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 $18,000 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 loose to the Combo boat with 138 hp.

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 443 pounds.  The 9 kw generator weighs 500 pounds.  All total, this equals 1,386 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 pounds, fully loaded.

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

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

Diesel

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!

OceanVolt


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.

Diesel

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). 

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.

Dual Drive

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 20kw 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 10 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.  

Electric Motor

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 electric motor, simultaneously.  This is excellent in a situation where more power is needed for a short time.

5) This system weights less than all the others.  

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 4,400 kw of solar panels to collect energy!

Dual Drive

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 system

Parallel Motors

Since we are building a Solitaire 1500, we must add  a new propulsion system, therefore, we could go with two parallel diesel/electric engine/motors (see right). 

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.

Parallel Motor Options

Beta 50 Hybrid

Beta 43

The Beta 43 engines are 43hp each and come with 10kw electric motors.  The advantage of this system is that it costs a combined $2,896 less than the Yanmar 4JH45 and is 34 pounds lighter.  However, it has a combined 4 hp less power. 

Stats Per Hull
Configuration - 4-stroke, vertical, water-cooled diesel
Max. output: 43 hp
Displacement - 2.197 ltr
Cylinders - 4 in-line
Dry weight without gear - 287 kg / 633 lbs.
Price: $26,263 x 2

Yanmar 4JH5E Hybrid

Yanmar 4JH45

The Yanmar 4JH45 engines are 45hp each and come with 10kw electric motors.  The advantage of this system is that the engines offer a combined 4 hp more power.  The disadvantage is that they cost a combined $2,986 more and are 34 pounds heavier.  Still, for the extra horsepower, at this point, we are leaning toward the Yanmar parallel engines.

Stats Per Hull
Configuration - 4-stroke, vertical, water-cooled diesel
Max. output - 45 hp
Displacement - 2.19 ltr (134 cu in)
Cylinders - 4 in-line
Dry Weight - 295 kg / 650 lbs
Price: $28,450 x 2

Combo Parallel & Hybrid

Combo System

Parallel Diesel/Hybrid plus 20 kw motor.

Once again, because we are building a semi-custom Solitaire 1500, we will need to add a new propulsion system. Our other choice is to go with one parallel diesel/electric engine/motor plus one 20 kw electric motor in the other hull (see diagram, right).

This Combo setup uses a Yanmar 110hp Diesel engine with a 20 kw electric motor in parallel, meaning they use the same drive shaft for propulsion.  This will be in one hull while a 20kw 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 advantage of this system over the Parallel are less cost, less weight, less fuel usage, less maintenance, and less money to install, yet it offers more horse power! 

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 20kw 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 20 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 20kw electric motor and become a 10 kw generator.
  • LIGHTER - 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.
  • CHEAPER- 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 48hp over the parallel system!
Quiettorque 20kw

As for fuel mileage, the 110 hp diesel is about the same as the 45 hp of the parallel or 50 hp of the diesel only system when we are running 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.  The only additional fuel loss comes when we engage the 20kw motor as a generator since that will put an extra load on the engine.

Stats Per Hull #1
Configuration - 4-stroke, vertical, water-cooled diesel
Max. output - 110 hp
Displacement - 2. Liters (122 cu. in.)
Cylinders - 4 in-line
20 kw electric motor
Max. output - 29 hp
Dry Weight - 389 kg / 858 pounds

Stats Per Hull #2
Configuration - 20 kw electric motor
Voltage: 48v
Max. output - equivalent of 29 hp
Dry Weight - 77 kg / 135 lbs
Batteries: 299 kg / 660 lbs

Propulsion System Diagram

If you would like to take a look at a diagram of our propulsion system, click the link below...

Comparisons

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

Weight Comparison

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

  1. Diesel Engines and Generator - The two 55 hp diesels weigh 443 pounds, each.  The 9 kw generator weighs 500 pounds.  150 gallons of diesel fuel weighs 1,121.8 pounds.  House batteries (11 kWh) weigh 220 pounds.  Grand Total: 2,727.8 pounds
  2. 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 1090 pounds.  The fuel weighs (1/2 tank for short passages) 551.7 pounds (full tank for long passages) 1,121.8 pounds. Grand Total (shorter passages) 2,516.7, (long passages) 3,086.8. 
  3. Parallel Engines with Motors - They two 45 hp parallel engines with 10 kw motors weigh 650 pounds each.  The 22 kWh main lithium batteries plus the tender/auxiliary battery of 11.kWh, all total, weigh 660 pounds. The fuel tanks only need 45% so that adds 497 pounds.  Grand Total: 2,457 pounds
  4. Combo: Parallel Engine & Electric Motor - The single 110 hp parallel engine with 20 kw motor weighs 858 pounds.  The single 20 kw electric motor weighs 135 pounds.  The 22 kWh main lithium batteries plus the tender/auxiliary battery of 11.kWh, all total, weighs 660 pounds.   We only need to carry 45% of the fuel capacity, so that weighs 497 pounds.  Grand Total: 2,150 pounds.

