Locked Prop vs. Freewheel vs. Boat Speed
- Thread starter Maine Sail
- Start date
The vortex (zone of turbulence) that follows all foils that are moving at any attack angle other than zero.
Let me help your intuition.
In fluid dynamics there are two components of friction:
1. shape or profile drag - caused by the cross sectional shape of the object - think of pulling a silloutte perpendicularly through the water. Doesnt matter if the silloutte is spinning or not.
2. Parasitic drag or skin friction - the friction due to the AMOUNT of fluid that is in contact with the surface of the object. Causes 'work' in the opposite direction of the object. The surface of the spinning prop comes in contact to MORE water than a non-spinning prop; hence, higher cumulative friction ... more 'work'.
Since a spinning prop has more (surface) contact with a viscous fluid, it does more work and that 'vector' is in opposition to the direction of travel .... slows the boat (or Ross' autogyro) down.
The autogyro doesnt crash because of the spinning blades coming in contact with a large mass of air, which slow the descent of the autogyro ... and develops lift in the opposite direction that the autogyro is going. Just like the autogyro, the spinning propeller does more work (than a non-spinning prop) due to parasitic drag ... and creates lift in the opposite direction of the boats travel .... spinning prop or autogyro - the same effect.
Spinning props slow a boat more than a non-spinning prop. If you want to 'feel' the amount of work being done ... just measure the temperature of the spinning bearings and lubricating oil - the temperature difference is the result work!!!!
Free wheeling
I too read that about the props in Boat US this month. Not sure I agree with it. I really can't tell any measurable difference with mine. ( 3 Blade ) I normally put it in reverse, as transmission wear is a bigger concern than any slight gain in boat speed.
I too read that about the props in Boat US this month. Not sure I agree with it. I really can't tell any measurable difference with mine. ( 3 Blade ) I normally put it in reverse, as transmission wear is a bigger concern than any slight gain in boat speed.
Yes, less cross sectional or less 'profile drag' because its not fully exposed to the slipstream ... the turbulence zone of the 'deadwood' is being 'pulled along' with the boat; hence, less parasitic drag on the prop if aligned vertically.
Ditto Tom's you have to have it in gear to get the folding prop to close.
Ditto Roger that the spinning prop/shaft/tranny can leave the moving tranny gears without the lube they have when being engine-driven. Each tranny maker should be the final word.
Ditto Roger that the spinning prop/shaft/tranny can leave the moving tranny gears without the lube they have when being engine-driven. Each tranny maker should be the final word.
I let it free wheel. The manufacturer indicates the gears and bearings get adequate lubrication. I don't want to submit a gear to stress forces induced by a locked propeller being pushed by the water. A mechanical lock on the axle would be best but I would not care to install one. I believe a freewheeling propeller creates less drag than a locked one but to me would be an unimportant consideration.
spinning prop
This is very confusing. Why would there be less drag on a fixed prop than on a
spinning prop? A system left alone wants to be in a state of minimum energy.
If it takes less force to drag a fixed prop then it would not want to spin.
Nonetheless, I usually put the tranny in reverse because I was TOLD it provides
better lubrication. Now I'm confused again. What difference does it make whether
it's in forward or reverse? Either way nothing turns inside the transmission (at least for my Yanmar 1GM).
Keeping it in neutral, on the other hand, makes the prop turn things inside at rpms vastly different from driving it with the engine. I can imagine that this might
compromise proper lubrication.
This is very confusing. Why would there be less drag on a fixed prop than on a
spinning prop? A system left alone wants to be in a state of minimum energy.
If it takes less force to drag a fixed prop then it would not want to spin.
Nonetheless, I usually put the tranny in reverse because I was TOLD it provides
better lubrication. Now I'm confused again. What difference does it make whether
it's in forward or reverse? Either way nothing turns inside the transmission (at least for my Yanmar 1GM).
Keeping it in neutral, on the other hand, makes the prop turn things inside at rpms vastly different from driving it with the engine. I can imagine that this might
compromise proper lubrication.
Your transmission, typically of most marine transmissions, doesn't shift gears like a car. Instead forward and reverse are permanantly engaged by gears and there are two clutches. These are simple cone clutches in small transmissions. You push the cones for forward together and the thrust of the prop then helps keep them engaged. Same for reverse.
If you put the engine in forward to stop the prop, only the linkage is resisting the drag of the prop. If it gets slightly out of adjustment, the clutch could slip and wear, depriving you prematurely of your most important gear. In reverse, the drag of the prop is holding the clutch engaged. You are also putting that stress on the least important and least used of the two clutches.
Larger gears tend to have different clutch arrangements and more powerful engaging mechanisms so it may not be so important which is used for locking.
