Hi Jamie,
Short answer. 12 x 10, 2 blade 800 Watts @ 4 Knots. 12 x 14, 3 blade 1,000 Watts @ 4 Knots. My preference is to switch to a 2 blade folding, likely Flexofold. It is a sailing yacht with sails and an auxiliary engine.
There are many caveats that have affected my decisions. Initial build was finances and space, the boat size, hull type and sea conditions. Others such as noise, peak system efficiency at cruise, monitoring systems, prop diameter (limited to 12), blade area ratio, blade width, blade thickness, rake, cupping, slip, P/D ratio. Motor gearing or not, number of bearings, packless seal or not, voltage v current v efficiency at load. Motor torque/amp, motor rpm/volt at no load and under load. Chosen S1, S2 and S3 motor ratings and supply voltage. Controller efficiency etc.
My boat is 8,000 lbs laden displacement, 21.25 ft LWL 26 ft LOA, Modified full keel with keel hung rudder. Prop in keel aperture, no (full flow) aperture in rudder. Need to cut out for that and folding prop. I have limited space in the engine bay as it is a double ender. Motor is 84V rated and 200 Amps at that voltage. It will run at any voltage below that with appropriate voltage v current reductions for load and optimising for efficiency which means choosing a prop for that as no gearing to adjust those parameters.
Suffice to say, and as others have said, it all depends what you want.
The long answer and info.
I'll take a step back and lay down my project criteria that I planned in 2011. I completed and sailed the basic install in 2012. Criteria were:
1) To replace the old 10 hp diesel with a basic EP system for the same cost or less as a new diesel and the same weight or less in the same place. I achieved this. If I could not have done it I would not have gone EP and would have bought a new 12hp or 14hp diesel intermittent duty cycle 3,500 rpm engine.
2) Continuous cruise speed to be 65% of hull speed (S1 rating). Short term duty (S2 rating) 80% hull speed. Intermittent periodic duty (S3 rating) 75% hull speed. All in calm conditions. Cruise efficiency is usually achieved at around half the motor rating current based on the system voltage/prop and around half the controller continuous rating assuming that the current sizing is equal to or greater than the motor S2 rating at the chosen system voltage (multiples of 2V possible if 2V cells and 1V via controller programmer). Nominal battery voltage is rarely achieved at the motor due to losses, battery SOC and the fact that a PWM controller is never 100% on as it is a chopped/switching voltage. My controller output voltage to the motor at full throttle is about 97.5% of nominal battery system voltage.
3) Use the minimum number of and the largest 8D sized Monobloc AGM batteries for energy density by weight/volume and cost considerations. This meant 2 x 12V and direct drive, as this was the best way to meet the constraints, and learn without costly mistakes! It was also a space/weight issue and that also meant an axial flux/gap open framed air cooled motor as opposed to an axial or radial air or liquid cooled IP65/67 system. IP ratings would need to include batteries, controller and motor if going that route in my view, as it would be a bigger boat with different areas of operation that I'd be considering.
4) Modular system so I could upgrade as finances allowed to enhance safety, range and system redundancy for my cruising grounds which are coastal, narrow inlets, channels and islands. Typical tidal streams around 1 knot. Peak streams and tidal gates around the Scottish islands can run to 8 knots. I've seen 12 Knots SOG.
5) Noise. By using a 24V system voltage my motor typically spins at 1/2 the speed of a 48V geared system and along with no gearbox or belt makes less noise compared to a 48V air cooled open frame. I would like a 2 blade folding prop in future for faster sailing and so the prop/motor does not spin. Regen on a small boat is a waste of time. If a larger monohull or Cat with hull speeds around 7 knots plus then I think it becomes worthwhile when voyaging and/or on a long tack for 24 hours for instance. If I needed extra charging for voyaging and inshore fast sailing with off grid ability - I'd use solar cloth sails, bimini, dodgers and stick on semi flex cells on boom, mast and coach roof as the Arcona 380Z does, plus regen but with a folding 2 blade for the quiet and speedier times. That all costs though.
My system has now grown (along with cost) as I've updated since 2012 for safety and redundancy.
