Trolling Motor & Battery Advice Required

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

  1. Milehog
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    Milehog Clever Quip

    Following with interest. Am considering building a 19' sailing dory with an outboard well. The thought of a 2 1/2 hp gas noisemaker appalls me. Torquedo and ePropulsion motors look good but are expensive, Also the weight of the battery is up high.
    A saltwater rated electric trolling motor with li-ion batteries straddling the centerboard case is appealing.
     
  2. Milehog
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  3. SolGato
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    SolGato Senior Member

    For anyone interested in an alternative to traditional trolling motors, Torqeedo or EPropulsion, check out the EP Carry at electric paddle dot com.

    I’m not in anyway affiliated, but have exchanged emails with the designer/owner.

    There’s lots to like about the design.

    As you can see he is using a large diameter R/C airplane like blade. He is able to do this because the motor has a bevel drive in the lower end, and unlike traditional trolling motors you aren’t dragging a big motor through the water with a small diameter prop behind it.

    I love the way the motor pulls up and out of the water, and unlike Torqeedo and EPropulsion you aren’t locked into an integrated battery system which is something I don’t like.

    And I think it’s fairly priced if you’re in the market for a lightweight efficient streamlined motor tasked to push around a small lightweight craft at cruising speeds.
     
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  4. Dejay
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    Dejay Senior Newbie

    PWM is just the simplest way to implement a throttle for the motor. They are cheap to implement, but as SolGato outlined apparently even there some manufacturers skimp.

    I think ideally for solar you'd want a MPPT controller DC-DC converter. That way the solar panels can run on the voltage where they generate maximum voltage, while the voltage for the motor can be different.

    Check out Will Frasers thread and videos: Electric propulsion design process https://www.boatdesign.net/threads/electric-propulsion-design-process.62309/
     
  5. SolGato
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    SolGato Senior Member

    Sure, PWM will greatly increase range and allow a motor to consume less energy.

    It’s like riding a bike. Once you get up to speed, in order to maintain speed you only have to pedal every once in a while allowing you to use less of your energy.

    But if you want to go full speed, you have to pedal all the time, using max energy.

    So if you plan accordingly so that your cruising speed is a speed you are able to reach while your controller is using its PWM feature, then you will be able to better take advantage of your batteries capacity or the power generated by your solar system.
     
  6. Dejay
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    Dejay Senior Newbie

    Interesting. I suppose you can't add a significant gear reduction in that bevel gear because then it gets big again.

    And you loose some efficiency with the bevel gear but probably less than a planetary gear with inline motor.

    But you can use a larger, slower motor up top that runs at more efficient RPM. Does he have any gear on top?
     
  7. SolGato
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    SolGato Senior Member

    With the shape of that Dory, I don’t think a Torqeedo or EPropulsion is going to do much for you in the speed department without consuming a lot of battery capacity. Look at the runtimes listed for those motors at full throttle and you’ll see they are quite short.

    What they do offer is a compact all in one lightweight package which is what appeals to many, especially if theft or security is an issue or you want to share between vessels.

    I think a regular saltwater trolling motor as you suggested would be a wiser fit if budget is an issue and you envision yourself needing to travel a long distance at times. Just remember that the important thing to consider is thrust in relation to the conditions you’ll be operating in.

    With sailboats, we tend to go out when there is good wind, and often good performing boats are designed to be light. If your hull has a lot of surface area above the water line, it can easily be pushed around by the wind, and maneuvering in those conditions is when you need that thrust.

    With Lithium batteries being more affordable, offering longer lifespans, allowing for quick charging, weighing a lot less, maintaining and providing a higher voltage while being discharged, and being available in different voltage configurations, it’s a much better way to go across the board.

    If you look at that EP Carry motor I mentioned, you can see they pair their motor with a very small lightweight 24V LiFePo4 battery.

    A 50ah capacity 24V LiFePo4 battery will be about the size of a normal car battery (Group 24) but weigh half the equivalent Deep Cycle Lead Acid battery, so they are easier to remove, install and transport to and from a boat.

    A battery that size powering a 24V PWM Trolling Motor running at 70% throttle will go for a good long time. And if you can go Brushless, you'll gain even more efficiency. Add some solar, and you’ll improve the efficiency even more.

    Personally I like to think more in terms of current than power when it comes to motors, batteries, etc.. If you think of a battery as a gas tank, and the rated discharge capacity of that battery as the amount of fuel it can hold (remember with lead acid you want to cut that rating in half ), with the amps drawn by an electric motor to maintain a particular speed for a duration of time like the traveling speed of a car to reach a destination, you can easily get an idea of range based on duration of use at a set amount of consumption (amps drawn from battery). That’s basically how these companies calculate their predicted range and speed and runtime figures.

    So to size a system, it’s best to work backwards and ask yourself what your typical use will be with regard to traveling distance and your ideal cruising speed. This will determine the size of your battery based on a motors efficiency.

    Some people just need electric propulsion to get a sailboat off the dock or in and out of the mooring field once an outing before recharging, in which case you could get away with a very small capacity battery, while others want to be able to cruise under electric propulsion all day long, which requires a larger capacity battery. If in the latter scenario that person typically wanted to travel to a destination where they would anchor and remain for a while before returning, you could add solar to allow the battery to recover while at rest, which means you could either make use of a smaller battery, or think of the solar as a range extender.

    In any case, if you’re not in a big hurry and speed isn’t an ultimate requirement, trolling motors will work well for many small boat applications. Just size your battery accordingly, make sure you have adequate thrust, and remember that choosing and matching the most efficient components for the system will result in better range and runtimes.
     
