Propeller Drag

Here is a study on propeller drag that I found on Michigan Wheel's website. It's written to answer the question of locking the prop verses allowing the propeller to free wheel while under sail. If accurate, it answers the question of free wheeling the prop for those with twins, with a transmission that can be free wheeled, who occasionally run on one engine. It also alludes to a MIT study that came to the same conclusion? I would like to see that!!

http://www.catamaransite.com/propeller_drag_test.html


Propeller Drag Test Results

Jig Drag = 12 pounds with no prop mounted. Measured at WOT. (wide open throttle)

Locked Prop Drag: 45-50 pounds: 50 -12 = 38 pounds of actual propeller drag when locking the propeller in reverse under sail.

Freewheeling Prop Drag: 20-25 pounds: 25 - 12 = 13 pounds of actual propeller drag when freewheeling the propeller under sail.

The locked propeller drag is 2.92 times more drag or a 292% increase in drag over a freewheeling propeller when you remove the test jig from the equation!!!!

As I said earlier, the test results are not even close. There's no need to worry about the .001's or a few pounds of drag here or there or even the rather "unscientific" method I used in this test. If a sailor cares to find an additional knot of boat speed in light winds, he must free wheel his propeller under sail. (note: I picked an average wind day with flat water and tried my props both locked and free wheeling while under sail. I have two props on my catamaran, and Main Sail's test was verified on my boat (it wasn't even close) --Rick)

Despite the Yamar advisory, which voids their warrantee if you lock their transmissions while sailing and despite Volvo's recommendation that you sail while their transmission are in neutral and despite the MIT study, the University of Strathclyde Ocean Engineering white paper, the Yachting Monthly data and this webapge, which all basically show the same thing, that a fixed prop caused more drag, there are still a few die hard types arguing the helicopter blade theory.


I have never seen as much misinformation on a sailing subject as we get on this one. I recommend you read this page carefully and that we all get this right. Unless a transmission manual specifically says to shift into reverse, all sailboat owners should be free wheeling their propellers while sailing. On second thought, if they ignore this webpage, it will be great for our economy as the diehards will have to buy new transmissions!

Most newer boats have water injected stuffing boxes with water injected from the engine cooling water. If you have a turning shaft without injected water you might burn the shaft seals which will be a lot more expensive than saving some fuel cost.

"If you have a turning shaft without injected water you might burn the shaft seals which will be a lot more expensive than saving some fuel cost. "

After several hours of free wheeling, I checked the temperature of my stuffing box and found it was warmer than the outside water, but far cooler than the ambient engine room temp (one engine working, one off). I couldn't get more than 6 knots of boat speed, so at what became my maximum speed, I don't see any concern that I might burn something by failing to lubricate the stuffing box from the engine cooling water (off at the time). My fuel savings came from going 6 knots instead of 8. I don't believe pushing my boat from one corner is as efficient as from both corners, especially when you add prop drag from the free-wheeling prop.When hauled out, I have tried to turn the props, trans in neutral, and have never been able to do so without extreme exertion. While traveling at 6 knots, the prop is turning, so there is more energy used to get that prop turning and thus more fuel consumption.
YMMV

koliver said:

"When hauled out, I have tried to turn the props, trans in neutral, and have never been able to do so without extreme exertion.
YMMV

Click to expand...

I wonder why all the resistance on you shafts?? Mine turn easily, I mean two fingers easy. Borg Warner Velvet Drives, 1 3/8" shafts, wet shaft glands with Teflon graphite packing.

I don't do it for fuel savings, only for fishing, slow cruising or running with a slower boat. 7.5 kts @ 2400 and 8.5 kts @ 2800 WOT on one engine.