Electric Cruiser

Hello. I apparently joined this forum some time ago, then forgot. Boat design has been on my mind a lot lately, though, and so I started reading here. Then I wanted to comment, and... well, here I am.

My wife and I would very much like to own a cruiser in the 34-40 foot range (11-13m), and we'd like it to be electric. Happily, there are more and more all-electric boats coming on the market, but the closest thing currently offered (no pun intended) is the Northman 1200, a beautiful boat. Configured the way we'd want it, the price approaches $300,000; too rich for our blood. The Northman 870 is priced more along the lines of what we'd be comfortable with, starting under $100,000, but it's not quite enough boat for extended cruising with the possibility of others on board.

I would love it if the Delphia Blueescape 1200 were available in electric from the factory, but it's not. And again, the price for such things seems to skyrocket as vendors want around $1000 per kWh for their battery systems. Larger systems are discounted to about $600/kWh, but that's still exorbitant compared to the cost of batteries for electric cars, and frankly I don't see that much difference between them short of perhaps an IPS67 casing.

As a child of the 70s, my mind quickly goes the DIY route. No matter what, knowing how much battery you want is going to be a key component of the design process, whether you're picking from the Northman catalog or converting an old CHB you bought off of Craigslist with blown engines.

A bunch of research on how far most people go on a cruise and why (50-60 miles at 6-7 kts for trawlers seems to be a really common number; more time than that at the helm is more than most people want to deal with). I connected with a guy who owns a Greenline 33, and he said that, topped out, his 7 kW motor pushes him along at 6 kts on 6.6 kW of draw. I did other inquiries and ended up figuring that about 60 kWh of battery was sufficient for the type of cruising I wanted to do.

The idea would be to cruise some 50 miles in a day, then either find a place to connect to shore power to charge back up or run an onboard generator for a couple hours to recharge when shore power wasn't available. Solar panels on the coach roof (no flybridge necessary on my dreamboat) would supply hotel power and a tad more in most situations.

For longer cruises (like this post ), the genset should be able to provide propulsion and hotel power as necessary. Turns out that 30 kW diesel generators aren't THAT much more expensive than 10 kW units. Which brings me to the motor. It seems to me that for mostly-slow cruises, with the occasional burst up to 12 kts, a 30 kW motor should do the trick. I don't know enough about hull design and hydrodynamics to say for sure, but I think 20 kW would do the trick with a right-sized prop, maybe 10 kW in the 6-7 kt range. I know a lot of the answer depends on a lot of things, but the idea was to use a 30 kW pod motor and not need its full power most of the time, hence the thought that a 30 kW genset would be sufficient even for extended cruising.

Back to the battery. It turns out that the battery pack for the Tesla Model 3 Long Range has a raw capacity of 80 kWh, with an advertised capacity of 75 kWh, and probably a realistic capacity when trying to get the longest life you can out of your system of 65 kWh or so. That's right on the money for my 60 kWh target. Speaking of money, those battery packs can be had from salvaged cars for around $10,000, which is around $130/kWh. As a bonus, most of the pack is only about 5" thick, about four feet wide and maybe six feet long. Designed into a hull, it could sit way down in the bottom. The weight is 1054 pounds (just shy of 500 kilos), which is a lot, but you're losing the weight of a diesel engine, so it balances out.

That motor linked above comes as a kit with motor, controller, throttle, and everything but the battery and the prop. The Tesla battery packs are an integrated system with BMS and charger built in. The motor kit comes with a cooling circuit that includes the controller, so the only other component you need is a cooler for the battery pack. Or so it seems.

I'll make posts (hopefully shorter) elsewhere asking about hull designs and whatnot. Am I crazy for thinking this straightforward-sounding electric drive system is feasible?

Cheers.

J.D. Ray

 

Welcome back to the Forum JD.

I must admit that I had not heard about the Northman boats before - here is a link to their 1200.
Northman 1200 | https://northman.pl/en/jachty/northman-1200-2/
Her displacement is 6,200 kg, and they mention the power options as two outboard motors up to 150 hp each, or two inboard motors up to 110 hp each - but I don't see any mention of an option to have electric motors?

