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Old 08-03-2021, 14:36   #1
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Electric motor for small boat

Good Day all,

I bought my girlfriend a 16Ft O'Day Javelin. Moored in the harbor, I need to motor out about a 1/2 mile each way to get out to the bay. An easy to operate electric is clearly the best way to go for her. I just need to verify that the system I am thinking of will be suitable.

This motor: https://www.amazon.com/gp/product/B0...ref_=pd_gw_unk

This battery
https://www.amazon.com/Mighty-Max-Ba...ustomerReviews

As there is no shore power I will add a 20W Solar panel and a charge controller, we will only be using this boat once per week so plenty of time to charge between uses. I will also add a battery box. Do you all believe this all will work as intended?
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Old 08-03-2021, 16:03   #2
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Re: Electric motor for small boat

The motor is low power, less than 1hp. So plan on using max throttle and it will have a hard time pushing the boat into a stiff wind... though if there is wind, then you would just sail it right? The battery's highest spec discharge is 35.2A for 1 hour, which you would exceed. At 55 Ahr in a Lead Acid battery, really only ~25Ahr usable capacity at 55Amps. That will allow you to run for 30min total in a day. I'm not sure how far you need to go or how fast that motor will push you.. assuming 4knts, that's a 2 mile total range. For $70 you can get a 109A-hr Lead acid battery of which I'd get 2 (i'd get 2 and run them at 24V and get a 24V trolling motor). That would more than triple your range for $140, just $30 more.
The solar would be a trickle charge at ~1.6 amps max. So it would take 62 hours of full sun to recharge 100Ahr... Figure 4 hours of full sun and that's over 10 days to charge with 20W solar. And that's assuming no cloudy, rainy days. Now if you only use the motor for 30min with your select battery, than 5 days of full sun would charge it... but that's marginal. I think you should increase solar to at least a 50W panel. Note that if you go 24V you need a controller that will output 24v or you could put a disconnect switching between batteries to allow 12V charging while not in use.



Also Note that the motor control arm you picked is not waterproof and will, at minimum, need to have a cover on it during storage.
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Old 09-03-2021, 05:11   #3
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Re: Electric motor for small boat

No, I don't believe it will work because the motor seems too small and the charging arrangements too little.

I do wonder why you consider a motor necessary? 1/2 mile is not far, and the boat is a nimble little thing that you can short-tack along even the narrowest of fairways. Looking at the Google map of Northport, NY I don't see an obvious need to motor. Ultimately you can paddle it, row it or even scull it.

Would it not be simpler and ultimately better in all respects to let your friend do what dinghy sailors always do when the wind is against them: tack, tack and tack again until she's in open water? With the wind aft of the beam there is, obviously no need to do anything other than sail.

FWIW, I've taught a great number of people to sail on Enterprise dinghies and never had a motor of any kind on any of them. Technique beats infernal combustion every time :-)

All the best.

Trentepieds
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Old 09-03-2021, 07:03   #4
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Re: Electric motor for small boat

Thanks guys,

Both of you bring up good points, I was a bit concerned that the battery was insufficient and the motor was questionable. I was unsure how to calculate the range and didn't trust the Amazon reviews of the battery of how much life and distance it provided. Now I'm getting a bit concerned if I put enough battery in it, it's going to be too much weight to put in the bow of this small boat.

The boat is for my girlfriend. I want her to be able to use it without me. I already have a 3.5 Mercury, but I'm not confident she can start it reliably. I need some kind of motor as the wind in the harbor can be fickle and has died on me more than once. I am in the south side of Northport harbor, so I have to go past the town dock, it's quite narrow over there, Not much room to tack even if the conditions allow.

I suppose my options now or either she's going to have to learn to deal with the Mercury or I moor it over in the harbor. Downside to the harbor mooring is getting to the boat, finding something she can handle itself to paddle out and it is less protected over there. No easy solution.

