Electric motor for E35II

[gareth harris]

A number of us seem to be considering an electric motor for the E35, and while a thread in the archive has some fairly forthright debate about the merits of an electric motor in general, this is my thinking about specifics for the model. The thoughts of others would be appreciated.

1HP = 736 W, but a diesel engine is generally about 40% efficient, with the very best about 45%, whereas an electric motor is usually about 85% - 90% efficient. The efficiency no doubt depends on where in the power range the motor is operating so direct conversion of the numbers is not very accurate.

There are two available options for US built electric motors that I have found online:

Sailboat Kits & Accessories

Electric inboard sailboat conversion kits. Convert your sailboat to clean, quiet electric with an easy to install kit.

www.thunderstruck-ev.com

https://electricyacht.com/shop/

Both have told me that their motors have been installed in an E35, but they could not provide customer feedback on how it had worked out. Both also said that 10kW is marginally under powered for an E35 but it is what most customers have chosen.

When trying to get out of trouble into the elements and against a current, a burst of high power could make a crucial difference; also, as the motor ages it maximum power would no doubt decrease somewhat, and it is never a good idea to run anything at its maximum for too long (Electric Yachts limits its 10kW motor to 20 minutes of output above 8kW), so there might be times when more than 10kW would be desirable.

Thunderstruck makes a 12kW motor which is water cooled, while the 10kW motor is air cooled. It weighs 55lbs so would be manageable, although it also needs a separate gear reduction which weighs 50lbs. With an air cooled system, running 5kW continuously at 90% efficiency would mean that 500W turning into heat, which is equivalent to a small room heater, so it would make the engine compartment extremely toasty, particularly in Pensacola in August. The only realistic place to mount a radiator would be under the cockpit, with airflow from the dorades that are left over from the A4 set up, but that system might work well.

Electric Yachts makes an air cooled 10kW motor and a water cooled 15kW motor, which weighs 125lbs as opposed to just 70lbs for the 10kW motor, and might be overkill on power.

Putting all of that together, I am leaning towards the Thunderstruck 12kW.

There would be room next to the motor (in the centre of the cockpit type Ericson 35) for one 48V 8D and a house 12V battery. The 12V capacity would not need to be high since it would be immediately recharged from the 48V system, so it would just have to cover large current outputs such as an anchor windlass or the refrigeration coming on. At least two 8D batteries would be needed in the place of the fuel tank.

Three 8D lithium batteries would give about 45 minutes at 10kW when needing to get out of trouble. Otherwise it would provide about three hours of 4.5kt cruising, or considerably more if using the motor to keep the sails full in a light wind.

A generator such as the:

Honda EU3200i Model Info | Super Quiet 3200 Watt Inverter Generator | Honda Generators

The Honda EU3200i generator combines power and portability. Lightweight and compact, this 3200 watt generator is the perfect portable power source for RV, tailgating, home back up, and more.

powerequipment.honda.com

would give about 4kts worth of continuous power in calm conditions.

Since an Ericson will perform well in anything more than 5kts of wind, it seems that the main drawback of the electric system is the inability to go a long distance in a hurry in dead calm. Motoring would also not be an option for long in a storm so storm sails would have to be carried in questionable conditions.

Gareth
Freyja E35 #241 1972

 

[Loren Beach]

Have a look at the many postings by : https://ericsonyachts.org/ie/members/p-gazibara.9607/

 

[Jerry VB]

1HP = 736 W, but a diesel engine is generally about 40% efficient, with the very best about 45%, whereas an electric motor is usually about 85% - 90% efficient. The efficiency no doubt depends on where in the power range the motor is operating so direct conversion of the numbers is not very accurate.

Typo: 1HP = 746W

The diesel engine efficiency is immaterial for this discussion because the diesel is rated for shaft HP (SHP), i.e. how much power the engine produces after it burns the diesel. The amount of energy produced by the diesel used vs. SHP output will be ~40%, with ~60% becoming heat that gets discharged by the water cooling system. Diesels produce a lot of waste heat, but the energy density of diesel is immense, overcoming the engine's inefficiency when calculating the range a typical diesel tank provides.