Re the Northman 870, here is a link -
Nexus Revo 870 | https://northman.pl/en/jachty/nexus-revo-870-2/
They do mention electric propulsion options -
"Engine types: outboard, diesel, Eco (electric drive), and Hybrid (a combination of an internal combustion engine with an electric drive.)"
And they mention the engine power as '25 - 115' - I presume hp rather than kw.

And here is a link to the Delphia BluEscape 1200
https://en.delphiayachts.eu/yacht/bluescape-1200
They mention a minimum displacement of 8,500 kg (more than two tonnes heavier than the Northman?), with a maximum engine power of 280 hp - but her fuel capacity is only 44 gallons, which is surely a mistake?

Re the Greenline 33 that you mention - the owner claims that " his 7 kW motor pushes him along at 6 kts on 6.6 kW of draw".
And 6.6 kw is only 9 hp - if his boat has a clean bottom then it might do that speed in flat calm conditions, but I am sure that the speed made good will drop off dramatically as soon as you encounter some head winds and seas.

And if you have a 30 kw electric motor in the boat, then using 20 kw (27 hp) to push a 12 metre long motor boat at 12 knots again sounds very optimistic to me.

 

There are a lot of hidden rocks in electrical boats (we design some of them). First, they are expensive to buy. Second, change of battery after 2 years is also expensive. Then, consider range, speed (slow!) and serviceability. And also, most of them still carry generator and fuel on board. Adding this all together, today electrical boat is good for 'marina cruiser', but not for real cruising, and also no way as a budget cruiser!

My concern is also advertised speeds and range of electrical boats. Those are usually given for flat water and ideal conditions. Given sea state, this advertised 7kts turn into 4kts...

 

I think for short cruisers from a fixed marina an electric boat can work great. If you only use the boat from one marina I think this is usable, for example for ferries. But that's not a "cruiser", you're pretty tied to specific infrastructure and recharge from land.
Modern batteries will last many cycles but need BMS and such. The cost for raw 18650 cells or new car batteries made of them is around $250/kWh now.

You should think about your intended use profile or "statement of requirements". For example if you only use the boat once a month for a weekend it is probably easier and more economical and environmentally friendly to use diesel propulsion.

Personally I dream of a purely solar powered trimaran. But that requires some specific design choices. The boat has to be long, very lightweight, it has to be a multihull and it has to have a ginormous roof area for lots of solar panels (50m² for 10 kWp solar). And you can't buy them and I haven't found plans for one either. Here is a sketch of what I have in mind, if you want I can share some of my ideas.

If you'd live on a boat and cruise constantly then electric with solar becomes attractive. 10kW solar panels are cheaper than a sailing rig. Electric motors need little maintenance. Overall system complexity is less than motorsailers or motorboats. But you're limited by the sun, season, latitude and by a relatively slow speed - I imagine I'd cruise around at 6kts. Someday, somewhere, in a sunny future

PS: Welcome to the forum!

 

Hello jdray, like you, I joined the forums a couple of years ago, then didn't spend much time here until recently. What a great bunch of people.

I also am interested in electric motive power. Mostly as auxiliary to sail. For world cruising, I think it has a couple of distinct advantages over traditional auxiliary power, not the least of which is the ability to refuel without a marina availible.

It sounds like you have done a lot of good research already. Dejay mentioned the need for an established infrastructure. On a purely power cruiser, you would best have your cruising ground well known. The East Coast ICW, should give you years of interesting places to cruise with little concern for availible infrastructure. You just have to pay for the hookup.

These are great times and that infrastructure is being built and added onto right now. There are designs being installed for dedicated electric recharging docks and more. Other places, like the caribbean may be more sporadic in finding plugin facilities. So know where you want to cruise.

