Propulsion Systems

Comparing four types of propulsion on the same catamaran

Four Propulsion Types

Diesel Engines

This system is used on a typical catamaran running two diesel engines
This is the system that most boats use, it is tried and true.

What we like:
1) Diesel engines are far cheaper than setting up Hybrid or Dual systems. They would only cost about $28,000 for the pair.

2) Diesel engines turning a prop are anywhere from 10% to 20% more efficient than a diesel generator supplying electric power to a motor turning a prop. The percentage range is based on the number of conversions of energy. If you go directly from a diesel generator, without going to the batteries first, you may cut down on some of the conversions,. In that case, there is less of a penalty, but there is some loss.

3) Diesels have more horsepower. Two 55 hp diesel engines have a combined 110 hp; this only lose to the Combo boat with 113.5hp.

Why people will tell you Diesel boats are better:
There is more energy per pound in diesel fuel than you may store in a battery. Then, as noted above, there are less losses in conversion and more horsepower.

What we do not like:
1) Weight: The engines each weigh 550 pounds (with gear box). The 9 kw generator weighs 300-500 pounds. All total, this equals 1,400-1600 pounds. But that is not the end of the story. Since we are reliant on diesel fuel, we have to carry more fuel on board, up to 158 gallons. That adds another 1,121.8 pounds. If we add the house bank of batteries (220 pounds) we get a Grand total of: 2,727.8 to 2,927.8 pounds, fully loaded.

2) More hours of listing and smelling a diesel engine run.

3) More time finding, transporting, and filling diesel fuel.

Hybrid Systems

Running two electric motors with one or two diesel generators.
(The motors can be pods or saildrives)

The electric system obtains energy from several sources, diesel fuel (a generator), solar power, wind generators and, occasionally, a little from sailing regeneration.

What we like about a Hybrid:
1) They are quiet, they don't smell, they take less maintenance, and they don't burn diesel for 3-5 hours a day when running off a large lithium battery bank which is typically charged by solar panels. In fact, on the days that we make a single day voyage, which are 90% of all passages, we won't burn any diesel fuel at all if we have electric propulsion.

2) Better range: in most cases you have additional range (even with conversion losses) due to recharging via solar panels that makes up more than the conversion losses while using making electricity from diesel fuel.

What we do not like:
1) Horsepower: They are underpowered; with only 30 hp per 20 kw electric motor, for a total of 60 hp. If you tried to add two more motors to get to 120hp, they use more power than your solar panels can recoup per day. In that case the batteries would only motor that boat for a short time each day. To extend your range, you would have to turn on a generator. Now you are back to the same noisy, smelly, high maintenance (on the generator), and diesel burning engine, wasting fuel at a less efficient rate due to losses in conversion. These extra motors would also cost a lot more money, about $50,000 additional!

2) Weight: With generators, larger battery, and diesel fuel for the generators, this is a heavy option.

3) Cost: This is the most expensive option, but a lot!

Parallel Propulsion

Running two engines, with separate electric motors and diesels engines on the same shafts.

Like the Hybrid, the electric system on a Parallel setup obtains energy from several sources, plus one more. They can use solar power, wind generators, diesel fuel (when acting as generators) and, occasionally, regain power from sailing regeneration. In addition, by adding high capacity 48v alternators to the diesel engines, we can recharge while the diesel engines are powering the boat. However, both propulsion systems turn the same shaft and propeller.

What we like about a Parallel propulsion:

1) While motoring with the electric motors they are very quiet and don't smell.

2) Anytime we are motoring off batteries we are not burning diesel. That is around 3 to 5 hours of silent motoring each day. In fact, on 90% of passages we won't burn any diesel fuel at all if we have electric propulsion.

3) We will have more range than a Diesel or even the Hybrid boat, because you can motor off electric when you have the battery power, then switch to the more economic diesels (versus a generator), which gets rid of some of the conversion losses. (See the fuel comparison below).

4) Redundancy of two diesel engines.

What we do not like:

1) Weight: Two diesel engines plus 10 kw motors, plus the lithium battery bank, and partial fuel tank is lighter than the Diesel boat and the Hybrid, but heavier than the Combo (see below).

2) Maintenance: Two diesels to maintain (like the Diesel and Hybrid) vs the one diesel of the Combo boat.

3) Smaller electric motors, which means we would need to run both diesels as generators to get maximum recharge, which wastes fuel.

Combo Propulsion

Running one parallel diesel engine and one separate electric motor.

