A Dual Diesel/Electric propulsion boat

 

A Combo or Parallel Diesel/Electric propulsion boat

First of all, what are the types of propulsion?  Well, a hybrid diesel/electric boat is generally talking about electric propulsion with a diesel generator.  A diesel boat is using diesel motors.  Then there is a combination of both.  Finally, there is Parallel Diesel/Hybrid.  That means, we have diesel engines and a electric motor system on board both running on the same shaft and prop.

We will talk about two versions, the combination and parallel systems.  The first uses pod motors (see image, top right), with separate diesel engines.  The other is a Parallel system, which uses diesel engines with an attached electric motor between that engine and the shaft drive (see image, bottom right).  The advantage there is you only need one prop for both the diesel engine and electric motor.

BUT!!!!

We know, you have some reservations.  But we understand the issues.  The main ones are the complexity of upkeeping and repairing two systems, the added weight, added drag, and the added cost to set them up in the first place.  These are all real issues.

However, let’s address each issue and why we may still go with a combo or parallel propulsion system. 

The Issues:

1) Complexity

We can start with complexity.  In truth, having both propulsion systems is not that much more complex… really, we mean it, stay with us here.  If we compare a diesel boat, you have the upkeep of the diesels, and likely you have a generator on board as well.  Then, you have your electric system for handling all your needs, things like house batteries, inverters, shore power isolators, MPPT controllers for your solar panels, etc.  So, what would we be adding to this system to get electronic propulsion?  Not much.  Mostly, we are just increasing the size of elements that already exist.   A larger batter bank, a larger solar array, larger inverters.  So, the complexity is not increasing so much as the size.  We would be adding the electric motors, but those are brushless motors, so very little to go wrong or maintain.  Now, with pods, we must also add a hydraulic lift system, which is a little added complexity, but we will also have other hydraulic systems onboard already (steering, most likely, and autopilot rams).  Therefore, we already have to carry the things needed to repair hydraulics, so that is not adding much.  As for a parallel system, you do not need the hydraulics, and you reduce complexity by eliminating the saildrives.

2) Drag while sailing

If you have pods, the issue is having four props instead of just two in the water, which adds drag.  Our solution to this would be to put the two pod motors on arms that swing up out of the water when not in use, thus, reducing drag while sailing.  Easy enough.  Here is a picture of such a system that was for sale awhile back.  We will be making our own version of this, though very similar (these are not available anymore).  However, if you go with a parallel system, then you have a single shaft drive and prop for both propulsion systems, and eliminate the drag of a saildrive!

3) Weight

OK, now we are getting to the real issues.  Weight is a big deal on a catamaran.  Therefore, a combo or parallel propulsion system is not going to work on just any catamaran.  You would need to have one that has a large enough load carrying capacity.  We plan to buy a cat with just that in mind.  Still, even on such a catamaran, you would still want to keep the weight as low as possible.  Therefore, if we go with pods, we would go with the lightest motors we can find, something like 41 pounds (each).  Of course, you need the arm and hydraulics, with adds more weight.  Still, this can be done for around 200 lbs, total.  

With a parallel system, you add only the electric motors and clutches, etc.  These add a bout 75 pounds each, or 150 pounds total, a little less than the pod system.

Of course, you must also add the weight of the large lithium battery bank.   However, it should be noted that we plan to add a large lithium battery bank for other reasons, like refrigeration, AC, etc.  And, we plan to have a larger solar array to charge these batteries.  We need these for all the other power requirements but the electric propulsion system can make use of them as well.  So, technically, all we are adding to the existing setup is the 150 to 200 lbs., depending on if we go with a parallel system or pod motors.  That is acceptable since we gain so many advantages.  We will go over these advantages further below.  But before we look at the advantages, let’s take a look at the final issue.

4) Cost

This one cannot be dodged, though it can be mitigated.  Let us explain.  As mentioned in the ‘Weight’ issue, we will already be adding the solar and batteries for other reasons.  Therefore, the added cost is only the pods and lift system for them.  Still, that is not an insignificant number.  The pods run $9,000 apiece and the hydraulic system another $2,000.  So, we are looking at $20,000 to add this electric propulsion system.  Ouch.  If we want to add a parallel system, it is even worse, as these run around $27,000 each (though this includes a brand new diesel engine).

However, now to the mitigation element.  That cost will be returned in fuel savings over time.  In fact, the pod motors will save us more than $85,000 in fuel and maintenance savings during our double circumnavigation.  If you want a full breakdown of why we save this much, you can read up on our website where we breakdown each passage to show the fuel savings.  As well as savings in gas, propane, maintenance, etc.   You can find that page here: 

http://svlynx.com/Boat_Hybrid.html

 

Advantages of a combo or parallel propulsion system:

 OK, we went over the issues, now for the advantages! 

