T-Pain Is On A Boat: Re: [Electric Boats] Props for EP

I have been watching the forum conversation on propellers and thought I should contribute my experience to the conversation. These comments are largely made from observation from the many boats that I have converted to electric power over the years. We keep tabs on how efficient each boat is and look at the record holders for tips on how to make all boats more efficient. The overwhelming observation is that larger propellers make more thrust and are vastly more efficient than smaller propellers. We routinely convert diesels with electrics 1/3 of the horsepower and get better performance. Many of these gains are made through swapping the propeller and I think if you truly want range out of your boat, you need to use a more efficient propeller. This is why we include a propeller with everyone of our Electroprop Propulsion Systems except Racers which require special consideration. One of these days we will get to play in a test tank so we can make propellers even more efficient for Electrics, but for now we need to use propellers from much larger diesels to complement our electric systems.

PROPELLERS

PITCH x REVOLUTIONS PER MINUTE x % SLIP DETERMINE BOAT SPEED

Pitch is the theoretical distance the propeller would move through the water if there were no slip. Easiest to imagine if you think of the propeller as a screw going through wood. The screw advances into the wood a distance for every revolution. That distance is the pitch of the screw.

Increasing the pitch is necessary to slow the propeller down, which is necessary to increase the efficiency of the propeller, because a slower turning propeller has less slip.

Choose the highest pitch available. In order to do this we typically source propellers from much larger diesel engines, because propellers are not currently designed with electrics in mind.

Pitch also determines speed so it you don't have enough pitch or rpm then you wont have desired speed.

Boat Velocity = Pitch x 12 inches per foot / 6076 feet per nautical mile x # of revolutions per minute x 60 minutes per hour x percentage slip

If we look at Kapowai we find that Pitch is 14 inches, propeller rpm is 800 to give us boat speed of 6 knots. Plugging these numbers into this formula we can find out the slip of Kapowai's Propeller.

14 /12 / 6076 x 60 x 800 = 9.21 knots per hour theoretical speed divided by 6 knots actual speed and we have slip of 65%

Expressed as a constant we find Pitch x .658 x slip = boat speed

When a car goes down the road there is no slip of the wheels on the pavement. If you discounted the friction losses, you could consider the car's wheels to be 100 percent efficient because there is no loss.

When likened to a car, a propeller acts like a car on ice, snow or mud where the wheels are spinning. The more slippery the medium is, the less efficient the power transfer of the wheels to the road is.

A propeller gets its traction from a liquid rather than a solid and therefore their is slip. If water was a solid, then the propeller could be considered as 100 percent efficient less friction losses. A propeller must have slip to propel the boat because the prop wash must be going faster than the water in order for the propeller to create thrust.

The amount of thrust that a propeller makes is equal to the amount of water that the propeller moves as according to Newton's laws of motion. Force equals Mass x Acceleration The propeller can either accelerate a lot of water at a slower rate of acceleration or a little water at a higher rate of acceleration to get the same amount of thrust. Large, slower turning propellers are more efficient because they accelerate more water, and don't have to accelerate that water as quickly as smaller, faster turning propellers do.

DIAMETER, BLADE AREA RATIO AND CUPPING DETERMINE POWER

The combination of diameter, blade area ratio, shape and cupping determine how much power the propeller will make. After choosing the highest possible pitch so we can slow the propeller down, we now concentrate on making enough power. There is a limit to how big a propeller should be on a boat. We keep putting larger propellers on boats and notice that the larger the propeller is, the more efficient the propeller is, but sooner or later we will find that efficiency will start to decrease once a propeller reaches a certain size. I remember reading somewhere that the theoretical size of the most efficient propeller is 1/3 of the beam of a boat, but as you can imagine there will be few takers for such a gigantic propeller, and the efficiency gains at some point will start to drop off due to surface friction of the water across the propeller blades. Nevertheless, the large propellers that come standard with a diesel are extremely inefficient as compared to those that we are using on electrics because by comparison, a diesel's propeller is tiny.

DIAMETER

My prop guy tells me its all about diameter. In fact, he says to me diameter, diameter, diameter. But I think that its more a relationship between diameter and blade area ratio. Diameter is usually limited on diesel engineered boats. Most sailboats today suffer from high drivetrain angles which are even higher if the yacht designer chooses a large diameter propeller, so diameter is often limited by the clearance of the propeller from the hull. Choose the highest possible diameter without getting too close to the hull. The common consensus is 10% of propeller diameter as clearance from the hull, with an absolute minimum of an inch or so, but I think it is better to get 20 % of propeller diameter with a minimum of 2 inches of clearance from the hull. The more clearance from the hull you get, the less the boat will walk sideways in reverse.

BLADE AREA RATIO

Blade Area Ratio is the percentage of the circle that the propeller is using. Higher blade area ratio propellers have bigger blades. Larger blades can move more water, so higher blade area ratios have higher traction in the water.

SHAPE

I call the standard propellers flapper blades for the shape of them. Typically the tip of the standard propeller is at the for to aft center of the hub and the propeller blades are at a consistent pitch. The high efficiency propellers that Electroprop is using have the tip about 2 inches astern of the hub. This increases the tip clearance and also accentuates the formation of the vortex. If you look closely at these propellers you will see that the pitch changes from the hub to the tip of the blade.

