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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    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
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    StormUK New Member

    Thanks @gonzo I'll take a look
     
  6. ondarvr
    Joined: Dec 2005
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    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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    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
  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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  11. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Based on a rough calculation from info at Post 9, and if the Bison is a 55 lb thrust, that Troll Motor system might work to get you a range of about 20 miles @ around 5 mph of river under zero wind conditions. But the total weight do the Electric System might be at least double the weight of the 3.5 horsepower petrol engine, and the petrol engine will get you higher speed and range which might be needed under windy conditions. If the wind conditions are such that there is a net push back on your boat equivalent to 5 miles per hour, no progress could be made Against the Wind with the electric setup.

    Will @ 9, thanks for your interesting info. I have found that the deformation will vary depending on the brand of model prop that you are using. APC props made for IC engines seem to be stiffer than those made for electric , and MAS props deformed more than the IC props for the situations I have measured. Reinforced carbon fiber props made by Bolly had little or no deformation, depending on wattage, some may still be around at model shops, as Bolly went out of business. Cutting off model prop tips seemed to lower efficiency for the 16x16 props I experimented with, but I made cuts at 3/4" intervals down to ONLY 13" diameter.
     
    Last edited: Apr 22, 2021
  12. portacruise
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    portacruise Senior Member

    Thanks for the interesting post, I also do similar things on a river, but slower at 100 Watts maximum, sometimes having to Portage Upstream. It helps to ride the Eddys and countercurrents Upstream against the current, sometimes I'll only be a couple of inches from the bank. I have found that a 6.5" Thai aluminum KKK prop is much better than the stock prop, but not quite as efficient as 10x6 range apc model props. But that might only be applicable to my particular set up.

    Post 252 has some useful data on model props and info on other alterations which help efficiency:

    Efficient electric boat https://www.boatdesign.net/threads/efficient-electric-boat.27996/page-17?fbclid=IwAR3GiokK4ire6MZ2ZPeLdDKQ5VOfxotlGDC96tubY50D62hhgmLYAquQWnE#post305142

    Here's an easy and cheap way without doing alterations to see if there is an improvement using an APC model prop, fits most troll motors:

    10x4P-LH | APC Propellers https://www.apcprop.com/product/10x4p-lh/
     
    Last edited: Apr 22, 2021
  13. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    A couple things to keep in mind, first is about PWM motors.

    Most PWM controllers only offer efficiency gains up to 70% or so throttle. After that they stop pulsing, so it’s best to run them at or below the threshold.

    With regard to prop experimentation, most trolling motors lack sufficient RPM to take advantage high speed props. It’s better to tune for the “sweet spot” of the motor controller as stated above if you want the best balance of speed and efficiency and or range. For example, running a Kipawa 3 blade prop on a MinnKota won’t give much more speed as a lot of people falsely assume, but what it can do is give you your top speed at a lower throttle allowing you to operate a PWM controlled motor more efficiently.

    Also, for those looking to squeeze as much speed as possible, consider adding a cavitation plate above the prop, especially if you are trying to push an inefficient hull or heavy boat through the water.

    Lastly, be very careful prop experimenting with brushless motors. Brushless motor controllers are matched to the loads that a motor will see with the manufacturers prop. If you try to run a more aggressive prop under more load, you run the serious risk of damaging the motor controller. Also keep in mind Lithium batteries tend to hold and maintain a higher target voltage, and when combined with a solar system and a charge controller in Bulk mode, it’s possible to have too much voltage for some motors (especially brushless), requiring that you ease into the throttle, or let the batteries fall to a lower resting voltage first before dropping the hammer.
     
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  14. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Just to see if I get what happens electrically: A more aggressive prop would move more water per RPM and so need more force and run slower. So it would run at less voltage but draw more amps.
    So you should measure the amps drawn under load and check if your motor controller / batteries can handle it.
     

  15. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    These are just general guidelines, but basically what you need to worry about is the amperage. If you are trying to swing too aggressive a pitched or diameter prop, the controller might see too much amperage as the motor struggles to spin the prop at lower RPM when the PWM is pulsing. This can lead to too much heat and burn up components in the controller. Trolling motors are by design made to move big heavy planing hull boats (fishing boats loaded with gear), so they typically operate at low RPM and use a low pitch small diameter prop.

    When you use a trolling motor on a boat hull that is efficient and lightweight, you are probably going to run out of RPM without the motor drawing it’s max current. This will allow for some experimentation of higher pitch and or larger diameter props, but you have to monitor the current draw under load.

    My advise to anyone is to test a motor under load under normal operating conditions with its stock prop and see what kind of amperage is drawn. If you are well under the manufactures stated continuous max, and find that speed does not increase much from 75-100% throttle, then you might be able to get away with a more aggressive prop. However keep in mind that you still need to consider amp draw in less than ideal conditions, like for example at a river mouth, upstream, in swell or gusty conditions, etc..

    If you study Torqeedo and their wide range of props, you can learn a lot about what they have learned by looking at how they have matched them to their motor offerings. They have probably done more than anyone else with regard to prop design for electric boat propulsion. They provide pitch and diameter specs for their props for their various size motors. Their motors with the planetary gear set borrowed from power tool industry (why their motors are so obnoxious) allow them to use more aggressive props due to the gearing. You’ll note that when they introduced their latest Brushless direct drive motor (much quieter), that even though it is more powerful than the planetary gear equivalent, and capable of higher RPM, they matched it to a smaller less aggressive prop.

    Anyway, every scenario is different and should be treated as such, however due to the low RPM operating speeds of trolling motors, it’s nearly impossible to make any huge significant speed gains. That’s why I recommend instead, tuning for efficiency and reaching max speed at a lower RPM by using a bigger thrust motor that will get you up to speed at a throttle position where the motor controller is still pulsing the motor in order to best take advantage of the PWM technology.

    On my Solar Powered Catamaran, I’m running dual 120lbs 24V Brushless PWM motors and I typically only draw half their stated Amp draw ratings. With solar, I can cruise along at 4MPH without pulling from bank capacity with my MPPT charge controllers providing about 30A. This because the motors are pulsing. Once I’m above 70% throttle, I can reach 6MPH but draw double that. However I’m feeding each of my controllers 60+ Volts, so even partially shaded or indirectly oriented, my solar still has enough voltage to provide fast charging, allowing the bank to recover quickly when at rest.
     
    Last edited: Apr 23, 2021
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