  8. Milehog
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    Milehog Clever Quip

    Thanks for the idea. I just emailed EP Carry about the suitability of it's use on my target boat, shortening the tiller to a stub and buying sans battery.
     
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  9. SolGato
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    SolGato Senior Member

    I don’t remember exactly what’s going on under the cover, but basically what he’s been able to accomplish is use a small lightweight DC motor to drive a large prop at slower RPM.

    Yes, there is some loss in the transmission, but if you look at the runtimes versus capacity of the battery, and speed achieved by various hull designs, you’ll see that he has come up with a well balanced and reliable design that meets the needs of the majority of small tender owners.
     
  10. SolGato
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    SolGato Senior Member

    I was told it can be purchased without the battery. Also if I’m not mistaken, I believe the tiller rotates up and over and can be stowed back out of the way when not in use. Take a look at the figures they have listed in their site for various hull shapes. You can see longer more streamlined hulls like a canoe will achieve higher speeds, while short inflatable dinghies will be slower. It is not a high thrust motor however, so keep that in mind.
     
  11. Milehog
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    The arm folded back might help to a degree. In the photo below the motorwell is immediately foreword of the mizzen mast, in fact the mizzen partner is in the rear of the well.
    [​IMG]
    [​IMG]
     
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  12. SolGato
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    SolGato Senior Member

    Ahh, I see.

    You might want to look into short shaft trolling motors for DIY application such as Kayaks, or even Minn Kota’s EM series which are designed to be mounted on the cavitation fins of an outdrive.

    A system like that could be mounted using a hinge, where it swings down and locks to deploy, and up to stow without the clutter of a tiller or shaft.

    Since the motor well is inboard, you’ll want to steer with the boats rudder anyways, so having a fixed position motor will work well for you with less moving parts.

    Also, the EM series and DIY motors have a wire remote with a potentiometer for variable forward and reverse thrust, so you can mount that anywhere you’d like to control and set your speed instead of having to work two tillers.

    Check out the EM (engine mount) series from Minn Kota and Caroute DIY brushless motor kits.

    I’ll post a photo of my motor mounts so you get the idea.

    My mounts are made of Ipe wood with one half mounted to the Hobie factory rudder gudgeon mount holes, and the other half hinged to it at the bottom that the stainless mount bracket for the motor then bolts to. This half doubles as a cavitation plate when the motor is swung back and deployed. I have different depth settings for navigating shallow water, and the motors can kick up incase they strike bottom or an object.

    Simple, and effective, with minimal moving parts.

    At the end of the lard that the motor is attached to, there is a Bimini pivot mount with a 1” stainless tube/arm that runs up and through a slot in the other half. Collars set along the length of the tube act as preset trim settings. I have 3, one for trailering and launching, one for shallow, and one for full depth as shown in the photos. To adjust you just pull on the arm, lift it clear of the slot and swing the motor up or down until you catch a collar. The weight of the motors and the forward thrust keep them in the water with the collar against the upper transom board.

    The motor housing mounts the EM Minn Kota motors come with are very cool, lightweight and indestructible. They would be very easy to retrofit to a drop in plug or hinged swing up mount. I’ve included a photo.

    I think a 24V 80lbs thrust saltwater EM series motor with a 24V LiFePo4 battery would be a good setup for your boat. The EM’s are PWM controlled but brushed. Brushed although less efficient, is more reliable and forgiving and you can buy all new internals like brushes, holders, thrust bearings, seals, etc. at reasonable prices from Johnston for the Minn Kota stuff. I’ve rebuilt and serviced quite a few.

    I used a 55lbs Minn Kota as an auxiliary to move my 20’ Trimaran around the mooring field. Worked fine except in strong winds when trying to turn into the wind. An 80lbs would have been perfect.

    I also originally built my Solar Catamaran with a pair of 80lbs EM motors which worked fine. But then I started adding weight like the Bimini hardtop and solar, and wanted to get out in ocean and swell, so I decided to go a little bigger when I had my brushless motors made, ultimately moving up to a pair of 120lbs thrust motors. They also happened to be slightly more efficient than the manufacturers 100lbs thrust model.
     

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    Last edited: Apr 23, 2021
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  13. Milehog
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    Milehog Clever Quip

    Now that has my remaining brain cells firing.
    The plans call for plugs, one cut away for the motor shaft and one solid, at the bottom of the well so the hull itself would require no modification.
    Thank you!
     
  14. portacruise
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    portacruise Senior Member

    Many years ago, I used a trick to get more speed from a $100 low- cost resistor speed control 12 volt Minn Kota rated at 30 amps Max by running it at 24 volts. First, I bypassed the resistors speed control, and ran the motor directly through an external 12-24V pwm controller. Then I experimented and found /replaced the standard prop with a smaller diameter and lower pitch propeller that kept the current below 30 amps at 24 volts when the pwm was at the full power 100% setting. That smaller propeller luckily happened to be the one that Minn Kota used on their high power expensive 24 volt model. So I was running at twice the RPM, and twice the volts but slightly below the 30 amps at maximum speed. It did increase the Troll Motor powered top speed of my boat by around 30% as I recall. When I disassembled the motor after about three years frequent running at 24 volts, and inspected it, there was no damage! So that's my amps are important verification story for Mister Sun Cat, haha!
     
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  15. portacruise
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    portacruise Senior Member

    Just a note that I also like the Electric Paddle motor referenced by SolGato, and have posted about it in the past. But there are some other boat Motors used by hobbyists that have an even lower appendage drag compared to the EP, based on the Asian long tail design. Rick Willoughby made a version like that which was used to break the 24 hour HPB distance record.

    Not to be considered an endorsement or recommendation for EP, as I have never owned one, and there are some minor things that I dislike about the design.
     
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