Weight Comparison Conclusion

It is pretty simple, the Combo boat beats the Hybrid's best case scenario by 367 pounds, worst by 936.8 pounds.  It beats the Parallel by 307 pounds.  And it beats the Diesel boat by 578 pounds.

The clear winner is the Combo system as the lightest option 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.

If you want a detailed look at everything we used in our comparisons as well as the results for each different passage calculated for each of the four propulsion systems, use the button below to view the Fuel Comparison Data

Or, just accept that the Combo system wins in fuel efficiency!

Purchase Cost Comparison

What about what it costs to add a Diesel, Hybrid, Parallel or Combo system to our boat?  And, what are the cost savings over time in maintenance and fuel?  Let's take a look!

Initial Cost to add Diesel Engines

Diesel System Components:

Two Yanmar 55 hp diesel engines: $9,000 ea.
One 9 kw Generator: $13,000
Two 11 kWh house LifePO4 battery: $3,500 ea.
One 8,000 watt Inverter: $4,000
Ten 420w solar panels $160 ea.
Ten MPPTs: $160 ea.

Other things of Note:
There will be additional costs for circuit breakers, wiring, etc., which could lead to another $1,000 approximately.

Total Price: $46,200

Diesel Engine

Initial Cost to add Hybrid system

Hybrid System Components:

Two Oceanvolt ServoProp Motors: $25,000 ea.
Two 20 kw Generator: $15,828 ea.
Three 11 kWh LifePO4 batteries: $3,500 ea.
Two 8,000 watt Inverters: $4,000 ea.
Ten 420w solar panels $160 ea.
Ten MPPTs: $160 ea.

Other things of Note:
There will be additional costs for circuit breakers, wiring, etc., which could lead to another $3,000 approximately.

Total Price to upgrade: $106,356

Hybrid System

Initial Cost to add a Parallel system

Parallel System Components:

Two 45hp Yanmar diesels +10kw electric motors: $51,300 ea.
Two Shaft clutches (for generator at anchor): $4,800 ea.
Two Smart Morse controllers: $800 ea.
Two 11 kWh of LifePO4 battery: $3,500 ea.
Two 8,000 watt Inverters: $4,000 ea.
Ten 420w solar panels $160 ea.
Ten MPPTs: $160 ea.

Other things of Note:
There will be additional costs for circuit breakers, wiring, etc., which could lead to another $3,000 approximately.

Total Price to upgrade: $78,100

Parallel System

Initial Cost to add a Combo Parallel & Electric system

Parallel System Components:

One 110hp Yanmar diesel +20kw electric motors: $38,812 ea.
Shaft clutch (for generator at anchor): $2,600 ea.
Two Smart Morse controllers: $800 ea.
One 20 kw, 48v Electric Motor: $8,990
Two 11 kWh of LifePO4 battery: $7,000
Two 8,000 watt Inverters: $8,000
Ten 420w solar panels $1,600
Ten MPPTs: $1,600

Other things of Note:
There will be additional costs for circuit breakers, wiring, etc., which could lead to another $3,000 approximately.

Total Price to upgrade: $72,402

Parallel System

Cost Conclusion

After calculating all our passages on our double circumnavigation, based on a gallon of diesel costing $5.00 we come up with some fuel savings for the Hybrid, Parallel, and Combo propulsion systems over the Diesel boat.  There are also some maintenance savings.  However, the initial cost of the diesel installation is less. 

That means, figuring in fuel and maintenance costs plus the initial purchase prices of the these four systems, the long term prices shake out like this:

1) Diesel: Purchase $46,200 Fuel used: $24,080.  Maintenance: $14,000 Total: $84,280.

2) Hybrid: Purchase $106,356. Fuel used: $35,236.  Maintenance: 1,000. Total: $142,592.

3) Parallel : Purchase $78,100. Fuel used: $21,306. Maintenance: $4,000. Total: $103,406.

