thrust question

[popsmazz1]

is there any way to compare thrust on an electric to hp.? I bought a 50 lb. thrust trolling motor and I want to use it instead of my 3hp. gas engine, I was curious to see how it would compare.

 

[adamsnez2]

Re: thrust question

google, simply.

"hp to thrust"

a million referrences i found

 

[asm_]

Re: thrust question

Actually, with out making a lot assumption on many variable involved in both system, it is very difficult to convert Thrust to HP.

With that said, assuming if you are talking about trolling electric motor vs 3HP small IC motor. 3HP would probably go a lot faster. Ignoring factor such as prop diameter, prop pitch and power curve band, etc... 3HP does generate roughly 2200W in electric term. A typical 50lb trolling motor only puts out around 400W of power. But, then again, electric trolling motor does have larger prop with almost none-existent prop pitch...

I did said it was complicated, didn't I?

B

 

[Silvertip]

Re: thrust question

Agreed! The troller would need to run wide open to keep up with a 3 - 4 HP outboard running at about half throttle. The outboard on a three gallon tank would last all day while the troller would kill a group 27 deep cycle battery in a few hours. Trollers are for trolling -- not as a primary power source.

 

[adamsnez2]

Re: thrust question

Agreed! The troller would need to run wide open to keep up with a 3 - 4 HP outboard running at about half throttle. The outboard on a three gallon tank would last all day while the troller would kill a group 27 deep cycle battery in a few hours. Trollers are for trolling -- not as a primary power source.

bullcrap @ the last sentence. they are LIFE SAVERS. my motor gave out one weekend and i trolled all the way back from the cabin to the boat launch, INTO heavy winds, INTO the current. they are a LIFESAVER! ahaha

 

[popsmazz1]

Re: thrust question

I tried the trolling motor today, on a fully charged battery it was only a tad slower than my old 3hp johnson, maybe like a 2hp. it"s on a 12ft alum. boat. I only use for primary power on a small lake. but I do need a 2nd battery, battery was dying on the way back to the dock. but it did a good job, and it has reverse, witch my 3hp. doesn"t.

 

[jtexas]

Re: thrust question

Thrust and horsepower measure different attributes. Horsepower measures work done in terms of distance and time (foot/lbs per minute). Thrust measures force applied to a load, in terms of pounds.

Thrust is dependent on propshaft RPM and prop pitch -- so motorguide and minnkota count on the motors spinning at their design RPM, and with such a low pitch they probably do. On a heavy boat, more of their energy gets wasted pushing water around; its still delivering the promised amount of thrust, but that thrust just doesn't move the boat as fast.

Horsepower = thrust (in ft lbs) x speed (in ft per second) divided by 550.

So, if 50 foot-pounds of thrust will move you at 8 MPH, that's the equivalent of
50 x 11.7 ? 550 = 1 HP.

If your 2hp motor also goes 8 MPH, keep in mind that 15, maybe 20% of that 2hp is wasted in the gearbox, then another 15% or so goes to prop inefficiency, so you might only be have 1.3 hp available. Then if the prop pitch is too high, it won't make it's full RPM, so it never delivers the full 2hp to the driveshaft in the first place.

These numbers are all pretty much made up, but they should be somewhat representative of reality. You could go out & get precise measurements and then you'd know for sure, but based on your experience I'd say we can conclude that a 50lbs thrust motor is about the same as 2hp gas engine.

According to my calculations, the difference in speed between a 2hp motor and a 3hp, is about 2.25 mph (see below).

 

[popsmazz1]

Re: thrust question

very interesting, I think my old 3hp, motor is probably not puting out 3hp. anymore, it"s a 1962. also it has a very small prop compared to the trolling motor, but does have more pitch.thanks for the info.

 

[jhebert]

Re: thrust question

Horsepower = thrust (in ft lbs) x speed (in ft per second) divided by 550.

Sorry but thrust is a force measured in pounds, not foot-pounds. Foot-pounds is a measurement of torque. Your adaptation of the relationship between torque and horsepower to boat speed and thrust is specious.

