battery-powering a 25 HP motor

system for my marine, to 23 hp..20 to 30 second run times...an easy install. If you are short on cash you could probably find a used one around a ship yard or on google. Used marine electrical items can be a looser though.
If money is an issue you might be able to do without VFD by bumping the motor as needed to get it around, my guess two bumps to start then full bore for 20 seconds then off. that should work in all but really tight maneuvering situations.
If this is a fishing boat and you need world class but affordable marine refrigeration give me a call..Ive done a few in my time. Freon and ammonia, brine, plate and blast freeze. I did two of H Salt Esq boats. including a 200' blast freeze processor.
(415) 927 7573 Phil Scott
limitations of the engine cannot be small. Direct diesel 25 HP, 1600 duty cycle. close to the problem. bowthruster. It would bowthruster rpm. (reliability/safety) of the concepts below, peak. use a VFD as feed the DC bus power of a suitable arrangement of the VFD, motor, Bob would be expensive, it requires an unproven sites, I have seen AC specially wound . defeats the saving transform 48 to 192 above), a standard engine. This applications, but expensive setup, is some these days. What have

Some considerations:
1. A 25 Hp VFD will most likely be powered from 208 vac 3-phase electric system power if standard USA equipment is used. This would result in a minimum dc bus voltage of about 300 volts dc. Modern VFDs have even higher dc voltages due to the circuits used to control harmonics and power factor on the ac lines.
2. If you want any life out of the batteries, the voltage will have to be controlled, but VFDs are not designed to regulate the dc voltage close enough.
3. Without some special circuit design, the VFD will draw its' full rated ac current as soon as the dc voltage drops (as it must for a battery to supply the surges). Perhaps you could turn the ac power off for the 3 second run time, then recharge the batteries after the run period. That would reduce the loading on the ac power source.
4. If the prime power source can supply the 25kW ac, then why not power the ac motor directly? A soft start motor controller can be used to reduce starting surges on the generator. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz

"large" industrial UPS units are fairly common. I would suspect any of the larger companies could tailor a product to fit your needs. for example a 20 kW ups configured for 15 kW (with room for additional modules) cost about $12,000 USD a couple years ago. its about cubic volume of a standard refrigerator (but different dimensions). an electrical contractor in your area will know which companies are available to you.
Here in the USA Powerware and Leibert seems to be the top 2.
The advantage here is you would be getting a standardized unit with mfr warranty.
the above mentioned unit gives me about 100 min operation at 50% load. it is 3 phase and continuous conversion (it always converts power to DC unless it is in maintenance bypass mode)
If I assume that your ship already has something like 220 V 3 phase (just too small to run the motor plus whatever its normal load is) that could be charging the batteries until needed. Alternetly if the existing gen could run the thrusters maybe you could run the remaining load from the UPS for the peroid that the thruster is engaged. this could prove to be a blessing... how many times has somthing onboard blown up durring the transfer from ship power to shore power? that would end it once and for all.
. the other method would be the "electric submarine" type of battery switching arrangement were you series batteries for high voltage operation and parallel them for high current operation. im not sure but I think the way it worked was that the batteries were seriesed for charging and switched to series-parallel for running.
the formula for HP to watt is 745.7 * hp = watt so you need 18.6 kW to RUN the motor (at max)
then, assuming zero losses, a 48 volt DC source would need to supply 387.5 amps to run at max.
3 min is far beyond the 30 sec marine cranking amps of a marine deep cycle battery rating.
I suspect you will need several parallel strings of batteries to get the required power without having batteries explode on a regular basis.
I also think your 3 min on and 1 hour to charge is way optimistic. you are thinking in terms of starting a car and not deep (and rapid) discharge of a storage battery. for example a depleted 200 A-H golf cart battery requires at least 20 hours at 10 amps (and usually a little more). in my experience 10 to 20 amps charging tends to boil the electrolyte in most batteries. this means you have to keep adding distilled water and worry about ventilation of fumes.
I'm guessing as to the size of your ship but if my guess is correct you really need an expert in marine power systems. It was interesting to try to consider the engineering features of your problem but the potential for disaster if a side thruster on a big ship fails when needed makes it far too important to be looking for free advice on the internet.
I suppose a small nuclear reactor is out of the question (just kidding)

. Sorry, I tried to resist but I have absolutely no will-power.. I just have to know why hydraulics are not an option!