The electric engine efficiency is important for figuring out how much heat will have to be "disposed of," as you rightly point out. I believe an electric motor is rated by input power, not SHP, so a 10KW motor at 90% efficiency will produce ~12HP at the shaft and ~1000W of heat (500W at half power as you point out).

Having said that, electric motor manufacturers assert that an electric motor, due to its torque at relatively low RPMs, can produce more thrust than an equivalent HP rated diesel. How much more is a bit hand-wavy.

 

[Kenneth K]

1HP = 736 W, but a diesel engine is generally about 40% efficient, with the very best about 45%, whereas an electric motor is usually about 85% - 90% efficient.

Yes, but a 25hp diesel still makes 25 HP (roughly) available to the user. The 40% efficiency means that it consumes 2.5 times more fuel in doing so versus a 100% efficient engine.

So, diesel: 25hp = 25hp.

Electric: 12kw = 16.3 hp (or, less if there are limitations about running at max power like the Electric Yachts motor).

(Jerry beat me to it.....) He also added the useful point, that if the electric motor is spec'd at input, rather than output power, there is a further reduction in actual power available.

 

[Mark F]

Something to keep in mind with an electric auxiliary is how the controller is programed. I've had two different controllers on my EP system - programed by different people and the performance difference is huge. My current Sevcon Gen4 gives me a full power draw of 60ish amps. My old Sevcon Milipak gave me a full power draw of 120ish amps. With the Milipak I would regularly see regeneration numbers of 4 amps at 50 volts (200 watts) with the new Gen4 I see half of that.

I'm not big on comparing efficiencies diesel to electric I don't know (care ;-) enough. Of course I know you have to start somewhere.

Personally, I would err on the slightly higher power side of a new install. I have a 5kw unit in my E27 and it works great, that said if I were doing a new install today I'm pretty sure I would go with a 10 kw motor/controller unit. The cost difference is minimal and added margin can't be a bad thing, right?

One issue I can envision with a bigger drive is not keeping an eye on consumption and more easily draining your battery bank, but paying attention is the remedy for that .

 

[gareth harris]

The diesel engine efficiency is immaterial for this discussion because the diesel is rated for shaft HP (SHP), i.e. how much power the engine produces after it burns the diesel.

Having said that, electric motor manufacturers assert that an electric motor, due to its torque at relatively low RPMs, can produce more thrust than an equivalent HP rated diesel. How much more is a bit hand-wavy.

I am glad that I asked the question since that is good food for thought.

 

[gareth harris]

My current Sevcon Gen4 gives me a full power draw of 60ish amps. My old Sevcon Milipak gave me a full power draw of 120ish amps. With the Milipak I would regularly see regeneration numbers of 4 amps at 50 volts (200 watts) with the new Gen4 I see half of that.

Personally, I would err on the slightly higher power side of a new install. I have a 5kw unit in my E27 and it works great, that said if I were doing a new install today I'm pretty sure I would go with a 10 kw motor/controller unit. The cost difference is minimal and added margin can't be a bad thing, right?

I have not yet looked at the hardware for charging the batteries from wind or a towed generator.

Electric Yachts puts its maximum output from each 8D at 100A, so when approaching 15kW a fourth battery would become necessary.

 

[p.gazibara]

I can shed some light, our 35-2 has done some 15,000nm with an electric drive. I built the system very frugally and simply, I think the total install bill was somewhere around $1600 including the first set of (4) light duty O'reily starting batteries. That did not include a battery monitor, I was pretty cowboy in the early days.