Anyway, appreciate your time
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Old 09-03-2021, 08:46   #5
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Re: Electric motor for small boat

I have a Cal20 that has a Minn-Kota 33lb thrust motor.
I replaced the stock prop with a Kipawa. Max speed went from 2.0-2.25kt.

https://www.kipawapropellers.com/

My boat is about 3,000lb with just me and gear.

The stock props are intended to shed weeds and accelerate fast. Kipawa are intended to be more efficient and have a slightly higher speed.

Even with a much lighter boat I wouldn’t expect you to be able to exceed 3.5kt, unless you wanted to mess with the pitch on the prop blades.

The battery you linked might be big enough. My bigger concern is that it might not really be a deep cycle battery.

The rule of thumb for solar panels is daily output in Ahr is 1/3 the Wattage of the panel using an MPPT controller and 1/4 using PWM controller. Using a PWM controller you would be fine with a 20W panel going out once per week. Bigger battery and going out more than once you’d want a bigger panel.
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Old 09-03-2021, 10:18   #6
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Re: Electric motor for small boat

I used to use a 30lb thrust on a O'Day Day Sailer to get down my canal to the bay. In to a 15 knot wind it would make little, if any progress. It would give me a chance to pull up the sail at the canal opening and improve my point but a little scary especially when you're getting pushed up against a sea wall. I know a 55lb would work better but I couldn't quantify how much better.
The 20 w panel should work depending on lighting conditions.
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Old 09-03-2021, 10:29   #7
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Re: Electric motor for small boat

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Originally Posted by sagablu View Post
Downside to the harbor mooring is getting to the boat, finding something she can handle itself to paddle out and it is less protected over there. No easy solution.
I've seen/heard of people using Stand Up Paddle Boards (SUPs) as boat to shore to boat "dingy" - should be easy to transport to/from launch and added advantage of fun/useful in it's own right. Downside is likely much more expensive than what you hoped.
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Old 09-03-2021, 12:14   #8
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Re: Electric motor for small boat

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Originally Posted by Rblakenyc View Post
I've seen/heard of people using Stand Up Paddle Boards (SUPs) as boat to shore to boat "dingy" - should be easy to transport to/from launch and added advantage of fun/useful in it's own right. Downside is likely much more expensive than what you hoped.
Stand up paddle board was actually my first thought If we moor it out in the bay where I don't need a motor. Unfortunately my girlfriend wants to use the boat in the spring and fall also, she's not too happy with cold water. I have a large windsurfing that I have used as a paddle board. Not particularly efficient but I own it and it would be sufficient for the job. Boats, girlfriends, nothing is easy!
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Old 09-03-2021, 15:02   #9
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Re: Electric motor for small boat

The early submarines used lead acid batteries and gas themselves when saltwater got into the system. Am I being paranoid but ne or two lead acid boats on a little javelin that may occasionally capsize is scary in my books. I agree with the theory of sailing or sculling to get home (or a tow. But in this modern era, are lead acid batteries and salt water a "threat"?
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Old 09-03-2021, 15:18   #10
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Re: Electric motor for small boat

Hydrogen is a byproduct of the charging process.
While a sub and the javelin dinghy boat travel on saltwater, the similarities end there.
A sub is intentionally closed. It may be vented a bit when on the surface but on the whole a sub is designed to be airtight most of the time.
A dinghy on the other hand would likely be very well ventilated and hydrogen being lighter than air would easily disperse.
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Old 09-03-2021, 16:07   #11
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Re: Electric motor for small boat

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Originally Posted by tidalwavenz View Post
The early submarines used lead acid batteries and gas themselves when saltwater got into the system. Am I being paranoid but ne or two lead acid boats on a little javelin that may occasionally capsize is scary in my books. I agree with the theory of sailing or sculling to get home (or a tow. But in this modern era, are lead acid batteries and salt water a "threat"?
Hopefully I won't turn the boat into a submarine, but capsizing is legit threat. As my girlfriend can be an aggressive sailor, we have actually capsized beach cats a couple times. Another downside to electric I didn't think of, although I don't think my gas engine would particularly enjoy a dunk either
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Old 09-03-2021, 21:51   #12
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Re: Electric motor for small boat

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Originally Posted by sagablu View Post
As there is no shore power I will add a 20W Solar panel and a charge controller, we will only be using this boat once per week so plenty of time to charge between uses. I will also add a battery box. Do you all believe this all will work as intended?