Don Macnamara in "White Sails - Black Clouds" (great old book if you can find it) describes surfing his 50 or so foot aluminum "Tara" down a wave in a storm under bare poles and hearing a roar and vibration that didn't stop when the wave went by. Turns out that the dragging prop had started the diesel!
At just the right point of sail, a close reach, I can feel my wheel bucking and vibrating slightly as the vortex from the keel hits the rudder. Close hauled, there is enough leeway that the vortex misses. Farther off the wind, the vortex is not strong enough to be felt.
At just the right point of sail, a close reach, I can feel my wheel bucking and vibrating slightly as the vortex from the keel hits the rudder. Close hauled, there is enough leeway that the vortex misses. Farther off the wind, the vortex is not strong enough to be felt.
But there is work being done to prevent the prop from spinning too.
More than once we've headed out of an anchorage in an early morning rush with the dinghy motor down and had to stop clamber into the dink and raise it. I have done this with it in gear and the motor actually begins to lift out of the water on it's pivot point! When I have done the same thing with it in neutral it does NOT lift out of the water. This seems to tell me less drag resistence in neutral?
That wouldn't surprise me with an outboard prop. There are circumstances where freewheeling produces less drag. With common prop configurations in speed regimes close to their design propulsion speed, drag will be less fixed in most configurations. Unless you have a rigerous way of measuring, odds are on your side securing the prop and you'll be putting less wear and tear on the machinery.
If I remember correctly the paper I read 10 - 15 years ago, freewheeling gains are also only likely to be realized with props that are very freewheeling. Outboards in neutral spin pretty easily, I've seen them turning on trailered boats going down the highway. Your inboard with stern bearing, shaft seal, transmission seals, oil viscosity in the slack clutches, etc. takes a lot more energy to turn the prop.
Obviously the two must produce close to the same drag, bacause there is 20,000 years of collective sailing wisdom here and no one can decide? Hasn't anyone just been sailing along and then lock the prop just to see the difference?
Yes on all my fin keeled boats, with fixed props usually three blade, they have always sped up. This is not however an accurate method of testing as wind, current and other factors change. Usually when one is trying to eek out that last bit of speed they are also tweaking the sails etc. so this can not be relied upon as an absolute or accurate test because the measurable parameters are not fixed and are in a constant state of change with many contributing factors.
On our old full keeler, when I was much younger, we locked it behind the keel because it was a two blade prop. Of course a two blade on a full keeler she did not do much when powering and with a full keel she didn't have much speed when in light air anyway...
I occasionally lock my prop but if I want to go faster I unlock it.. My prop on the CS spins quite easily with a PSS seal & a two year old cutlass and will spin beginning well before two knots of speed through water. I will be measuring the exact "start spinning speed". On my C-36 & C-310 it began spinning at closer to 1.5 knots. I've actually had my prop spinning on the 310 while at a dock in Portsmouth NH with about a 2 knot +/- current.. I agree that some boats have more resistance than others and that friction resistance would cause more drag.
So if resistance/friction has an effect on boats speed wouldn't infinite, no prop movement, be the most resistance? How do you move up the curve of friction resistance, starting at very freewheeling being more efficient, to the point of most friction, right before it stops spinning being the least efficient but then when it stops spinning it goes back to the least drag?
I bet that rigging a prop to hang off the stern on a shaft and measuring the drag on that assembly locked and free spinning will be the easiest way to answer this question. That way the props could be changed for different pitch and blade configurations.
Roger, on my boat when on the hard I can hand turn the propeller if I hold the blade tips but if I hold the shaft I can't apply enough torque to make it turn.
Roger, on my boat when on the hard I can hand turn the propeller if I hold the blade tips but if I hold the shaft I can't apply enough torque to make it turn.
Changing angle of attack, conservation of energy, and other complex aspects of lift and the rotating foil that a prop is. The performance if aircraft props is directly applicable because their area and pitch are adjusted to match the differences in speed and fluid density. Aircraft with fixed pitch props do glide farther if the prop is stopped. However, it isn't done in practice because the pitching up of the aircraft to slow it enough to stop the prop after an engine failure loses more altitude and distance than the drag reduction gains.
If you really want to know, put a heavy weight spring scale into a tow line and have some one tow you on a calm day and clutch in and out of gear. You might find that your boat is in one of those narrow regimes where resistance goes down when you clutch to neutral but I would be against it. You'll probably find that there is only a narrow range of speeds where resistance goes down.
But when you are out of the water you have no water lubricating your cutlass or stuffing box. Mine is harder to turn on the hard too but still not difficult and I can spin the shaft by hand..