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Old 29-11-2019, 10:42   #1
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Assistance in choosing an electric motor....

Hello all,

So after spending an enormous time digging, I've decided to come and ask for input.

I am looking at the feasibility of replacing a Lehman Diesel (~85-90HP) with an electric motor on a sailboat (~55', ~54,000lb displacement).

Before we go too far, I know it is an unpopular idea. I'm not interested in hearing how difficult or not-feasible this is with batteries, charging etc.... I am interested in making an off-grid, green, live aboard and willing to sacrifice speed and convenience for that effort.

More importantly, I am not interested in reaching Hull speed (7-8kts for my boat). More like max of 4kts is what i am thinking and even that i'm willing to reduce further.

If i try and do a HP for HP sub to electric, i'm looking at something like 40-50kW electric motor which has been difficult for me to find in the BLDC realm.

I'm curious about Motenergy ME1115 or similarly sized motors and how undersized a 15kW to 30kW max. I'm also curious about peoples recommendations on a bigger electric motor and where to find one for my application.

Thanks folks
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Old 29-11-2019, 11:05   #2
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Re: Assistance in choosing an electric motor....

Greetings and welcome aboard the CF, MarsRover.
You might look to what the EV automakers are using.
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Old 29-11-2019, 12:05   #3
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Re: Assistance in choosing an electric motor....

Insufficinet information for a useful recommendation.

We need to know the maximum size prop you can swing so we can figure torque needs, for example.

Pushing even a 27ton barge at 4 knots doesn't take much power, but do you want more for emergencies? If so how much?

What duty cycle are you looking for? What range? What power supply?

A electric propulsion system is a SYSTEM. It is not picking one from column A and one from column B if you want it to be efficient and actually work.

We really need more understanding of your design requirements--if you have even thought that far.

Assuming your waterline is 50 feet, and a prop of 55% efficiency you are looking for a SLR of

4 knots /50^(0.5)= 0.566

SLR = 10.6665 / (LB/SHP)^0.33

0.566 = 10.665/(55000/SHP)^0.33

SHP = 8.2 HP

The power needed at the prop is only about 8 HP (6KW) to move that boat at 4 knots on flat water with no wind. Add a little for losses along the way, and maybe a 10KW motor will work for you.

If that is REALLY all the performance you want, why are you looking at 40 or 50 KW motors?
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Old 29-11-2019, 12:18   #4
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Re: Assistance in choosing an electric motor....

Seen a few use these guys

https://www.thunderstruck-ev.com/sev...t-kit-5kw.html

that one pushed a Triton around pretty well
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Old 29-11-2019, 15:56   #5
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Re: Assistance in choosing an electric motor....

Thank you for your replies!

As for my 4kts, and 35-45kW motor statements, I was just indicating that I don’t need to get to full hull speed. But yes, I’d like to kick more than that if I can afford/fit it for emergencies. 35-45kW was just my starting point in looking to see what it would take to repower at the level of the currently dead motor.

For more background in hopes you all can help me out: I am looking at purchasing a 1983 Hudson force 50 with a dead Ford Lehman 85hp motor. It needs some work for sure so I’m trying to figure out what it will take to repower with electric, batteries, solar. I am a mechanical engineer so I understand well the inefficiencies and massive requirements. Not worried about that. What I don’t know is how “underpowered” is reasonable. I also don’t claim any experience in repowering a boat so any and all guidance is welcome. Also if you’re interested in telling me what you think the boat might be worth I’ll post photos Sunday after I go see it.

Thanks all

Ben
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Old 30-11-2019, 10:18   #6
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Re: Assistance in choosing an electric motor....

I understand your desires and applaud them! Battery & solar technologies are the future. I am considering the same transition, but it is a much bigger step than manufacturers seem to admit.


That said, my 33' Nonsuch displaces about 15K lbs, pushed by a Westerbeke 38hp diesel. The recommended electric is 10hp and weighs 2/3 less. Keep in mind that the weight & space you save by giving up the diesel will be largely taken up by batteries. On the other hand you give up enormously difficult to service/replace wet exhaust hoses and associated parts, along with a large fuel tank and all the associated hoses and filters for that.



Here's a link to Ocean Planet Energy. You have probably already visited it, but just in case: https://oceanplanetenergy.com/electric-propulsion-2/



Some YouTube vids.


https://www.youtube.com results?search_query=ocean+planet+energy


Good luck with your transition and please keep us informed of you progress.
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Old 30-11-2019, 10:54   #7
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Re: Assistance in choosing an electric motor....

Without doing a rigorous breakdown here's a rough estimate based on my research into electric for my boat. FYI I'm also an engineer but electrical but my focus was on digital logic so don't count me as the ultimate expert on motors.

I think the 85 HP diesel was an appropriate motor for the Force 50. Certainly not overpowered but adequate I think.

To get and equivalent power from electric is not an exact science. There's a lot of very different opinions on the useful torque, especially at low rpm of electric vs diesel. I'll ignore that and just say, when you account for the parasitic loads on an ICE like pumps, alternator, transmission and convert the HP to kW then your estimate of 40-50 kW is in the ballpark.

But 99% of the time, especially when motoring at 20-50% of theoretical max hull speed AND in calm conditions IE no strong wind or current, you will probably only used 5-15 kW at most.

The problem is, what happens when you're trying to enter a dock with a 2-3 kt current against you or trying to go under a bridge against a 15-20 kt headwind or whatever? In the years I've owned my boat I've only needed to use the full power of my engine a few times but those few times I really needed it. So how much is that worth and how much do you want to spend to get that peace of mind?

