T-Pain Is On A Boat: Re: [Electric Boats] Re: Electra Glide Data and updates

Hi Scott and Mike,

Thanks for your commentaries on my post regarding Propellers.

First we need to recognize that there is no scientific proof for many of the concepts that we are talking about.

Getting an efficient propeller on a boat is an art form rather than a science. The proof comes from actual results. The explanations come from postulations of why the propellers that exhibit very high efficiency are obtaining these efficiencies. Trends then verify postulations when propellers are compared with only one change. If we see an improvement when pitch increases then we follow that change as long as we receive positive results.

Every boat is different, every propeller shaft installation is different, every propeller shaft angle is different, every hull is different. All we can look at are the results and follow any gain made in efficiency.

A data bank for test results is a great idea. It is easy to do with a smartphone taking video so we should be able to start

Standard test data should include:

Observations done should include:

- low tide or high tide so no currents are involved.

- two passes in opposite directions and the two passes are averaged

- unedited video evidence that pans from GPS data to current meter which also shows voltage so power can be calculated.

- no wind or waves present

- make and model of propulsion motor

- gear ratio used

- motor and propeller rpm data

- type of reduction system: gear, belt or direct drive

- make and model of boat

- Voltage and Current

- Boat Speed by GPS.

- Customer Estimated Displacement

These requirements are necessary so we can establish a baseline. Over time and with multiple observations this group can change the way that boats are converted and new boats are built. We can't leave it up to the experts from the past to come forward and admit that their formulas are incorrect for electric. We must find out for ourselves what is working in the field through bonafide evidence.

Electric propulsion requires new thinking with new teachers based on what works rather than what is predicted to work. There are way to many variables to put into any kind of formula.

The main difference between diesel and electric is that diesel relies more on power to get results, whereas electrics depend more on efficiency to get desired results. Calculating overall system efficiency includes all the energy losses multiplied by all the propulsion losses multiplied by hull losses. Electrical Propulsion Systems are dependent on each component within the system.

It has been my experience that the single largest gains that can be made with an electric propulsion system come from increasing the size and perfecting the shape of the propeller. I believe in the advancement of the efficiency of the propeller…... my company is The Electric Propeller Company. Whats more, propellers are relatively inexpensive to buy and many prop shops will return a propeller with at most a small fee if done within a reasonable time period. High Efficiency Propellers can be purchased for between $400 and $600 for boats between 30 and 35 feet long. High Efficiency Propellers come with the added bonus that the tip is 2 inches aft of the center of the hub, so they enjoy added tip clearance too.

There really is no end in the joy that you can have with electric boating, especially when you hone in on a good propeller.

Here are my notes on your post in italics:

For background, I primarily rely on Dave Gerr's "The Propeller Book" for making propeller calculations and recommendations. Dave is a respected naval architect and his book is used by many. I have also waded through a reasonable amount of more scholarly (PHD type) work on propellers and feel I have a reasonable grasp of the theory and practice of sizing propellers. I am a degreed engineer and capable of understanding higher math and theoretical concepts, at least at an undergraduate level.

Dave Gerr's Formulas do not apply to electric boats. The basic concepts of design apply to all propellers but the difference is: electrics are optimized by torque, whereas diesels are optimized by peak power at full rpm with an unstable torque curve. The two types of propellers are vastly different because of this. Diesel Propellers have very little pitch so have to turn very quickly to make desired speed, whereas electric propellers can turn very slowly and do most of the work with torque rather than rpm.

1. Diameter is the number one consideration in optimizing for efficiency.

Diameter is only part of the formula. It is possible to have a smaller diameter propeller that is more efficient. Pitch, Blade Area Ratio, Cupping and Propeller Shape are part of the efficiency formula too. Increasing the diameter also increases the tip speed. When tip speed increases, so does turbulence.

2. Reducing shaft angle will increase efficiency.

Agreed. The company RE-E- Power which has now gone out of business, did a lot of tank testing and found that shaft angle was not as important as the shaft being parallel to the hull surface, something that a pod can accomplish, but a shaft drive can't. It makes sense because the laminar flow of water will be parallel to the hull so if the drivetrain is on an angle to the hull, then this will create more turbulence. Getting additional tip clearance helps reduce the effects of the propeller not being parallel to the hull surface.

Here is where he is off base:

Building Efficient Electric Propulsion requires discarding all you know about diesel propulsion and making systems that are completely off base with the norm that we are used to.

