1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Good energy efficient motor to use with 12 volt battery?

  1. May 6, 2015 #1
    I am designing a fishing pontoon boat at home. I am wanting to build into the pontoons a small water jet engine on both sides. Does not need to be jet ski type speeds. Just looking for an energy efficient motor that will exceed trolling motor speeds and still be small. I am more of a do it all design and create inventor. Electrical is one of my lesser experienced areas. So the question is.

    What is a good motor (i.e. Drill, grinder, sander, etc.) that is cost effective and can be found easily that will work on a 12 volt marine battery and can be enclosed inside a pontoon?

    Would I have heat concerns with the motor?

    What are specific criteria I need to look for? Voltage and what not.

    Will I need a separate regulator of sorts for voltage control?

    This you tube video inspires me that I can do this

    What say you the physics community?

    Thanks a bunch for your input, I really do appreciate it.
  2. jcsd
  3. May 6, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Not enough info by far.

    How much power do you need to move the pontoon at the required speed? One way to estimate that might be to tow it at the required max speed (loaded with people and into wind etc) and measure the tension in the tow rope.

    Power (in Watts) = Tension (in Newtons) * Velocity (in Meters per second).

    Then you need to estimate the efficiency of the jet unit. That's quite hard unless you have manufacturers data. If you assumed it was 50% efficient then you could multiply the figure above by 2 to get the motor power needed.

    Then factor in the motor efficiency. Perhaps 60% for a brushed motor and 80-90% for a brushless motor?

    That gives you the power that the battery has to deliver to the motor (assuming no losses in the speed controller).

    Next thing to check is the likely duration. If the battery is 12V divide the power by 12 to get the current. Then work out the duration (in hours) by dividing the battery capacity (in AH) by the current (in A). Suppose you needed 1500W (2 horsepower). The current would be 1500/12 = 125A. A 100AH battery would run for 100AH/125A = 0.8 hours or 48 mins. However it might need to be a special battery to run at 125A continuous. At that sort of current you might not get anything like the full 100AH capacity. It might make sense to use more batteries in series to reduce the current.

    You also need to a graph for the impellor power vs RPM. That will give you data needed to work out the motor constant (rpm/volt).

    Then you might have enough data to select a motor. However it's quite likely the sums will be out and you will probably have to experiment. One advantage of a regular prop is that if you get the sums wrong you might be able to change the prop diameter or pitch to correct the error rather than change the motor. With an impellor it's not possible to change the diameter and there might not be many pitch options available?
    Last edited: May 6, 2015
  4. May 6, 2015 #3
    Thank you so much for the informative and quick reply.

    The truth is that this will most likely be a backyard put together by experimentation design. I am designing and building a 1 man pontoon system that can be hauled in a van or back of truck in pieces and assembled on site within 15 minutes. It will measure 72" x 60" pontoon tip, to pontoon tip. My idea is to the keep the deck clear of all things but a seat and fishing gear. With a couple of small water jets installed into the pontoons that will require only an electrical hook up to the battery is my ideal situation.

    I guess I am mostly looking for a point in the right direction to start with for experimentation so as to not blow up a motor or 3 by hooking up to too much current. As for power ratios I am looking for something on power with or a bit faster than trolling motors so that a need for a big gas engine is not required. The desire is to market this boat as a reliable and affordable option for those people needing a boat that does not require a trailer or truck to pull. It would be useful also in emergency situations where a gas engine is not practical or access to that hard to reach spot is limited. The boat will also be equipped with portable solar chargers to prolong power consumption. I suppose I could use trolling motors but I was looking for a smaller unit with increased RPM's if possible.

    Thanks again for the reply
  5. May 6, 2015 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Just had a look at a cheap electric outboard..
    http://www.force4.co.uk/flover-33f-electric-outboard.html?gclid=CPbhi4j8rcUCFcrpwgodj6sAQA#.VUp7amdFDmQ [Broken]

    The data for that suggests about 33lbs thrust from 12*30 = 360W.

    You can certainly get brushless motors for model aircraft or boats that are around 360W. You can get motors that will turn a large prop slowly or a small prop quickly depending on the motor constant. Some motors will handle up to 50,000 rpm but at that speed the prop will have to be very small. If you want max efficiency you normally want to turn a large prop slowly rather than a small prop quickly. It might even pay to use a gearbox to reduce the rpm so it can turn a larger prop. The problem is they will draw too much current if you hook up the wrong size prop or impellor or use wrong gear ratio. You need to match the prop to the motor and that's difficult without data on the prop you want to use.

    If you can find data on a prop that includes a power vs rpm chart we might be able to work out a suitable motor to turn it.
    Last edited by a moderator: May 7, 2017
  6. May 6, 2015 #5
    Awesome thanks so much I will research that info.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook