn if you can achieve this level with a surface prop.

    The two bladed prop had such low efficiency that I did not bother even trying to measure it. It was under 50%.

    The best surface prop efficiency I have seen is around 70%.

    Rick W
     
  5. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I recall reading somewhere an article with an excerpt from an interview with Renato Sonny Levi, where he was talking about his work on new propeller shapes and sections which would yield efficiency gains. It was written by A. Soccol, if I'm not wrong. So he was probably referring to the research work you have described.

    Time ago I was meditating about surface props for displacement speeds. I concluded that aeration, centrifugal water particle scattering and water splashing (together with the acceleration of water added mass) during blade entrance were the enemies to fight if one wants to improve efficiency. You are probably the right person to ask how correct are these conclusions?

    Mr. Brunello, is there a site where more info could be found about this work? I'm very curious about it.
     
  6. Guest625101138

    Guest625101138 Previous Member

    I found this link:
    http://www.icmrt07.unina.it/Proceedings/Papers/c/44.pdf
    I am assuming this relates to the work.

    The best open water efficiency obtained is only 70%. The same as I have seen before. This is well below what can be achieved with a submerged prop designed for maximum efficiency. Certainly high eighties is possible and maybe over 90% with care.

    Rick W
     
  7. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    I too am interested in developments here and post this to get notification of future posts...
    Thanks to all contributors....
     
  8. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    I think the key here is the big difference in loading. In typical displacement boats and ships the thrust loading coefficient is high, since the propeller diameter has practical limits and speed is low. Just the most basic propeller theory limits the efficiency well below the figures you are giving and above that comes the restrictions caused by cavitation and actual propeller design. Thus even 70% could be a very good value in many cases.

    In Rick's case the loading is very low due to limited human power and very large diameter of the propeller. Also cavitation is not a limiting factor, thus high efficiencies are possible.

    How big would a 85% propeller be for the actual case we are discussing? How much thrust is needed at the average speed of 8-10 kn?

    Joakim
     
  9. Guest625101138

    Guest625101138 Previous Member

    Joakim
    The significant point is that the upper limit of efficiency for a submerged prop is over 90% while the upper limit for a partially submerged prop is about 70%.

    The overall design of hull and prop to achieve the efficiency potential wrests with sound engineering knowing what contributes to losses. The submerged prop starts with a big advantage from an efficiency perspective.

    Rick W
     
  10. Joakim
    Joined: Apr 2004
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    Location: Finland

    Joakim Senior Member

    Rick
    It is clear that submerged propellers have an advantage in most cases. I just wanted to point out that 70% is not at all a bad efficiency in many real displacement boats and ships. Are there any papers that refer to actual 85% efficiency on any propeller that would be usable in a "normal" displacement boat/ship? These propellers have normally wide blades designed to avoid cavitation and vibration and can not typically have a very high efficiency in any condition.

    Here is one set of curves for "obtainable open water efficiency": http://www.gidb.itu.edu.tr/staff/sogut/den322/Notes/Ship_Prop.pdf
    It doesn't show anything above 70%.

    High efficiency propellers don't look or behave at all like "marine propellers" and thus comparing those to surface propellesr designed to replace marine propellers is comparing apples to oranges.

    Far better efficiencies are possible with lighter loadings of planning boats or very low drag boats like yours. I don't know where to put this solar boat, since there isn't much information about it.

    Joakim
     
  11. Guest625101138

    Guest625101138 Previous Member

    Not sure if you would regard a Wageningen B-series with an area ratio of 30% as a "normal" boat propeller but such a propeller has an open water efficiency nudging 90%. All but the smallest of props with high aspect blades will get close to 90% open water efficiency at their best operating point. The challenge is then to place it in a open water condition while pushing or pulling a boat.

    Rick W
     

    Attached Files:

  12. Guest625101138

    Guest625101138 Previous Member

    If the solar boat is going to achieve 20kts with 3kW it will have a lightly loaded prop. It will also need to be a well designed hull of very low weight.

    The attached shows the sort of prop efficiency possible under such circumstances.

    None of the partially submerged prop data I have seen would come close to matching that efficiency.

    Rick W
     

    Attached Files:

  13. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Well that's quite close to something that could be used in a small sailboat with a small engine where you can afford to have a rather big propeller compared to power, but the operating conditions are far from the optimal efficiency J at which the thrust is very low.

    If you used Waegening B-series based propeller optimator for "normal" displacement boats/ships, you will not get anything close to 90%.

    If the solar boat can really reach 20 kn with just 3 kW, it could use a very small propeller with good efficiency. Even 15 cm diameter would give ~80%.

    Is 20 kn possible with 3 kW? The smallest racing boats (3.1 m) can reach 32 kn with an unmodified 15 hp outboard at total weight of 160 kg. According to Savitsky method even those would need more than 3 kW of effective thrust at 20 kn, thus at least a 4 kW motor. Would a slender hull do better?

    Having 3 kW of solar power requires close to 20 m2 of surface area, which would add weight and aerodynamic drag and also probably spoil weight distribution of a planning vessel.

    Joakim
     
  14. brunello
    Joined: Jul 2008
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    brunello Junior Member

    Pictures of Tank testing in Naples by Flexitab

    The data discussed in my previous message is not available online and is part of a research program which is still very active today, I attach a few pics to show what we are working on. We have definitely beaten the fully submerged twin prop set up, especially at the higher speeds (due to shortage of knowledge on the lower end, where we are still closely matching).

    We think that quasi propulsive efficiency, or what you can actually achieve with a drive system physically installed onto a vessel, is what really matters.

    Diameter constrains are a physycal limit for submerged props, as shaft & brackets drag & turbulence ahead of the disk are. The cyclic loading of a conventional prop in the up and down position is a fact, as the angled flow into the prop disk is. All factors that, in practice, bring actually achievable efficiency down.

    The Flexidrive H Drive, developed for heavy displacement vessels, has no appendage drag (yes, it does matter) except for rudders, and the thrust line is horizontal as the axial flow into the prop disk is. What we get in the cavitation tunnel and in the isolated prop tests, is exactly what is delivered to the vessel, when installed.

    There is a lot of work to be done, I know, but note that we went straight into two very reputable universities, Naples and Genoa, regularly doing commercial work, explained our goal and challenged them to choose one of their best self powered ship models, with twin submerged counter rotating props. Their wrote the final reports, they made all tests, they were the first to be totally amazed.

    Flexitab is making a huge economical effort to make these advances and patent them worldwide. While I was the one with the original concept, it would have been impossible to make it work without Sonny's knowledge and huge experience, which never interfere with his free thinking approach to design. Look at the props and you will see that he did not just scale up something.

    We are now in a very advanced stage of CFD virtual replay of the trials, which is matching very well with all test data.

    brunello
     

    Attached Files:

    1 person likes this.

  15. Guest625101138

    Guest625101138 Previous Member

    Thank you for the photos.

    I see a fundamental flaw in your work in that you are setting up the props to give the very best efficiency possible from new technology and drawing comparisons with old submerged prop technology.

    Existing ships propellers are the result of evolution in a period when fuel was in great abundance and efficiency was not a significant driver.

    To make a realistic comparison with a submerged prop you need to start with a clean slate and target highest possible efficiency with that technology. It has a fundamental advantage that has not been fully exploited.

    Rick W
     
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