1) Fuel Savings

We already mentioned the huge fuel savings.  In most short or mid-length passages we will not even need to turn on our diesel engines.  With 3 hours of electric motoring available when we depart and regaining another 3 hours from solar along the way, we will seldom need to fire up the diesel engines.  And, on longer passages, we save more fuel (over a hybrid system) by having the more efficient diesel engines to run instead of using a generator and converting the power, which causes up to a 20% energy loss. Between the two systems, each having its advantages in certain situations to save fuel, we come out way ahead.

2) Less noise and smell

In most cases, we won’t need to use our diesel engines and that means that we don’t have to here them or smell them while on a voyage.  That is a big plus as neither of these are desirable.  We will have to live with them when it is necessary on a long passage, but those are few and far between compared to the massive number of shorter passages where we are noise and smell free.

3) Instant power available

Diesels have to be warmed up before use.   This wastes power.  When we are entering or leaving an anchorage, we do not have to fire these up and wait for them to warm up before using the throttles.  With the electric pods, we can use them as soon as we can drop them in the water (about 30 seconds).  If they are already in the water, they are ready to go.  Another use of instant power is during a tack.  Catamarans are great, but they can have a hard time in certain situations when you tack.  If needed, you can always push the bows around through the wind with a little bit of electric thrust.  And, since you do not need to warm them up, you can drop them into the water just before a tack, push the bows around, then just raise them back up to get rid of the drag.

4) Thrust at any RPM

Electric motors do not have a sweep spot in RPMs, like diesels.  With your diesel engines you have a minimum RPM that you need to keep them happy.  This works fine when you want to motor for long periods at 5-6 knots, you set them at around 1,800 RPMs and you are good to go with excellent MPG.  However, if you want to run them at a slower speed, that will not work.  With electric motors, you can do a couple of things.  One is maneuver in a marina without having to use higher RPMs.  This gives you better control of the boat.  Also, you can use lower RPMs while sailing to motor sail at lower wind times.  With a diesel, you would have to just drop the sails and go with the 1,800 RPM setting.

5) Drogue effects in heavy seas

While you are in large swells, you can use the electric propulsion as a drogue system.  You have the pods power you up a swell face, then spin backwards on the way down, slowing the boat slightly.  This evens out the speed of the boat, which is more comfortable (and safer since you do not want the bows to plow into the next swell at great speed).

6) Extra propulsive power when needed

This is an area where the hybrid system loses out to a diesel.  Generally, unless you are going to run four electric motors, the diesels have more power.  However, where power is really needed, in the case of a combo propulsion system, you could run both!  That means, on a pod/diesel boat, you get 110 hp from diesel power and 60 hp from electric for a total of 170 hp!   You will not need this often, but there are situations where you need all the power you can get.  There are several emergency situations with current or lee shores, but here is another example.  From Koblenz to Wiesbaden, on the Rhine River, the river is running at nearly 9 knots.  That means you would need all the thrust you can get to make that 40 mile stretch of river.  We do plan to travel up the Rhine, across the canal to the Danube, and down to the Black Sea, to this is important.

7) Regeneration when sailing

While sailing, especially during high winds, you can use the electric pods to regenerate power.  This will not happen all the time, but on occasion, there is some energy to gain this way.

8) Less Maintenance (than a diesel boat, but not a hybrid)

How can two systems cause less maintenance than one?  Well, in the case of comparing to a Hybrid only boat, a combo or parallel propulsion system has additional maintenance.  However, compared to a diesel boat, there is actually far less.  This is simply because we will be running the diesel engines far less often, which means less miles on them and longer periods between maintenance.

9) Redundant propulsion system safety

Backups on an ocean crossing boat is paramount, and with a combo or parallel propulsion system we have another kind of backup, two methods of propulsion.  If we lose an engine, we can still fully maneuver in port with the electric motors, and visa versa if we lose electric power, we can still use the diesels.

 

Conclusion:

If you are a boat that plans to stay for long periods in remote places (which we do), with a larger crew (which we will have), then you have greater electric needs in water making and refrigeration, at minimum.  Due to that, you need a larger battery bank and more solar panels to charge them.  Once you go that route, adding in the pod motors is not a huge hit in weight.  As for the cost, in the long run they will save you loads of money in fuel savings.  In addition, you gain all the comfort advantages from electric propulsion on the majority of your passages.  You also get the fuel savings of the diesels on long passages.  Of course, you need a boat that can handle the extra weight, so smaller performance cats are probably out.  However, if you are going with a 45-46’ comfort oriented, production cat, then this is a viable option with a host of advantages and very few disadvantages.  It would also be possible on a larger performance cat of at least 50' LOA.  Obtaining one of those boats is our plan and, unless we are convinced otherwise, this is the propulsion system we plan use on S/V Lynx.