Trolling motor for outboard

[Daveinet]

Just thought I would report back on one of the successes of my vacation. About a month ago, I was having issues with my outboard. The float stuck, which I cleaned, and then seemed to be flakey again, which I was able to get it running again. However the outboard has always been a pain, because it is a long shaft motor, and will not lift out of the water without removing it, due to the engine well size. So for having problems, it was finally an excuse to plunge into the undocumented area of electric propulsion. Being one to not take a simple no for an answer, I dug into it a bit, and found that all who had tried unsuccessfully had not really done their homework and had chosen woefully under sized motors. From my research, everything I was reading was telling me that about a 100 lbs of thrust was equivelant to a 4 HP gas outboard. I found an 86 lbs thrust motor on Ebay, took the plunge and bought it. Bought two brand new 175 minute reserve capacity batteries from Napa, and wired the boat. I will have to say this was one of the best decisions I have made regarding the boat, and was a life saver for the week long trip. With 6 people on board, and loads of extra food, we were able to cruise along solidly over 4 mph. Kind of funny, but the speed setting of 1=1mph, 2=2mph, 3=3mph, 4 =3mph, 5=4mph+. I don't have an amp meter hooked up, but the motor spun effortlessly at what ever speed setting I used, which makes me think it could use a taller prop. I need to get my amp meter hooked up to know for sure. The batteries seemed to last forever. The first two days had no wind, so we spent a lot of time motoring, just to cool off. The meter never came off 10, which was the highest number. I measured the battery voltage at the end of those two days, and it still measured 12.46 volts, which is considered 80% charged.
From a functional standpoint, electric is sooo much nicer. Being able to run the motor and not hear any sound is almost like sailing. You just glide through the water. Docking or getting into the bay area is much easier to control. Just click the motor on and off, or into reverse with the flip of the wrist. Waiting for the line up of boats to use the ramp, I could just float without noise or exhaust, with an occasional bump of the motor to keep the boat from drifting or rotating. Then when I did need to turn, the motor can be turned 90 degrees, and spin the boat quickly without any forward motion. And most importantly, if I am in the water DW has no trouble twisting the tiller to come pick me up. Lifting the outboard into place and getting it started would have been a trick.
As far as power, comparing to my 4 HP Suzuki, I would say that it doesn't have the top speed, but does seem to start the boat moving forward from a dead stop noticeably better than the gas outboard. If I'm coasting backward, and need to change direction quickly, it does it with authority. I only had it in winds of about 12 knots, and they seemed to have no affect on the speed. Maybe it accelerated slower, but over all, wind seemed to have little affect.
I did finally find a chart on the Torqeedo website which shows what they claim as equivalent thrust to a gas outboard. It shows a 3 HP being equivalent to 68 lbs thrust electric. The complete chart is here:
http://www.torqeedo.com/en/compare?pids=1403-00|1140-00|1142-00|1234-00|1232-00|3205-00
Looking at these numbers, it all looks pretty reasonable to compete with a gas outboard. I've got about 400 bucks tide up in the motor and batteries. Just purely from a cost perspective, it seems the way to go, let alone the ease of use. Not to mention, but the batteries are up front, so it gets the weight off the rear of the boat, and puts some ballast near the keel, right where you want it.

 

[Jackdaw]

There really is no way to straight-convert Thrust to horsepower. It's because HP is a simple scaler (having a magnitude) quantity and thrust is a vector quantity with both a magnitude and a direction. Any simple conversion you see has to normalize the conversion by assuming values for things like drag, max speed, and weight of the load.

At the end if the day, you end up seeing if it works. I used a 55lb troller to push my First 235 around on windless days. It worked for that but no more. She would make no progress in more that 7 knots of headwind. Bigger would have been better but it was the only one on craigslist at the time. Your lightweight DL probably does better. One thing for sure, it's hard to beat the silence and instant-on power of an electric motor.

 

[McIntoshUSA]

We use a 55# trolling motor on our dinghy and love it.

 

[Daveinet]

There really is no way to straight-convert Thrust to horsepower. It's because HP is a simple scaler (having a magnitude) quantity and thrust is a vector quantity with both a magnitude and a direction. Any simple conversion you see has to normalize the conversion by assuming values for things like drag, max speed, and weight of the load.

