Assistance in choosing an electric motor....

[moseriw]

I think the mean problem is not the engine or the batteries. One will figure that out somehow. (Btw. professional solutions use always 2 motors)

No the problem in fact is the charging. And it is easy to figure it out.
You want 40 Kw? OK, say 4 knots for 6 hrs gives you 24 NM and costs you 240 Kwh.
You only need 10 Kw? that is 60 kwh

Ok does not matter what you want you have to provide the max for the system for a minimum of 3 hrs when it gets really rough = 120 Kwh

One can stuff up the boat with batteries - so this is not a problem.

BUT! How do you think you can produce enough energy to recharge?
Oh.. no problem.. Use a genset of 40 Kw and you are again at fossile energy.

Manufacturers should consider that it may be a good Idea to build their boats with enectric propulsion and an apropriate generator as the ratio is far better than with diesel or gas (40% max). Electric propulsion is > 90% (Motor only)

Resume: to have an headache free electric propulsion system one needs an always running genset aboard and the price for a 40 kw genset is twice the price of a standard diesel engine.

Hmm.. how many miles do you want to go by year? 20.000? If yes then it may be worth calculating it and hopefully it makes sense.

[moseriw]

Oh I beg your pardon you did not want the problems - you want THE solution. Yes but this works only in the very stupid manmade management bizz and not in nature itself.

[fred4936]

I went with a Thunderstruck kit in our Tartan 27. It has worked very well for the past 8 years. Your boat is considerably larger, so a generator to charge the battery bank is not unrealistic. But I would err on the side of more power, not less. There are times when you really need it. The instant power of the electric has been a useful characteristic!

[misfits]

I bought a ME 1115 from Thunderstruck, very pleased with it. At 48 volts, 2400 rpm works out to be 13 hp. My boat displaces 4 tons, your boat displaces a wee bit more. You may want to consider something larger than 10 KW. Electric is more efficient when compared to it's ICE counterpart. I can move along at 3.5 knots using 15 amps out of the 48 volt bank. 700ish watts that's 1 hp.

Point being don't base your selection on ICE HP. For my size boat probably could have gone with a 5 KW motor.
Chat with some of the vendors. They are knowledgeable & very helpful.

[DMCantor]

Keep in mind that an engine that will push you at 4 kts in flat water and calm winds won't remotely get to that speed in big waves or with 35kts of wind on the bow. It's important to have enough reserve power.

[misfits]

Originally Posted by DMCantor

Keep in mind that an engine that will push you at 4 kts in flat water and calm winds won't remotely get to that speed in big waves or with 35kts of wind on the bow. It's important to have enough reserve power.

I agree. One of the reasons I went with a 10KW system despite many vendors using the 1KW per ton of displacement rule of the thumb.

[john61ct]

Originally Posted by fred4936

I went with a Thunderstruck kit in our Tartan 27

Fred, was it you using lead, 6x 8V T890 Trojan batteries to get to 48v?

[chris07732]

Many posters repeat that someone converting to electric needs to be concerned with having "enough" reserve power :to get off a lee shore, to deal with head winds, to combat waves. I think we all understand the need, but does anyone have some basis to calculate those needs.

[StuM]

Originally Posted by chris07732

Many posters repeat that someone converting to electric needs to be concerned with having "enough" reserve power :to get off a lee shore, to deal with head winds, to combat waves. I think we all understand the need, but does anyone have some basis to calculate those needs.

Not without knowing a lot more about the use case.


If you're just day cruising local harbour areas in benign conditions you have entirely different requirements to passage making.


Sailing location and prevalent weather are also key factors.

[mitiempo]

Originally Posted by Yes maybe

I have been looking at comprehensive electric solution - solar, wind, Lithium batteries electric motor propulsion for my next boat. Not an engineer rather, A grade electrical contractor 35yrs.

Some of links found so far:
https://oceanplanetenergy.com/electr...ic-propulsion/
https://electricyacht.com/electric-sailboat-motors/

I would think you will need a 40 to 60 KW elec motor for 55K lbs boat. Possibly water cooled. Don't think the heat exchanger would get hot enough to make hot water. But could be a possibility, to make some limited amount, or help heat the tank.

From my perspective the elect motor, speed control and safety requirements are not the major issue. It's the number of LiFeP04 batteries need - especially in the case of 55K lbs boat.
Best option found so far is : Battle Born 12V 100amp hr

Quick numbers: 72 or 96V DC (higher voltage would be better, but it all gets to difficult to achieve with existing batteries available)

Take the lower 40 KW 72V option $15,000 plus,plus; 72V div by 12V = 6. You need strings of 6 batteries to make 72V @100 amp hr
40KW for 1 hr (think of dragging anchor in 40+ knots at night, even 1hr may see you on the rocks..)
1 hr = 420+ amp hr
I won't go through all the math. = 5 (more would be better) x 6 strings of batteries = 30 batteries = $28,500 plus, plus.

A lot of batteries for minimum propulsion. If just crossing seas you can do it. If living and cruising around the coast (and seas) in crowded real world conditions. Need very big genset... for emergencies

Maybe go smaller say: 30KW @72V = 24 batteries, 20 KW @48V = 16 batteries

FYI, Im looking at solar panels from the dodger past the transom...

It's not impossible, just very limited time between battery full charge cycles....

Battleborn 100 AH batteries are limited to 4 if in series - so 48 volts max.

[valhalla360]

If you understand you will be limited to 4kt and you understand it will hurt resale value being an oddball boat with poor performance under power....

Think tractor. Get an oversize low pitch prop. Use a reduction gear if needed to get the torque needed to turn it and then run the calculations based on max continuous KW motor output for 4kt (maybe add 10-20% more power as a margin for error)
- This should get you to your 4kt cruise speed.
- Docking or against heavy wind/waves, the big low pitch prop will act like a tractor giving you lots of thrust (force not power) for tough conditions. You still probably will slow down a bit in bad conditions but that will keep you moving with less HP. It's similar to using a low gear in a truck to climb a hill.

If in doubt go bigger on the prop.

[sailingharry]

You've heard a lot about the problems with generation, recharge, capacity, range, etc. That wasn't your question -- you asked about size.


Even if you want to go 4 kts, even if you are willing to wait days for a recharge, etc, even if you only need 10kW to get your desired speed, I would make a strong case for putting in a 40-50kW motor. Electric motors are better than diesel at low speeds -- there is no penalty for running a 50kW at 10kW. So you get all your range that you get with 10kW. And motoring against heavy winds and seas, which do require a large engine, is an avoidable situation -- just don't do it.


But, here's my reasoning. While I have sat in my Sabre 34 going head to wind at 3 kts with my 27hp Volvo wide open, it is much more common for me to use full throttle in short bursts -- kicking the stern in a turn, stopping the boat while approaching a landing, etc. You really can live on nearly zero range/capacity (see Sailing Uma), but you do need those moments/seconds where you will be very limited if you can't call up full throttle in a burst.


Build your source (batteries, chargers, solar, etc) for your desired range and speed. But build you propulsion for the most you'll ever need, even if it doesn't affect your battery bank.


Of course, once you have a motor that can actually push you at 6 kts, you'll find you don't run at 4. My Sabre, I couldn't bring myself to motor at 2/3 throttle at 5 kts for very long -- soon, I was at 90% doing 6.5kts. And paying for it at the pump.

[Mick of 42]

This company has been around since Jesus wore shorts... if only I had the money...

They have a great inverter integrated motor combo and sell everything you would need.


This motor would more than replace the current engine power-wise. Electric is very torquey from zero rpm.


https://www.elcomotoryachts.com/prod...ctric-inboard/