Large 2 blade efficiency and high rpm electric motors

[bdgWesternMass]

I have been pondering sailboat drive systems. My previous question here regarding propeller efficiency led me to believe that the largest and slowest (highest torque) 2 blade prop was the most efficient.

I notice that the market for electric motors seem to have very small props with high RPM.

I imagine the high RPM creates more HP but wonder if this is more efficient then a slower and larger propeller.

I guess what I have been trying to figure out theoretical best approach to propeller driven water craft with no consideration of power source.

If that makes sense to anyone I would love to hear thoughts.
If not simply why we aren't seeing large blade electric motors...

Thanks,
Ben

[BillKny]

Originally Posted by bdgWesternMass

I have been pondering sailboat drive systems. My previous question here regarding propeller efficiency led me to believe that the largest and slowest (highest torque) 2 blade prop was the most efficient.

I notice that the market for electric motors seem to have very small props with high RPM.

I imagine the high RPM creates more HP but wonder if this is more efficient then a slower and larger propeller.

I guess what I have been trying to figure out theoretical best approach to propeller driven water craft with no consideration of power source.

If that makes sense to anyone I would love to hear thoughts.
If not simply why we aren't seeing large blade electric motors...

Thanks,
Ben

These questions are all well treated in David Gerr's book . Reading that book will be far more efficient use of time than this thread...

And of course, a propeller doesn't "know" what kind of engine is turning its shaft, so any propeller design is just looking at available prop shaft speed and torque.

It is not a question of what you "believe", it is a question of physics. It is well known that ALL OTHER THINGS BEING EQUAL a larger, slower turning prop with few blades is more efficient. This changes a bit at high speeds, but very few electric boats are high speed craft.

AS far as I know, nobody makes a high horsepower, slow turning electric motor. In order to get a slow prop shaft speed you will need a reduction gear. This raises complexity, and cost, while reducing efficiency.

Now... comes engineering instead of physics.

1.) Can you actually FIT a much larger prop?

Since very few boats are designed (yet) to be electric from the ground up, most boats have limited space for a propeller. Tip clearance, and other spacing needs always limit the possible size of a propeller in the real world.

2.) The reduction gear has its own loses.

Does the gain in propeller efficiency offset the losses to the gear train?

3.) A large prop will slow the boat while sailing.

Many electric boats are sailboats, certainly any that want to actually pretend to be "cruising" boats with long ranges. A prop wtih large surface area (i.e., one that is efficient when motoring) will always slow the boat down when sailing. What do you want, a motor boat? or a sailboat?

[bdgWesternMass]

Originally Posted by BillKny

These questions are all well treated in David Gerr's book . Reading that book will be far more efficient use of time than this thread...

Great thanks for the book reference.

Originally Posted by BillKny

It is not a question of what you "believe", it is a question of physics.

Yes, I understand that enough to reach out to random people on the internet and see if they know about physics Honestly, I only expressed my belief so people could correct my understanding....

Originally Posted by BillKny

AS far as I know, nobody makes a high horsepower, slow turning electric motor. In order to get a slow prop shaft speed you will need a reduction gear. This raises complexity, and cost, while reducing efficiency.

If I find a good one I will report back...


Thanks for your thoughts,

Ben

[tkeithlu]

For a particular horsepower, a slower electric motor is going to be bigger, heavier, and more expensive. It will also have more momentum/inertia all its own and will need larger gears. That makes it an unlikely find among sailboat designs. The designer is going to look for the right compromise that lets the electric motor throw the prop with little or no gearing.

Billkny's #3 point rules - you are going to loose far more in the long hours of sailing than you gain in the short time of docking by going to a large propeller.

[Nicholson58]

Originally Posted by bdgWesternMass

I have been pondering sailboat drive systems. My previous question here regarding propeller efficiency led me to believe that the largest and slowest (highest torque) 2 blade prop was the most efficient.

I notice that the market for electric motors seem to have very small props with high RPM.

I imagine the high RPM creates more HP but wonder if this is more efficient then a slower and larger propeller.

I guess what I have been trying to figure out theoretical best approach to propeller driven water craft with no consideration of power source.

If that makes sense to anyone I would love to hear thoughts.
If not simply why we aren't seeing large blade electric motors...

Thanks,
Ben

Small prop and high rpm is lower cost per HP delivered. Physics and economics is boss.

We have 1983, 3 blade X 24 inch adjustable variable pitch @ 500 rpm max. Quiet, reliable, adjustable thrust at any boat speed. Full thrust in reverse.