Like the Parallel system, the electric system on a Combo setup obtains energy from several sources, plus one more. They can use solar power, wind generators, diesel fuel (when the diesel is acting as a generator) and, occasionally, regain power from sailing regeneration. Because we are going with larger 25kw motors, we can recharge at 10 kw while the diesel engine is powering the boat and because we are adding a clutch we can use it as a 12 kw generator while at anchor.

Also, while the diesel is running, it will supply enough energy to run the electric motor in other hull without pulling energy from the batteries. In fact, while just cruising, we could use 5 kw for the motor and still send 5 kw to the batteries.

What we like about a Combo propulsion system:

1) While motoring with the electric motors they are very quiet and don't smell.

2) Anytime we are motoring off batteries we are not burning diesel. That is around 2.6 to 4 hours of silent motoring each day (the range depends on if we are using our auxiliary battery as well). In fact, on 90% of passages we won't burn any diesel fuel at all if we have electric propulsion.

3) Our range is about the same as the Parallel system and we will have more range than a Diesel or Hybrid boat because we can motor off electric when we have the battery power then switch to the more economic diesel (versus a generator), which gets rid of some of the conversion losses. (See the fuel comparison below).

4) We will have more horsepower available than any of the other options because you may choose to power the boat from the diesel engine and one electric motor, simultaneously. This is excellent in a situation where more power is needed for a short time.

5) This system weighs less than all the others.

6) The larger 25kw electric motors will regenerate at a higher amount, offer more horse power than the parallel system, and recharge the batteries from our single diesel at more than the two 10kw motors of the parallel system running two diesels.

Why people will tell you this option is worse:

Redundancy: A Parallel propulsion system will have two diesel engines where the Combo will only have one. However, the Combo system does still have the two electric motors to propel the boat if the diesel goes down, plus the sails. That should be enough redundant propulsion systems for safety. The biggest downside is that if the diesel engine is down we will not have a second generator; however, we do have 4,785 kw of solar panels to collect energy!

Parallel Propulsion

Now that we have talked about the four systems, let's take a more detailed look at our two favorites, the Parallel and Combo setups.

Parallel Motor Features

PARALLEL SHAFT DRIVE - Both the electric motors and diesel engines drive the same shaft and propeller.
REDUNDANT POWER - If either the diesel or electric system has an issue, the other can still power the shaft.
RECHARGING - While on diesel propulsion, the electric motors can charge the batteries.
REGENERATION - While sailing, the prop can turn the electric motor, regenerating power to charge the batteries.
GENERATORS - While at anchor, the engines can be used as 7 kw generators.
LIGHTER - Because we are removing a generator and carrying less diesel fuel, the overall parallel system is much lighter than a diesel boat.

Nanni N5.50 + Combi motors x 2

The Nanni N5.50 engines are 50hp each and come with Combi 10kw electric motors. The advantage of this system is that you have two diesel engines with a combined 100 hp or two 10kw electric motors. The disadvantage is that this will be heavier, cost more, regenerate less energy from sailing, have less electric propulsion at only 20 kw, total, of electric propulsion.

Stats Per Hull
Configuration - 4-stroke, vertical, water-cooled diesel
Max. output - 50 hp
Displacement - 2.19 ltr (134 cu in)
Cylinders - 4 in-line
Dry Weight - kg / 548 lbs
Combi 10kw electric motor (in parallel)
Max. output - 13.4 hp
Dry Weight - 341 kg / 150 pounds

Combo Parallel + Electric

Parallel Nanni N4.80 Diesel/Combi Hybrid plus Combi 25 kw motor.

As good as a Parallel system might be, our current choice for our new propulsion system is to go with one parallel diesel/electric engine/motor in one hull and one electric motor in the other hull (see image, right).

This Combo setup uses a Nanni 80hp Diesel engine with a Combi 25 kw electric motor in parallel, meaning they use the same drive shaft for propulsion. This will be in one hull while a Combi 25kw electric motor is in the opposite hull. We will store the batteries and dive compressor in the same hulls as the electric motor to offset the weight of the parallel diesel in the opposite hull.

The advantages of this system over the Parallel are far less cost, less weight, less fuel usage, less maintenance, and less money to install, yet this Combo system offers more horse power and better regeneration!

