# How much motor do i need?



## himesrun (Jul 14, 2009)

Hey Everyone,
I was given a jon boat by my aunt a few years ago and now i want to finally get it on the water.The boat is in great shape for being nearly as old as me with no old repairs or dents. It really just needs some fresh paint. I took it on a long float down the Juniata river near Bedford PA over the 4th and everything looks solid on it. Closer to where I live however most of the lakes where I'm going to have it are electric motors only. I've never owned a boat before so I really know nothing about...well...anything. #-o I want to get a trolling motor this weekend but I'm not sure what size to get. I'm guessing it doesn't have to be very big. The boat is a 1977, Sears, flat bottom, 14', 32" accross the bottom. It seems rather shallow at only about 14" though. As far as weight goes the boat is basically bare with only two seats, oars, fishing gear, etc., no decks. So what do you all think? I know nothing about pounds of thrust, shaft length or anything like that so any info I can get will be great.
Thanks in advance.
Paul


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## Jim (Jul 14, 2009)

:WELCOME:

Thanks for joining! :beer:

The biggest electric motor you can get is a 12 volt 55 pound thrust if you are going to use one battery. That is what I would go for.


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## russ010 (Jul 14, 2009)

I agree with Jim... You'll want a 36" shaft, 55# thrust motor.

Not sure how much you're wanting to spend, but a Minn Kota will run you about $250.


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## bassboy1 (Jul 14, 2009)

While the 55 pound would be the ideal option (that size boat will scoot pretty well with a 55), you probably wouldn't be unhappy with a 30 pound thrust. Personally, that is what I would do. A 30 pound Minn Kota (spend the extra 10 or 15 bucks on the Minn Kota over the BPS "Prowler" - you won't regret it), will run you 110 or so.


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## russ010 (Jul 14, 2009)

ya know... after reading bassboys post, it triggered something....

I've got a 1232 at my parents house. I tried a 42" 55# on it and it wouldn't move for crap. I also had a 36# thrust MotorGuide with a 28" shaft and it pushed better than anything else would.

You'll stay out on the water longer with 1 battery and a 30-36lb thrust motor than the 55 too


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## Victor Coar (Jul 14, 2009)

I bought a BPS Prowler 2 mos ago along with a minn kota on board charger and an optima battery. it works pretty well. I haven't run outta juice yet...Its a 55lb thrust not really fast but I like it.


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## Zum (Jul 14, 2009)

I see speed and trolling motor used all the time.
The standard props on trolling motors aren't made for speed...period.
I bet a 30lbs TM on that size boat will go as fast as a 55lbs TM.
It's going to be somewhere around 4-6mph.
Thats all they do,unless you can get your boat on plane and it's hard to do with a trolling motor on the bow,speed and electrics aren't ever going to be the issue.
The reason people go the a 24V or 36V has alot to do with time on the water and not speed.
Sorry going off the topic.
A 30lbs should be fine with a good battery or batteries being equally important.


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## russ010 (Jul 14, 2009)

Zum said:


> I see speed and trolling motor used all the time.
> The standard props on trolling motors aren't made for speed...period.
> I bet a 30lbs TM on that size boat will go as fast as a 55lbs TM.
> It's going to be somewhere around 4-6mph.
> ...



agreed... but for that boat, my guess your top speed will be more like 3-4mph...

I have a 71# on the front, and 2 55# on the back of a 1546 and the fastest I go is 6-7mph by myself.

On my 1236, I had a 40# on the front, and 2 55s on the back and never got over 4.6mph


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## bassboy1 (Jul 14, 2009)

Zum said:


> The reason people go the a 24V or 36V has alot to do with time on the water and not speed.


Torque too. I guarantee a 30 pound thrust is not going to do squat on a 20 foot fiberglass bass boat. Just not enough thrust behind the prop. 

In most cases, the very same prop is used on both the 30 and 55s. Not sure if a different prop is on the 24 volts, but that is a different story. So, at a given RPM, that prop is going to push your boat at a certain speed, no matter what motor is turning the prop. As long as the motor can turn the prop to that RPM, the boat will go that speed. I am fairly certain that the max RPMs of both motors are similar as well, so, on a light boat, both motors will get about the same speed. Okay, if that is the case, why are there different thrust motors? Well, that is for bigger boats. On a larger boat, getting it up to speed takes a lot more effort. Thus, a smaller motor will bog down, and not be able to turn the prop the RPM needed to get the boat to a certain speed. A bigger motor can turn the same prop under the heavier load, and can thus get the bigger boat up to about the same speed as the smaller one. 

Now, there will be some difference, but _most of the time_ it is unnoticeable. 