[roryboy]

Originally Posted by Nicholson58

We have 1983, 3 blade X 24 inch adjustable variable pitch @ 500 rpm max.

500 RPM shaft speed is paddle wheel speed.

[Nicholson58]

Originally Posted by roryboy

500 RPM shaft speed is paddle wheel speed.

Rory,

Roxy is 40 tons, 58 LOA. The engine is a a British Leyland 6 cyl tractor engine. Max 1400 rpm with 2:1 reduction. Cruising engine RPM is 950 so prop runs 450. Motoring speed is about 7.5 knots.

[roryboy]

[/QUOTE] Max 1400 rpm with 2:1 reduction. [/QUOTE]

So... max Prop shaft speed is 700 not 500 RPM.

[Nicholson58]

Max 1400 rpm with 2:1 reduction. [/QUOTE]

So... max Prop shaft speed is 700 not 500 RPM.[/QUOTE]

Carefully read my post. 1400 Max. Boat reaches 7.5 knots at 450 prop speed. Anything higher is wasted fuel and heat + wear. It will overheat at constant max output. We use max at low prop pitch for maneuvering at low boat speed in tight spaces only for a short burst as necessary. Normally we never go over 500 prop speed ever. I appreciate that this is difficult for owners of high rpm small engines, light boats to follow but as the boat gets large, things change. My 115 HP 6 is 3-1/2 feet long and two feet wide, 2-1/2 feet tall. It is high torque low rpm. There is a stand up walk in engine room. It is quiet and vibration free. My cutlass lasts forever.

[bdgWesternMass]

Thanks for this detailed response of your experience. I would like to ask one more question to clarify

Originally Posted by Nicholson58

I appreciate that this is difficult for owners of high rpm small engines, light boats to follow but as the boat gets large, things change.

Does weight pay a significant role in your performance?

[Nicholson58]

Originally Posted by bdgWesternMass

Thanks for this detailed response of your experience. I would like to ask one more question to clarify



Does weight pay a significant role in your performance?

We weigh 40 tons. 47.5 feet water line. We don’t line people on the rail. If the engine was double weight, no matter. People stepping aboard do not roll the boat. We carry 7000# of fuel and water.

[roryboy]

[QUOTE Carefully read my post. [/QUOTE]

Which I did. You have given 3 different RPMs for your "max" prop shaft speed. Hence the confusion.

I appreciate slow turning propellers and it sounds like you have a nice design working for you.

[Adelie]

Originally Posted by BillKny

These questions are all well treated in David Gerr's book . Reading that book will be far more efficient use of time than this thread...
A little bit snark followed by a pretty good summation


And of course, a propeller doesn't "know" what kind of engine is turning its shaft, so any propeller design is just looking at available prop shaft speed and torque.
The prop doesn't "know' why kind of motor is turning it but it can "see" the available torque at a given RPM.

It is not a question of what you "believe", it is a question of physics. It is well known that ALL OTHER THINGS BEING EQUAL a larger, slower turning prop with few blades is more efficient. This changes a bit at high speeds, but very few electric boats are high speed craft.

AS far as I know, nobody makes a high horsepower, slow turning electric motor. In order to get a slow prop shaft speed you will need a reduction gear. This raises complexity, and cost, while reducing efficiency.

Now... comes engineering instead of physics.

1.) Can you actually FIT a much larger prop?

Since very few boats are designed (yet) to be electric from the ground up, most boats have limited space for a propeller. Tip clearance, and other spacing needs always limit the possible size of a propeller in the real world.

2.) The reduction gear has its own loses.

Does the gain in propeller efficiency offset the losses to the gear train?

3.) A large prop will slow the boat while sailing.

Many electric boats are sailboats, certainly any that want to actually pretend to be "cruising" boats with long ranges. A prop wtih large surface area (i.e., one that is efficient when motoring) will always slow the boat down when sailing. What do you want, a motor boat? or a sailboat?

My feeling is that electric propulsion using the existing prop is good for getting in and out of harbors/marinas, setting/retrieving the anchor and getting out of the way of ships that might run you down.
And it would be best if you got a folding or feathering prop to minimize drag.

If you want to make serious distance with an electric motor, solar panels & battery you need to accept a boat speed of about 3kt. The motor should be an outboard with a huge prop. SeanD has been experimenting with this. He managed 2kt using about 150W and 1kt on 70W in a 26' or 28' multihull being pushed by a DIY set up.
There is not a commercial product for this yet.