1. Maximizing propeller area will increase efficiency.
It will tend to increase static thrust and provide better maneuvering performance. However, this is not the same as efficiency under way. The optimum propeller for efficiency will have blades only wide enough to absorb the power of the prime mover while maintaining reasonable face pressure, that is to say at a level below cavitation. Making them larger beyond this point only increases propeller drag. Again, it does improve maneuvering thrust, so there is a trade-off, based on your goals. With a heavier boat, you probably want to go with a 3 blade propeller, though do not need to get crazy with a high blade area one.

Read Nigel Calder's article "In Praise of Big Propellers"

Larger propellers will increase Propeller Drag which is why we advocate zero thrust operation of the propeller by keeping the motor on at a very low setting. I also call this locking into the natural energy of the ocean.

We have noticed significantly higher efficiency from some of our boats with 4 bladed propellers but our highest efficiency propeller remains a 3 bladed propeller.

Obtaining the highest efficiency is all about slowing the propeller down so there is less slip. You can only slow a propeller down if you have enough blade area ratio to push the boat, and pitch to get to desired boat speed.

2. Going to a higher pitch reduces slip and increases efficiency.
First off, this is completely untrue.

Words like completely untrue do not belong in a discussion that has no scientific data apart from actual results. In order to make a statement like "completely untrue" you should review the data first. Higher pitch can obtain speeds with lower rpm if their is enough power in the propeller which comes from blade area ratio, number of blades, diameter and cupping.

Slip is a ratio between the theoretical advance of a propeller through a solid and the actual through water. If diameter is held constant, the slip will be more or less the same with different pitch propellers at any given boat speed.

This may be true with propellers that have a similar propeller shape. Alter the shape, cupping and pitch and the numbers will be vastly different.

The math is pretty simple. Also, slip and efficiency are not the same, though they are related.

Also, slip changes with speed, which he does not seem to acknowledge.

Force per kilowatt also changes with speed.

According to the experts, there is an optimum pitch/diameter ratio for any given boat speed. For sailboat speeds, 0.6 to 0.8 is generally considered to be the sweet spot.

Using diameter instead of blade area ratio, cupping and pitch is based on how diesel's work and not on how electrics work. Diesels use very small bladed propellers with minimal torque requirements. Electrics have the capability of using very high blade area ratios with high pitch. We have found that the optimum pitch to diameter ratio is more like 1.2 or even as high as 1.5:1

For higher speed vessels, higher ratios are in order. James has been making the claim that going to very high pitch increases efficiency for years, yet has never offered any data or analysis to support it.

The reason for this is we consider the propeller and gear ratios that we choose to be proprietary information. That will all be changing in the near future as we become an open source company for the enabling of this technology. We have provided data on our various systems in the form of watts to knots which verifies our claims. There are many posts on this forum with data from Electroprop customers that can be referenced to.

3. 3. Higher speed propellers create a wider wash cone.
I have never observed this in any kind of controlled situation and have never read anything that makes such a simplistic claim. It is more complicated than this, though the concept of a narrow "wash cone" being more efficient is essentially true.

Propellers accelerate water in a cone shape. I agree this should have further study. We have to look at why the Electroprop propeller is more efficient than a typical sailing propeller then we postulate the reasons. The propellers that we use have a different pitch at different distances from the hub center. We could prove this theory with some water die in a test tank.

4. 4. Diesel propellers are very inefficient and the ones he installs are 50% more efficient.

In some cases I think that the propellers we use might be as high as 100% more efficient than the propellers that they are replacing in particular, the atomic 4 propeller. There is no other explanation. We know the drive systems are between 80 and 85% efficient so the improved efficiency must come from the propeller. We regularly replace diesels 3 x the horsepower. 6 electric horsepower has proven to have better performance than 35 h.p, Atomic 4. The only explanation is the added performance must come from the propeller.

This can only be true in cases where the existing propeller is, indeed, an very inefficient one. Yes, this is sometimes the case. The most common case is the propellers used on Atomic 4's. There are intentionally undersized and inefficient to allow the engine to turn up to a higher speed. It is a compromise dictated by the design of that engine and transmission. We always strongly encourage customers to change an A4 propeller to something better. Same in cases where the diesel propeller is undersize, or has a really low or high pitch. In most cases, the propeller installed is already as large as the space will allow. Also, they are usually ok on pitch. In most cases, there is little to be gained by changing propellers. We evaluate on a case by case basis.

In summary, though James is partly right, he is also partly wrong.

Judgemental words are not appropriate for learning from a trial and error physics problem based on evidence from a wide range of test platforms. There are right turns and wrong turns in the road towards ultimate efficiency. We may find an aspect to propeller design in the future which changes everything we know about propellers today so we must keep an open mind towards any kind of advancement.

