Why Not Use AC Power for Thrusters and Windlasses?

[markpierce]

24-volt thruster works fine for me.

[Dockhead]

Originally Posted by minaret

I think you should get a barn and some plans and have me come over and build a hull for you.

Sounds great!

But don't pack your bags just yet -- I have to earn the money to pay for it first

I'm hoping to do a very adventurous trip in two years through the Belamor Canal and White Sea to the Arctic Ocean, then around the top of Norway back into the North Sea. It will be done in my present boat, but if it's as fun as I hope it will be, then maybe the new boat will be ready in time for the next one of these.

[Dockhead]

Another aspect to think about is the fact that Lithium batteries have huge charge acceptance rates, so can be charged by short runs by large generators, which greatly adds to efficiency and convenience. Downside is weight, bulk, and cost of larger versus smaller gensets.
So it might make sense to have a larger three-phase 380volt generator, maybe 10kW or 15kW. Or a large DC generator putting out 48 volts.
Maybe best of all would be to run the thruster and windlass off 3 phase or 48v DC power, rather than single phase 230v power. At 48 volts, a lot of the disadvantages of DC power start to go away.
And with the large charging capacity you need to make this work, you get large inverting capacity.
Your house bank is 48 volts which you convert to 12v or 24v for the smaller DC consumers. But you can use AC power for most electrical devices on board.
By the time you get this far, it is tempting to start thinking about hybrid propulsion. I know some on CF have successful experiences with this, but I think that is just a bridge too far for me. Considering the remote places I want to go with this boat, I don’t want to be an alpha tester for an avant garde propulsion system.

[ranger58sb]

Originally Posted by Dockhead

Fantasizing about having a new boat built, I was thinking about the best way to drive the thruster and windlass.

My boat uses 24v DC power for this, which works fine with a large battery bank and large alternator.

Hydraulic would be better of course, but expensive and awfully messy (!) if you have a leak.

So why not use 230 volt AC power instead?

In the past, cruising boats didn't usually have that much AC power on board, but these days we have inverters which could power such devices, with AC generators for backup.

What would be the downside to this? Seems ideal for a boat with robust AC power system, which more and more larger cruising boats now have.

I've read of several boats who use an AC-powered thruster and/or windlass; doesn't sound completely uncommon. And many of us run the genset all the time while underway, anyway -- at least in specific months around here -- so immediate power could be generally available.

But I'd imagine a cost/benefit comparison would let you rank the several options: AC on genset, AC from inverter, hydraulic, 12v, 24v, or OthervDC with conventional batteries, LiFePO4, etc.

I'm guessing on some boats, a 24vDC system is less expensive, on others maybe an AC-driven system would be more effective for less cost (assuming AC power is appropriately "amortized" over all the other systems it runs, too). IOW, I suspect there's probably not a universal answer.

-Chris

[Telemark43]

DC Motors produce their maximum torque at stall, when they are NOT turning at startup. Note that diesel electric train engines use DC motors to get the train moving. AC motors product their maximum torque when the phase angle is slightly shifted from their nominal operating speed, and, at stall, there is little available torque. Accordingly, a windlass which needs its torque at stall to pull that huge anchor out of the mud... DC motor... not so sure about the load requirements for a thruster (don't have one !)

[carstenb]

Dockhead

I spent 30 years in the hydraulics industry so I feel I know a bit about it. Hydraulic systems do not have to leak - that is a fallacy.

One of the business units in the area I managed, built power units for many different applications, amongst others, patient lifting tables for MR Scanners.

Hydraulics in a hospital????

Yep, we made thousands of them and never had a leak complaint. With todays soft-seal fittings, you should never see a leak, assuming it has been assembled properly.

If I were to design a system for you, I'd put the main pump on a power take off from the engine and run the system from that. I'd also put a small pump and DC motor off-line so you'd have emergency hydraulic power if your engine didn't start (or you didn't want to start the engine for some reason)

There are some benefits to using hydraulic- your winches for example become bi-directional. If yo want you can also get infinite speed control on your winches or your thrusters.

Regarding expense? I really don't think it would be all that expensive if you made everything hydraulic.

carsten

[Dockhead]

Originally Posted by carstenb

Dockhead

I spent 30 years in the hydraulics industry so I feel I know a bit about it. Hydraulic systems do not have to leak - that is a fallacy.