What I have deduced by all the stories I have read is that Motorguide props tend to have more pitch than Minn Kotas, for whatever reason. That is why Russ010's little boat runs faster with the little motor, than the big one. The 28 pounds of thrust is more than adequate for the boat, and fully capable of turning more pitch of prop, which pushes the boat faster. The Minn Kota apparently has slightly less pitch, so while the motor has plenty of power left, there is no means of getting that power into the water, as it can go as fast as the prop can pull water at a given RPM. 

Once you get past all that, you get into hull speed. Hull speed is the maximum speed a hull travels in displacement. Thus, where the weight of the boat is displacing an equal amount of water. Once you reach the hull speed, the boat starts to get up on plane. On jon boats, that is usually 5 - 7 mph, whereas a barge is going to need to go MUCH faster before it thinks about getting on plane. To jump up on plane is going to take much more power than is feasible on electrics, due to the fact that you have to carry batteries, which will weigh the boat down. So, once you have reached the hull speed, you have pretty well maxed out the speed you are getting. Any more thrust, and you are just slipping the prop more. 

I see a lot of electric only guys with 3 or 4 trolling motors, of different thrusts and brands, going at once, when in reality, some of them are only burning juice, as they can get to the hull speed with 1 or 2, and if you have mixed pitches, some are actually slowing you down. 

If you really wanted to max out the speed, you should get a 55, and start working on adapting different props to it, to find the max speed (they don't sell thousands of trolling motor props with different pitches like they do outboards, so you would be adapting props from old small outboards more than likely). Otherwise, just get a 30, go fishing, and not worry about crunching numbers to obtain performance. I highly doubt you are planning on doing much racing in a 1432 with an electric on it.


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## Victor Coar (Jul 14, 2009)

> I bet a 30lbs TM on that size boat will go as fast as a 55lbs TM



wow really? I don't know anything about this stuff I bought the best I could afford... what is the difference then? is it that they are the same speed but can push more weight?


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## russ010 (Jul 14, 2009)

bassboy1 said:


> Not sure if a different prop is on the 24 volts, but that is a different story.



nope... they are the same as a 12v


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## russ010 (Jul 14, 2009)

Victor Coar said:


> > I bet a 30lbs TM on that size boat will go as fast as a 55lbs TM
> 
> 
> 
> wow really? I don't know anything about this stuff I bought the best I could afford... what is the difference then? is it that they are the same speed but can push more weight?



no... it's the boats hull speed. The 30# will push the boat to the max, and the 55 will do the same, but the boat is only going to as fast as it's hull will allow. That's why just because you get a bigger motor doesn't mean you will go faster. 

You can get a larger motor and use the lower speed settings and this will allow longer battery life


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## bassboy1 (Jul 14, 2009)

Victor Coar said:


> > I bet a 30lbs TM on that size boat will go as fast as a 55lbs TM
> 
> 
> 
> wow really? I don't know anything about this stuff I bought the best I could afford... what is the difference then? is it that they are the same speed but can push more weight?


For the most part. Now, I haven't actually experimented with this, but crunching the numbers, and working with an understanding of the principles behind these motors, and looking at the results others have had, that is my best understanding of it. In actual practice, there are almost always other anomalies that play into effect, without being noted when running numbers. 



> You can get a larger motor and use the lower speed settings and this will allow longer battery life


I respectfully disagree. Remember, both props have the same pitch. In the time between writing my last post, and this one, I have found the answer, and it is a 4 inch pitch. More on that in a minute.

So, at the same RPM, that prop will push the boat the same speed, no matter what thrust is behind it. Now, I am 98% sure that both motors max out at the same RPM, one with more torque than the other. If I am wrong, this theory is shot to heck, however. Both motors would have to be on the same speed setting to achieve that speed. Thereby, the bigger motor is eating up more power for the same performance. 
Same principle as me towing my boat with the chevy 7.4l 454, whereas you tow with your FIL's 5.7l 350. Our boats are approximately the same weight. Both trucks will pull the boat just fine. However, you use less fuel, as there is less engine sucking it down, under fairly light load (our boats are nothing for either engine). Provided both trucks have the same tranny and rear end gear ratio, the both engines have to turn the same speed to pull us, but yours turning at a given speed eats less fuel than mine. 

Minn Kota trolling motors have a 4 inch pitch 2 blade prop. Thus, when putting the prop in the water, and spinning it, under ideal conditions, and no slippage, the slope on the blade would push the prop forward 4 inches, for every revolution. Thus, at the max RPM, the max boat speed is going to be around 5-6 mph. Coming straight from MK, they design their props for acceleration. This follows the same principles as putting a prop with less pitch on your outboard for pulling skiers, when hole shot is more important than top end. If you put more pitch on it, you would have more top end speed, but acceleration, or hole shot in the case of the outboard, would be compromised dramatically. I don't know the pitch of the Motorguide, but if I had to guess, it would be a higher pitch, because they tend to run a tad faster, and if you experimented with it, you would find that it would accelerate a bit slower.


