Trolling Motor & Battery Advice Required

Discussion in 'Electric Propulsion' started by StormUK, Feb 24, 2021.

  1. StormUK
    Joined: Feb 2021
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    Location: United Kingdom

    StormUK New Member

    Hi,

    I am currently building a 12ft plywood skiff which I will be using to sail our local river. I am brand new to boat building and boating in general. I am thinking of using an electric trolling motor as its primary means of propulsion. I quite like the idea of quiet running, less to go wrong over combustion engines (I hope) etc.
    My query is what would be a good size of motor to use and what size battery to go for?

    Boats dry weight (according to the plans) 59kg/130lbs
    We only have about 20miles of navigable river to play with and a speed limit of 5 mph.

    Is a trolling motor even the way to go?

    Any advice is gratefully received. If additional info is required just ask.

    thanks
     
  2. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Any trolling motor will move the boat, you need to decide how long you want it move and how quickly.

    On my 17' boat I can use my 80 lb thrust, 24 volt motor for more than 8 straight hours of trolling at very low speeds (below 2mph) and not be out of battery life.

    Even on an efficient hull these motors don't typically achieve 5mph or above, they were designed for below that speed.

    The higher the thrust, the better control you will have in wind and waves.

    You don't really need more than 55 lbs of thrust for that boat, which keeps you in the 12V range. You could probably run all day on two good sized deep cycle batteries.

    The cheapest trolling motors don't use power as efficiently, so if you want extended run time get one with PWM, each brand will have its own name for it.

    Batteries are an entire subject all to themselves, the prices range from $80 to $800. You can get a "good enough" battery at Auto Zone for about $120
     
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  3. StormUK
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    Location: United Kingdom

    StormUK New Member

  4. gonzo
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    gonzo Senior Member

    An AGM battery will be a bit more expensive, but they last longer and can be charged faster.
     
  5. StormUK
    Joined: Feb 2021
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    StormUK New Member

    Thanks @gonzo I'll take a look
     
  6. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    The Bison looks like a Chinese knockoff of a name brand. It doesn't list many specs, so it's probably a low end model.

    It may work OK, it just won't have PWM circuitry, which extends your run time.
     
  7. StormUK
    Joined: Feb 2021
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    Location: United Kingdom

    StormUK New Member

    Thanks for the advice, it's much appreciated.
     
  8. Dbparke
    Joined: Jan 2021
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    Location: Montana

    Dbparke Junior Member

    I built two 9'6 cheap roto kayaks with 30lb 12v trolling motor. Small 20amp lawnmower battery lasted about 4hrs. We used them for our montana river trips. Motor up than float down. Very slow upstream, since the motor was hidden, people would watch us scratching their heads how we were floating up stream..
    Ordered some aluminum weedeater/mud/trolling props. Which gave us about 5-6 mph and extended the battery life.. The stock prop pitch is made for battling wind.
    The bass pro trolling motor was only 89 bucks, prop 14.99, batt 20.00. took out the rotary switch mounted with knob. The 30lb trolling motor is .4-.45hp.
    To do it again I would find a aftermarket .5 hp elec motor and make a lithium 12-24vdc battery from 18650 battery cells for faster charging.. Or a 1.2 hp 4 stroke mini bike motor v drive setup.
     
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  9. Will Fraser
    Joined: Feb 2014
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    Location: South Africa

    Will Fraser Senior Member

    StormUK I have an almost identical situation as yourself. I have only recently started testing out my trolling motor but the results should give you an idea of what to aim for.
    My boat is a 13ft Miracle, 12ft at the waterline and perhaps slightly heavier than your skiff. I use a Minn Kota 40 Riptide with 13V, 44Ah LiFePo battery (5.6kg).

    With the stock prop I get the following results. Range calculation is based on a 90% (515Wh) discharge.
    3mph at 167W, 9.25 miles
    3.3mph at 200W, 8.5 miles
    4.6mph at 360W, 6.6 miles

    I am busy experimenting with model aircraft propellers. The first prop, an 11x5.5, flexed badly so I gradually trimmed it down. Efficiency peaked at a diameter of 7.5"
    The blades still flexed about 30deg but speed was up 10% (on all speed settings except full power) and power required down by 5%.

    Even a small solar panel can noticeably add to range.
    Using the stock prop performance as an example, a 50W solar supplement will give
    13.2 miles at 3mph
    11.5 miles at 3.3mph
    7.6 miles at 4.6mph.

    Likewise the aircraft prop will give 15 miles at 3.2mph.
     
    Last edited: Apr 15, 2021 at 5:14 AM
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  10. Dbparke
    Joined: Jan 2021
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    Location: Montana

    Dbparke Junior Member

    When I built my kayaks I used mud motor props and they worked great on my 30lb trolling motor.. They were only 12-15 bucks each so I bought all 6 types to play with. Just checked website to see if still around. Looks like they have 50 types of props now..
    https://www.mudmotorkit.com/product-category/parts/propellers/
    They'll work much better than air props and have a nice selection and for what they are the price is great...
     
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