One of the business units in the area I managed, built power units for many different applications, amongst others, patient lifting tables for MR Scanners.

Hydraulics in a hospital????

Yep, we made thousands of them and never had a leak complaint. With todays soft-seal fittings, you should never see a leak, assuming it has been assembled properly.

If I were to design a system for you, I'd put the main pump on a power take off from the engine and run the system from that. I'd also put a small pump and DC motor off-line so you'd have emergency hydraulic power if your engine didn't start (or you didn't want to start the engine for some reason)

There are some benefits to using hydraulic- your winches for example become bi-directional. If yo want you can also get infinite speed control on your winches or your thrusters.

Regarding expense? I really don't think it would be all that expensive if you made everything hydraulic.

carsten

Great info, thanks!

Now still another option to think about

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[FamilyVan]

Some food for thought and I'm afraid I pose more questions than answers with this post.

Some years ago I skippered a small research vessel, 60 feet. She had 2 main engines as well as 2 generators. She was at the time a very nice new vessel and even had a remote control with a 300 meter range you could drive her with. She had 2 generators, a 40kw working gen and a very nice little quiet running John Deere/Kubota 20 kW for at dock.

She had a hydraulic system (I can't recall what engine or Genny it ran off of). The hydraulics ran the crane, the net hauler and the A Frame.

For some reason, I wasn't involved in the design process and have no idea what that reason is, she had a DC thruster.

I can only assume that the thruster wasn't hydraulic because the DC system was considered to be superior to hydraulic for such a small vessel, either cheaper or better and I sincerely doubt cost was a factor.

Has any one else encountered a vessel with a hydraulic system that used an electric thruster and know the logic behind this design?

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[Dockhead]

Originally Posted by FamilyVan

Some food for thought and I'm afraid I pose more questions than answers with this post.

Some years ago I skippered a small research vessel, 60 feet. She had 2 main engines as well as 2 generators. She was at the time a very nice new vessel and even had a remote control with a 300 meter range you could drive her with. She had 2 generators, a 40kw working gen and a very nice little quiet running John Deere/Kubota 20 kW for at dock.

She had a hydraulic system (I can't recall what engine or Genny it ran off of). The hydraulics ran the crane, the net hauler and the A Frame.

For some reason, I wasn't involved in the design process and have no idea what that reason is, she had a DC thruster.

I can only assume that the thruster wasn't hydraulic because the DC system was considered to be superior to hydraulic for such a small vessel, either cheaper or better and I sincerely doubt cost was a factor.

Has any one else encountered a vessel with a hydraulic system that used an electric thruster and know the logic behind this design?

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One problem with hydraulic - I think - Carsten will correct me if I'm wrong - is that the revised range of the pumps is limited. So if you have the pump on the main engine, you have to review up the engine to use the thruster - so you really have to use the generator engine for this, not the main engine. The big yachts I've been on with hydraulic systems all had the pumps on the generator. This creates a single point of failure - can't start the gennie, and suddenly you've got no windlass and no thruster.

With DC or AC power, you can have redundant power supplies.

Sent from my D6633 using Cruisers Sailing Forum mobile app

[minaret]

Originally Posted by Dockhead

Sounds great!

But don't pack your bags just yet -- I have to earn the money to pay for it first

I'm hoping to do a very adventurous trip in two years through the Belamor Canal and White Sea to the Arctic Ocean, then around the top of Norway back into the North Sea. It will be done in my present boat, but if it's as fun as I hope it will be, then maybe the new boat will be ready in time for the next one of these.

You just let me know when to dust off my grinder. I've hung my own shingle and have an ace crew who'd love a big build. Several of us worked with Janicki on BMW Oracle, for instance. I built Carl Schumacher's last design as well. Be happy to build you something big and beautiful in carbon!


BTW, sounds like a hell of an Arctic journey. Good luck, should be an amazing voyage!

[carstenb]

Originally Posted by Dockhead

One problem with hydraulic - I think - Carsten will correct me if I'm wrong - is that the revised range of the pumps is limited. So if you have the pump on the main engine, you have to review up the engine to use the thruster - so you really have to use the generator engine for this, not the main engine. The big yachts I've been on with hydraulic systems all had the pumps on the generator. This creates a single point of failure - can't start the gennie, and suddenly you've got no windlass and no thruster.

