Propellers
Originally created on November 02 2007
Borrowing again from David Tether’s E-Motion website and to not try to re-invent the wheel (he was partially responsible for bringing the electric wheel to the marine propulsion world), the following reading is interesting. I also recommend reading ave Gerr’s Propeller Handbook, The Complete Reference for Choosing, Installing and Understanding Boat Propellers available at International Marine.
“E motion Hybrid motors are built to turn big propellers. We install 3-bladed props with our systems, and they are generally one-two inches larger in diameter and pitch than those turned by a diesel engine sized for the same boat.A big, slow-turning, 3-bladed prop pushes more water more efficiently than a small, fast-turning one. There’s less slippage and less thrust lost from water spinning off the propeller tips. For the same reasons, a big prop regenerates electricity more efficiently, producing more electrical power under sail to recharge the batteries.
A large propeller also provides superior low-speed maneuverability. Especially when docking, a big prop pushing a lot of water provides immediate response – including instantaneous reverse – at any speed from near zero rpm to our motors’ maximum of 1000 rpm.
What about the drag?
Sailors spend a lot of time and money trying to squeeze the last 1/8th knot out of their boats. Many people don’t understand how we can justify adding something that apparently increases drag.
But there are several ways to minimize drag with our system:
1. With a fixed blade prop, keep the motor running at a very low speed so it turns slowly while sailing. This is the recommended option for regeneration. The prop essentially corkscrews through the water and minimizes drag, while consuming only a small amount of electricity. When the wind picks up or the boat slides down a wave, the prop is forced to spin faster by the water rushing by it. The prop turns the motor rotor and produces electricity to recharge the batteries. (See Electric Sailing.)
2. Install a feathering prop. The blades automatically align themselves parallel to the water flow and greatly reduce prop drag. Feathering props are somewhat less efficient than fixed-blade props for both propulsion and regeneration and considerably more expensive, but they will cut drag to a minimum. Regeneration is still possible by switching the motor briefly into reverse to lock the blades open. The prop will then rotate under sail, turn the motor rotor and produce electricity to recharge the batteries.
3. Install our motor with a retractable outboard drive leg such as the Sillette Sonic, if the boat configuration allows. The big advantage is that the prop can simply be swung out of the water under sail, and prop drag is eliminated. But there also are a couple of disadvantages. Regeneration is reduced because of resistance in the drive leg’s bevel gears and U-joint. And the largest allowable prop diameter in the manufacturer’s current line is 16 inches, which is smaller than the 18-22 inch props used with our 9 kw and 16 kw motors. The smaller diameter can be compensated for to some extent by increasing the pitch, for example, by using a 16 x 20 prop. Drive legs that accept larger props can be custom-built to order, but at greater expense.
4. Install a folding prop. Probably the most common form of prop drag reducer, the folding prop is the least desirable for our system because it eliminates regeneration. The blades simply fold back as they’re designed to do when water flows by them under sail. There’s no way to lock them open as can be done with a feathering prop. But in certain circumstances a folding prop may make sense. For example, a racing skipper uninterested in regeneration or efficient motoring could choose a simple two-blade folding prop to minimize drag.”
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