Bow Thruster rebuild sanity check

[accomplice]

Our boat came with a Vetus 160kg 250mm 24V bow thruster. While all systems on boats are prone to failure and require periodic maintenance, our thruster has been particularly cursed.

Having recently redone most of the thruster wiring in the forward sail locker, where the motor lives, I was disappointed to discover that the motor had stopped working. After great effort and significant expense I have managed to remove the motor from the boat and bring it to a motor shop for rebuilding. If the installers had used Teff-gel or equivalent, the stainless bolts securing the motor in place might not have become one with the pot-metal housing. The metal of the housing crumbled as heat and torque were applied.

To make a very long story shorter, the motor is not readily repairable. If I want an operable bow thruster, I essentially have two options -- replace the motor and contactors with a new Vetus motor, or fit a different motor. I'm inclined to use a different motor as parts for the Vetus motor are difficult to obtain (e.g. when I called Vetus to inquire about new springs for the brushes, I was told that I would have to buy a new motor to get new springs) and the quality of build is less than desired (IP21 enclosure on the motor, disintegrating electrical connections, crumbling housing).

I have a plan, and I wanted to run it by others to see if it is crazy (I know I'm crazy, but is the plan?).

The motor is labeled 7kW, 24V, 2600RPM, 500A max. I doubt it draws anything near 500A as there is a 400A fuse on that circuit that has never blown. (7kW input at 90% efficiency at 2600RPM is ~205in-lb torque out) It is a series wound motor. The motor is attached to a mounting flange which has an unusual 6 bolt pattern at approximately 225mm diameter. The shaft on the motor is a 13 Tooth-7/8". I believe it is 16/32 30degree. It was coupled downward to a similarly splined shaft which drives the right-angle-drive unit. I do not know the reduction ratio in the right-angle-drive.

Incidentally, I recall that Vetus claims the duty cycle on the motor is no more than 5 minutes continuous, 5 minutes per hour. The motor itself is labeled S2 - 6min duty.

My tentative plan is to replace the motor with a permanent magnet brushed DC motor, the Motenergy ME-1004 -- rated for operation up to 48V, for 200A continuous and 400A for up to 1 minute (I cannot remember ever using the thruster for a minute continuously!). It is rated to produce 218 in-lb of torque at 200A, although the performance curves only show 140A@150in-lb@48V. It has a 2.5" long keyed shaft of 1" diameter.

My plan is to replace the shaft coupling with a jaw type coupling of sintered iron and rubber spider. While I can readily source keyed jaws of aluminum or stainless, I have only been able to find iron or sintered iron splined shafts without resorting to custom manufacturing. My plan is to replace the face mounting with a 1/4" sheet of 6061 aluminum with 4-holes drilled for the motor mount, 6 holes drilled for the drive mount, and 1 big hole for the shaft.

The shaft on the old motor extended approximately 2-5/8" below the mounting face, and I believe was 0-1/2" of the lower shaft. The shaft on the new motor is 2.5". The spider is 0.61". I will have some (unknown) longitudinal adjustment possible before the jaws are secured to the shafts.

My plan is to try to salvage the reversing contactor from the Vetus. If it is not salvageable, I will replace it with a 400A reversing contactor.

My questions/concerns are these:
* Will I be satisfied with the thrust produced by this arrangement? Driving the motor at only 24V, it will spin significantly slower than the rated speed on the old motor (~1700rpm vs 2600rpm) -- but I do not know the actual speed the old motor was spinning nor the torque required by the prop at speed. I believe that the ME-1004 is capable of producing sufficient torque, albeit at higher current than its performance curves show, but I expect to be within its ratings, particularly at the lower (1700vs 2600) speed. I don't have the ability to really change the pitch of the thruster prop or the gearing of the right-angle-drive.

* What should I coat the coupling with to inhibit corrosion? Should I grease the shafts where they enter the sintered iron coupling, or is that a waste of time? I intend to clean and paint the coupling to inhibit corrosion. Note that the (much larger) coupling on the main auxiliary is a painted mild steel, not to mention the engine block, transmission, etc. Should I replace the set screws on the coupling with stainless, coated with Teff-gel? or simply grease them very well? The nature of the jaw coupling is that it comes apart by physically separating the motor's shaft from the driven shaft -- which should make disassembly so much easier in the future.

* The ME-1004 is not wet-rated, but it is in a dry location. However, the previous motor in a dry location clearly exhibited corrosion. The motor is cheap enough (~$500) that replacing it is not nearly as painful as replacing the Vetus motor (~$3k), and parts are readily available. Should I be worried about corrosion of this motor?

* Is there a different motor I should consider? Although I'm inclined to replace our autopilot pump with a brushless motor because it gets so much use and chews through brushes every couple of years, the thruster gets so little use that the extra expense and complexity of brushless hardly seems worth it. Also, our highest DC voltage onboard is 24V. Our 220V AC system only has a 5kVA inverter -- so I'm not inclined to switch to an AC motor -- not to mention the need for reversing. I'm not aware of any high power, low cost DC-DC boost converters.

* Is there some inherent flaw in this plan?

Thanks.