4) Combo: Purchase $72,402. Fuel used: $12,766. Maintenance Total: $3,125.  Total: $88,293.

As you can see, in the long run, the Diesel boat still comes out the cheapest, but only by around $4,000 over the Combo boat.  For that small amount of money, we get the peace of mind of endless electric 'fuel' so that we will never run out on a passage.  We get rid of propane on board, and gas for the tender, making the boat safer.  

The Diesel boat also requires more gallons of fuel, which means, we must find, buy, and occasionally transport and then pour this fuel from Jerry cans.  During our long voyage twice around planet Earth, we get to use 2,262 gallons less diesel fuel with the Combo setup over the Diesel boat.  That is a lot of Jerry Cans!  We also spend about 6,000 hours less time smelling and hearing the diesel engine running with the Combo boat. 

Finally, our catamaran weighs less the entire time by hundreds of pounds and has a redundant propulsion system if the diesel has an issue! 

Therefore, in the long run, though the Diesel saves us $4,000 over the Combo, all the numerous extra benefits Combo propulsion are just too good to pass up. 

As for the $4,000 extra, this is all assuming that the price of diesel does not go up during those 13 years we plan to sail!  Most likely, it will.  If the price does increase... even by a couple of dollars a gallon, here are the recalculated numbers:

1) Diesel: Purchase $46,200 Fuel used: $33,713 Maintenance: $14,000 Total: $92,913.

2) Hybrid: Purchase $106,356. Fuel used: $49,330. Maintenance: 1,000. Total: $156,686.

3) Parallel : Purchase $78,100. Fuel used: $29,829. Maintenance: $4,000. Total: $110,929.

4) Combo: Purchase $72,402. Fuel used: $17,873. Maintenance Total: $3,125. Total: $93,400.


As you can see, now the Combo is about equal in long term coast to the diesel only boat, yet you still have all the benefits of the electric portion of the system. 

Now, if your overall voyage is much shorter, the diesel system will save you money over any of the other system, including the Combo, simply because it is far cheaper to install initially.  So, if you are not going around the planet, it might cost you more to get the benefits of the Combo propulsion system.  

However, we are planning to go around the planet, twice!  Therefore, there is no question which is the best option for S/V Lynx; the Combo propulsion system wins the cost battle due to the small difference between it and the diesel, coupled with all the benefits you gain with a hybrid boat. 

Horsepower Comparison

What about how much horsepower we obtain with each system?

  1. Diesel Engines - there are two 55 hp diesels                                                                              Total: 110 hp
  2. Hybrid motors - There are two 20kw electric motors, which supply about 30 hp each.    Total: 60 hp
  3. Parallel. 2 Engines & 2 Motors - Two 10 kw motors (13 hp ea.).  Plus two Yanmar 45hp diesels.     Total: 116 hp.
  4. Combo: Parallel Engine & Electric Motor - One 20 kw motor (29hp).  Plus one Yanmar 110hp diesel.  Total: 139 hp.

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 beats the Diesel with 6 additional horsepower.   

The Combo beats the Diesel by 29 horsepower.  So, of the three other systems, the Combo system boat has the most horsepower. 

The winner, obviously, 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.

Parallel System

Safety Comparison Conclusion

Both the Diesel and Hybrid boat are not as safe as the Parallel or Combo boats as they are the only two offering both the total elimination of explosive fuels.  They both have the needed horsepower for certain situations.  And, they both have two propulsion systems. 

Final Conclusion



The Combo system ties or wins in nearly every category we have listed: better fuel economy, more horsepower, less weight, and added safety.  It only looses to the Diesel in overall cost, and only by a small amount.  And, like the Hybrid and Parallel system boats, the Combo boat offers us all the comforts of electric propulsion and far less times than any of the other systems where we must obtain diesel fuel. 

Combo System


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.

Therefore, the Combo propulsion system is our winner, by a long shot.   The initially cost to install is steep at $72,402, which is $26,202 more than buying the diesels and a generator. 

However, that extra initial cost is paid back in fuel, propane, and maintenance savings over the thirteen year voyage, assuming diesel prices slowly rising. 

That means, in the long run, using the Combo system only costs us a maximum of $4,000  more than a Diesel system, and may cost about the same depending on diesel prices in the future.  Then, it gives us a whole lot of added comfort and safety during the entire circumnavigation of planet Earth.

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

Combo System 2

Propellers for S/V Lynx

Your propellers are an important part of your propulsion system.  The right propeller will add speed, save fuel, regenerate more power, and do better in reverse or stopping.  That's why we looked at all the options out there and chose a reversing propeller


There are, basically, we had four types of propellers to choose from, (see below).