The first batteries were $75 each so I didn't really care if I torched them as they were used to prove the system. They were enough to get us out of the locks and under the bridges in Seattle into the Sound and back. The second set of 4x Group 31 deep cycle batties got us to Tahiti. We left Cinderella there for 4 months and the batteries were left uncharged (my fault) and when we got back they were each reading between 0 and 2.4V, my charger would not register them to charge. Fortunately someone had a dumb old charger that brought them back to 12V but they were severly compromised. They worked well enough for us to do another lap of the Society Islands and Tuamotus while we waited 3 months for our set of Firefly Group 31 replacements to arrive from India.

Those batteries were installed in 2018 and are still happily running the boat. The Fireflys are rated for heavy discharges, more than a typical AGM and that is how we got away with only using 4. Typicaly 8x would be the minimum requirement if you wanted to pull over 200a regularly.

With 8kw, Cinderella will do a tick over 6kn on flat water. The Yanmar 2gm20 it replaced could only get us to around 5.5kn, so I was happy. The shaft power is nearly identical between the old yanmar and the system I installed. The electric system is a bit more powerful when considering the gearing down of the motor to 2.7:1.

The biggest issue we have is that my home grown motor mounts can't handle a sustained 8kw (we start to smoke the drive belt). It has gotten us this far, but at the moment I am hesitant to put more than 4Kw though it. That moves the boat along at 4 kn no problem and is sufficient for docking. I am having more robust motor mounts made to resolve that issue.

The thunderstruck kit solves the mounting problem and gets you a more efficient brushless motor. Our motor is air cooled and the batteries would run flat before the motor overheated, so I never really worried about it. If you are regularly running the motor for long periods in tropical places, a liquid cooled motor might be a good idea.

I purchased a new motor controller 3? years ago and was hoping to use it for better regeneration under sail, but never got around to fitting it as life got busy. The trick with regen is that full batteries don't like to accept much charge, especially if they are lead-acid (absorb state charging can be up to 58V on a 48V system). If we sailed too fast (over 5 kn), it would cause the voltage to spike up to the 60V max allowed by the controller which would shut the system down to prevent damage. To solve this, I also purchased a resistor array that closes at 60V to dissipate any excess energy and keep the battery charging going, but again never had the chance to install it. This is because we never really used regen after I fitted the solar panels, the solar was able to keep us topped up until we got to NZ winter. (We found that a marina berth was the best way to live aboard during the winter. Rowing to shore when its howling 40+ kn isn't very fun. Stepping off onto a dock has it's advantages!)

As for sizing, ideally you size the motor for efficency (max efficiency at fully loaded prop spinning around 1000rpm I believe). So a larger motor generally does not produce a better system, you end up creating more loss as the motor isn't as efficient when spinning more slowly. This may be negligable depending on the controller, but its what I have read.

I would start by comparing the current engines's power at the shaft (NM) to that of the electric drive you are considering. Don't forget the gearing of the transmission, most ICE are geared down at least 2:1 so try to match that as well if the power is about the same.

The prop is another big piece of the puzzle, as it is what actually turns the work into propulsion. The theory is that you want to size the prop as big as possible up to some figure (1/3 the beam or something like that). Most boats are limited by the underwater appendages (keel/rudder/etc). A bigger prop pushes more water at a slower speed, and is easier to keep from cavitating because... slower speed. This doesn't really work for ICEs because of their power curve, if the prop is too big, the engine can stall going from forward to reverse when running the engine too slowly. Propping an ICE vs an elecctric drive can be very different, so unless you change your prop during your conversion, you will likely be limiting the overall electric drive efficiency. We "kind of" got away with this as we are running a 2 blade max prop which means we can vary the pitch to match the motor a bit. A 3 blade fixed prop is proably the worst and I would strongly consider a prop change if you were to go to electric (same is done if swapping ICE by any reputable shop, as engines are much more suceptible to damage from a mismatched prop). Elecric drives just take a hit on efficiency.

These limitations of ICE are why most large trains and submarines are running electric drives powered by gensets. Electric motors do all the high-torque requirements and gensets can be run at their optimum efficiency rpm to rechage the batteries.

