Swapping a Perkins 4.236 with Electric Motor

[Zigmo]

Good Day All,

I am a proud new owner of a 50' Sailing Trimaran. It's 11 Short Tons and currently powered by a Perkins 4.236.

It's a Hurricane damaged boat that still floats but needs the electrical and motor re-worked. This is my first boat but I'm mechanically inclined and ready to dig in. I've worked on cars and have converted a few to electric.

The Perkins is in rough shape. It runs but it's been sitting in salt water (splashed in not leaked) since last September. Instead of replacing/overhauling it, I would like to explore replacing it with a 3 Phase AC motor. The boat has plenty of space for panels and batteries. The overall cost would be more than a replacement Perkins but it would have some benefits.

PROS:
1) "Free Fuel" The boat will be used for day and short term charters in the USVI. Meaning no trips with lengthy motoring needs. I've calculated that I have enough space for a 2.4kw system without compromising leisure and work space.

2) Low/No noise - The motor would be virtually silent compared to the diesel engine.

3) Less Maintenance and less mess. All the consumables required for the diesel would go away. No oil, coolant (Not entirely true but much cleaner), Fuel, Filters etc..

CONS:
1) Weight - While the motor would be substantially lighter, the batteries would quickly make up and then exceed the distance.

2) Range - a non-issue since I would need to motors less than an hour a day. But this would still limit the utility of the boat if longer trips are required that require motoring for longer lengths of time.


My questions are around the relationship with prop size, torque, HP and RPM. As mentioned before I am new to boats and not sure how all these metrics need to be balanced.

Here is the motor I am looking at:
Siemens Azure AC Induction 3 Phase Motor 1PV5135 4WS14, EV West - Electric Vehicle Parts, Components, EVSE Charging Stations, Electric Car Conversion Kits

At 61hp contentious (160hp peak) it's comparable to the Perkins @2000-2200 RPM and way more power than the 4hp per Ton rule I've read on this forum.

Questions:
1) I'm assuming doing direct drive is out of the question. What is a normal RPM at the prop?
2) What are the Perkins torque specs? I can't find them anywhere online.
3) Electric motors have instant torque. Is this an issue for a boat?


Thanks in advance. I would love to hear your thoughts or concerns.

[Zigmo]

Edit: The boat is 22 short tons.

[Stu Jackson]

Originally Posted by Zigmo

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2) Range - a non-issue since I would need to motors less than an hour a day. But this would still limit the utility of the boat if longer trips are required that require motoring for longer lengths of time.>>>>>>>>>>>>>>>>>>>>
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Wouldn't this be the only question that applies and one that ONLY you can answer?


If not, then it's not a "con" at all, should be off the list, and you can just proceed to your sizing questions.


If it is, then one gets into resale value and overall potential use of the boat with whatever you install.



If the [reduced] range is such that it precludes "normal" use of the boat compared to others of its type in those particularly local waters, then you end up with an outlier of a boat, that may well prove to be less than useful to you and even less so for anyone else.



Good luck.

[GrowleyMonster]

Don't underestimate the amount of storage you need. My boat is way way smaller than that and my bank is 10.5kw/hrs. You really should figure on double that, TBH. You are going to be consuming a lot more power than I do when I purr along at 2.5kts. I think a 20 kw/hr bank would be more suitable. Don't make the mistake a lot of e-boat newbies do, and design for the minimum storage and power needed.



Also on a boat that size, forget about 48v, the standard voltage for smaller boats. Higher voltage will be much more efficient. Actually, the drive motor from a wrecked e-car and a couple of new battery packs might be the way to get your hand in the game. Typical electric cars run 144 to 288v. I think the Tesla cars are higher, actually. Higher voltage means lower current to perform the same work. Lower line losses. Smaller cable, and longer cables are not a deal breaker. Different electrical standards, though, which is why small boats use 48v, because 50v is arbitrarily considered the dividing line between lethal/nonlethal voltages. IMHO 48v is inadequate for the power levels you will need.



Most electric motors like to turn at around 2400 to 3600 RPM at full rated power. Obviously you will want a reduction gear of some sort. The good news is you do not need mechanical reversing, or a clutch. A very heavy duty belt drive would allow you to experiment with reduction ratio to get optimum efficiency. Spreadsheets, formulae, they only get you in the ballpark and are often wrong wrong wrong. Actual empirical data tells the real story. First, figure on a prop change. Largest fixed prop you can swing, pitch equal to or slightly less than diameter. Then determine the desired initial reduction ratio. Probably between 3:1 and 4:1 I am guessing. Totally guessing. A big slow turning prop is more efficient than a small blender blade prop turning at a kazillion RPM. You can use whatever ratio you need to make it turn with the power available and the desired motor speed. The trick is to open up the throttle so that the prop spins at freewheel speed, reducing drag when sailing. On a long sail passage you can go into regen mode and actually produce electricity, with a slight speed loss. No, it isn't a lot, but it is something.


On my boat I was using an enclosed iron gearbox made by Baldor but you will need a bigger one, if you want to go with a gearbox. I am now experimenting with direct drive, a thrust bearing, and a smaller prop. I may end up going back to my 2:1 gearbox.



Best bang for the buck in batteries is 6v golf cart batteries. My 220ah batts cost me $85/ea at Sams club and they are almost 4 years old, still going strong. But if you go with lead, you better know how to maintain them. And be able to equalize. If you are scared to hook together a big Variac and a welding machine rectifier to manually charge your batts from shore power, you might want to consider one of the several types of lithium batteries. The plusses are lighter weight for the same capacity, easier maintenance, lower minimum charge level and so more usable storage, and longer life. The cons are really just one... cost. Even figuring higher usable storage and longer life, LiFeP04 batts can't touch golf cart batteries price wise. But if you have deep pockets, for a nice big bank for a large boat I would go with Lithiums.


Your boat will have a higher resale value with a diesel than with an electric drive system. However, obviously it will have a higher resale value with an electric motor than with no engine at all. Two headed coin there. And of course resale value isn't very important if you never sell the boat.



Can you mount an outboard? Just askin. And a Beta/Kubota engine of appropriate size wouldn't cost much more than electric. Electric isn't for everyone. It has a lot of strong points, yeah, but so does diesel.


I suggest you actually get some time onboard an electric boat before you whip out the credit cards. And maybe putter around with a smaller eboat conversion first. You can convert an old Catalina 27 or similar size boat for under $3k including batteries, and get your hand in the game and have a lot of fun, too. A 22 ton tri electric repower is a really big comittment and you would be definitely taking the path less traveled.


Of course, a boat that size could easily mount 2kw of solar panels, giving you more free fuel. Hmmm.