Advice on the correct power rating for my Autonomous boat

Discussion in 'Propulsion' started by mitchellleary, Mar 30, 2017.

  1. mitchellleary
    Joined: Jul 2015
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    mitchellleary Junior Member

    Hi all,

    I haven't been on for a while and my Autonomous Boat project has evolved somewhat since I last posted. Some issues I have overcome and others have presented more questions.

    Originally I had planned to build a 5metre boat which would have a battery bank to give continuous power to the engine and in turn, the batteries would be powered by solar power.

    My boat is still a mono-hull, sealed, kayak-shape but I have scaled down the boat to only 1metre(maybe I'll go back to 5m in the future).
    The issue that this has presented is in the size motor that is required to power my boat. Originally, someone recommended a motor to me(it could have been someone from this forum but not sure) this one: http://www.goldenmotor.com/ (the 2-3KW version).

    Now that I've scaled my boat down and with its capacity for solar cells on the deck and batteries inside, a power draw of 2-3KW is now too much so I am looking for some advice from the super-brains of this forum who can help me to choose a motor with a lower power rating but that would be powerful enough to power my boat.

    The details/requirements of my boat are;

    - 1 metre in length
    - Monohull, shaped like a kayak with a V-hull
    - Total weight capacity 40kg
    - Be capable of powering through waves and currents satisfactorily. The boat doesn't need mega speed, but needs a decent propulsion to travel against currents where necessary.

    I guess my question is; what is the smallest rated power(a ballpark figure) that I could safely power my autonomous boat at a decent speed with its max weight of 40kg onboard? I ask because now that I am not able to use the 2-3KW motor, I genuinely don't know. Is it 600W? 100W? 1KW?

    I welcome all advice and opinions and am very grateful for your time replying to me!

    Thanks in advance.
     
  2. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hi Mitchell,
    I do realize that you are not sure about the speed you should design your boat for, but still I think that you should specify a ballpark design speed in order to allow qualified people to respond to your question.

    In alternative, since your model is 1 meter long, IMO you can search the internet for technical data of existing RC models.
    For example, this Graupner model: https://www.graupner.com/Tug-Boat-Nordic-Scale-1-/-75-Premium-Kit/21016/ is 1.04 m long, 0.22 m wide and weighs 10 kg. The motors are powered by 2x 6V 7Ah batteries via a 40 A controller (see the specifications page).
    So at full throttle it gives 6x40=240 W power input to motors. Assuming 60% motor efficiency and 30-40% propellers efficiency, you get 50-60 W effective power.
    The speed is not specified but looks decent: https://www.youtube.com/watch?v=c1AhVVRLZF0 . The "hull speed" formula 1.34*sqrt(LWL) gives Vmax=2.48 kn (1.27 m/s) max speed, and by the looks of the bow and the stern waves I'd say that the model attains the Vmax with the installed power.

    Cheers
     
  3. mitchellleary
    Joined: Jul 2015
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    mitchellleary Junior Member

    Thanks Daiquiri,

    I really appreciate your input and this was just the kind of advice I was looking for.

    My question would be(and this is some information I didn't include with my initial post: my boat will operate at sea); could the tug boat in the video you posted operate against strong currents and waves at sea? In your opinion? Was it operating at full throttle in the video as well?

    Thanks again and anyone else who can also contribute to the discussion is welcome to do so.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I think that if it was operating at full throttle, then it was pointlessly wasting a huge amount of power to wave drag. A tug-type hull has a practical speed limit which cannot be exceeded regardless of installed power.
    The wave train the tug model was creating in the video indicates that it was running at maximum practically attainable speed - the so-called "hull speed", equal to 1.34sqrt(LWL). In order to reach that speed a model of that size and weight probably needs some 20-25W effective power (100 W electric power), so any further power input can only increase the waste to heat, cavitation and turbulence, with no further increase of the boat speed.

    A lighter and better-shaped boat can perform much better. Check this measured resistance of a 1-metre class sailboat:
    http://radiosailingtechnology.com/i...surements-on-an-international-one-metre-yacht

    [​IMG]

    Evidently, with the same power input you could arrive to as much as 4-5 kt with this hull type. But it weighs (or bears) only 4 kg (the tug boat is 10 kg).

    Sea currents can flow at 1-2 kt, so you'd have a safe speed margin in any case.
     