With DC or AC power, you can have redundant power supplies.

Sent from my D6633 using Cruisers Sailing Forum mobile app

Dockhead - if you use a variable volume pump, it will supply as much or little oil as you need in the system and if you do what I noted, which is mount a small pump with a DC motor in an off line loop, you have the possibility of havinng hydraulic power even if you do not start the engine.



you are correct when you say that you do need to rev up the engine when using the pump mounted on the PTO. If this really bothers you, just use a DC mmotor/pump combination

[Dockhead]

Originally Posted by carstenb

Dockhead - if you use a variable volume pump, it will supply as much or little oil as you need in the system and if you do what I noted, which is mount a small pump with a DC motor in an off line loop, you have the possibility of havinng hydraulic power even if you do not start the engine.



you are correct when you say that you do need to rev up the engine when using the pump mounted on the PTO. If this really bothers you, just use a DC mmotor/pump combination

OK, I see. But if the thruster has 10 -- 15 horsepower, then it would have to be a pretty big DC/motor pack, wouldn't it?

Revving the main engine during docking maneuvers can't be done, because you have to control the propulsion separately during such maneuvers. I guess that's why the hydraulic pumps in such systems are always mounted on the genset, not the main engine.

I guess a genset mounted pump plus backup electric would be very robust. But it sounds like a lot of complexity and expense to get the same level of robustness you'd have with an electric thruster with redundant power sources.

I guess you could skip the electric backup and just name your genset as mission critical, so then we get back to the familiar system which I've seen in use many times, which is maybe a pretty good way to do it.

Most elegant of all I guess is total hybrid drive. I notice Northern Lights are selling a complete system -- maybe these are already better developed than they used to be? Then the thruster is trivial -- you use the same power sources and infrastructure your main propulsion is using.

[valhalla360]

I don't think this is a technological problem. If you have 10kW of AC power readily available, you can design a system that will work.

The problem is it makes little market sense.
- Below 50-60' most people only have 12-24v DC. Even those who have a bit of AC power available have no where close to 10kW AC on tap at a moments notice.
- As you get to the really big yatchs hydralic systems start making sense and they need much larger thrusters that don't make sense on smaller boats.

That leaves little market for a small AC powered thruster.

[Dockhead]

Originally Posted by valhalla360

I don't think this is a technological problem. If you have 10kW of AC power readily available, you can design a system that will work.

The problem is it makes little market sense.
- Below 50-60' most people only have 12-24v DC. Even those who have a bit of AC power available have no where close to 10kW AC on tap at a moments notice.
- As you get to the really big yatchs hydralic systems start making sense and they need much larger thrusters that don't make sense on smaller boats.

That leaves little market for a small AC powered thruster.

Yes, but they exist -- you can get Vetus thrusters and probably others in AC. Look like 3 phase, though.

Also, there are plenty of hydraulic thrusters in sizes appropriate to 50' - 60'. My current thruster in my current boat is 10 horsepower (Sleipner), and there are even smaller ones than this in hydraulic from Vetus.

[Dockhead]

So looking at the problem holistically, looks to me like there are a few options:

1. Conventional shaft drive, large batt bank, 24v thruster, just like I have now. Big advantage that this is highly developed, well trodden path, reliable, anyone can fix any part of it.


2. COnventional shaft drive, hydraulic pump on generator, hydraulic thruster. Advantage more efficient and unlimited duty cycle, disadvantage depend on generator.

3. Conventional shaft drive, large batt bank, large inverter capacity, AC thruster. Advantage more efficient; multiple power sources. Disadvantage not well trodden path.

4. Hybrid drive and high voltage DC thruster to match drive motor. Large LiFePo bank. One large one small generator. Advantages: many, many. Very high efficiency, much synergy between systems (including ship electrical systems). Disadvantages cost, not well trodden path, who can fix it.


So it looks to me like 2 and 3 are half measures without any overwhelming advantages over 1. 4 is really tempting but not ready for prime time? Very costly?

I guess after all this thought, there doesn't seem to be a really good reason to do it any way other than the old-fashioned conventional way.