In Gear or Out

It seems there is a great amount of mixed opinion about whether to "fix" the propeler or to leave it spin freely when sailing. as yet i have found no literiture on the subject of drag caused by these practices and wondered if any body could point me in the direction of some?

 

Its covered in an old issue of wooden boat.

Basically IF you can have the prop freewheel AT its top speed (requires ball bearings in stern bearing box , loose packing or dangerous ceramic seal) its not too bad.

Any reduction in the free prop speed will create more drag than the locked prop.

The best for the boats that can, is a locked 2 blade prop , behind the deadwood if you can.

FF

 

From Dave Gerr's "Propeller Handbook":

 

If you are a sailor get a feathering prop. If you are a motor sailor run the engine.

Rick W

 

trouble is a MAXPROP here is 8000, , was for Nimbus,(
I did calcs, and unless youa re doing massive miles, the time saved is so little, BUT the noise of a spinning shaft will drive one insane, , so if you are building, build a shaft brake

 

Here is the article from Wooden Boat. You will need to open the attached file to see the graph.

Sorry I don't know how to post it properly.

WHAT ABOUT PROPELLER DRAG?

Does propeller drag actually produce appreciable negative effects-how does it relate to the power developed by the sails? As an example, the specific thrust factor for a 10-knot wind ranges from 0.4 to 0.7 pounds per square foot of sail area, depending on the point of sail with beam reaching usually being the most efficient and beating the least efficient. Under reaching conditions, a boat with 700 square feet of sail and a thrust factor of 0.6 would be developing 420 pounds of effective thrust. A fixed three blade prop exerting 80 pounds of drag would drain off about 20% of this sail power. On a weather leg, the sail thrust would be 280 pounds and the 80 pounds of propeller drag would dissipate 30% of the sail power.

One of my early cruising boats was a 26' Sea Bird yawl equipped with a very tired 25-hp gasoline engine turning a 13" three-bladed propeller located in a large aperture in the rudder. Part of the reason' for the large aperture was a stern gland at the end of a tube that projected 4" beyond the stern post. She was a fine little vessel but was slow in coming about and generally a bit heavy-gaited. To improve performance, I installed a shorter stern gland and filled in almost half the rudder opening. The big, fat-bladed propeller was replaced with a 12", two-bladed propeller, 3" in width, that could be totally hidden behind the deadwood. The effect was astonishing. Not only did the sluggishness disappear but the boat sailed at least a knot faster. And performance under power was also surprisingly improved; the large prop had been lugging down the tired engine so that efficient power could not be developed.

But the old engine was a big headache, so I pulled it out and replaced it with a 5-hp British Seagull outboard mounted on a sturdy stern bracket attached to the boomkin. The prop and shaft were removed, the rudder aperture completely filled in, and with the relatively heavy engine gone, the boat floated on better lines rather than her previous stern squat. Again, the effect was astonishing. She was even more alive, faster and a pure joy to sail. And the little outboard did its job just fine for narrow channel powering and tight docking situations.

An interesting area of discussion is the practice of allowing a solid propeller to free-wheel in order to minimize drag. Some sort of clutch arrangement is usually necessary to disengage the shaft from the gearbox and engine to prevent damage to the transmission. But it seems that this matter of whether a rotating or stationary propeller provides less drag is not a simple yes or no and depends on a number of factors.

Under sail, if the prop is allowed to freely rotate, a condition can theoretically be reached, dependent on hull shape, boat speed, and propeller characteristics, where there should be no drag. This condition is given by the following formula:
Edward L. Delmar-Morgan reports experiments where the maximum drag occurs at about 38% of the rpm for no drag. The drag for a stationary, non-turning prop is about 83% of this maxi-mum drag figure. The accompanying curve illustrates the results of applying this drag formula to actual tests on Delmar-Morgan's 33' LAURA sailing at a speed of five knots and carrying a three-bladed propeller 15" in diameter with 12" of pitch.