  5. Mermaid Co
    Joined: Oct 2012
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    Mermaid Co Junior Member

    Design will have to make sure that it can propel through wind,wave and current. So minimum speed criteria for this autonomous boat shall be carefully checked.If it is designed for salt water,fresh water no issue.

    How a hull behaves at that speed depends not only on hull length, as is commonly used to calculate hull speed, but also, displacement, length/width ratio, bow and stern shape and several other factors. Most significantly, the traditional 1.34 x sq. root of the water line length formula was derived experimentally by William Froude for heavy displacement conventionally shaped hulls such as military and cargo ships, not planing hulls or kayaks which are usually classified as semi-displacement hulls.

    However because a kayak is so light it doesn't necessarily follow the hard stop rules of the 1.34 rule of thumb.if you go to Gerr's power required formula's you can see the difference.

    you can use gerr's formula to compute power requirement.

    http://www.mermaid-consultants.com/power-prediction-of-boats---gerr-method.html
     
  6. daiquiri
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    daiquiri Engineering and Design

    We are talking about a 1 meter long boat, operating in fully laminar and surface tension-dominated flow regime. Power-prediction methods developed for ships and recreational vessels are not valid in these conditions. And Gerr's formula gives questionable results even for full-size vessels, as was shown in several discussions in this forum.
     
  7. mitchellleary
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    mitchellleary Junior Member

    Thanks both Daiquiri and Mermaid Co. Both very useful replies.

    Daiquiri - Thank you very much for your examples as they help me to understand something that is a complex area for me. Much appreciated.
     
  8. Jim Caldwell
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    Jim Caldwell Senior Member

    One meter probably will not give you enough solar panel area.
     
  9. mitchellleary
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    mitchellleary Junior Member

    Hi Jim,

    Thanks for the message.

    With the right battery configuration and motor, we are going to have solar cells custom made which will cover the boat's surface. This should give us continous power. The motor won't be powered directly from the solar cells.
     
  10. Mermaid Co
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    Mermaid Co Junior Member

    One more interesting part is, Have you considered how good is the directional stability of this motor powered kayak shaped boat?
    I think they are intrinsically directional unstable in following wind and sea. So motor shall be remotely steered continuously to maintain desired direction.
     
  11. mitchellleary
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    mitchellleary Junior Member

    Thanks for the comment. It's very interesting because this is something that I had considered.
    Do you have any further advice on this?
    I was thinking to have the GPS adjust every few minutes to compensate for the directional instability.
    Are there any other considerations that you think are unique to a kayak-shaped boat at sea? (the boat shape is loosely-based on a kayak shape. It has better hydrodynamics than an actual kayak).

    thanks again!

     
  12. Mermaid Co
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    Mermaid Co Junior Member

    Keel strip,fin,skeg,rudders are few means which will increase directional stability of this kayak boat but off course with some complexity in system and 5 to 10% of additional power.

    In normal directional unstable ships,helsman will periodically change rudder angle or azimuth thrusters or an automatic steering apparatus is provided .In this system command rudder angle signal is produced from a deviation signal representing the difference between a predetermined course and the ship's heading by means of a operational circuit and a rudder angle,a compensating circuit is provided before the operational circuit and the gain of the compensating circuit is increased when the deviation signal is in the small range, thus preventing the occurrence of directional instability.How this system will be integrated in small boat with simple system that is a question.

    So if you want to use GPS and avoid any zig-zag motion of this autonomous boat,coding will require which will integrate propeller or rudder angle with determined course and deviation in real time. There is always a lag between rudder or propeller response and actual deviation.So that should be accounted well.

    This boat might be small but basic principles are still same.All these considerations depends on whether you want to scale up the idea or just keep it that small for fun.
     
  13. mitchellleary
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    mitchellleary Junior Member

    Thanks Mermaid Co! A very interesting reply. It gives me alot of information to look at.

    I originally was going to work with a 5 metre version but decided to scale down for now to keep the expenses of the project down more than anything.
     
  14. Jim Caldwell
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    Jim Caldwell Senior Member

    Mitch, sorry but you are not going to produce even 100 watts with today tech. in that size range. That may do as a proof of concept trials.
     

  15. mitchellleary
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    mitchellleary Junior Member

    Thanks Jim.
     
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