If I were designing a new boat for electric propulsion, I would maximize prop diameter and make the shaft angle as shallow as possible. Look at a trawler. Of course, these work against each other, so it all ends up being a trade-off. I might do some other things with pitch and blade area, but this is more at the margins.

As we dig into this further we will find that pitch, blade area, and perhaps most importantly blade shape are more important than diameter of the propeller. There have been significant advancements in propeller design since most of the boats that we have been converting were built. Propellers are now machined with CNC machines to new shapes that have slowly improved with the years, as propeller companies follow increases in efficiency. When I visited Torqueedo in Germany I was shown all of the different designs of propellers that they tried before arriving at the ones they currently use. None of those propeller look anything like a standard sailing propeller built over the last 40 years. The torqueedo propeller is part of what makes the Torqueedo a viable alternative to small outboards.

Increasing efficiency has lots to do with other parts of the propulsion system too, and how the propulsion system is geared. If you gear for speed, this will come at a cost for efficiency at all the lower speeds because the motor then runs on current rather than on voltage. This also plays into pitch and rpm. Finding the most efficient propeller solution has to include the current capability of the motor, the currents of the motor and the gearing involved. In general, all losses of an electric drive motor are current related and no losses are voltage related. The windings of the motor also matter when chasing efficiency as a motor wound for higher torque will have higher efficiency but will have less rpm. Slowing the system down is the best way to increase efficiency which includes slowing down the motor so there are less speed related frictional losses in the reduction unit, and slowing down the propeller shaft so you have less speed related friction losses of the thrust bearing and cutlass bearing. This also plays into slowing down the propeller so you have less friction losses from the water across the blades, and slower tip speeds for less tip turbulence.

What really matters more than anything else that we have discussed here is propeller placement. On many boats fore and aft placement of the propeller is limited to an aperture or by a rudder. On some boats there is open water behind the propeller and the prop can be slid back an inch or more, providing more clearance from the hull and from the strut. Normally, prop manufacturers recommend the hub be very close to the cutlass bearing, but we are finding that slower turning propellers can be moved aft, away from the cutlass bearing and still be adequately supported. Even 1/4 to 1/2 and inch can make a difference to vortex formation so take some if you can. If you move the propeller too far away from the cutlass bearing you can experience shaft whip which you will notice as a new vibration under heavy power and cause premature wear of the cutlass bearing, so don't take too much!

A propeller creates a vortex, and any obstruction to that vortex robs the propeller of its efficiency. Boat manufacturers can make the changes we wish we could, so reach out to them and when they get on board you will find we can increase system efficiency could improve by another 30 % of so as they go to extended shaft tubes and eliminate the strut which causes significant amounts of turbulence in the vortex every time a blade passes it.

One thing is obvious, we are on the leading edge and the road before us is a great place to enjoy.

Hoping everyone out there is having a great holiday season!

Happy Electric Sailing in the New Year!

James

James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103

805 455 8444

james@electroprop.com

www.electroprop.com

On Dec 25, 2015, at 10:09 AM, mike@electricyachtssocal.com [electricboats] wrote:

To Electricboat Group
From Mike Gunning Electric Yachts of Southern California/Pacific

I have asked Electric Yacht's prime engineer to review and share his thoughts on the subject of propellers and how to make the electric boat as efficient as possible. This thinking is based on naval engineering plus the antidotal sample set of at least 300 conversions. I will present his thoughts with the understanding that everything is a compromise when designing a boat and its propulsion system. You need to understand the specific character of your boat, how you use your boat, and where you use your boat.

First: let's talk about the boat that you and I are updating.
The original naval architect that designed your boat used his skill and training and the boat builder applied that engineering as they built the boat. Prior to World War Two, most recreation keel boats were not powered to drive the boat to hull speed and that sailor used the motor system within that limitation.

Since the vast majority of the boat we have converted were built from 1965 through 1990, we see two basic engines – Gas motors through the early 1980s and diesel from that point on. We also see many ICE repowers being not the first but the second and even the third repower. We see engine installed that are underpowered but mostly overpowered with installations from poorly done to very well done.

Second: The boat you and I are converting are not the same as the boats being built today.
Today one can see that most new boats are overpowered (estimated they are being power being 140% to 150% of what is required to reach hull speed) and engine compartments are highly compromised for the sake of the interior layout.   Hull shape and keel design has changed radically along with the weight of the boat.