Yes, I never liked the idea of thrust rating, as the test conditions are not quantified. With the right prop, nearly any amount of thrust can be achieved, but at what speed? It doesn't quantify the max current rating of the motor, so who knows what type of "gearing" could be used. Being this is a cheap China import, product support is vague. The manual does not even specify current.

As you have stated, at the end of the day, it is what works, so hence the reason for me bothering to report how this specific acts. Because my gas outboard was a 4 HP, it is pretty easy for me to compare performance to that. As it turns out, its pretty close, and seems to be very consistent with the performance information from Torqeedo, as I pegged it to be equivalent to a 3.5 HP gas, which compares to everything that is published.

The real point of the thread is if one chooses to purchase this specific motor, expect it to perform similarly to a 3.5 HP gas motor, although it may not hit the same top speed, due to prop pitch.

One other thing of note, I did try to drag four adult "dead" bodies in the water. That did slow it down quite a bit. I could pull two bodies without much impact.

 

[zeehag]

torqueedo t 1003s. excellent and even lets youknow how far you can go before recharge is needed and has a solar charger as well.
good stuff. i have owned mine or 2 years , 3 summers, now.
good choice

 

[KrustyKrab]

One other thing of note, I did try to drag four adult "dead" bodies in the water. That did slow it down quite a bit. I could pull two bodies without much impact.

 

[Daveinet]

So this part weekend I got caught in a very serious headwind. Enough to put the rail in the water running reefed main and no jib. (was trying to secure the bimini and running on autopilot) The main channel that opens up into the lake tends to channel the wind into a compressed area which increase the wind speed. Running straight into this headwind on the electric motor was, shall we say, interesting? I guess technically I was moving right along, but I did not feel particularly secure. I forgot to turn my speedo on, so I don't now how fast I was going. Faster than one typically goes when docking. Slower than I normally would go past the no wake buoys. I've not had the 4 HP gas engine out in such winds, so I have difficulty comparing. I will say that I would not have wanted to depend on it on Lake Michigan in those conditions. Coming back in, I was probably making 1 knot, sailing on just the poles. I had to use the motor to stop, once stopped the boat would pick back up and start moving again - fast enough to easily steering the boat.

Something odd I've noticed. This motor does not do well in reverse. It generates a lot of prop walk, and not a lot of motion. The gas engine had no prop walk, and would easily steer in reverse for hundreds of feet. Not sure what the difference is, except this prop is about twice the diameter.

 

[walt]

Comparing gas and electric is difficult but..

I’ve seen a ballpark measurement a few times where the current from a 12 volt trolling motor at peak is on the order of 83% of the thrust. Not at all exact but ballpark.

So your 86 pnd thrust electric motor (assuming 12 volts) would take 71.6 amps and at 12 volts this is 860 watts or 1.15 hp.

Does 1.15 hp power use for an electric outboard match 4 hp from a gas outboard..

You can get a Hobie TI up near 10 knts so its somewhat is a good platform to compare gas to electric. I have a 2.5 hp gas Suzuki outboard on my TI and it gets this boat up to about 7 knts. On a forum, someone with the 1000 watt torqeedo on the same boat only gets the TI up to about 6 knots. I don’t remember the exact number but the Torqeedo claims the equivalent of more than 2.5 hp.. Yet in a fairly controlled experiment, the 2.5 gas outboard takes the TI noticeably faster - leading me to think some of the Torqeedo claims came from their marketing department. Could be wrong however.

Since you have the 4 hp gas outboard and the electric trolling motor, try a much more controlled experiment. I like electric, use it on my dingy but some of the claims/ comparisons seems to be a little off.

 

[zeehag]

ifound the torqueedo to be most satifactory in real time situations. i use mine in winds and current and find it slow but persistent, where my gas powered obs failed miserably with carb issues and other problems when needed most. other issues being fail head gasket, fail lower end unit, .... this all adds up, torqueedo doesnt have these issues.
torqueedo keeps on keping on ..until a wire breaks then fix it and keep on going.
charging at 10 percent per hour beats the crap outta the price of gas these days...and the reliability cannot be beat.
so what yer slow.. why do you wanna be first into the bar anyway.