Thrust is a force. Horsepower is power. The two are related by

Power = Force x (Distance/Time)

In order to convert thrust into an equivalent power, we must know the distance and time. You may recognize that (Distance/Time) is also known as speed. Therefore to know the equivalent horsepower for a particular thrust, we must know the speed at which the boat is moved by a certain thrust.

The best approximation for an electrical motor for horsepower is from its energy consumption. If the motor draws, say, 50-amperes at 12-volts, that is 600-watts. The relationship between watts and horsepower is

1-HP = 750-watts

Thus a 12-volt electric trolling motor that drew 50-amperes at 12-volts would be using the equivalent to 0.8-HP in electrical energy. This is its energy input. The motor is not perfectly efficient at conversion of electrical energy to mechanical energy. If we assume an efficiency of 80-percent, then the propeller shaft power output of the trolling motor would be around 0.64-HP.

 

[jtexas]

Re: thrust question

Sorry but thrust is a force measured in pounds...

Thrust is a force. Horsepower is power. The two are related by

Power = Force x (Distance/Time)

...

Sorry, right, should have been "Horsepower = Thrust (in pounds)..." instead of foot pounds.

Since as you say, "thrust is a force", substitute "thrust" for "force" in your equation, and yours is almost the same as mine.

We know that 1 horsepower by definition is equal to 33,000 foot pounds per minute, or 550 foot pounds per second, or 746 watts.

I made a speed assumption (8 mph) precisely because you can't convert thrust to HP without knowing the speed. Obviously my example of 8mph wasn't a coincidence...I solved for mph given 1HP and 50lbs, and concluded based on experience that it was a reasonable answer.

If you think my theory is specious, it probably is. It's just too much trouble to try to defend it.

So, on to personal experience: I replaced my 36lb Motorguide with a 46lb motorguide, and they recommend a 50-amp circuit breaker both motors. Same current draw, different output?

Course, if the recommended breaker is 50 amps, then that's the max you should ever see in normal operation (at start-up). I guess you could run it at 50 amps all the time, but that's not a reasonable assumption, in my experience. Steady state expect maybe what, half that?

Puts you at about 0.4 hp assuming 100% efficiency, 1/3 of a HP at 80%. That might be the answer. It might even be all you get from a 2hp gasoline engine, in which case, we reach the same conclusion..........

 

[Silvertip]

Re: thrust question

Bullcrap huh? What size troller were you using, what make & model, and how far back to the dock was it? Whatever you elect to tell us, lets look at some numbers. A 50 - 55# troller on full speed draws about 50 amps. A group 27 deep cycle has roughly a 150 - 180 minute reserve at a 23 amp draw (23 amps being the battery manufacturer standard test rate and listed on the capacity label). So for this example lets give you the benefit of the doubt and take the 180 minute rate (3 hours where I went to school). Since the actual draw of the motor is 50 amps (thats over twice the test rate of 23 amps) that cuts the reserve to 90 minutes. So in this example that motor would kill the battery in an hour and a half. I'll even give you more benefit of the doubt by saying 2 hours. A 3 or 4 HP outboard will definitely outlast and out push the troller by a bunch on a 3 gallon tank. How do I know this? Been there and done that.

 

[RicMic]

Re: thrust question

JTexas, the recommended circuit breaker size has absolutely nothing to do with the maximum current draw of the device, in this case a trolling motor, circuit breakers or fuses are there to keep the wire from melting down and are sized by the maximum draw of the wire size,type and conditions.

 

[jtexas]

Re: thrust question

"absolutely nothing to do with"?

It's my assertion that the manufacturer's breaker recommendation is based on the max expected current draw of the motor in normal operation (not stalled by vegetation or whatever), not on wire size. The manufacturer knows the current draw of the motor, not what size wire is in your boat. It's up to you to select wiring that will handle the 50 amps.

The motor needs protection, too. Otherwise, you could put 4/0 cables and a 100 amp breaker and the wires would be fine. But the motor, if it gets stalled by fishing line, will be toast.