Thrust-Trolling vs Thruster

scott011422

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I'm attempting to learn some things and am having trouble wrapping my head around a few concepts. Strictly talking about usable thrust and how it relates to a boats movement.

On one hand we have an 80lb 24vdc trolling motor. pulls 54 amps at roughly 1000rpm and produces 80 lbs of thrust.
On the other we have a Thruster. same 24vdc and 80lb thrust, it however produces 3500rpm at almost 200amps.

Why is the thruster so much more power hungry and what does it give me? I've asked a couple of thruster manufacturers and they basically say it just does. Higher rpm = more thrust. Yet when I tell them the trolling motor produces the same thrust at a lower rpm and a fraction of the amperage, they just skoff at me.

I gave one Dealer the example, "if a boat were floating in a dock against a scale, That scale should read 80 lbs with either the trolling motor or the thruster" He told me I was an idiot and hung up on me..

What am I not getting? Any input?

Thanks
 
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GA_Boater

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Welcome aboard.

I call won't you names :smile:, but two different designs. A trolling motor pulls or pushes a boat along the streamlined hull. A thruster is used to push the boat at right angles to normal hull movement.

If you have a boat in the water and push from the stern, a couple of fingers can move the boat forward. Try the same thing on the side and it requires much more effort on your part. Same thing with a trolling motor vs a thruster.

Prop size and pitch also come into play, which makes the thrust, RPM and current draw almost meaningless. Forget the scale analogy because it is not measuring thrust.
 

lmuss53

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But, 80 pounds is 80 pounds, I think the question is how can they both be rated at 80 pounds and one be considered more powerful, and draw more current to produce the same effect.

If you see the new thruster devices designed for pontoon boats they are really only a glorified trolling motor, mounted a little toward the midship. I have been getting similar results on a pontoon boat for years with electrically steered trolling motors.

Of course the 80 pounds pushing from the rear will move the boat easier because it is moving the bow straight thru the water. It is less effective from the side, but it is the same 80 pounds of push.
 
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scott011422

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Prop size and pitch also come into play, which makes the thrust, RPM and current draw almost meaningless. Forget the scale analogy because it is not measuring thrust.

True, But why design a system to use 4x more power if it doesn't have to, to accomplish the same output? Could you please explain why the scale example is flawed? That is pretty much how its measured in lab for testing. A scale measures forces acting upon it.Thrust is a measure of force. Force is measured on a scale (or transducer same thing)


scott011422 - What are you using as a reference for a thruster drawing 300 amps?

I am using a Vetus BOW3512D. I did notice that I mistakenly typed in 300 instead of 200. However, once you hit about 170ish lbs of thrust, most are 300 amps or above. [h=1][/h]
 

Silvertip

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Motor design has a major effect on its rpm, speed control and current consumption. Outrunner motors used in RC airplanes can easily draw nearly and in some cases more than 100 amps yet wouldn't push a pontoon six inches. Prop design (pitch, diameter and shaft speed) along with the motor design determines thrust. If you compare what's being called a "Thruster" for a pontoon with a standard trolling motor, there is likely little difference. Now then, load up the "Thruster" and you will see high current loads. It's the way electric motors work. Just like a steam engine, they develop maximum torque at zero rpm. The closer you come to stalling that motor the more current it will draw until the fuse/breaker pops or the motor burns up.
 

Scott Danforth

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dont forget tip clearance and efficiency

put the trolling motor in a tunnel like a thruster and you will see high motor current as the motor is actually forced to work
 

scott011422

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Lets try this a different way. Are there any advantages to running a propeller with a lesser pitch and a high rpm over the same propeller with a higher pitch and a lower rpm if the resulting thrust is the same? And lets not factor in peak hp and torque curves and all of that. its moot., Keep it simple, just the mechanics of the propeller's effects on the water.
 

Scott Danforth

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depends on the propeller blade design. some blades are more efficient at a high-pitch, low RPM than others.
 

Silvertip

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Since you are electing to ignore certain applicable factors -- "in theory" you could pick two props (one high shaft speed low pitch) and one high pitch with low shaft speed) that provides the very same -- WHAT? You are attempting to use THRUST as a baseline measurement for a number of different props and are ignoring their intended purpose. If you ignore boat speed, a barge for example would require a large diameter, large blade area, but relatively slow shaft speed. A planing hull on the other hand (since a boat doesn't have a transmission) must rely on the prop to get out of the hole, cruise efficiently and have a reasonable top speed. This is exactly why you see so many varieties of props. Some folks care less about top speed but want to get on plane quickly (skiers for example). Others are in search of every MPH they can squeeze out of their boat and could care less if it takes half the width of the lake to get on plane. In fact, you can pick props from three different manufacturers all with the same diameter and pitch and none of them will have identical performance -- or THRUST.

In the case of trolling motors, current draw is crucial since a battery is not an inexhaustable source of energy. Prop it too high and it will draw excessive current making a day of fishing very short. Prop it to low and the battery may last much longer but it would take forever to get anywhere. The motor "design" would need to change to switch between these two "theoretical" props to achieve efficient prop speed. In your last post you asked about advantage/disadvantage to prop speed and pitch again is a poorly worded question. Of course there are advantages/disadvantages. High shaft speed says probably gasoline engine running it's guts out to move a barge. Low shaft speed high pitch says diesel engine at much lower rpm (e.g., ocean liner with engines that run at 450 - 900 rpm wide open). Electric motors require a delicate balancing act between current consumption and application. The idea of any propulsion system is to load it for "normal operating conditions and best overall economy". If normal means pushing a barge, ski boat, pontoon, cruiser or battle ship, all of them require very different propulsion systems.
 

scott011422

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SilverTip: All good points and info. Any idea on the Thruster question? Ie, Why is a 200 amp 3500 rpm 7" thruster better at moving a boat than a 54 amp 1000rpm 7" trolling motor when they are both producing 80 lbs of thrust?
 

Silvertip

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Once again -- it's design and application. I suggest you go back to the spec sheets for the Vetus thrusters and pay attention to what you read. Note that maximum run time is 4 minutes before bad things happen. A low rpm troller will run for hours at full power. Trolling motors are almost always pulse width modulated (PWM) these days where the thrusters appear to be straight DC motors (as they say -- more like an automotive starter motor). Hence the high current draw . Also note the six blade props vs two or three on a troller. It appears there is "gearing" involved with the troller so what those ratios are also has an effect. Ducting a prop vs unducted obviously has some benefits or they wouldn't use them. But just looking at the overall design should shed some light on why 80# in one application requires much more electrical current than 80# in another. These are simply two very different applications requiring two very different approaches to get 80# of thrust. If you compare draw bar pull on an old steam tractor and an equivalent pull on a modern tractor, which one would you choose? They both pull the same amount but one will outperform the other in the speed and efficiency at which that job gets done. Done here. Can't 'splain it any other way -- at least any other way that would not put the people to sleep who are not already sleeping.
 
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