I firmly believe that a daysailing yacht has the most to benefit from a electric conversion. No more engine maintanence/services/oil leaks/diesel to lug/spill. Simple, clean, quiet and if you plug in at the dock, no big solar array is required. The second group (cruisers) also have huge benefits as a large propulsion battery bank can be used to suppliment the house bank when living off grid on anchor/mooring. It also prevents you from needing to get to a gas station or carry tons of extra spare engine parts, allowing you to stay in that remote wonderland that much longer.

The "weekend warrior" is probably the worst candidate for an electric conversion. When you sail by the weather, electric drives are great. Just wait for optimal sailing conditions and enjoy life. When you sail by work timelines... you can find youself in sticky situatons where a big diesel engine is required (assuming it doesn't quit on you).

 

[gareth harris]

I can shed some light, our 35-2 has done some 15,000nm with an electric drive.

I searched your entire posting history before I created this discussion, and you contribution here is exactly the kind of feedback I have been looking for.

When I spoke to a salesman from Electric Yachts he told me that a propeller for the E35 should be either 12" or 14" with 11 degrees of pitch. He did not know where the maximum efficiency of the motor lay, but I certainly take your point about a low rpm on a larger motor potentially being inefficient in a system where range is crucially limited - I plan to talk to one of the engineers once I have figured out what questions to ask. I need to replace my propeller because some previous owner put a 3/4" shaft into a 1" stuffing box with tight hose clamps to keep the boat afloat; and I am planning to install a two bladed folding propeller.

Something you did not talk about much is whether you have ever felt restricted by power. I bought Freyja in Long Beach, and when I first entered San Diego Bay it was at night in el nino, and I found myself heading directly into a breaker zone where the waters would ordinarily have been calm. The A4 had enough power that I could pull a fast 180 and push out into the massive swells and breeze. would 10kW do the same in your estimation?

An electronic charge controller ought to solve the problems of regenerating under sail, and be a small part of the overall expense.

Gareth
Freyja E35 #241 1972

 

[p.gazibara]

I searched your entire posting history before I created this discussion, and you contribution here is exactly the kind of feedback I have been looking for.

When I spoke to a salesman from Electric Yachts he told me that a propeller for the E35 should be either 12" or 14" with 11 degrees of pitch. He did not know where the maximum efficiency of the motor lay, but I certainly take your point about a low rpm on a larger motor potentially being inefficient in a system where range is crucially limited - I plan to talk to one of the engineers once I have figured out what questions to ask. I need to replace my propeller because some previous owner put a 3/4" shaft into a 1" stuffing box with tight hose clamps to keep the boat afloat; and I am planning to install a two bladed folding propeller.

Something you did not talk about much is whether you have ever felt restricted by power. I bought Freyja in Long Beach, and when I first entered San Diego Bay it was at night in el nino, and I found myself heading directly into a breaker zone where the waters would ordinarily have been calm. The A4 had enough power that I could pull a fast 180 and push out into the massive swells and breeze. would 10kW do the same in your estimation?

An electronic charge controller ought to solve the problems of regenerating under sail, and be a small part of the overall expense.

Gareth
Freyja E35 #241 1972

I wouldn't quite call it "restricted by power," more "forced prudent seamanship." If I had any doubts, we would heave-to and wait for light before making such an entrance. Cinderella is our home, and prudence is the best insurance policy.

I'm betting that an aged A4 doesn't make much more than 10kw at the prop shaft, so yes, you would be fine. As for how long you could do it, that is entirely up to your battery bank.

From Wikipedia about the yanmar 2gm20 I replaced:

The continuous rating output at the crankshaft is 11.8 kW, at 3400 revolutions per minute (rpm). The maximum output at the crankshaft is 13.4 kW, at 3600 rpm


Cheers,

 

[gareth harris]

The salesman at Thunderstruck openly admitted knowing nothing about matching the motor to a propeller, so can anyone here make any recommendations for a source?