Combo System Features

PARALLEL SHAFT DRIVE - In one hull, the electric motor and diesel engine both drive the same shaft and propeller.
REDUNDANT POWER - If either the diesel or electric system has an issue, the other can still power the boat.
RECHARGING - While on diesel propulsion, the 25kw electric motor can charge the batteries at up to a 14 kw maximum rate; we are figuring about 10 kw.
REGENERATION - While sailing, the props can turn the two 25 kw electric motors, regenerating power to charge the batteries and again, at a higher rate than the Parallel's 10 kw motors. At 9 knots of boat speed, stats show this will be around 1 kw per motor!
GENERATOR - While at anchor, the parallel diesel engine has a clutch to disengage the drive shaft. Therefore, the diesel can turn the 25kw electric motor and become a 12 kw generator.
LIGHTEST OF ALL - Because we are removing a diesel engine and a separate generator while carrying less diesel fuel, the overall Combo system is 100s of pounds lighter than all other options.
Maintenance - Also because we are removing two diesel engines from the boat, we will have less maintenance than any of the other propulsion systems.
Less Expensive- It is less expensive than a Hybrid or Parallel system. Only Diesels are initially less, yet they will cost about the same over time due to fuel burn and maintenance savings.
HORSEPOWER - These have an additional 23.5 hp over the parallel system!
BETTER FUEL MILEAGE - the 80 hp diesel has about the same fuel mileage as a 45 hp diesel when they are both running on one diesel engine at the same horsepower for cruising speed. By comparing the power output vs rpm, and then how much fuel these engines burn a that rpm, if any of the diesels are pushing the boat at the same horsepower, the different size engines use nearly the same gallons per hour. However, the Combo system has a larger electric motor, so it will recharge our batteries faster. To get the same speed, we would have to run both diesels on the parallel system. Also, we can run the diesel as a lower RPM because the load is put on it by the Combi 25kw electric motor as it sends 12kw to the opposite hull Combi Electric Motor.

Stats Per Hull #1
Configuration - 4-stroke, vertical, water-cooled 4JH80 Yanmar diesel
Max. output - 80 hp
Displacement - 2. Liters (122 cu. in.)
Cylinders - 4 in-line
Combi 25 kw electric motor (in parallel)
Max. output - 33.5 hp
Dry Weight - kg / 568 pounds
Combi 25 kw Combi electric motor
Voltage: 48v
Max. output - equivalent of 33.5 hp
Dry Weight - 77 kg / 150 lbs.

Stats Per Hull #2
Configuration - 25 kw electric motor
Voltage: 48v
Max. output - equivalent of 33.5 hp
Dry Weight - 77 kg / 135 lbs.
Batteries: 22kw, 48v 200 kg / 440 lbs.
Aux Batteries: 11kw, 48v 220 lbs.

Conclusion: Parallel or Combo?

It is plain to us that the Combo system is far superior. For a further more detail look at why, take a look at the comparisons of four propulsion systems, below: Diesel, Hybrid, Parallel, and Combo.

Comparisons

Below are comparisons between the different propulsion systems: Weight, Fuel , Horsepower, and Safety.

Weight Comparison

What about how much each system weighs? Comparisons include, engines, motors, fuel, generators and batteries.

Diesel Engines and Generator - The two 55 hp diesels weigh 519 pounds, each. The 9 kw generator weighs 500 pounds. Being a diesel only boat, we would need to go with 150 gallons of diesel capacity. That much fuel weighs 1,050 pounds. House batteries (11 kWh) weigh 186 pounds. Grand Total: 2,774 pounds.
Hybrid Motors and Generators - The two OC SD15 motors weigh 42.5 pounds each. The two 20 kw diesel generators weigh 542.5 pounds each. The 44 kWh lithium battery bank plus the tender/auxiliary battery of 11.kWh, all total weighs 744 pounds. Typically, we would only need to carry 50% of our fuel capacity, so that weighs 350 pounds. Grand Total: 2,264, or 2,614 pounds if we are full of diesel fuel for ocean crossings.
Parallel Engines with Motors - The two 45 hp parallel engines weigh 548 pounds each. The two 10 kw motors weigh 90 pounds each. The 22 kWh main lithium batteries plus the tender/auxiliary battery of 11.kWh, all total, weigh 558 pounds. Typically, we only need to carry 50% of our fuel capacity, so that weighs 350 pounds. Grand Total: 2,184, or 2,534 pounds if we are full of diesel fuel for ocean crossings.
Combo: Parallel Engine & Electric Motor - The single 80 hp parallel engine weighs 568 pounds. The 25 kw parallel motor weighs 150 pounds (with brackets). The single 25 kw electric motor weighs 135 pounds. The 22 kWh main lithium batteries plus the tender/auxiliary battery of 11.kWh, all total, weighs 558 pounds. Typically, we only need to carry 50% of our fuel capacity, so that weighs 350 pounds. Grand Total: 1,761, or 2,111 pounds if we are full of diesel fuel for ocean crossings.

