Hybrid Propulsion Reasoning



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Myths and Truths

Is Hybrid Propulsion viable on a Catamaran?

If you believe this Yachting World article, the time is here for a serial hybrid propulsion system for sailboats, or "Volt in a Boat".  But if you talk with builders and experienced sailors they say the technology is not there yet.  This is page a detailed discussion of some points folks have made when I discuss this idea with them, including some links to some helpful information available on the web. 

What is a Serial Hybrid Propulsion System (SHPS)?

It is an electric motor powered by a large battery array with a diesel DC generater that recharges the batteries when necessary. Unlike a parallel system (used in a Prius), there is no connection between the diesel motor and the propeller with a SHPS.  Since most motors require 48v, there would need to be a dedicated Engine Battery (EB) array to propel the boat.  For the lower 12/24 volt circuits on the boat either a step-down voltage converter from the EB or the usual 12/24v House Battery (HB) array connected directly to solar panels (to avoid on step-down conversion losses).  The EB's would be hooked up to the generator, shore power and the inverter.  With the AC loads all coming off the EB, the HB capacity can be smaller than a typical HB setup.  While the windlass and winches will still need to be on HB, that should be easily replenished by a decent size solar array.  There still needs to be 48v->12v step down converter to charge from EB to HB if there isn't enough sun to charge, but that will hopefully be rarely needed. 

In theory, a well designed SHPS would allow someone to fully charge their batteries while they sail, then use that charge to motor into the anchorage, cook, run light AC and/or laundry that night, motor out to set sails, and then recharge again.  Sounds kind of like a perpetual motion machine, but it should be doable as long as electrical loads are within reason and lithium batteries are used conservatively. 

So why isn't everyone doing this?

Well the main reason is probably cost as it will cost tens of thousands of dollars for a high quality SHPS, but there also fundamental misunderstanding of the technology which has led to safety concerns.  If one reads this this CruisingForum thread, they can see that many boaters don't really know what hybrid means and there are long memories of poor past implementations which have poisoned the well.  Many skeptics also get focused on marketing exaggeration/optimism from electric motor vendors, which kills credibility for those arguing for hybrid propulsion. 

This OceanVolt White Paper is an example of vendor overeach.  The piece is pretty good until the last paragraph where this gets stated:
"In a typical real-life scenario, owners notice that they are able to motor roughly three times further with the same amount of fuel, when using the generator and electric engines, compared to only diesels.  This is exactly the same principle that is applied in today’s shipping industry – all modern ships have diesel generators powering electric engines due to this efficiency gain."
The only way that first sentence could be true is if you are going 1-2knots OR they are counting regen while sailing.  However the second sentence implies to me that they are stating the former, travelling less then 3knots.  There is research showing that a powerboat can go three times as far on in diesel-electric mode than diesel alone but only at a very slow speed where the diesel is not even remotely efficient.  That actually is backed up by research (see the Nigel Calder stuff below). 

However when folks in the CruisingWorld forum read that improperly qualified statement, they think that OceanVolt is stating that SHPS is three times as efficient as diesel at all speeds which everyone knows is not valid.  If hybrid propulsion really was 3-times more efficient at all speeds, every boat on the water would be using hybrid propulsion.  After hearing clearly false claims like that, Diesel-only folks will just think that Hybrid guys believe that they can defy physics.  So they think that if Pro-Hybrid guys buy those claims, they couldn't have any common sense, so why listen to them at all and there is nothing a pro-hybrid guy can say to regain credibility by that point.

Kinda sounds like most political discussions these days doesn't it?  In that Cruising World forum cited above, anti-hybrid folks usually (1) fail to differentiate between power and speed and (2) don't get the idea of when or how the backup generator would be used.  While the arguments are similar to the electric car (EV) vs ICE debate from 10yrs ago, they are big differrences.  While no one argued back then that EVs can't be fast as ICE, there was always the range debate.  Electric only vehicles do have a range problem, but hybrid cars like the Volt (which is the same kind of electric drive with gas/diesel backup generator as an SHPS) do not have a range problem.  Unike a car, speed will be an issue in a boat b/c there is an inordinate power draw once the boat hits hull speed which makes a marine electric motor untenable unless the boat planes.  The most important difference between the auto and marine hybrid debate is that if the electric motor fails in a car one can usually walk, call for a tow truck, or get a ride home within someone else.  Whereas in a boat if your motor fails when you need it, it could be fatal

Then there are the poor implementations of the past which many have heard of and those memories last.  A classic bad, real life example was the idea of putting a AGM powered SHPS in a Lagoon 420 around 2007.  That system was doomed from the start: undersized motors for a heavy (25k lbs) boat (10kW instead of at least a pair 20kW motors), an undersized generator (8kW instead of 20kW), little chance of regen since those elephants need >15knot winds to break 8knots and even if they could do regen, the AGM's can't accept that much current that quickly.  The design was dangerous and should not have travelled further than 3miles from shore, but it is often cited as proof that hybrids can't work on an bluewater boat.  What that 2007 fiasco really demonstrates is that some folks bought some marketing Koolaid and didn't understand the technology or how to implement it correctly.  Oh and by the way, that was 12 years ago folks! 