This means I now have:
16 A 240 AC shore supply with 900 Watt generator via 10 Amp breaker. Separate 6 Amp breaker for other shore loads.
Separate House bank. 2 x 130 AGMs running @ 24V with bi directional inverter/charger. 400 Watts charge. 700 Watts continuous inverter. A separate 300 Watt charger for a combined charge rate with charging redundancy of over 0.2C on bulk. Plus a small 60 Watt panel with MPPT to run my 4G router, inbuilt USB GPS and live real time monitoring and website reporting. At my latitude I get an average of 2.5 times panel rating in the summer sailing months of May, June, July with a flat panel. Min x 1 and max x 5. It just about runs the complete monitoring system. All other boat house loads such as nav etc runs off this bank. Chargers temp compensated, 4 stage or more programmable digital. If I had a 48V system I'd consider battery balancers and separate 12V chargers but no redundancy that way if a charger fails. I have remote control and monitoring of charger/inverter on/off and current limits via the net using my mobile phone on the boat and ashore. DC to DC 24v to 12V for house loads.
LiFePO4 propulsion only batteries with CB, BMS, hi/lo voltage and temperature cut off relays. Higher and more stable voltage than AGM. 2 x 12.8V. Better for direct drive due to Peukert effect being larger on 24V v 36 or 48V or more geared systems. I use 2 x 200Ah which gives me more range than the old 260Ah AGMs and better cost per nautical mile over time. Range at cruise is now 21nm as opposed to 11nm with AGM of a higher Ah. Another reason for the separate house bank is that I don't want any loads in the winter on the Lithiums as no charging below freezing. Batteries stay on boat at around 60% SOC and idle. 50 Watt tube heater in engine bay to keep moisture at bay. Chargers 1 x 650 Watt and 1 x 400 Watt for redundancy and charge rate over 0.2C. I can limit current by a rotary switch so as not to overload generator. Shore supply no issue unless using fan heater on 6 Amp circuit.
Hybrid range extender mode. Fixed and plumbed in Honda 1kVA generator giving 900 Watt continuous. I chose this over the supposed 2kVA Honda which is actually 1,600 Watt continuous. Same engine just more RPM and more cost and noise. 900 Watts is enough for me and that can let me motor at 4 knots after charger efficiencies are taken into account and leave a little for house battery charge. All without depleting Lithiums. Petrol consumption 1/2 l per hour at that.
Redundancy if generator fails. This is the clever bit I like. If the generator failed the house bank can supply the Lithiums with charge for a time, if needed in an emergency, via the inverter and propulsion bank chargers. Also even though the generator outputs 900 Watts continuous I can still have 1.7 kW peak AC onboard @ 240V or 1.6kW continuous for a time. That runs my 600 Watt kettle or toaster or 700 Watt George Foreman grill as well as charger/s ;). It's called power assist and is seamless and automatic via a fast transfer switch.
All this has come down to what is cost effective at the outset and what I could afford over time. I'm happy with my range under battery of 21nm @ 4 knots. 2 x 5l of petrol gives me 20 hours motoring @ 4 knots using genny if needed. I treat the genny as a standby as I often visit marinas. I carry the petrol in my old propane locker open to sea. I have a 9l bio diesel tank there too, extra 5l can, for the blown air heating. This is Scotland! I won't have gas (propane) onboard. I use a Bio Ethanol stove for cooking with set top oven for breadmaking. I use an 8l Peltier solid state (inefficient) 20 below C cooler which is way cheaper and more reliable than a compressor type.
So about the only item without redundancy is the brushed DC Lynch motor. Could skip the Curtis/PGDT controller in an emergency, but hey it is a sailboat. Brush life around 3,000 hours and as these motors have been going longer than any others of this type or the variants, I think they are to be trusted. I also like their rpm/volt and torque/amp figures as they are better than most, short of going to an IP65/67 YASA or Emrax liquid cooled or Zero IPM IP 65/7(?) air cooled motor. Now if only I had a bigger boat and loads of money....
Comparative costs.
15p per nautical mile for AGM v 10p per nautical mile for LiFePO4. Includes purchase cost, AGM cycles to 50%, Lithium to 80% DOD, excludes charging source costs per kWh. Marina electricity cost usually 0.20p/kWh.
In the UK marine propulsion diesel is around £1/l as is petrol. With my old 10hp diesel engine I'd use around 1l/hr @ 4 knots = 0.25p/NM if using diesel. If using my electric propulsion in serial hybrid mode, using the 1kVA Honda portable generator, I use around 1/2l/hr @ 4 knots = 0.125p/NM for petrol.
Electric motor sailing in light airs and just pure silent sailing with no spinning motor/prop are both a joy. It is a sailboat after all, but the electric systems give me small luxuries onboard.
That's enough from me!
John R.
ELEKTRA
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