I have a motor like that. Different brand 30a not 55a, on a 16' lightweight aluminum fishing boat. It will be fine for what you want to do. It will take you 15 minutes to go half a mile.


Battery is marginal. I would use two.


20w solar is also marginal, I would try for 50w.
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Old 10-03-2021, 04:21   #13
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Re: Electric motor for small boat

A trolling motor with a variable speed throttle instead of a five speed will almost double your battery run time. At half speed the variable uses half the energy as the five speed.
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Old 10-03-2021, 07:46   #14
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Re: Electric motor for small boat

Continuously variable throttle only saves power if it is running a PWM controller which will save significant power at part throttle. If the throttle is really running a variable resister, part throttle won't save much power.

On Minnkota it's called "Digital Maximizer™" Don't know who else has that technology on their trolling motors. I would pay extra for it.
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Old 10-03-2021, 11:08   #15
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Re: Electric motor for small boat

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Originally Posted by zstine View Post
The motor is low power, less than 1hp. So plan on using max throttle and it will have a hard time pushing the boat into a stiff wind... though if there is wind, then you would just sail it right? The battery's highest spec discharge is 35.2A for 1 hour, which you would exceed. At 55 Ahr in a Lead Acid battery, really only ~25Ahr usable capacity at 55Amps. That will allow you to run for 30min total in a day. I'm not sure how far you need to go or how fast that motor will push you.. assuming 4knts, that's a 2 mile total range. For $70 you can get a 109A-hr Lead acid battery of which I'd get 2 (i'd get 2 and run them at 24V and get a 24V trolling motor). That would more than triple your range for $140, just $30 more.
The solar would be a trickle charge at ~1.6 amps max. So it would take 62 hours of full sun to recharge 100Ahr... Figure 4 hours of full sun and that's over 10 days to charge with 20W solar. And that's assuming no cloudy, rainy days. Now if you only use the motor for 30min with your select battery, than 5 days of full sun would charge it... but that's marginal. I think you should increase solar to at least a 50W panel. Note that if you go 24V you need a controller that will output 24v or you could put a disconnect switching between batteries to allow 12V charging while not in use.



Also Note that the motor control arm you picked is not waterproof and will, at minimum, need to have a cover on it during storage.
Im not challenging your specifications or science nor your knowledge .
My question is , have you ever actually taken a 55 pound thrust trolling motor for a spin with any kind of deep cycle battery on ant type of watercraft ?
I have a 35 pound thrust unit picked from a garbage heap needed a propeller that was acquired for 10$. The battery I used with it was a used Walmart 75$ at todays prices. It sat on an 84 Avon 11.5 inflatable with inflatable keel. I once rescued a 24 ft pontoon boat which had spun off a propeller on the Tombigbee River.. I was tooling upstream. The pontoon owner was overly excited about being three miles upstream of a lock n dam. I tied up alongside, got him relaxed and tugged him about 3/4 mile into an empty marina slip which of course got the marina hag all excited too lol. Relax people ! There were at least another dozen unrented slips. Two hours later tugged him over a mile to his marina slip. My 85$ Harbor Freight solar panel keeps it amply charged even by moonlight. That same panel is now 69$ today. Went for a ride upstream a couple more miles at about 4 knots and didnt get stranded.
Judgementally , due to my experience with my 35# motorguide I would question the quoted scientific jargon on paper.

The OP is going a 1/2 mile. One way.
I would imagine he would not attempt the trip in a gale ...
Oh he has a sailboat ...
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