So what's happened in 12 yrs?  Well several things: This interview with the owner of Top Secret a Voyage 580 Hybrid, is a nice testmonial from a Hybrid believer. 

Myth vs Reality

Given how much has changed in the last few years, lets discuss point by point what we have heard as reasons not to go with hybrid and my counter arguments. 
  1. Electric motors are not strong enough - Myth

    Electric motors are very strong, they can easily out torque diesels at low revs, however they cannot drive a boat as fast b/c the motors draw inordinate current after certain RPM without a CVT.  This is one reason that the Toyota Prius (parallel hybrid) uses electric motor to get the car moving and tapers it's use off and the ICE takes over as the car gets going faster.  Commercial hybrid Powerboats do the same thing, using the electric motors for the low to mid propulsion needs and the diesels to push the boat past hull speed and/or where the diesel can be at its best RPM.  This tow-off video demonstrates that an electric motor has power.  But anyone who's seen a tractor vs a pick-up tug-of-war knows this is just about torque.  The pickup wins everyday when it comes to a drag race and so will a diesel on the water unless there is a nuclear reactor powering the electric motor.  However if you need to drive against an oncoming sea or current, well it's gonna be pretty even until the speed of the oncoming water reaches a point that the electric can't spin fast enough, then it will be toast.  But that's going to be around 7knots on most boats.  While the fastest natural ocean current is 5.3knots, there are plenty of high traffic cuts like Woods Hole and the Cape Cod Canal that can get up to 5knots. This just means that you have to time your transit with the tide, like sailors used to do. Waves and wind in the ocean move faster, but one can still make headway only up to a point no matter how good an engine you have.  I don't have enough data to say this is not an actual problem, but the Voyage 480 (which should have 10kW bigger engines btw) did fine in a 25+ knot headwind with 4-6ft seas.  So in summary, electric motors can't move a boat as fast as a diesel w/o burning an unsustainable amount of battery, but they are plenty strong enough for 7knots on flat water and can still move the boat in big seas and stiff winds within reason

  2. Lithium Batteries are dangerous - Myth

    This is super common belief thanks to phones exploding and the Boeing battery fires.  First off marine batteries are not the same chemical make-up as car and phone batteries, they are Lithium-Phosphate (LiFePO4).  Secondly the Boeing fires were caused by a manufacturing and design issue for a custom battery (Wikipedia article is the easiest to read on this) and the Samsung problem was caused by over aggressive charging firmware on the phone.  The upshot is that quality assurance standards for lithium weren't fully baked which allowed defective equipment in the field, that is now a thing of the past.  Essentially as long as correct protocols are followed (see these Battery University list detailed practices) and there is more than one sensor on the battery, lithium batteries have proven safe.  The main goal on HE (Lithium Nickel Manganese Cobalt Oxide or NMC) batteries like the ones used in cars is to use the batteries between 20% & 90% charge levels so that charging current does not need to be throttled (one wants to keep the charge level within this range anyway to preserve the battery capacity).  However with most LiFePO4 batteries you need to charge to 100% to get full cell balancing in the battery, the 20% is still the lowest you want to go, but that does give some more range.  (see Victron Forum).  The SV Delos lithium page has some good FAQs and this Boat Galley Lithium article sums it up pretty well:
    "In years past, fire was a concern with certain types of lithium batteries. With LiFePO4 technology, that is a thing of the past.  In fact, LiFePO4 batteries are safer than wet-cell lead-acid batteries that can spill acid. "

    Even with all the evidence to the contrary there are still ill-informed opinions on places like CruisersForum where comments like "On boats they're usually lithium ferrophosphate. Slightly less energy density, but safer" evokes a response of "Not necessarily, once EP is involved" WTF? not necessarily if the batteries are used by motors? Not necessarily if used improperly yes, but that has to do with bad design and/or stupid users ignoring warning and/or guidance, not the technology.  That's sort of like arguing gas pumps aren't safe when you smoke cigarettes while pumping gas. 

    Marine Lithium batteries still aren't cheap, but they are definitely a lot safer than their counterparts used on land.  There are some companies like Torqueedo are using high density car battery technology in a boat.  Given the strict EU standards on safety, I am pretty sure they are taking steps to ensure the batteries are as safe as the are in cars, but they will probably need to be used more cautiously to make sure they don't fail. 

  3. Safety/Reliabilty - Fixable

    While new technology is exciting, the fact is that things fail at sea, and you need to be able to rely on your motors to get you out of trouble .  Given that fact, having a cutting edge propulsion system does not sound like a very good idea.  At first glance there are a lot of things that can go wrong in a blue-water SHPS boat, but while the chance of failure is never going to be zero, each risk can be mitigated:
    1. Electric motors rely on complicated electronics - which cannot be easily repaired at sea.  If you engine room floods and your boat has a diesel hooked up to the prop, after you drain it one can usually resurrect a diesel.  That is highly unlikely with the OceanVolt inverter but that's why it is good have two motors in a catamaran.  However to avoid situations such as this, one needs (1) to make sure the electronics are high enough that they are protected from water and (2) that the electronics are protected from surges within the boat.