In the sailing world, there is a concept called "hull speed". It is defined by a formula based on waterline length: 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). Hull Speed is a physical phenomenon where, as a displacement boat moves through the water, the bow causes a bow wave to build as the water is pushed aside by the advancing hull. Fluid dynamics describes the acceleration of the water past the hull, coming out of that bow wave, to rush aft and fill the void left by the advancing stern. This causes a second wave to lift as the rising water runs into the still water behind the boat and a series of waves are created. The faster you move the water out of the way, the longer that wave length gets until the wave length equals the waterline length of the boat. At that point, the boat has reached Hull Speed. Going faster, begins to lengthen the wave such that the stern is falling into the trough of the wave and you are effectively driving the boat uphill, until you go fast enough to achieve planing speeds.

The significance of this is in the power curve over speed. The maximum efficient speed over power to achieve it is theoretically Hull Speed.

There are a lot of other factors that influence this, hull shape, wind and wave conditions...

You might use this formula to get an idea of where your power plant should be.

Good luck.

-Will (Dragonfly)

 

Yeah I think for cruising with an electric motorsailer it really makes sense to go electric. You can harvest quite a lot of power from your sails to do "regenerative breaking". By shaving 1 knot off your top sailing speed you can probably regenerate and store enough power to motor with 4 knots later.

 

Hi Will.

Yes, I'm familiar with hull speed, at least conceptually. I started reading Robert Beebe's "Voyaging Under Power (4th Edition", but life got in the way about a quarter of the way through and I haven't finished it yet.

Hi Dejay.

Indeed, requirements are near and dear to my heart. The basics are as follows:

• Provide a comfortable platform for four adults to cruise near-shore waterways (e.g. Puget Sound or Dalmatian Islands).
  • Two double berths; "island" style berth for the master stateroom.
  • Staterooms have standing headroom exceeding 6'2" for at least four square feet of floor space (headroom may be reduced slightly for VIP stateroom; required in the master stateroom).
  • At least one dry head; standup shower with 6'2" headroom.
  • Staterooms have privacy doors.
  • Galley has at least a two-burner cooktop; microwave oven; refrigerator; sink.
  • Salon has dinette that seats four adults comfortably.
  • Cockpit has roof as a sun shade (retractable fabric shade OK); seating for six.
  • Transom swim platform with stowable ladder.
  • Space for two persons to lounge in the sun on the foredeck on sunpads (my wife may have more articulate requirements on this item).
  • Sufficiently tall gunwales and readily-available grab bars so passengers and crew can move about the boat while underway without feeling like they might get thrown overboard.
• Electric propulsion with generator backup.
  • Electric propulsion sufficient to move the boat at 12 knots.
  • Battery sufficient to provide a range of 50 nm at 6 knots (flat water, no wind, modest payload).
  • Generator large enough to provide power to propel the boat at 6 knots as well as hotel power.
  • Generator fuel supply sufficient to power the generator for 72 hours at the minimum power output level to meet the propulsion and hotel power requirement.
• Supplemental power
  • Sufficient solar power infrastructure to provide all hotel functions of the boat for a 24-hour period with 10 hours of full sun.
  • Hotel battery (12 V) sufficient to provide 24 hours of hotel functions. Battery may be used for ancillary boat functions such as bow thruster, powering electronics, etc.
  • Battery charging infrastructure sufficient to charge propulsion battery with solar power when the hotel function battery is full and charge the hotel battery from the genset.
  • Shore power connection of 50A to charge both propulsion and hotel battery sets.

These to me are the core requirements. I don't know if they're achievable, or if they are, at what cost. And while my wife will have certain aesthetic requirements, none of them will be at the level of finery that one sees in multi-million dollar yachts. We appreciate those trim levels, but don't need them to make us happy on the water. But show her a boat with 80s-style upholstery in it, and her immediate reaction is, "What's my redecorating budget?" God forbid I show her a boat with honey oak cabinetry.

The genset fuel supply combined with a fully-charged battery should provide around 500 miles of range at slow cruise. That's the entire length of the Adriatic Sea or a trip from Olympia, Washington to Campbell River, British Columbia and almost all the way back. That, to me, is a sufficient range to do all the cruising I might ever want, and depending on the goals for the particular cruise, we should be able to do a lot on little or no diesel consumption. My one regret is that such a boat couldn't cross the Atlantic, and therefore I couldn't take it from the PNW to the Adriatic. But increasing the range of the boat for that sort of crossing (about 4x) seems irresponsible for the one or two times we might endeavor to do such a thing.