CUPPING

The wing of a plane lifts a plane because the top surface of the wing is longer than the bottom surface of the wing. This creates a low pressure above the wing which creates lift and keeps an airplane flying. Cupping is very similar with the forward side of the propeller blade longer than the aft side of the propeller blade, causing a low pressure on the front of the propeller, which accentuates the thrust by creating a vacuum on the front of the propeller.

PROPELLER WASH CONE

Prop wash leaves a propeller in a cone shape. Most of the water is accelerated directly astern, but some of the water is affected by the centrifugal force created from the spinning of the propeller. The faster a propeller turns, the wider the cone will become. Very high efficiency propellers accelerate water in a tighter cone than a regular propeller which will have a larger cone. It is simple trigonometry to determine that if the water goes outward instead of backward, then the outward component of the force does not propel the boat forward which decreases propeller efficiency. You can notice this effect just by looking at the propeller wash behind your boat. It will likely be several feet wide, whereas the propeller is usually only a foot to 18 inches wide.

CREATING AND SUSTAINING THE VORTEX

A propeller creates a vortex. You can visualize a vortex as the water going down your drain or one of those pictures of a hurricane from outer space. The vortex on a propeller starts in vacuum ahead of the propeller. The vortex starts as far ahead of the propeller as is allowed by the design of the boat. Usually there is some interference in vortex formation, either from the keel, or the strut or the hull of the boat itself. In order to optimize formation of the vortex you need to clear out the obstructions. This is typically not possible, however, on Snow Lily, the boat builder used an extended shaft tube and the result was our most efficient propeller to date.

The vortex can also be obstructed by what is behind the propeller. If the rudder is right behind the propeller, it splits the vortex in two and the turbulence created by the vortex back-feeds the entire vortex robbing it of efficiency. Again we look at Snow Lily where the drivetrain was offset so the propeller wash largely went to one side of the rudder.

BIG SHIP DESIGN

There is real world proof of how to build efficient boat drive systems traversing our oceans every day on big ships. All we have to do is try to emulate ship design as much as we can. If we can get a little bit here, or there, it really can add up to huge efficiency savings and consequently increases in range. Large ships use horizontal drivetrains, very slow turning shafts, high pitch propeller blades and the high blade area ratios.

Horizontal drivetrains are more efficient because the force is directly astern which propels the boat. Boats with drivelines at a large angle lose much of their force to lifting the boat, rather than pushing the boat foreward.

There is a maximum theoretical size of a propeller of about 23 feet because the tip speed ends up so high which causes too much stress on the propeller and cavitation which can damage the propeller.

CAVITATION

Shake up a pop bottle then take off the lid and, well, you know what happens. The air that has dissolved in the soda pop comes out and it bubbles everywhere. Water is the same way. There are disolved gaseous molecules in all water. Its how fish breathe. If there is too much vacuum created by a propeller, then these dissolved gasses appear and can ignite at very high temperatures causing pitting of the propeller. Believe it or not, the temperatures experienced from cavitation can be as high as 700 degrees Farenheit. Cavitation occurs when the propeller is starved of water and the vacuum is so great that the air molecules are pulled out of the water.

ELECTRIC OPERATION

With an electric motor it takes very small amounts of energy to turn a propeller while the boat is sailing, in the order of 50 watts for an Electroprop - kind of like leaving a light on - so we advocate large propellers for efficiency and to leave the motor on to create zero thrust and eliminate the drag of the propeller. Large propellers lock your boat into the natural energy of the ocean and increase the overall efficiency and range of an electric boat.

CONCLUSION

Each boat is different so finding the absolute best propeller is a trial and error process.

Choose the largest pitch, largest diameter, highest blade area ratio propeller with cupping that you can fit onto your boat and you will see increases in efficiency and electric range. The limitation to how large you can go with your propeller is usually the current capacity of the motor and the physical limitations of the boat. Your existing propeller gives you a starting point to go from. If you notice that at full throttle, the motor draws its continuous current rating then you should stay at around the same size of propeller. If the boat can fit a larger propeller, you may have to change the gear ratio so your motor will be able to make the power by coming up to rpm without overheating.

There are so many variables that employing someone knowledgeable, with systems experience, is the usually the most cost effective and efficient solution.

Happy Sailing!

James

www.electroprop.com

James Lambden

The Electric Propeller Company

625C East Haley Street,

Santa Barbara, CA

93103

805 455 8444

jlambden: Skype

www.electroprop.com

james@electroprop.com

On Dec 20, 2015, at 7:20 AM, Mike biankablog@verizon.net [electricboats] <electricboats@yahoogroups.com> wrote:

As Myles mentioned cost is a big factor in experimenting with props. Some boats more than others. Much easier to do with an outboard or trailerable boat. But, on a 30 foot sailboat like mine it becomes a much expensive process. Right now the boat is on land for the winter so changing the prop would be pretty easy. Though I'd have to wait until spring to check the results. If it turns out to be a mistake and I want to go back to the original prop it would cost another several hundred dollar just to get the boat pulled by the boat yard. Which is why without seeing more definitive data I am reluctant to experiment with a new prop. Since my original prop is working well I remain in a "if ain't broke don't fix it " frame of mind. Still I would love to have an idea of what prop specs might improve things should it need to be replaced.

Capt. Mike
http://biankablog.blogspot.com

On Saturday, December 19, 2015 10:55 PM, "tkgc9@yahoo.com [electricboats]" <electricboats@yahoogroups.com> wrote:

Hello all. I have been lurking and digesting as much as possible.
I have a pearson p30 with an Electric Yacht 10 kw system.
Looking at Prop changes soon.
Thanking for all the valuable info