Practical considerations about adding electric drive

[FatBear]

Say a person had a 40' boat with twin 375 HP engines. It can cruise at 20kts and tops out at 24kts. But say sometimes you wanted to go that fast, but usually not. Or maybe you wanted to scream out to a location and then troll for hours. And let's say you have become particularly averse to noise.

It occurred to me that it might be possible to add an electric motor to each prop shaft and use them and a generator for your harbor maneuvering and your trolling.

I've seen a similar thread about a get home engine where an electric is considered, but I'm talking about having two electric "mains" that can be selected as an alternative to the two diesel mains and which will run for as long as the generator and/or battery bank will last.

I assume that the transmission needs to be able to free-wheel. Obviously, the generator needs to be able to supply adequate power continuously for propulsion and other loads. (It is an all electric galley and there are lots of electronics.) What else would I need to consider?

How would one add an electric motor to a prop shaft? I know it's been done before, but I've only seen kludgy looking setups; nothing that I would want to risk. Is the hardware to do this kind of thing commercially available?

Thanks.

 

[High Wire]

It’s all custom work. The hp needed to turn the shafts through cutlass bearings is large when compared to what is available via generator. Small potatoes for a 375 hp diesel but a show stopper for twin electric motors powered by a generator. Then you also need some way to connect and disconnect when needed. If it was practical there would be already boats like this since submarines were powered this way since WWI.

 

[O C Diver]

Probably close to 20 years ago there was an article (maybe in Passage Maker magazine) about a twin engine boat that was converted to twin electric drives off a single engine generator. I believe the equipment was by Siemens. Anyway, what you want to accomplish Probably could best be done in that same large single diesel generator driving twin electric shafts. For redundancy and maybe increased efficiency in displacement mode, maybe a secondary smaller generator.

Ted

 

[Lepke]

It requires a big generator or you're limited to very low speed.
I owned a couple diesel electric tugs, 2 engines & generators and 2 electric motors connected to one reduction gear. I could run on one engine at slow speed. I loved it, so much I sometimes dream about it, but it's not very fuel efficient. It's cheaper to run a diesel directly to a shaft than thru a generator and motor. The cost of adding a motor and generator is more than the fuel you might save. The electric motors weight was a couple tons and the cables between the generator and motor were the size of my arm.

It cheaper to run the existing diesels at low speed or run one engine.

 

[Comodave]

PMM did an article about a trawler being converted to just generators and electric motors a long time ago. It was going to be the new big thing but quietly disappeared.

 

[pjtemplin]

1HP = 746 watts (and that's just the conversion equation; nothing in life is 100% efficient). You'd need 280kw to match cruising speed, but obviously you're not trying to do that. 15kw gets you 20HP. What could you do with a 20HP outboard? 15kw should be reasonably "in scope" for what you could cook up.

 

[Insequent]

There is plenty of information available on this. But not that many examples in practice as efficiency and cost intervene...

At the small end the Beta hybrids have been around for quite a long time and have a niche in English Narrowboats used in their canals.

https://betamarine.co.uk/he-hybrid-propulsion/

There was Isara Yachts offering Yanmar versions of up to 110HP for each engine using Beta Marine's tech, in a "hybrid catamaran" but not sure how many of these have been built.
https://www.worldwidecatamarans.com/WORLDWIDECATAMARANSIsara.pdf

Then people like Twin Disc have done a fair bit of work in this area and could easily come up with a solution to whatever situation you had. I doubt it would be cheap though.....
https://twindisc.com/goelectric/#marine

My interest has been similar to the OP in that I would like to be able to run just one of the large diesel engines (just 200 HP in my case) for normal cruise. If I had transmissions that could take an electric motor on an auxiliary output shaft of the gearbox, and for my old boxes that's not an option, then it could work. Run one diesel and power both props for low speed cruise. The engine that was running would no longer be just lightly loaded, and might operate with a little better efficiency.

For higher speed cruising start the second diesel. In practice this might need a 2 speed gearbox as well. It isn't all straightforward, and it would be bound to get expensive.