I chose the incident on San Diego Bay as an example of how you can get into trouble even when you think you are doing everything right. The main channel is deep and wide enough to be used routinely by aircraft carriers, and sailboats routinely cut the corner around Point Loma in waters I was familiar with. On the night in question however the extraordinary conditions exposed a hidden reef or shoal which was not charted and only became apparent at close range on a moonless night. That is when it is nice to have power.

Gareth
Freyja E35 #241 1972

 

[p.gazibara]

That would be a common response even from most engine manufactures. Prop size is dependent on the load and the motor torque curve. The prop shop would be the guys to ask.
Electric motors will handle a wider range of props because the torque is there from 1rpm to max rpm. You will just take a hit on efficiency.

-p

 

[johnnyL]

@gareth harris, did you ever finalize your electric motor and propeller size?

I’ve gone with a direct drive setup using an 8kW QS273 hub motor. I’m curious, what size propeller did you end up using?

Also, does anyone know the maximum prop diameter that will fit on the Ericson 35-2? I’ve heard that larger diameter props with more blades improve regeneration due to the increased torque. Has anyone tested this theory or seen noticeable regen improvements with a larger prop?

Lastly, has anyone upgraded the shaft size on their Ericson 35-2 to accommodate an electric setup and larger prop? If so, what modifications were necessary?



~Johnny
S/V Fuzzy Logic
E35-2

 

[Kenneth K]

... accommodate an electric setup and larger prop?

I don't know a lot about electric conversions but an 8kw motor, at 10.7 hp, develops about half the power of the M-25 diesels in most Ericson 35s. It seems you'd need to put on a smaller, not a larger, prop.

 

[Mark F]

Hi Johnny,
Yes a larger prop should help in many ways with an electric drive on your E35 - provided it's not too big (hull clearance) and too aggressive of a pitch (overheat issue). Higher max speed and better regen. I got an improvement in both switching to larger, steeper pitched three blade props rather than a two blade.

 

[johnnyL]

I don't know a lot about electric conversions but an 8kw motor, at 10.7 hp, develops about half the power of the M-25 diesels in most Ericson 35s. It seems you'd need to put on a smaller, not a larger, prop.

My QS273 motor operates in a range of 8kW to 20kW, depending on battery voltage, amperage output, C rating, and phase amps. The system runs between 48V and 120V, with the potential to draw 200+ amps, depending on the controller and battery configuration.

Since I'm using direct drive, I have higher efficiency with fewer parasitic losses compared to a traditional reduction drive. Additionally, the hub motor produces more torque because it is physically larger and designed as an outrunner motor, meaning the stator remains stationary while the outer shell (rotor) spins. This allows for a greater lever arm effect, increasing torque at lower RPMs, which is ideal for driving a large propeller efficiently.

I control the phase amps through the motor controller, which adjusts the power delivery and torque characteristics. This allows fine-tuning for efficiency, acceleration, and regeneration performance. My setup also benefits from regen while sailing, where the propeller helps recover energy at higher sailing speeds, further improving overall efficiency.

With the right prop size, this system balances power output, torque, and sustainability.

~Johnny
SV Fuzzy Logic
35-2

 

[johnnyL]

Hi Johnny,
Yes a larger prop should help in many ways with an electric drive on your E35 - provided it's not too big (hull clearance) and too aggressive of a pitch (overheat issue). Higher max speed and better regen. I got an improvement in both switching to larger, steeper pitched three blade props rather than a two blade.

Hey Mark,

I just picked up a four-blade 24D 22 Pitch propeller for stupid cheap, very lucky, in my opinion. Now I’m looking to see if I need to modify my strut, log, or shaft, or if I can just use some kind of sleeve or bushing to adapt it to my current shaft. Also, thanks for the input, it helps confirm the research I’ve been doing on larger propellers and their benefits.

~Johnny
SV Fuzzy Logic
35-2