Standard Prop

Standard

Standard propellers have fixed blades, meaning, whether they are in use or not, they blades are still extended.  

Prop Feathering

Feathering

Feathering props turn the blades.  That way, when not in use they can be turned sideways to the flow of water. 

Folding

Folding

Folding props, as the name suggests, fold the blades down when not under power.


Autoprop

Reversing

Reversing props fold down and then reverse the blade so that the leading edge is always used in forward or reverse.

Standard Props

Pros

  1. Lowest price - These are the price winners, but a long shot!  They can be five times less than folding or reversing props, and ten times less than feathering props!
  2. Extremely low maintenance - With no moving parts, these have little maintenance compared to any of the others.

Cons

  1. Fixed Pitch - These props, once setup by the manufacturer, cannot have the blade pitch adjusted later on by the owner.
  2. Poor in Reverse - Because these props do not use the leading edge while in reverse, they are not as efficient as feathering or reversing props when stopping or backing up. 
  3. Poor Regeneration - Just like in the reverse situation, the leading edge is not used when regenerating.  This affects the efficiency of regeneration if you are using them on a hybrid boat.
  4. Excessive Drag - With the blades always extended, these cost a lot of drag while sailing, up to one knot slower, per prop!

Feathering Props

Pros

  1. Rotating Blades - These props turn the blades so that they are perpendicular to the flow of the water while not in use, reducing drag greatly.
  2. Good thrust Reverse - These open the blades based on the torque of the shaft.  Therefore, when in reverse they blades rotate the other way making them also good in reverse.  This also adds to the boats maneuverability in close quarters.
  3. Adjustable Pitch - Both forward and reverse pitch may be set to optimize the performance of your engines on your particular boat.

Cons

  1. Expensive - There is no denying it, these props are expensive.  They may cost twice as much as folding props.
  2. High Maintenance - They have more moving parts and are more difficult to service, and more expensive in parts and labor, if you have someone else work on them.
  3. Fouling - With the blades still extended, even when feathered, they are more likely to catch on lines, seaweed, ropes, etc.  They also cause a little more drag than folding props.

Folding Props

Pros

  1. Reduced drag - Folding props collapse down and offer less drag while sailing compared to feathering props and far more than standard props.  This also means they catch on less things, like lines, seaweed, etc.
  2. Reasonably priced - These are half the price of feathering props (though standard props are much less than either).
  3. Lower Maintenance- With fewer moving parts, these are easier and cheaper to service or have serviced.

Cons

  1. Fixed Pitch - These props, once setup by the manufacturer, cannot have the blade pitch adjusted later on by the owner.
  2. Poor in Reverse - Because these props do not use the leading edge while in reverse, they are not as efficient as feathering props when stopping or backing up. 
  3. Poor Regeneration - Just like in the reverse situation, the leading edge is not used when regenerating.  This affects the efficiency of regeneration if you are using them on a hybrid boat.

Reversing Props

Pros

  1. Good in Reverse and during Regeneration - Because they reverse the blades in forward or reverse, that means they always use the leading edge for efficient maneuvering, or for regeneration capabilities while sailing with hybrid system.
  2. Reasonably priced - Similarly priced to folding props and, also like folding props, these have fewer moving parts, which makes them easier and cheaper to service or have serviced.
  3. Automatic Pitching - Since the blades adjust their pitch based on the RPM of the shaft, these props can adjust to work efficiently at various speeds.  This is very useful in a parallel engine/motor situation (like we have on one engine of S/V Lynx)

Cons

  1. Fouling - With the motors are in neutral, the blades are still extended, just like a feathering prop.  That means they are more likely to catch on lines, seaweed, ropes, etc.
  2. A little more drag - Also because the blades are still extended and curved, they cause a bit more drag than feathering props while sailing, and slightly more than folding props, but have far less drag than standard props.

Prop Comparison Conclusion

Brunston's Eco*Star Reversing propeller
The Eco*Star is known for its regenerative capabilities.  Since we have two electric motors, both using regeneration, this is critical.  But that is not the only reason we went with this choice in props.  Since we have one Parallel Diesel engine/electric motor, and the diesel engine needs to run at a faster RPM while the electric motor will run at a slower RPM, we needed a prop that would automatically adjust the pitch based on the RPM so that both engine and motor using the same prop are efficient when powering the boat.  The Eco*Star is that prop.

Eco*Star

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