 

[Daveinet]

You can get a Hobie TI up near 10 knts so its somewhat is a good platform to compare gas to electric. I have a 2.5 hp gas Suzuki outboard on my TI and it gets this boat up to about 7 knts. On a forum, someone with the 1000 watt torqeedo on the same boat only gets the TI up to about 6 knots. I don’t remember the exact number but the Torqeedo claims the equivalent of more than 2.5 hp.. Yet in a fairly controlled experiment, the 2.5 gas outboard takes the TI noticeably faster - leading me to think some of the Torqeedo claims came from their marketing department. Could be wrong however.

The limitation of this comparison is that one can not assume that the Torqeedo is propped for a faster speed. In otherwords, you could put the Torqeedo on a toothpick and still top out at 6 knots. Seat of the pants feel tells me that the 86lbs thrust trolling motor has less prop slip at low speeds than the 4 HP gas engine. If I'm backing up with the gas engine, and then switch to forward, I can go full throttle on the gas engine, and it still takes the boat some time to change direction. With the electric, it changes rather quickly, and you feel the acceleration. However top speed on the trolling motor is less, as the motor just tops out on RPM.

BTW: Max current from nearly stopped to WOT was ~40 amps.

 

[walt]

Some actual experience with changing props on the Torqeedo used on a Hobie TI (from a forum and every one understands everything you read on forums is 100% accurate lol)

The T1003 has a two blade prop and is rated to do 5 kts top speed @ 1200
RPMs. The T503 has a three blade prop and is rated to do 4 kts top speed @ 700
RPMs. (This rating has been changed, it used to say 5 kts, the same as the
T1003). Anyway, if you put the 3 bladed T503 prop on the T1003 which spins much
faster it should give a considerably higher speed, right? At my dealer's
recommendation, that is what I did. No noticeable difference in speed

Also, prop slip has more to do with hull efficiency than prop efficiency. Prop slip just says how much the prop "screws" compared to the speed of the prop in the water.

Zero prop slip sure sound good until you realize that for zero prop slip, the prop is screwing at the same speed as the water and therefore there is NO thrust.. there is no acceleration of water. You must have prop slip in order to get thrust. Prop slip sounds bad (something is slipping!!!). Its just related to thrust. Say you are doing 4 mph so the prop is generating some thrust. If you want to go faster, you need more thrust - so the prop must operate at a higher "slip". This does not imply that the prop is now operating any worse.. just means the prop is screwing faster since it has to accerate water out the back at a higher rate.

don't have an amp meter hooked up

sounds like you hooked up the amp meter? Is that 40 amps you mentioned at wide open for the trolling motor at 12 volts or 24? At 24 volts, that would be 960 watts (1.28 hp) - which would be more consistent to what Ive seen the past.. but cant be sure.

 

[Daveinet]

Yes, I have an amp meter that I bought a long time ago, but had no use for till now, so it is permanently in the boat. The motor is 24 volts.

I find it interesting that the Torqeedo changes operating RPM when changing props. This really should not be. RPM on a DC motor is primarily determined by voltage. Unless there is a lot of resistance in the windings, the motor speed should not change dramatically under load, unless the power supply is not stiff enough. Wonder if peak current is limited electronically.

 

[walt]

I dont know much about electric motors so am speculating - could be wrong.

This link is interesting
https://evmc2.wordpress.com/2012/04/...-the-universe/

The Rated HP is typically the torque level at which the motor can be run continuously without exceeding the temperature at which the winding insulation beaks down

When the load on a petrol motor increases and rpms drop, torque also falls. For an electric motor torque increases.

A gasoline motor has a peak torque value for a given RPM (3600). It’s easy to peg that as a HP rating – measured torque at a fixed RPM. An electric motor produces torque in response to load. You load it, the RPM drops, and it draws more current and produces more torque.

If I understand this correctly, if you were to put a higher pitch prop on an electric motor, the power would increase because the load increased. Sounds good but the parasitic power loss in the motor goes up by the square of current so you would have to be careful of burning the motor out.

On the other hand, if you had a gas outboad that had a prop on that allowed it to operate at its peak power RPM and then put a higher pitch prop on, the outboard would simply operate at a lower rpm and the power would drop.

Looks like gas and electric behave differently for prop selection.. As I mentioned.. Im just speculating also.

One other thing interesting about the parasitic power going up with the square of current. When you go from 12 volt to 24 volts, the current is 1/2. So for a given wiring in the motor, the parasitic power disipation would be 1/4. This is why higher hp electric motors are also higher voltage.