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Old 03-30-2018, 07:59 PM   #21
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Pete: I like the way you think! All you mentioned will be researched. I actually was looking at the ez way out that will not really solve the issue. Wires, connections, BRUSHES! YES, BRUSHES! Can't believe I didn't think of that because we sell DC powered machines with up to 250 hp. Looks like i'll be squeezing into tight spots soon.
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Old 03-30-2018, 08:32 PM   #22
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Sounds like a conversation with the thruster manufacturer is in order.
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Old 03-30-2018, 08:49 PM   #23
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I have a sidepower thruster that I converted from 12 volrs off the house battery to 24 volts next to the thruster. Here are some things to check:

As already mentioned:
Load test batteries
Check cable connections at batteries and thruster
What size cables and how long are they?
Check voltage drop at thruster. 15% or more will be a problem.

Have you checked the thruster propeller(s)? Fouled props will eat a lot more amps with less push. Is the tunnel clean? A fouled tunnel slows the push, requires longer duration, and requires more operating amps.

Ted
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Old 03-30-2018, 09:17 PM   #24
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I need to clear up some stated misconceptions. A simple, series wound, brushed DC motor does NOT tend to overheat with reduced DC voltage. It does not act like an AC induction motor in this regard. Lower DC voltage, due to cable loss or battery drop will result in lower amperage draw. It will turn slower and produce less power.
Now, the use of larger copper wiring will tend to sink heat a bit, but I tend to doubt serious help will result.
At long duration 24V operation, the windings are simply running hot. You want either a larger motor, less duration, less voltage, less prop pitch, or better cooling. Is the motor totally enclosed with zero fan cooling??
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Old 03-30-2018, 09:22 PM   #25
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Originally Posted by diver dave View Post
I need to clear up some stated misconceptions. A simple, series wound, brushed DC motor does NOT tend to overheat with reduced DC voltage.
I would like to understand the physics (?) behind that, starting with why ohms law doesn't apply.
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Old 03-30-2018, 09:55 PM   #26
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I need to clear up some stated misconceptions. A simple, series wound, brushed DC motor does NOT tend to overheat with reduced DC voltage. It does not act like an AC induction motor in this regard. Lower DC voltage, due to cable loss or battery drop will result in lower amperage draw. It will turn slower and produce less power.
And I may have been one of the promoters of that incorrect information, my apologies!! I was thinking from the perspective of AC induction motors where lower voltage means higher amp draw and greater potential to overheat. But as you point out, the DC motor we're looking at behaves differently. Thanks for setting the record straight!!
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Old 03-30-2018, 10:16 PM   #27
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Next boat will have hydraulic thrusters. Contuous duty.
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Old 03-30-2018, 11:48 PM   #28
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I have never needed more than 5 to 10 seconds on my stern thruster. It is rated for 3 minutes use. Donít see need for continuous duty and the expense of hydraulic thrusters unless I hit the lottery and buy a Fleming 58 then all bets are off.
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Old 03-31-2018, 12:17 AM   #29
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Isn't pulling up 150 feet of anchor chain rode just as taxing as using the bow thruster for a longer period of time? Maybe even more?? Do you guys pull up the anchor in short spurts and then rest? Or go for the glory?
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Old 03-31-2018, 05:41 AM   #30
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When talking to the Mfg ask their opinion of max on time.

Many thrusters are short duration only.
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Old 03-31-2018, 06:34 AM   #31
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Isn't pulling up 150 feet of anchor chain rode just as taxing as using the bow thruster for a longer period of time? Maybe even more?? Do you guys pull up the anchor in short spurts and then rest? Or go for the glory?
It depends on the size of your windlass and the size of your thruster. In my case, my thruster draws 3 times the amps (when they were both 12 volts) compared to the windlass. That number is also effected by the depth of water you're anchored in. The deeper the water, the more chain you're lifting off the bottom, and then the greater the load and amp draw.

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Old 03-31-2018, 06:46 AM   #32
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I would like to understand the physics (?) behind that, starting with why ohms law doesn't apply.
The AC motor is the unusual item, not the DC. Most devices tend to be linear, that is, the more drive, the more output. Lamps, resistors, transformers, brushed motors (ac or dc). As you know, there are some items that act in a non-linear, even inverse way. Switch mode power supplies and AC induction motors are both great, everyday examples. SMPS are constant output devices, so when input Volt drops, the input Amps will rise. Very predictably, until the controls run out of headroom. AC induction motors are a close example also. They want to run at a near constant rpm. Not as constant as a true syncronous motor, but almost. If loaded then, you will see it act as a constant power device, trying to keep speed. As the V reduces, the Amps will increase. P=V*I.
In the induction motor, as you then reduce V, I increases and therefore copper loss, which is I*I*R, will rise as a squared function. Thus, the burned out reffer motors at 200V, when designed for 240.

Back to brushed. As V lowers, and load torque is held, the rpm drops and the current also drops. Input power is dropping as a squared function. Winding heat is dropping quickly also, since it is still I*I*R. R is constant. Many a DC motor is speed/power controlled by dropping its supply V, like the vent/ac blower motor in your car.

Back to the OP issue. First off, I don't know what winding temperature is being reached, nor the method and set point of the interrupter. A IR gun would be useful to shoot the armature. It seems to me that the industry needs more/better heatsinking on the motor. Slapping on a truck 24V starter motor is not the best idea. You really want some fins, or other decent method of cooling the armature and rotor. At least a forced air system, or something other than a pile of mass, with little cooling potential.
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Old 03-31-2018, 09:14 AM   #33
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Modern electric thrusters have fans as an integral part of the motor assembly. I did hear of one person where the fan blade was loose and not turning properly with frequent thermal timeouts. Tightening the nut made a big improvement.

