Surface propeller and low speed displacement boat

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

  1. Joakim
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    Joakim Senior Member

    Could you give us some numbers? If you can not give any for your propellers, at least you could give the ones you are comparing with. I would like to have the speed, power/thrust, efficiency and the number and diameter of propellers for the ship in question at the nominal operating conditions. Also the size of the ship (LWL, BWL, draft) and wake factor would help.

    Then Rick could try to find out a submerged propeller system, that would be clearly more efficient.

    From the pictures I would estimate, that the propeller diameter of SPP propellers are about 1/3 of BWL, thus 1/2 submerged would have draft of 1/6 BWL.

    For most ships the submerged propellers are only 1/2-2/3 of draft, which could be 1/3-1/5 of BWL. Thus the submerged propellers could be 1/10-2/9 of BWL, thus clearly less than 1/2 of the SPP propellers. 1/2 submerged the SPP will still have more than double the "actuator area" than the submerged one and thrust loading coefficient will be less than 1/2. Cutting thrust loading coefficient to 1/2 gives about 10% edge on the theoretical maximum efficiency. I think this is the key to the possibility of having better efficiency in spite of the inferior peak efficiency of SPP propellers.

    Increasing submerged propeller diameter is typically not an option. Draft is limited and having propeller below keel is a security issue. You also need quite much space between the propeller and hull and bringing propeller closer to water surface makes cavitation worse.

  2. Guest625101138

    Guest625101138 Previous Member

    The original question was for a 3kW solar boat to do 20kts. A submerged prop for this application can achieve 90% if such a boat could be built. In my view the only possibility would be a hydrofoil and it would need very exotic panels. It may not need full 3kW collection because the cruise speed nominated was 8 to 10kts so a battery could could be used for short duration bursts.

    I know there are many factors that contribute to reducing the prop efficiency but they can be reduced with careful design.

    The test data I linked to above shows the surface prop peak efficiency is 70%.

    With regard to heavily loaded surface prop I can see some serious design issues regarding cyclic loading of the blades so scaling up to large applications will bring many more issues to resolve.

    Rick W
  3. Joakim
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    Joakim Senior Member

    All we are doing is guessing, since the original question did not specify much about the boat. 3 kW could be max solar power, max total power or power needed at 20 kn or 8-10 kn. In any case a submerged propeller can have a very high efficiency, if rpm and diameter can be properly selected.

    I was more referring to the post by Brunello, which discussed SPP in very different enviroment.

  4. Alberto78
    Joined: Dec 2008
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    Alberto78 New Member

    Dear All
    Thanks for Your marvellous help. I was abroad for one week and I can not reply really fast. Today I check by Bp-delta method described on Propeller handbook by Dave Gerr the open water propeller design.
    I have find 0.86% efficiency 2 blade 0.3 DAR 20” pitch and 17” diameter with 13.1knots boat speed target and with 700 reduced rpm on 3kw motor with a final thrust of 47 Kgf.

    ALso other specifications:


    “ 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?
    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.

    From preliminary computer mono hull drag simulation by Michelet, a program for hull resistance of slender displacement forms, (in brief displacement 300Kg, Water line 8mt, beam line 0.4mt, draft 0.20 mt ) we have find:
    • 8 kts 11kgf drag resistance;
    • 15Knots 30Kgf drag resistance ;
    • 20Knots 50kgf drag resistance.
    Now after some modification on solar panels and battery we will reach a final 200Kg displacement weight with pilot. We do not run an new simulation right now.

    Rick Willoughby

    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.

    We will install around 1.5 Kwp PV solar panels, standard efficiency around 16%. In order to reach 3 kw peak power we use the batterypower with solar panels.


    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.

    With optimised blade design, in ‘surface’ mode, it is possible to reach any comparable result? In my opinion the target to reach is to optimize the propulsive efficiency so maximize( the propeller efficiency*hull-propeller installation).
    And more, in order to reduce the blade area with the surface propeller it is correct to move near the cavitation propeller speeds or there is a efficiency ratio between the blade load and cavitation limit?

    To all

    I enclosed a picture to show the boat from first testing in june 2008 without solar panels and with a standard trolling motor fitted in the center of boat. The new propeller, open or surface, will be fitted at the stern.

    Anyway I take a decision and I will test the best open water propeller designed for this boat and another configuration with the best surface propeller specifically designed for this boat.
    I am very glad to public on this forum my next experiences and I hope You could help me also from this point.
    In order to test the two configurations I will report motor current drag Vs the GPS boat speed during the same day, an empiric way I know but what I can do.

    Any help, considerations, advise, collaborations for open water and surface configuration will be the best.