Thoughts? Feedback?

Cheers.

JD

 

Have you seen Silent Yachts' line of solar power catamarans? They start around $2M, but they're the first of their kind. Someone else, Sunreef I think, jumped on the bandwagon and offers one now, too, but not as... ahem... affordably. To me, the SY50 is a near perfect boat.

 

This is why I mentioned the hull speed formula.
For a 40 footer, 8 knots is generally full steam ahead. It isn't that an electric motor can't push a 40 footer at 12 knots, but that's a planing speed, for a boat that size, that requires enough power to drive to the top of the bow wake, and that will burn right through your battery charge.
At 8 knots, you will get where you're going in good time and use your battery charge at a much more efficient rate. Of course, planing is very efficient, once you get there. In that case, go for 18 knots, because twelve is barely planing. Again, depending on other factors.

Modest. I think quite doable.

I like your list. One thing you might look into is Alibaba for a manufacturer who can sell you a dozen or so AGMs for factory cost. Get a few fellow sailors to go in on it and really get the cost down. Alibaba usually lists minimum order size and price range. I'm not recommending it and I haven't done it, but I have browsed through with the idea in my head to give it a try.

-Will (Dragonfly)

 

Math doesn't lie. I've based my conceptualization on the Greenline 40, which, somewhat due to its vertical bow, has an LWL of 38.29 feet, giving us a (crunch, crunch, crunch) hull speed of 8.29 kts. C'est la guerre.

 

As far as I know (but that might have changed from the last time I looked) no third party has made a Tesla pack function with all the needed functions on the original hardware. There are some third party solutions but none for a full unopened pack.

 

How much energy would it take to get a 40', eight tonne boat like the Greenline 40 up on plane? I realize a lot of that has to do with hull shape. Is there a straightforward answer that can be had without just asking Greenline?

 

By the way, if I could just write a check for a boat, and it came with an electric powertrain, it might end up being the Fountaine Pajot MY 40 power cat: MY 40 | Fountaine Pajot https://www.fountainepajot.com.au/power-range/my-40/

 

Re the amount of power to get the Greenline 40 up on the plane - they mention a minimum of 2 x 170 hp diesel engines on their website, hence I would guess that this is effectively the minimum power required.
THE GREENLINE 40 I Classic Cruiser I Motor Yacht I Hybrid Yacht https://www.greenlinehybrid.com/yacht/greenline-40/

They also mention in the brochure (which can be downloaded from the site) that the maximum speed with the larger 220 hp diesel engines is 22 knots.

That Fountaine Pajot cat looks very nice - but they make no mention of any electric drive options, just a choice of a pair of 260 hp or 370 hp diesels with IPS drives.

 

I should have posted a link. Here's the one I used after finding them on YouTube: https://www.naturalyachts.info/northman-1200-electric

I reached out to them via email and asked a bunch of questions. I got these responses (they use "sail" perhaps inappropriately):

They also attached a spec sheet, which is attached here.

There's a lot to like about the boat. 19 km/h (~11.4 mph) on an 85 hp (which I take to mean 64 kW) motor seems reasonable.

Siemens makes a motor, the Azure series, that's water cooled and will do brief periods of 100 kW with good cooling. New Electric uses them in their marine drivetrains. I considered one of them mated to a pod drive system like the Volvo IPS. The advantage to that is your motor is inside the hull. The downside is that space is taken up inside when a straight pod drive wouldn't, but it's not a bunch. The batteries are flat enough that they can be buried in what would otherwise be infrastructure (floors, probably, wherever you need to put them for good weight distribution). The "engine room" should only take up ~4 feet near the stern, and house the genset as well as the pod drive and other electronics. The only thing in there that would need regular maintenance is the genset.

Thoughts?

JD