By the way, the biggest issue with going electric and what stopped me is range. Unless you add a large diesel generator or a HUGE battery bank you are going to be limited to maybe 20-50 miles max under electric, even at slow speed.
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Old 30-11-2019, 14:44   #8
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Re: Assistance in choosing an electric motor....

Quote:
Originally Posted by billknny View Post
.......

Assuming your waterline is 50 feet, and a prop of 55% efficiency you are looking for a SLR of .........
Please remind me what SLR stands for?

I feel I should know but right now it escapes me
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Old 30-11-2019, 15:10   #9
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Re: Assistance in choosing an electric motor....

Quote:
Originally Posted by Wotname View Post
Please remind me what SLR stands for?

I feel I should know but right now it escapes me
Did some clever googling. Stands for Speed to Length Ratio
S/L Ratio = hullspeed (in knots) divided by the square root of the waterline length (in feet)

Also, did some searching and discovered max hull speed:
Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)

which puts my prospective boat at max hull speed of 8.36kts or so. Go to psychosnail website it has a cool calculator for anyone looking.... but here's what i found:

Acronyms that took me some time to figureout:

SHP - Shaft horse power
LWL - Length at water level of the boat
SLR or SL Ratio - Speed to Length Ratio




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Old 30-11-2019, 15:22   #10
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Re: Assistance in choosing an electric motor....

Quote:
Originally Posted by MarsRover View Post
Did some clever googling. Stands for Speed to Length Ratio
S/L Ratio = hullspeed (in knots) divided by the square root of the waterline length (in feet)

Also, did some searching and discovered max hull speed:
Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)

which puts my prospective boat at max hull speed of 8.36kts or so
Note that this forumla is a very simplified approximation.

The actual hull speed of a boat is determined by a number of other variables including but not limited to:

- wetted surface area
- hull shape
- displacement
- waterline when heeled

Also, some boats can significantly exceed their hll speed when surfing a wave downwind or especially in the new, surfboard shapes race boats when they are planing.
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Old 30-11-2019, 15:28   #11
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Re: Assistance in choosing an electric motor....

yes i figure it is conservative only for flat water but it is a good starting place!

Thanks!

I still think a 30kW or more electric motor is what i am looking for. That'd put me at about 70% of the theoretical max hull speed (what ever that REALLY means in real life as pointed out above....)

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Old 30-11-2019, 15:43   #12
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Re: Assistance in choosing an electric motor....

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....
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Old 30-11-2019, 16:31   #13
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Re: Assistance in choosing an electric motor....

Thanks Yes Maybe. I think i'm looking for more of a DIY build it myself system.

The cost of the "out of the box" systems are insane and i'm confident i can figure this out myself (with online help of course ) for thousands less.

For example: found two potential candidate motors:

ME1002 :https://www.thunderstruck-ev.com/mot...me1002-en.html

and this crazy thing: https://www.alibaba.com/product-deta...4a185304ckyai8

The later, i'm worried only because it is so small/cheap/light compared to other options i've looked out....don't know what i'm missing. I'd like to go with a lower Kv because i think torque is more important than RPM for this application. though.... it looks pretty close to this MUCH more expensive one....http://mymobilemms.com/OFFTHEGRIDWAT...ss-Motor-100hp)

any one know anything about these?
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Old 30-11-2019, 17:36   #14
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Re: Assistance in choosing an electric motor....

Quote:
Originally Posted by MarsRover View Post
Thanks Yes Maybe. I think i'm looking for more of a DIY build it myself system.

The cost of the "out of the box" systems are insane and i'm confident i can figure this out myself (with online help of course ) for thousands less.

For example: found two potential candidate motors:

ME1002 :https://www.thunderstruck-ev.com/mot...me1002-en.html

and this crazy thing: https://www.alibaba.com/product-deta...4a185304ckyai8

The later, i'm worried only because it is so small/cheap/light compared to other options i've looked out....don't know what i'm missing. I'd like to go with a lower Kv because i think torque is more important than RPM for this application. though.... it looks pretty close to this MUCH more expensive one....40KW BLDC brushless motor 100hp [202/80 motors 40kw] - $1,699.00 : OFFTHEGRIDWATER.CA, RC UAS UAV DRONE Systems)

any one know anything about these?
I couldn't get your link to work - 40KW.

Yes quite possible to source elect motor, motor control and drive line.The ThunderStruck site has items you can put together and then do the engineering to make it work...Motenergy ME1002. Voltage requirements - still think you will need 72V or more with 55K lbs.. = more batteries, sorry
Not sure you will get the Nm needed with lower voltage than 72 and KW of 30. More voltage (and KW) = more Nm.

I haven't found anything on the issues or service life of either commercial units or DIY yet. Its possible too early or people aren't reporting the problems, with these systems.
So its early days, buy some items, put them together and see what happens..

I was using the commercial offerings as a quick guide to what's possible.

Batteries (the bigger problem)- you can source (alibaba) your own cells and put them together. If you choose LiFeP04, spend a lot on BMS and motor/wiring safety - use the best tinned wire, lugs and circuit protection and lots of it. You don't want Mr heat, arc and fire on your boat.
Obviously the lower voltages will need bigger cables etc.

This young man is very honest in his appraisals - almost comical by today's standards.


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Old 30-11-2019, 18:03   #15
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Re: Assistance in choosing an electric motor....

I would think with all of the electric tow motors about plus smaller electric non riding package movers around your choices would be great. At one point I was thinking of toying with an electrical vehicle and went to a small tow motor shop and they had a bunch of old stuff in their back lot. A pickers dream.
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