    Best regards from Italy

    Attached Files:

    • 022.JPG
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  5. masalai
    Joined: Oct 2007
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    masalai masalai

    Best wishes for success in 09....
  6. Guest625101138

    Guest625101138 Previous Member

    I am estimating the boat at 8m long and 420mm beam. I expect you might be able to build it with panels and low weight motor/battery for all-up weight of 200kg.

    With a good prop you could expect top speed of 16kts with 3kW. It will take more like 5.9kW to do 20kts.

    Rick W
  7. Joakim
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    Joakim Senior Member

    I don't know what is your design goal, but I think a well designed planning hull would be more efficient above ~15 kn.

    With the data you have given, I would forget the surface piercing propellers. Submerged ones will certainly outperform them.

  8. Guest625101138

    Guest625101138 Previous Member

    The planing hull will be better above 15 kts as you point out but the long slender hull will have a huge advantage in the 8 to 10 kt region and I think this is a realistic cruising speed. It should achieve 8kts with only 400W at the prop. 10 kts will require 760W. So with 1.5kW of collection it will be able to store energy even at 10kts when in good sunlight.

    Rick W
    Last edited: Dec 25, 2008
  9. Frosty

    Frosty Previous Member

    Brunello your pictures of the twin surface prop boat appears to have the rotation opposite to a normal rotation, Is there a reason for that?

    And why did you choose the tunnel rudder?

    Sonny Levi would sit on my boat almost every day for a cup of tea, He talked constantly of slow moving surface props -even a tug.

    My semi displacement cat 14 ton 500Hp total is very satisfactory and achieves speeds in excess of manufacturers claims with conventional props and 600HP. Against all (experts ) advice to the contrary Sonny was correct.
  10. FoggyBottom
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    FoggyBottom Junior Member

    Folks, this is a great discussion. Regarding the actual hull design, don't all hulls produce a lift component?

    If they do then this lift has an associated drag?

    Is it possible to deterime at what speed this vessel wil start to plane and generate additional drag due to lift?

    From an efficiency perspective (not speed) would it be more efficient to have an underwater hull (fully submergered) with a strut to allow the soar panels above the water? In this case batteries could be used as ballast.

    When a boat planes and the boat rises above its static freeboard; is there a linear loss of engine output to counter act the amount of lift generated?

  11. Joakim
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    Joakim Senior Member

    Not all hull forms produce lift. They can produce sink (downforce or whatever is the correct term) as well. Producing lift causes some drag, but at higher speeds reducing wetted surface is more important thus planning or foiled vessels have a clear edge.

    Lift is porpotional to the square of speed. In the semiplaning region (Fn from 0.4 to 1-1.3) lift is already important, but hydrostatic force is also. Above that hydrostatic forces can be mostly ignored, if the hull form is planning.

    Close and above Fn=0.4 "wave making" resistance becomes very important. A submerged hull will have more wetted area and thus more frictional resistance, but it can avoid wave making resistance. Submarines usually are more efficient under water due to this. Slender hulls have very much reduced wave making resistance, thus they don't have the problem of very rapid increase of total drag around Fn=0.4.

    According to Savitsky method drag/lift is equal to tan(trim angle).

  12. Guido
    Joined: Oct 2011
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    Guido Junior Member


    Dear forum users,
    I'd appreciate very much your opinion on a new semidisplacement/low planing speed boat project.
    I'd be very grateful in you could spend few seconds in filling in the questionnaire I prepared at following link:

    As soon as the poll will be finished, I'll share with you the main overall results.
    Thanks for your collaboration.

    P.S. Sorry if in the questionnaire you'll find some italian words, it's a limit of Google tool I've used to prepare the poll.
  13. Frosty

    Frosty Previous Member

    I notice in your pics that the rotation is opposite to conventional , is there a reason for that. Still no reply to this obvious difference in propulsion rotation.

    Alberto you have a refreshing outlook on surface propellers, many people just dont understand them even though they think they do. I knew sony levi very well and I built my semi displacement cat with surface props because of him. I experienced the 10% increase in performance as expected in surface propulsion, but the smoothness has to be experienced.
  14. sottorf
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    sottorf member

    indeed a very interesting development. You mentioned you tested the propellers on a scaled mdoel of 73m vessel. What speeds were you targeting and in future which sized vessels will you be aiming at for the application of this drive?

  15. daiquiri
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    daiquiri Engineering and Design

    Brunello never replied to any of our posts back in time when this thread was young and fresh. I sincerely doubt he is still following this after 4 years but, hey, who knows? Miracles happen, sometimes... :)
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