 

[twistedtree]

The Greenline Hybrids are built to accomplish exactly what you envision. Quiet maneuvering, and slow trolling or puttering about. You should check out the Greenline threads here on TF. By all measure they accomplish the goal, but you need to be realistic about the limitations.


First, any notion of gaining fuel economy with a hybrid in a boat is a fools errand. At the risk of being too blunt, anyone who thinks it will be more efficient failed (or never took) high school physics. But you didn't list that as a goal which is a good first step.


Even if you can shut down your main engines, if you have to run a good size generator you have lost the whole quiet-ship goal, or at least greatly diminished it.


The only quiet-ship operation will be running the boat on batteries, and you time doing that will be for a limited time, and very slow. You mentioned maneuvering and trolling, so your goals match the "slow" part. But I doubt you will be able to troll all day. A few hours may be realistic.


The generator will need to be big. There is a reason why your boat has two 375hp engines. That's equivalent to a 560kw generator. As mentioned earlier, you won't need that much for your described use, but it will give you a sense of the different in power requirements between propulsion and normal house electric needs. Because of the large generator need, even for the more modest electric capabilities of the Greenline, they don't have a generator, and instead use the main engine.


Note that one thing I head Greenline owners consistently praise is having 24x7 house AC power with ample capacity. Personally, I think that's where the most quiet-ship gain can come from. But that can be accomplished in any boat with an inverter/battery/charger system that's completely independent of hybrid propulsion, and a lot less expensive.

 

[ScottC]

I feel compelled to jump in here.

I have a hybrid boat. I enjoy cruising on electric when I can for the silence, the precision in maneuverability while docking/locking and the 24x7 house current.

But...

Unless you are an engineer/mechanic and like to tinker A LOT, I would think twice about sinking what would have to be a serious amount of $$$$ into the conversion you are contemplating. It risks becoming a money pit. And, if you get tired of the whole thing, it could negatively impact the resale value of your boat.

Hybrids are complex and can be expensive, despite what the marketing literature would have you believe. With hybrid, you have not one, but TWO complex systems to maintain. And TT is right, there is no economy reason to do this, unless you spend the majority of your time in canals where you can "fill up" on electric at your overnight marina, run for the next day on electric at 4 knots to the next marina and "fill up" again. Most marinas in Europe still include un-metered electric in their overnight fees, but this will change if more and more boats go electric/hybrid.

I like Greenline and if I were to buy a Greenline again...and had no desire to cover the Looonnngg distances we like to cover at sea, I would simply order one of their fully electric versions. No more hybrid.

My Volvo is 22 years old and for some years, I have had cash burning a hole in my pocket to buy a new car...but I hesitate. Why? I want the simplicity and benefits of PURE ELECTRIC. But, I also want the range of hybrid/conventional. Once a pure electric car can achieve the range of my gasoline-powered Volvo, I will be ready to add that car to my shopping cart. Tesla is almost there. Europe will not allow the sales of pure fossil fuel cars after 2030. So, I do want to invest in the right direction, but I would like to skip over hybrid and go pure electric. Maybe in another year or two...

 

[ScottC]

Forgot to add my last thought...
I've seen some articles here and there in the marine journals lately discussing surprisingly effective electric outboards. I mean serious outboards - not just fishing boat trolling motors. Could adding "infrastructure" on the stern of your boat to accommodate one or two of these be something to consider? I'm sure they're not cheap, but the investment and disruption to your existing setup would be less...and you could re-sell the outboards if the whole experience turns out to be disappointing.

 

[Mako]

Second the idea that the most practical way to achieve this would be with an electric outboard. Here's an outboard with (what they state has) equivalent thrust to a 50hp motor:

Elco Motor Yachts EP-50 96 Volt DC 50 HP Electric Outboard Motor for sailboats and small boats, pontoon boats, fishing and trawling in environmentally sensitive lakes and rivers - the reliable alternative to Torqeedo electric outboard marine engines.

