Thruster Stopping

South of Heaven said:

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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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.

Ted

MYTraveler said:

I would like to understand the physics (?) behind that, starting with why ohms law doesn't apply.

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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.

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.

twistedtree said:

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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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.

You all know more about electricity than I do but I’d never let ignorance hinder me from sharing my opinion. I’m 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 won’t put out its rated power. I don’t think that this would really cause it to heat up more quickly.

However, a thruster is spec’d for a boat based on it power. If your thruster isn’t 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, I’ve never had my thruster cut out due to thermal overload. I do use my thruster frequently, but I don’t use it a lot. If that makes any sense. There have only been a few times when I’ve run the thruster close to 20-30 seconds and that was in intermittent pulses. The longest pulse I’ve 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 isn’t running at its rated power or maybe you are just getting used to a new boat.

MYTraveler said:

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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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.

Ted

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.

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.

I've not yet ever timed out a thruster.

Sometimes I wish we had one to try...



-Chris

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.