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post #1 of 62 Old 04-29-2019 Thread Starter
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Lightbulb Crazy Propulsion idea for next cruiser... Thoughts...

Hello all,

I've been dreaming about electric propulsion for my next cruiser. Looking at something like a Jeanneau Sun Odyssey 379 or 389
(38 foot). My plan is the set this up as my last boat to last me well into and past retirement. I'm sold on the idea of the lower maintenance of the electric drive and the simplicity of a one fuel cruiser. A large bank of lithium cells will allow all the conveniences of an induction range, microwave and possibly even A/C on the odd night at anchor. No need for diesel or propane tanks.

There are a few offerings for electric sail drive systems on the market and I've gotten the white sheets from most of them.
My initial idea was a 14KW sail drive and a bow thruster for better maneuverability as I'd be in the back row of the tight marina.

Then I had a thought. Why spend the additional 10K on a bow thruster install? Instead of one sail drive, how about 2 smaller units (7KW) installed port and starboard of center. My thoughts are 1) that smaller drive motors would be better at regenerating at lower speeds than one large unit and 2) while in the marina, two props give you the ability to steer the bow by reversing drive on one prop. Catamaran level steerage skills on a monohull may be possible.

Any thoughts????
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post #2 of 62 Old 04-30-2019
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

Have you done the simplest audit of the daily energy requirements of your boat?

How are you planning to recharge these lithium batteries.

Even if you were to cover every available horizontal space with solar panels you will not get anywhere near enough to run A/C.

I know of a few that have gone the electric power route and they have all fitted generators.
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

You would lose the prop wash effect, since your prop would no longer be behind the rudder (unless you have one of those new twin-rudder boats).

This is completely out of my area of expertise, consider me nothing more than a guy at the bar chiming in, but... With twin props if you're counting on regen while under sail wouldn't one of the props be out of the water if you were heeled at all? That would cut your regen in half.

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post #4 of 62 Old 04-30-2019
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

Asymmetrical dual prop thrust on a mono hull is still not going to perform like a catamaran or stinkpot. The primary purpose of the keel is to interfere with pushing aside. The bowthruster gets away with it, as it provide thrust exactly perpendicular to the keel and most often at a further leverage distance.

Aside from your question, the overall design thought may be from the future. I don't think there is such a lithium setup that is going to do all that you want, at least not for very long. Then, you have to recharge it. Hopefully, such options exist at some point. For now, the motivation to get away from diesel maintenance are a valid motivation. Albeit, you won't go very far, without a fuel burning generator. You're not going to get battery powered air conditioning and induction cooking at the same time.

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post #5 of 62 Old 04-30-2019
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

Up until last month, we had an electric stove/oven. The change has cut our generator time by about 50%.
We have a fairly large solar array and a wind gen, and we couldn't possibly meet our energy needs with both because some days there just isn't enough sun and/or wind to keep up with even half our energy needs.
As TQA mentioned, your going to need an auxiliary charging system unless you are just day sailing, and plug in every night. Calculate the energy requirements of an electric stove, motive power, and a thruster, not to mention A/C, and you are going to find them astronomical even with a generator. In the end, it's just simple math; can you replenish the energy you consume with any system other than shore power? If not, you are going to get stuck w/o power sooner or later.
If I were to put a thruster on a small sailboat, which I wouldn't do for any reason, I'd go to a hydraulic unit. Electrical systems are the weakest and most susceptible to problems on any vessel.
As for twin screws on a small sailboat, the props are not going to be far enough apart to give you the control you expect, even with twin rudders.

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Re: Crazy Propulsion idea for next cruiser... Thoughts...

If you are starting from scratch, perhaps you could come up with something like the pivoting Volvo drive units. You could swing the stern around like there's no tomorrow.
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

postdoc, it doesn't hurt to get creative in your thinking, and electric propulsion is wonderful if it suits your usage, but basically you're looking at the following solution as a good proposal:
  • I assume you're buying a used boat;
  • Remove and sell off your diesel propulsion engine, replace with a suitable electric motor setup;
  • Install a generator set powerful enough to quick charge your huge lithium bank and powerful enough to power your electric propulsion motor to at least 50%;
  • Install induction stove top, not "electric" resistance stovetop;
  • Mount as much solar as you can without the boat starting to look ridiculous;
  • Forget about the thruster. Use the money to pay for the genset (in a silent box) and perhaps some boat handling lessons to improve your confidence while docking.
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post #8 of 62 Old 05-01-2019
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

Interesting idea and concept. I've had an EP boat for over ten years now and keep discovering new ways to use it. Things you would never do with a diesel. For example in light winds I crank the motor control a tiny bit to turn the prop just enough to eliminate prop drag. You could do the same with your forward and aft prop idea. Gives you a nice bump up in boat speed and automatically starts regenerating once the wind picks up and no folding mechanical prop issues.

Another idea I've been thinking about. I have a electric outboard for the dingy. I was thinking it would be great to have something like it that would drop down from the bow on a rail for when one needed a bow thruster and pull it up out of the water when under way. I saw something similar being used on a power boat.

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post #9 of 62 Old 05-01-2019
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Re: Crazy Propulsion idea for next cruiser... Thoughts...

Until there is a significant technical breakthrough, all electric boats are going to be limited to daysailing or marina hopping. Just compare the amount battery storage you would need to equal the energy in say 10 gallons of diesel fuel.

If you are going to have a large diesel generator onboard, I don't see the big advantage over a direct drive diesel for propulsion.

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Re: Crazy Propulsion idea for next cruiser... Thoughts...

If you're serious about making your boat more maneuverable, you could install an electric pod motor that rotates 360 but locks forward when sailing. I imagine that would be fairly expensive, though.

As far as a general electric conversion goes, you have to be willing to accept both the benefits and limitations the system will impose on you. For my electric conversion, I went with lithium batteries, and saved about 100lbs net after pulling all the diesel components out.

Benefits of electric propulsion include much higher reliability and motor endurance, instant power, regeneration under sail, easy diagnostics and part replacement, universal recharging, and almost no or no maintenance, depending on the motor you choose.

The only real limitation is range, but it's a pretty big limitation. Expect ~1/10th the range of the diesel you replace, assuming you don't have a generator. That being said, a lot of sailors rarely use more than 1/10th of their diesel tank on any given outing. Personally, I'm generally daysailing, so I just need enough to get out of the marina and back in, plus a little more for emergencies.

If you're in a slip on shore power the vast majority of the time, an all electric setup makes sense. If you're making passage and anchoring a lot, you're going to need a both a generator and a very large battery pack to make that kind of system viable. Both electric cooking and electric propulsion take an enormous amount of energy to run. I'm not saying it's impossible, but you'll need much more electric generation than an average boat to make it work.
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