    2. Lightning strike could take out propulsion - As noted on the Marine Lightning site, catamarans are prone to catastrophic lightning strikes, insurance figures have the odds at 2:1 vs mon-hulls.  The fact is that there is no such thing as lightning prevention on a boat, only lightning mitigation.  However if one follows the Marine Lightning guidelines, chances of catastrophic failure are greatly reduced.  I've been in a house hit by lighting with lightning rods on the roof, no damage when the electricity has an easier path to ground.  Top Secret, Voyage 580, took a hit late in 2018 and lost use of 3 out of 4 OceanVolts engines and most of its navigational electronics.  Not good, but they had no lightning protection whatsoever and on the bright side they still had use of one engine.  The most important thing to do on a cat is to make sure that there is an easy way for current to get from the sky to the water without touching any of the boats systems and that defense is nicely laid out on Ewen Thompson's site.  It basically means a constructing faraday cage that goes to the water and disconnect any mast electronics.  For the same reason you put your laptop and phone in the oven, you need to isolate the sensitve electronics for your electric motor.  In the case of an OceanVolt system, disconnect the Inverter (whcih has all of the sensitive electronics) to isolate it from the motor (where charge can come up the prop shaft) and flip the main shutoff switch to cut it off from the batteries (which should be protected by the lightning rods). 

    3. Electric motors are not as fast as diesels so you cannot outrun danger - As stated above they actually have more torque, so we shouldn't have to worry about getting pushed into a lee shore. However speed is an issue since the RPMs usually top out lower speed.  This means you're not going to get the boat to 10knots to outrun a storm on a motor without a lot more kW than most folks will ever want to put on a boat. 
      This is where we have to rely on having a performance cat (which can sail more than 10knots), being retired and not racing (e.g. not in a hurry to get somewhere).  Plane tickets and charter deadlines should not be an issue.  As long as we don't have to be somewhere by a certain date, we should never have to leave port if things look grim.  So rely on weather forecasts and worst case, if something comes up by surprise, we'll always have a sea drogue if things get too nasty and we need to ride things out. 

  4. All those Batteries Add Weight - Myth

    Well that was definitely true 10years ago, but with lithium instead of AGM, the weight difference is basically less than a passenger.  A quick rough estimate excluding House Battery reduction:

      Total Engines Electronics Fuel Generator
    Remove 1396lbs 2 Yanmar 29hp SDs970 Quattro 12/300066  0 6kW Fischer Panda AC360
    Add 1346lbs 2 OceanVolt SDs290
    Quattro 48/10000 128
    2 OceanVolt controllers 50
    30kWh 6-Victron 24-200 HE's538 14kW Polar PowerDC340
    Net gain (in lbs) (50)lbs (680) 112 538 (20)

    So that's a net loss of weight when you have a hybrid system.  In addition, since you may not need as much diesel fuel with all the regen, one might get away with 30-40gals less fuel, which is another 250lbs right there. 

  5. You can't go very far with an electric motor - True but..

    ...that doesn't apply to a SHPS because a SHPS is more than just an electric motor with batteries.  It is true that you typically can't go more than 15-20mi @5knots on just the batteries, but with the correctly designed SHPS you can travel at slightly less than hull speed for as long as you have diesel fuel.  A DC generator is more efficient than an AC generator, and if it is properly tuned will burn less than the same amount as a diesel driving the motor directly again up to hull speed.  There is a loss going through the batteries, but if both motors are at their peak efficiency, that is the only loss in the system.  You may not be able to go as fast as a diesel, but if we need to go faster than 7knots with a sailboat, we shouldn't buy a sailboat

  6. Cost - True Problem

    The technology costs more, and will for the foreseable future.  This additional cost is definitely less when you are building the boat b/c you aren't getting things like two diesel engines, 12v inverter/chargers, etc that you are going to put up for auction.  For all of the estimates we have so far, the additional cost building from scratch will conservatively be $45k and the fact is you are probably never going to get that back unless fuel prices double or triple.  By using electricity as much as possible we are trying to take advantage of the regen while sailing, but we won't be sailing all of the time.  Sure if one cuts their fuel consumption 80% and uses electrical for cooking, there will be diesel and propane savings, but even if you are living on the boat full time it will take a long time to make up that initial cost.  Exaggerating things a bit and saying folks use their two diesels or generator 1.5hrs a day, puts diesel costs around $100/wk (definitely high, but some people do use diesel just to charge batteries) and propane at $2/wk, it would take at least 9yrs.  That's actually faster than our solar system took to payoff, but I doubt we'll have the boat that long. 

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