[Lepke]

I have a power boat and use ac for everything, winch, etc. With my inverter I can run anything if necessary. 240 ac is going to use many times fewer amps than 24v dc. Smaller wiring, relays, etc. Many ac motors reverse.
No thruster, but dc winch was constant trouble until converted to ac.

[accomplice]

Originally Posted by Lepke

I have a power boat and use ac for everything, winch, etc. With my inverter I can run anything if necessary. 240 ac is going to use many times fewer amps than 24v dc. Smaller wiring, relays, etc. Many ac motors reverse.
No thruster, but dc winch was constant trouble until converted to ac.

How big is your inverter? I agree that AC is preferable for many reasons, a big one being the smaller wire gauges -- and another being the cost and weight of multi-HP DC motors.

We have a 5kVA inverter, which is more than adequate for the smaller AC loads (microwave, hairdryer, entertainment systems, etc.) but inadequate for the large AC loads (air conditioning, water maker, and potentially bow thruster). It would seem less than desirable, although potentially doable, to have to fire up the genset in order to dock. Making systems cascade (genset->thruster) strikes me as inherently less reliable as a failure of either system would disable the thruster. On the other hand, we dock so infrequently - a few times a year usually -- that perhaps I should consider it. The cost of adding another big inverter and an AC motor far eclipse the cost of a new DC motor, but would offer inverter redundancy and potentially the ability to run the large AC loads without requiring the genset. Of course, I'd then have to reconsider my battery capacity

[Jammer]

How did your motor replacement project turn out?

[accomplice]

Overall, the project was a big success.

The mounting configuration ended up being a little different than I originally anticipated. I don't recall exactly what the shaft was, but it took a different $27 half-coupling to match.

I ad reused the old contactor, which recently failed. I should've replaced it last year when I did the other work. It looked corroded when I put it back in service, but it worked. A couple of weeks ago it stuck "on" to port. A new contactor ($159) arrived today. I hope to put it in this weekend.

The new motor works well and produces sufficient thrust. It *may* have less thrust than it used to, but if so, the difference is barely perceptible. (If I were to run the motor on 48V it would clearly have more!). Not that I stand on the switch for minutes at a time, but now duty cycle is no worry -- it could run until the thruster batteries wore down.

When I get around to it, I intend to build a plexi-glass splash screen over the motor. Although it is in the forward sail-locker, covered with a plywood panel and with all the stuff that accumulates in a sail locker, it could potentially get wet at times (e.g. retrieving docklines in a pouring rain, pulling out sea anchor when seas are up, etc.). The old motor didn't have that, but my hunch is that the new motor will last longer if I do install one.

I'm happy with my decision to go rogue on this one. My only concern is that I cannot speak to the longevity of the motor in this application. Will it last a decade like the old one? I cannot say yet; however, at $525 for the motor, I can buy 9 of them before I could pay for a new Vetus motor.

[Jammer]

Thanks for the update!

[Zach]

Hi,


I'm interested to hear where you found your contactor?



Anything overly special required to interface between the controller?



I've got a project that started out hydraulic and turned electric that the vetus sold a take off motor from someone going the other direction, but without the contactors and control board on the stern thruster. Bow thruster has them... Stern thruster, I got a "Deal" but after receiving the hardware learned that for no price were the circuit boards, mounting brackets or contactors available short of buying a new electric motor.

[accomplice]

Originally Posted by Zach

I'm interested to hear where you found your contactor?

I replaced with one just like the original Vetus used. It is an Albright SW202. Google it and you'll find many sources.

Anything overly special required to interface between the controller?

Controller? You mean the joystick switch?
Nothing special. The joystick completes a circuit that energizes one solenoid or the other on the contactor. I switched the +24V side of the contactor, and connected the other pole of both contactor solenoids to 0V/GND. The switched side is protected by a small (5A?) fuse. Since it is reversing, the motor poles get open/open (switch off), +24V/0V (port thrust) or 0V/+24V (starboard thrust).

My joystick switch is on the way out too. Any marine-capable 2-way joystick switch should work.

I used beefy 2AWG cables from the contactor to the motor. There were already large gauge cables to the old thruster that I reused. There are big (400A?) fuses protecting the high current wiring.

As for the mounting bracket, I got a 1/4" square aluminum plate and drilled holes to mount it down to the thruster base, and other holes to mount the motor to the top of it. I put in a few small holes to the side to hold the contactor. I could've rounded the corners of the Al plate, but didn't as there was plenty of room for the corners to stick out.

The drive I mated using two Lovejoy L-100 half-couplers with a spider in the middle. It can tolerate slight misalignment.

I've got a project that started out hydraulic and turned electric that the vetus sold a take off motor from someone going the other direction, but without the contactors and control board on the stern thruster. Bow thruster has them... Stern thruster, I got a "Deal" but after receiving the hardware learned that for no price were the circuit boards, mounting brackets or contactors available short of buying a new electric motor.

Yup, and it was hard for me to get a new vetus motor without buying the drive, mount, etc.

Most of the projects I get into end up being more work than I initially thought. This was one of the few that didn't.

So you have a stern thruster with no motor or control?