Sue,
Not sure whether you are into marine diesels, but my MWM diesel is a so called direct reversible engine. There is no clutch, no reduction gear. It drives the propellor by direct connection to the crankshaft. That means that is is off during manoevring, and therefore not a viable power source for hydraulics in this case. The genset is a 15 HP unit, not sufficiently powerful.
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diesel is a so reduction gear. It not a viable HP unit, not ..thats sure a small gen set for such a big boat. Im surprised. Your cheapest alternative in this then, since you can surely use back up generator capacity might be to add a 25 hp gen set with a hydraulic take off and run hydraulic thrusters... thats going to be cheaper than batteries and a new 25 hp DC thruster... and will give you generator back up.
Used marine hydraulics can be fairly reliable. and you should be able to pick up a used gen set cheap too.
Phil Scott
bericht bowthruster system for my limitations of the engine cannot too small. Direct to: 25 HP, 1600 max duty cycle. close to the problem. bowthruster. It control over integrity do realise that current levels, in the use a VFD as feed the DC bus power of a rectifier arrangement standard AC expensive, it arrangement, and it is an sites, I have seen requiring specially . defeats the transform 48 to case above), a DC engine. This applications, but expensive setup, there is some these days. What have will-power.. I just have

you may be surprised how cheap some dc motors around that size you're talking a lot of battery, tho. curtis controllers are quite good but i forget just how high the voltages may go. most of my experience has been in the 36 to 72 volt range. keep in mind that you can run a higher voltage motor, say 50 HP at 240 volts and get 25 HP at 120 volts, same current but less voltage. also running the field somewhat high will aid. good luck, sammmm
.

My thoughts almost exactly - which was partly why I asked. I love hydraulics - particularly as electrics, especially batteries, and salt water mix all too well. The hydraulic motor would be tiny and almost zero maintenance. As for the hydraulic power pack - second hand ones of those are easy to find and practically given away.

I am also looking at using a DC motor to power a bow thruster in a 22m steel barge and have found these motors - Recently visited the factory and witnessed one churning out 7 kw output at 36v 225 amps and it was not even up to full power at 48v 600amps or getting warm!! - these motors will go up to 72v and one guy runs his at 1000A. Motor is tiny - 8 inch diameter and weighs 11kg. A recommended controller is a Millipak 4QD at which may not be quite big enough for your vessel, but it does not need the separate contactors that the Curtis ones need. How big is your ship??? I reckon 12kw - 15Hp is enough for my vessel. Batteries - 4 x 12v 105 Ah is considered by Lemco to be sufficient with a 48v charger running all the time. Silette Sonic, who make sail drive legs, suggest using these motors with their legs.

Interestingly, Royal Navy ships are moving away from hydraulics to electrics because hydraulics are not reliable or maintenance free!!

Interesting! I was looking at the same. Have you seen the LEM 2*2? It is really just two LEM 200 flanged together.
The manufacturer admit that they have no controller to run it, but that should not be a problem.
My ship is roughly same length as yours, but weighs about 100 metric tons. Have a look at it on (unfinished website, in Dutch, but the pictures translate fine).
I would admit that my aversion to hydraulics is perhaps irrational and a little subjective.
One consideration is, that an electrical installation can be made almost completely reversible. That is a consideration since the ship is national heritage registered. With hydraulics there are more and larger ductings to worry about.
I am exploring VFD regulated submersible pump motors now - haven't found any low-voltage ones yet. They come in the right power ranges, and could drive a propeller directly. I could clamp one to an existing rudder with only the power supply cable to feed through the hull. No idea how much that would cost though ...
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Interesting - Ik sprek een beetje Nederlands!!! Mine is a Euroships new build being built in Waalwijk by Triton Jachten. My displacement will be around 65 tonnes, and reckon that 12kW will be enough - a friend of mine has a 22m klipper with only a 5.5kW 3 phase motor for his BT. Working on a website - - but no pics yet - to follow shortly.
The LEM2*2 would really give you sufficient power.
I have the Silette Sonic sail drive leg in a tube and just need to mount something to drive it - have considered 400v 3Ph from my genset, hydraulic and now DC. DC would be the simplest installation - just a 240v cable and a control cable to run.

Use a hyd motor with a remote hyd pump powered by electric motor runs off house batterys (may need to add to your bank) a manual valve with power beyond spool or solenoid with dc coil ( pump can be driven by used forklift motor)

Look at the latest golf cart solutions. They are a mass market thing.