Therefore decide in your specific repower:
·         - Do you want to reach hull speed?
·         - Do you want to impact the sailing character of your boat for the motor propulsion?
·         - Is cost an issue?
·         - Is range more important than speed?
·         - Is it a race boat in protected water or a passage making sailor crossing an ocean?
·         - Do you have a limited requirement for motoring?
·         - How do you charge the battery technology that you decide to use?
- Do - Do your want to update a boat that is so unique that only you will want it or be more conventional for resale?

Third: Let's do a test of similar boats with identical hull shape and compare across a number of propellers. The boat that I would recommend is the Catalina 30 which had a production run of over 7000 and of which a significant number have been converted to electric. There may be some differences in propeller angle or slight differences in individual boat along with the impact of hull bottom condition and the specific of water in which the boat is located. The boat has a fairly modern hull and well proven and well understood. This would be the best we can do without a true scientific study with multiple propeller configuration in a labratory tank.

To this effort, I will ask our 15 or so Catalina 30 conversion clients to participate. I would also ask anyone else who has converted their Catalina 30 to do the same. I will create a data sheet that will be completed which will give us basic facts and also some antidotal information by each participant. I would hope to see propeller that mimic the propellers that we see in our community. As we all understand what the sailing character of that well know boat is, we can compared it to the individual boat that we own or are converting.

I would like to see the marine propulsion builders participate in this. I know there are very knowledgeable sailors and engineers on this forum who can then participate and comment on the data. If the data sample is not enough to make a valid analysis or if the data is sperious, I believe we will at least have additional antidotal information. It might spur on research with a marine engineer within a controlled test environment.

Note from our engineer:

For background, I primarily rely on Dave Gerr's "The Propeller Book" for making propeller calculations and recommendations. Dave is a respected naval architect and his book is used by many. I have also waded through a reasonable amount of more scholarly (PHD type) work on propellers and feel I have a reasonable grasp of the theory and practice of sizing propellers. I am a degreed engineer and capable of understanding higher math and theoretical concepts, at least at an undergraduate level.

Here is what James gets right:

1. Diameter is the number one consideration in optimizing for efficiency.
2. Reducing shaft angle will increase efficiency.

Here is where he is off base:

1.   Maximizing propeller area will increase efficiency.
It will tend to increase static thrust and provide better maneuvering performance. However, this is not the same as efficiency under way. The optimum propeller for efficiency will have blades only wide enough to absorb the power of the prime mover while maintaining reasonable face pressure, that is to say at a level below cavitation. Making them larger beyond this point only increases propeller drag. Again, it does improve maneuvering thrust, so there is a trade-off, based on your goals. With a heavier boat, you probably want to go with a 3 blade propeller, though do not need to get crazy with a high blade area one.

2. Going to a higher pitch reduces slip and increases efficiency.
First off, this is completely untrue. Slip is a ratio between the theoretical advance of a propeller through a solid and the actual through water. If diameter is held constant, the slip will be more or less the same with different pitch propellers at any given boat speed. The math is pretty simple. Also, slip and efficiency are not the same, though they are related. Also, slip changes with speed, which he does not seem to acknowledge. According to the experts, there is an optimum pitch/diameter ratio for any given boat speed. For sailboat speeds, 0.6 to 0.8 is generally considered to be the sweet spot. For higher speed vessels, higher ratios are in order. James has been making the claim that going to very high pitch increases efficiency for years, yet has never offered any data or analysis to support it.

3.                 3. Higher speed propellers create a wider wash cone.
I have never observed this in any kind of controlled situation and have never read anything that makes such a simplistic claim. It is more complicated than this, though the concept of a narrow "wash cone" being more efficient is essentially true.

4.                 4. Diesel propellers are very inefficient and the ones he installs are 50% more efficient.
This can only be true in cases where the existing propeller is, indeed, an very inefficient one. Yes, this is sometimes the case. The most common case is the propellers used on Atomic 4's. There are intentionally undersized and inefficient to allow the engine to turn up to a higher speed. It is a compromise dictated by the design of that engine and transmission. We always strongly encourage customers to change an A4 propeller to something better. Same in cases where the diesel propeller is undersize, or has a really low or high pitch. In most cases, the propeller installed is already as large as the space will allow. Also, they are usually ok on pitch. In most cases, there is little to be gained by changing propellers. We evaluate on a case by case basis.

In summary, though James is partly right, he is also partly wrong. If I were designing a new boat for electric propulsion, I would maximize prop diameter and make the shaft angle as shallow as possible. Look at a trawler. Of course, these work against each other, so it all ends up being a trade-off. I might do some other things with pitch and blade area, but this is more at the margins.

I hope this helps.

Happy Holidays!