Weight Comparison Conclusion

It is pretty simple, the Combo boats beats the Hybrid's 503 pounds. It beats the Parallel by 423 pounds. And it beats the Diesel boat by 1,013 or 663 even on ocean crossings!

The clear winner in weight is the Combo system as the lightest option. It is hundreds of pounds lighter than any of the others! And remember, keeping the weight down is absolutely critical on a performance catamaran!

Fuel Use Comparison

When we compared all four of the propulsion types, using different length passages in different situations. We discovered that the Combo propulsion system is the obvious winner. This is simply because it has all the advantages of each system. It can function as a diesel boat, a hybrid boat, or a parallel boat, using whichever is the most fuel saving system at that moment. In fact, it pretty much ties or wins in every scenario we tested.

For Patrons, we offer detailed looks at everything we used in our comparisons as well as the results for each different passage calculated for each of the four propulsion systems. Just use this link or the button below to view the Fuel Comparison Data.

Fuel Comparison Data for Patron Crew Only

Horsepower Comparison

What about how much horsepower we obtain with each system?

Diesel Engines - there are two 55 hp diesels Total: 110 hp
Hybrid motors - There are two Oceanvolt 20kw electric motors, which supply about 30 hp each. Total: 60 hp
Parallel. 2 Engines & 2 Motors - Two Combi 10 kw motors (13 hp ea., 26 total). Or two Yanmar 45hp diesels for a total of 90 hp.
Combo: Parallel Engine & Electric Motor - One Combi 25 kw motor (33.5hp). Plus one Yanmar 80hp diesel with 25kw electric motor. Total (diesel plus the 25kw motor): 113.5 hp. Also, on only Combi electric motors we get 25kw + 25kw. That means she has 67 hp while using electric, which is 7 hp more than the hybrid boat.

As you can see, the Hybrid boat is woefully underpowered in comparison to the other three options. Horsepower is the category that eliminates the Hybrid from our consideration since that system does not have enough power to motor us out of a dire situation or get us up current while motoring on swiftly flowing river.

The Parallel system has 20 less horsepower than the Diesel.

However, the Combo system has a little more horse power than the Diesel boat, by 3.5 hp. That's not much, but we will take it.

So, of the three other systems, the Combo system boat has the most horsepower, slightly more than the diesel, and also slightly more electric propulsion horsepower than the hybrid.

Since it beats both diesel and electric horsepower, while having to push a boat that is hundreds of pounds lighter than the diesel, hybrid system, or parallel systems, the winner is the Combo system boat.

Safety Comparison

What about the safety with each system?

Safety Consideration:

The 3 other options are safer than the diesel boat:

1) The options with electric systems have no need for propane, eliminating any chance of an explosion from a leak.

2) The options with electric systems get rid of storing flammable gasoline by going with an electric dinghy.

Safety of the other systems over the Hybrid boat:

1) The Diesel, Parallel or Combo systems eliminate the danger and limitations of an underpowered Hybrid boat whenever more horsepower is required.

The Parallel of Combo over the Hybrid or Diesel boats:

1) Both of these have two separate propulsion systems, Diesel or Electric. If either is down, the other may propel the boat. The Diesel and Hybrid each only have one propulsion system. The Hybrid's electric motors must function or there is no propulsion, the Diesel must runs it's engines.

Safety Comparison Conclusion

Both the Diesel and Hybrid boat are not as safe as the Parallel or Combo boats which offer the total elimination of explosive fuels while having the required horsepower in certain situations. And, they both have redundant propulsion systems (electric and diesel).

Final Conclusion

The Combo system wins in every category we have listed: better fuel economy, more horsepower, less weight, added safety, and less cost in the long run. And, like the Hybrid and Parallel system boats, the Combo boat offers us all the comforts of electric propulsion with far less times than any of the other systems where we must obtain diesel fuel.

The reason it beats the others is simple. The Combo a can be a diesel boat, a hybrid boat, or a parallel boat, depending on which is best at that moment. And, it only has the weight and maintenance of one diesel engine.

Therefore, the Combo propulsion system is our winner, by a long shot. The initially cost will be around $12,000 more than the diesel boat, but worth it since the combo system should make up that money over time due to fuel savings while giving us all those other advantages.

That is why we decided that to go with with a Combo Hybrid Parallel Diesel system.

Electric System Diagram

If you would like to take a look at a diagram of our Combo Hybrid Electric System, click the link below...

Combo Hybrid Electric System

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