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## Zum (Jul 14, 2009)

You going to get a trolling motor,battery and don't forget charger.
The more Ah a battery has the more time you will have on the water.
A 12V TM draws about an amp per pound thrust.(at full throttle)
If your battery is 40lbs thrust and your battery has 120Ah,you will get around 3,probably 2.5 hours run time,thats at full throttle.Going half speed or fishing your time will increase because your not going full throttle.
Never run your batteries all the way down,kept them topped off with distilled water and charge them back up as soon as you can.
If you take 2 batteries and hook them in parallel,you will get twice as long on the water.
https://www.kipawapropellers.com/
I havn't used these props and only have read what other people have said about them.
Trolling motors,IMO,aren't made for speed,but you can diffenitly enjoy the day using one.


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## Victor Coar (Jul 14, 2009)

:shock:


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## Zum (Jul 14, 2009)

Victor Coar said:


> :shock:


Why you shocked?
Hopefully these posts don't confuse you...go get a TM and enjoy.
OOPs thought you were the starter of this post.


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## Victor Coar (Jul 14, 2009)

Sorry, I just wasn't prepared for all the quality information. there isn't an emoticon of someones head exploding.


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## himesrun (Jul 14, 2009)

Wow! 
Thanks for the response guys. I had no idea I would get so much to chew on. There were a lot of good points but the one that hit me first was that, "no, I wont be doing much racing on the water". That said it would be nice to get back to the the ramp quickly in a pinch. It sounds to me like the difference in top end speed wont be that noticable between a 30# and 55# though. 

Am I right in thinking that a shorter prop on a shallower boat would perform better than a longer one? It seems to me that a shorter prop would push the boat more or less in a more horizontal line and a longer prop would do more to push the bow up. Maybe you want the bow up? I dont know, Im just thinking out loud now. 

Again, thanks for the info guys


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## Jim (Jul 14, 2009)

Victor Coar said:


> Sorry, I just wasn't prepared for all the quality information. there isn't an emoticon of someones head exploding.



Exactly! :mrgreen:


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## Zum (Jul 14, 2009)

> Am I right in thinking that a shorter prop on a shallower boat would perform better than a longer one? It seems to me that a shorter prop would push the boat more or less in a more horizontal line and a longer prop would do more to push the bow up. Maybe you want the bow up? I dont know, Im just thinking out loud now.


I take it that you are putting the TM on the stern?
The bow is never going to come up with a 12V electric.
I think your talking about the shaft length?
If so,the motor/prop part,should be under the water about 10 inches or so.
The rest is out the water so you don't want to have to reach way up to control the TM,so on a smaller transom like that boat,you don't need a real long TM shaft.
So you need a 12V trolling motor,a good Deep cycle battery(big Ah)and charger.


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## bassboy1 (Jul 14, 2009)

The height is adjustable, so if you get a longer shaft motor, you won't have to lower it down all the way - the handle would just be a little big higher. It really depends all on how you want the handle, depending on your seat arrangement, as to what shaft length to get.


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## stinkynathan (Jul 15, 2009)

Also, the longer shafts are generally used for boats with more freeboard and/or ones that operate in heavier seas. You'd be surprised how long of a shaft you'd need to keep the prop submerged on a comparatively tall walleye boat in a 2-3' chop.


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## himesrun (Jul 16, 2009)

Ok, got it i think. Yes i was talking about shaft length and not prop. I had an extra long day at work and was about asleep when I posted that.Thanks for all the info. I'll update this post and let you know what I get and how it works.


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## TrackerTom (Jul 17, 2009)

I have had several setups that were electric only and have chased speed quite a bit. I have run a 12/32 and a 14/42 as electric only boats. One big motor definitely beats multiple smaller ones. Personally, I love the 24 volt stuff. Here's an example. I ran the 14/42 with a 30 on the front and a 40 + a 50 on the back thinking that the 3 props would do better than a larger motor and off of some talks I had with Minn Kota. The best it would ever run on GPS was 3.4 mph. I bought a Minn Kota 80 lb maxxum and it would do 3.9-4.0 on only that motor so compared to 130 lbs of combined thrust the 80 beat the heck out of it. I was able to run 4.7 with the 80 and the 50 running. From what I have heard from some local electric only guys. The difference between 24 volts over 12 is a lot, but the difference between 36 and 24 is not as great. On the 12/32 boat that I had a 50 lb. pushed it nicely. Longer boats are faster then shorter boats when the width is the same, but wider boats soak up power quick and run slower. My last test has been with my 16/54 and I have only tested it with the 80 on the back. The weight was still there from my outboard, but it was tipped all the way out of the water. The best it will do is 3.4 with that motor by itself, though I think it would pick up quite a bit with a 24 volt on the front too and the outboard weight missing. I hope all that info helps you guys. 

You do know that your boat would be exceptionally fast with a 250 Optimax on it


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