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Repowering a 55' sailboat

2885 Views 1 Reply 2 Participants Last post by  Roaminaway
I am in the process of repowering my 55' sailboat (ketch) with electric. I’ve spent a great deal of time on this site and others trying to understand many of the trade-offs and ways to approach the conversion. I wanted to post my tentative plan here for feedback from people who’ve been through it before. ANY and ALL feedback is welcomed and greatly, greatly appreciated! 😊

Boat Details:
  • Former motor: 120 HP diesel engine (substantially more power than needed, even at hull speed).
  • Length: 55’ (49’ at waterline)
  • Beam: 16’ (14-15 ft at waterline)
  • Hull speed: 9.4 knots (calculated, and seems about right from experience)
  • Prop: 24” diameter, feathering, with adjustable pitch (currently 24”, I believe)
  • Transmission: 2.57 reduction (inline gear reduction), also acting as thrust bearing (I believe)
  • Moorage: Washington state (but need to move from Columbia River to Puget Sound)

Planned Usage:
  • Electric motor primarily for moving in and out of marina
  • Mostly day sailing, but with occasional trips to Hawaii / South Pacific
  • Potentially add a 5kw diesel generator for continuous motoring at 4 knots (calculated guess)
  • I’m planning on keeping the feathering prop, as I don’t intend to attempt regen until I’m beyond day sailing and will be taking her to Hawaii or the South Pacific.
The Plan:
  • Motor: Hyper9 HV or AC-51/35 running at 144V
    • I can’t figure out how to get enough power running at 48V
  • Transmission: Keep the existing 2.57 reduction in-line transmission
    • Ideally a 5:1 reduction would allow the motor to run at higher speeds and I could use the entire motor range at existing 24” prop pitch (but I have the existing 2.57 reduction)
  • Prop: Lower to 14” pitch
    • Minimum” recommended” OK range (from Propeller Handbook, Dave Gerr), (0.65 pitch ratio), but could go as low as 13” with additional efficiency loss)
    • Assumes 45% slip (calculated based on Propeller Handbook, not measured)
  • Battery Bank: 45 CALB 3.2V 100Ah cells, in series
    • 100Ah & 144V = 14.4kW
    • Alternatively, buy a used Nissan Leaf battery and tear it apart. (Same 14.4kW assuming batteries are only at 60% - but extra work and an extra hundred pounds or so. Savings is substantial, though. Price goes from $7-8,000 for batteries to around $1500.)
Details and Calculations:
  • Range on batteries (given the current plan of 14.4kW and stopping at 10% remaining charge):
    • At 4 knots approx. 2.5 hours of runtime = 10 nautical miles
    • At 9 knots approx. 20 minutes of runtime = 3 nautical miles
  • Range with 5kW Diesel generator: limited only by diesel (at 4 knots)
    • Anything above 4 knots will take > 5kW and will use battery
    • Anything below 4 knots will charge battery
  • Prop pitch will influence motor RPMs
    • 24” pitch at 4 knots requires 1100 motor RPM (9.4 knots = 2700 RPM)
    • 18” pitch at 4 knots requires 1500 motor RPM (9.4 knots = 3600 RPM)
    • 14” pitch at 4 knots requires 2000 motor RPM (9.4 knots = 4600 RPM)
  • Motor efficient RPM range:
    • Hype-9: 1000 – 6000 RPM
    • AC-51: 1000 – 6000 RPM
Hope and Dreams:
  • My Dad, Uncle and Grandpa bought the hull and custom built the boat. It’s beautiful! I took over the boat a few years back. I’d like to repower the boat partially to make it easier for me (I’m more comfortable with electric motors than diesel), and partially to add sweat equity into the boat in order to share in the family effort to build the boat. One of my sons will also be joining me in the effort.
  • It’s in the Columbia River now, and I’ll need to move it out the mouth of the Columbia, then up and around the Olympic mountain range, and into Puget Sound. I have a slip for her in the southern part of the Sound.
  • Although likely use is day sailing for the next few years (once or twice a month), eventually I’d like to take her to Hawaii, and beyond. But that will likely be several years from now.
  • Is changing the pitch to 14” (on a 24” prop) a good or bad idea?
    • It would give me closer to the full range of the motor (4600 RPM at hull speed vs effective top range of 6000 RPM for the engine)
    • I’d lose a little efficiency of a higher pitch, but wouldn’t have to buy another inline gear reduction, or figure out how to do a belt system or the like. It just seems simpler, and the gear reducer is attached to the driveshaft already.
  • Is the 3.2V CALB 100Ah battery the correct choice? (CALB 100Ah Battery | CALB Batteries | Stealth EV )
    • Can I put that many in series (45 – 48 cells) to get 14.4kW hours at 144V? If so, do I need to use any special connector bars since I’ll be pulling 500 Amps, or are the thick copper ones they ship with sufficient?
    • Should I buy them on Alibaba for less than $5,000 for 50 batteries or get them from someplace a more traditional store (EV West, Stealth EV, etc )?
    • Should I go with an OEM battery and just rip it apart?
  • Which BMS should I use? I’m thinking the Orion BMS2, but they are expensive. (Comparison | Orion Li-Ion Battery Management System)
  • Which motor should I buy?
  • Should I add a 5kw diesel generator for cruising longer distances?
    • By my calculations I can push the boat approx. 4 knots with 5kw.
  • I don’t have any idea on a charging system. At all.
  • Oh, my. What am I missing? When I started writing this thread I felt like I knew a lot more than I feel like I do now! :-O
Everyone, thank you so much for your help in the forums. I’ve read so much, and I’m so pleased to have this resource available. So, even if you can’t help with these particular questions, I still want to say “Thank you!”
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You have given this a lot of thought. I am a marine systems tech, who sailed away from Victoria, BC, in 2011. Made it as far as New Zealand, before running out of money. Now planning to sell the boat, and return to Canada.