I think the bottom line here is that electric thrusters are designed to work 90% of the time, or even 95% or 99%. In most situations, between your general boat handling skills and pulsed use of the thrusters, you will never experience a thermal timeout.

The issue only arises in that 1% or 5% situation where the you-know-what is hitting the fan, and you need every ounce of thruster you can get for every possible second you can get it. That's when you don't want your thrusters to time out, yet that's exactly when they are most likely to do just that.

But the electric thruster tradeoff is very suitable for most boating, as demonstrated by the pervasive use of electric thrusters, so you really need to ask yourself what sort of boating and situations do you expect to be in, and what will you avoid like the plague.
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Old 03-31-2018, 10:42 AM   #34
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But the electric thruster tradeoff is very suitable for most boating, as demonstrated by the pervasive use of electric thrusters, so you really need to ask yourself what sort of boating and situations do you expect to be in, and what will you avoid like the plague.
When I was spec'ing my boat, I had intended to go with a hydraulic thruster, but not because I ever expected the time-out issue to be a problem in normal maneuvering situations. Instead, since my boat is primarily for (long range) fishing, I wanted the ability to stop my boat and have it keep a designated heading (the Simrad AP-50 can control a bow thruster for that purpose). My manufacturer talked me out of it, in part by telling me that hydraulic lines get old and need to be replaced, or rupture, and that would be a real mess. I don't know why I didn't tell him to build an enclosed conduit for that line. But the additional cost I was quoted, which now seems artificially high, was $40K (especially considering that both the crane and stabilizers are hydraulic, so a pump is already in the equation). I only have a few regrets in the way I spec'd the boat, but that is certainly one of them.
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Old 03-31-2018, 11:10 AM   #35
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You all know more about electricity than I do but Iíd never let ignorance hinder me from sharing my opinion. Iím a US Citizen after all.

On a DC motor like a thruster, I think if you have a bad wiring connection, a bad battery, or a too-long wiring run, all you will get is a motor that wonít put out its rated power. I donít think that this would really cause it to heat up more quickly.

However, a thruster is specíd for a boat based on it power. If your thruster isnít putting out its rated power, then you may need to run the thruster longer to get the desired effect. It would be like having a thruster that is too small installed in your boat. So secondarily, a wiring problem could result in overheating simply because you hare having to run the thruster longer.

BTW, Iíve never had my thruster cut out due to thermal overload. I do use my thruster frequently, but I donít use it a lot. If that makes any sense. There have only been a few times when Iíve run the thruster close to 20-30 seconds and that was in intermittent pulses. The longest pulse Iíve ever used is maybe 4 seconds long with at least as long a period of rest in between. Usually, the thruster gets pulsed for about 2 seconds.

So, if you have to run your thruster for longer periods with pulses longer than a few seconds frequently, then maybe the thruster isnít running at its rated power or maybe you are just getting used to a new boat.
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Old 03-31-2018, 09:23 PM   #36
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When I was spec'ing my boat, I had intended to go with a hydraulic thruster, but not because I ever expected the time-out issue to be a problem in normal maneuvering situations. Instead, since my boat is primarily for (long range) fishing, I wanted the ability to stop my boat and have it keep a designated heading (the Simrad AP-50 can control a bow thruster for that purpose). My manufacturer talked me out of it, in part by telling me that hydraulic lines get old and need to be replaced, or rupture, and that would be a real mess. I don't know why I didn't tell him to build an enclosed conduit for that line. But the additional cost I was quoted, which now seems artificially high, was $40K (especially considering that both the crane and stabilizers are hydraulic, so a pump is already in the equation). I only have a few regrets in the way I spec'd the boat, but that is certainly one of them.
When I was doing the refit and repower on my boat, converting the unreliable thruster to hydraulic was a high priority. Everything in the engineroom was going to be over $10K, and that was with me doing the install and fabricating all the brackets and mounts. Never even got to the thruster motor, hydraulic lines, and multiple station thruster controls. Saw the costs peaking between $15K and $20K. Ended up converting to 24 volts for less than $3K. Solved all the problems.

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Old 04-01-2018, 06:03 AM   #37
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On a new build Hyd everything makes lots of sense.

On a retro fit the first item , like a thruster will seem really expensive ,

but the rest ,windlass, stabilizers, dink hoist,steering, auto pilot , stern capstan will not be much different from electric.
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Old 04-01-2018, 07:06 AM   #38
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I had a 24V bow thruster, probably similar to the OP, in a 50ft Selene. The only time it let me down was because of failed batteries. You do get some warning (not recognized at the time) because the operating cycle does get shorter and shorter, till the unit finally lets you down when you really need it. The thruster had two dedicated 4D batteries with dedicated charger, fed by shore power or the generator. Batteries and charger were located within a few feet of the thruster. Thruster units place a high power drain on the batteries. At least with a 24V set-up the amperage draw is halved for each battery vs a 12V set-up. But you are still looking at somewhere in the range of 20 amps per battery. The only maintenance I ever did was to clean the motor carbon brushes once, while checking for wear; to keep the oil feed topped off; and to ensure the impeller blades were free to turn and that the tunnel was not bound up with barnacles.
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Old 04-01-2018, 08:59 AM   #39
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I've not yet ever timed out a thruster.

Sometimes I wish we had one to try...



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Old 04-01-2018, 09:07 AM   #40
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For our new build I thought about hydraulics, but dropped it fairly quick. The cost was the main reason. I have never had any issues with short pulsing DC thrusters in the past. With that said, I understand why people with large boats under construction would use them. Hydraulics are proven reliable work horses, but keep an eye on those hoses, fittings, and seals, and replace as needed. When they spring a leak, it is not a pretty sight.
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