Nigel Warren has reported on a series of tests on the drag of a two-bladed propeller 12" in diameter with an 8" pitch trailed at various shaft speeds. At a water flow ~peed of six knots, a locked shaft condition gave 20 pounds of drag. A free-wheeling condition at 200 rpm produced a maximum drag of 60 pounds, then decreased to 15 pounds at 750 rpm. By varying the water flow speed at the prop, his tests also proved that maximum drag occurs when the prop is turning slowly and acts as an effective brake. This drag is dependent on four factors: (I) speed of the water into the prop; (2) ratio of pitch to diameter; (3) blade area; (4) frictional shaft torque (stuffing box, bearings, gearbox).
For most practical purposes, unless you want to get involved in lengthy experimental measurements on your own boat, it seems better to lock the propeller in a stationary position (preferably a two-blade hidden behind the deadwood or a feathering/variable-pitch installation). In order to have decreased drag characteristics with a free-wheeling prop, you would need to be sure that the boat is sailing at a speed where the propeller rotation is at least greater than about 60% of the rpm required for no drag.

There are some efficient folding propellers on the market that are practical for use with smaller engines. Variable-pitch props are a fine way to go, allowing pitch and rpm to be cranked in for various sea and wind conditions. Under sail the blades are feathered fore and aft for minimum drag. Although the adjustable-pitch propeller is expensive, the overall power installation is reasonably priced since it eliminates the need for a reduction gear, clutch arrangement, and reverse gear. Fuel costs should be lower because there would probably be more sailing and less powering with a minimum drag prop. It seems like a needless waste in performance for a cruising sailboat to drag around a too-large prop when there are other more serviceable alternatives.

 

FF
I said in half a line what took your article to say in a short story. A "sailor" would simply not tolerate a prop dragging. On the other hand kick-up outboards and shafts do not have the required thrust capability. Hence the only choice for a serious sailor is a feathering prop.

Rick

 

"Hence the only choice for a serious sailor is a feathering prop."

For a cruising boat the more efficient choice of a 2 blade prop, locked and shadowed by the deadwood isn't that bad.

We cruise such a boat , and can move the prop about 1/4 inch in either direction by hand at 6k.
Meaning the prop is only creating surface area drag , not form drag , or we sure wouldn't be able to move a 19 inch wheel by hand!

The simple skin area of the prop IS higher than a prop that folds flat , but a feathering prop would probably be about the same?

FF

 

Like I say Fred serious sailors would not tolerate a non-feathering prop.

Rick W

 

Rick,

I agree for racers but on a cruising boat of 18,000 lbs with perhaps 250 sq ft of immersed hull , do you think the resistance of 250.5 is going to be noticed ?

 

Fred
Depends on the relative Cds. If Cd of prop is 2 and the Cd of the hull is 0.01 then the prop is still a big component. No point in comparing the area. You have to compare drag.

Serious sailors sail with the prop feathered. Others motor sail with a fixed bladed prop. By my definition a sailing boat has a feathering prop and a motor sailor usually has a fixed bladed prop. Only my definition - not universal. If you have a sailing boat why would you want to lumber along with the brake on.

Rick W

 

"If Cd of prop is 2 and the Cd of the hull is 0.01 then the prop is still a big component."

However hidden behind the deadwood , and locked to the point where it has no rotational effort (can be moved a bit by hand) I cant imagine it has a high CD. Although it sure does when spinning!

Any racer would PANIC! on our boat,

100lb Herrishoff , 60lb Plow,60H Danforth, 45 Bruce , 200 ft of 3/8 chain ,
spare parts for Mr Volvo , and the tools to install them.

200G fresh water, sun shades , a circulating hot water heat system and all the rest , would be hard for the usual racer that emptys the toothpaste to only a race days worth.

FF

 

Fred
How often does the motor run on a typical cruise to help make way?

Rick W

 

"How often does the motor run on a typical cruise to help make way?"


WE do different types of cruising.

In summers we sail in New England , and due to the power requirements of cold plate refrigeration (ICE CREAM!) we move the boat on the third day in port , to the next port. Since ports are close the required 2 hours of engine time frequently gets us almost to the next port.

If we get a fine sailing day we buoy the anchors and day sail for a few hours with Zero engine time.

On a trip from New England to Florida , on the ICW ,its a 8 to 10 hour motor experience as bridges wont allow a sailing pass. The few large areas where sailing is do able we do, but its hardly every day.

In the Bahamas there are many day movements that allow the engine to be secured after 2 hours and sailed the rest of the day. The harbors are Very! tight so the engine is required to be safe anchoring 10ft from the next boat.

On passages , NY- Bermuda or Bermuda -St. Thomas we sail, but do have to run the engine every 3rd day , to keep up the reefer.

I think this is a fairly normal routine , although the folks with electric reefers have to operate the engine 300% more.

FF

 

Fred
I am impressed. You do actually "sail" that heavy weight. Have you got a photo of it under sail?

Rick