I am at the hypothetical stage, of planning a car conversion, when I get back. (And maybe a boat conversion farther in the future)

Either the Hyper9, or AC50 would work. I like the Hyper9, mostly because they have a matched controller, and the higher voltage adds to efficiency.

I think keeping your gearbox is a good choice, for your application. What is your current engine's cruising RPM? If it is a Lehman or Nissan, I'm guessing around 1800 RPM.

24" pitch sounds like a lot, but I am used to smaller boats. I would suggest getting the batteries and charging system figured out first, followed by removing the diesel, and fitting the motor.

Worry about the prop after the boat is moving under electric power. As it is a feathering prop, you can adjust the pitch after you get a sense of the performance and have real world RPM to base things off. Prop calculations are complicated, requiring assumptions that affect the result, but may not be accurate. Most 'prop guys' can tell from experience, what the pitch should be, from the basic boat stats. Knowing your actual motor RPM and your target RPM, should allow you to get the correct pitch, without too much trouble.

Check out Will Prouse on Youtube, DIY Solar Power. He reviews a lot of different lithium iron phosphate battery manufacturer's products, often cracking open the case, to inspect the internals. He also discusses battery management systems.

If you are considering a battery pack, from a salvaged EV, take a look at Damien Mcguire's Youtube channel, EVBMW. He gets into the fine details of hacking and repurposing EV systems.

My general impression is that putting together a propulsion system with something like a Hyper9 and lithium iron phosphate batteries is expensive, but reasonably straight-forward. Using a system from a salvaged EV can be pretty inexpensive, but tends to be complicated, especially for those of us without knowledge of CAN bus.

I'm not sure a diesel generator is going to have the benifit that you expect. 5kW converts to an equivalent of about 6.7 horsepower. 5kW won't move your boat at 4 knots.

Your 120hp diesel is equivalent to a 90kW Hyper9. If you are running it easy, just over half power, at 50kW (67hp), you will need 50kW hours of capacity, for every hour of range that you want. I can't see a battery pack of less than 150kW hours being practical.

And that brings us back to the question of new, or salvaged batteries.
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