It's input power is 22kW-hr so you'd need a 25-30 kW generator to run it. Or about 100kWh of lithiums in 96v packs to operate in Red October mode for a few hours. I'd bet you could put this together for about $20-25k with the outboard bracket bolted to your transom.

How serious is this endeavor to you?

 

[FatBear]

How serious is this endeavor to you?

It is not immediately serious. It is an idea I had that seemed good, but sounds like it is probably more expensive than just buying a brand-new boat that is exactly what I want.

But I'm going to read through all of those options on the Twin Disc site. They are certainly interesting.

Thanks, everyone, for all of the feedback.

 

[jimfrens]

I recently piloted a 115' dinner cruise boat (1966 Bender Shipbuilding) with 2x Detroit 16V71 motors. At some point in the past, electric motors had been installed parallel to the transmissions with a chain drive arrangement (a large sprocket had been added to each drive shaft just aft of the coupler). A large Yanmar-powered generator was installed (in addition to a smaller Detroit-powered generator) to run the electric motors and recharge the 24V batteries which also served as start batteries for the Detroits. For running on the diesels, the drive chains were removed. For running on electric, the chains were installed and transmissions left in neutral. Supposedly the electric motors had the ability to drive the shafts at 1,000 rpm limit. (The Detroits topped out at 1200 rpm.) I did not have the opportunity to see the boat run on electric, unfortunately. On the helm was a second set of 'throttles' for the electric drive system. A very interesting idea that could be added to many boats, given the space for the generator.

 

[Choices]

We have done this with push boats as an experiment.

If I remember correctly, goal was 20k hp for one minute. Exercise was to see how much electrical bump you could get on top of conventional diesels.

The experiment was funded for millions. I don't think we ever built it, because it just wasn't feasible, even if free. Too.much stuff for zero benefit. We did several electric ferries etc.

The technology is close, but not practical. I have said this before, maybe three guys in Texas could troubleshoot it, two in California, one in Florida. Lots of downtime.

 

[AndrewAdams]

Switchable electric drive in Willard 40

My 36,000lb Willard 40 gets 4.5kts from a 5kw electric motor mounted above the main shaft. One starts the genny, puts the transmission in neutral, fits the 3 belts into v-grooves on the motor shaft and main drive shaft pulleys, turns a bolt to lift the electric motor up on its tabernacle, thus tightening the belts, and flips a switch and off you go. Other owners have done a little extra wiring and can do reverse too. Like everything else on the Willards, it's only as complicated as it needs to be, and no moreso.

Andrew

 

[Mako]

AA, that’s very cool [emoji1303]

 

[Bmarler]

My 36,000lb Willard 40 gets 4.5kts from a 5kw electric motor mounted above the main shaft. One starts the genny, puts the transmission in neutral, fits the 3 belts into v-grooves on the motor shaft and main drive shaft pulleys, turns a bolt to lift the electric motor up on its tabernacle, thus tightening the belts, and flips a switch and off you go. Other owners have done a little extra wiring and can do reverse too. Like everything else on the Willards, it's only as complicated as it needs to be, and no moreso.

Andrew

with that infrastructure already in place, at this point it would be easy to switch to a 3 phase motor and a small vfd to control it. easy variable speed control and reverse as well.
my boat has an enormous sprocket on the shaft, and a chain and sprocket that fits the front pulley of my onan genset so i get forward drive from the genset. needless to say it's emergency use only. it came with the boat and i haven't tried it, but it works in theory. i'm tempted to build a bracket to hold an electric motor instead and do the vfd setup. an electric actuated clutch could make it a push button activation. then you'd have backup electric drive from the genset or battery bank.

 

[Mako]

... "my boat has an enormous sprocket on the shaft, and a chain and sprocket that fits the front pulley of my onan genset so i get forward drive from the genset."

That sounds like the ideal, simplest emergency get home. Like you said, you've never had to use it and that's why it's for emergencies. On another note, you may want to use it sometime in order to work out any kinks.