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!
