Captain/Owner Sleeping at the Helm

[StuM]

Originally Posted by fivecapes

https://youtu.be/5i0WBv4ygNo?t=80

Free wheeling all the way down?

In what way does that have anything at all to do with comparative drag between locked and free-wheeling boat propellers?

[fivecapes]

Nothing...just showing that free flight plane props free wheel when the rubber is spent vs fixed, ie the opposite of what some are advocating here.

[HeinSdL]

Originally Posted by StuM

Forget about the "swirly water" for a minute. (Whether a fixed prop or a free-wheeling prop causes more "swirly water"
is subject to debate. I haven't seen any conclusive research either way.

There are many techniques, practical and theoretical, which prove spinning prop = less drag than fixed prop. I find the swirling water thought process one of the more eloquent theoretical techniques. A fixed prop will leave a rotating body of water in the wake of the boat. This requires energy which is taken out of the boat speed. Subsequently unlocking the prop will mean the prop starts to rotate reducing the amount of kinetic energy left in the wake (the prop slices through the water with less effort) and the boat speeds up. I know in this particular reply I'm preaching to the converted but I was really itching to make that point!

[Wotname]

Originally Posted by waterman46

Not quite. A stationary object has zero dynamic energy. The prop could exert a huge torque, but if the shaft is locked, no energy is created or stored by that force. That torque can have zero retarding effect on the boat.
.......

StuM has already given you a good theoretical explanation of where you went wrong but here is a simple pictorial explanation which refutes your claim.

But first, let's recap energy - you are correct in saying "A stationary object has zero dynamic energy". However a stationary object can have stored potential energy. In the case of the locked prop on the boat, the action of locking it transfers kinetic (or dynamic) energy into stored potential energy.

Consider the diagram of a floating sail boat alongside a jetty.

Lock the prop shaft inside the boat.

Now attach a rigid lever to the prop blades as shown.

Step off the jetty onto the end of the lever.

Clearly the boat will heel somewhat and then settle into a new position without any further movement. At this point it has no kinetic energy but it does have stored potential energy. To prove the energy is stored, step back onto the jetty and the stored energy is released (as kinetic energy) as the boat returns to it's original position.

You will see that when applying a force to the prop blade, the energy is now stored in the new position of the hull.

It doesn't matter if the force applied to the prop blades is your body weight or the pressure of moving water.

Locking the prop requires a force to hold it stationary and the only available energy is the kinetic energy of the boat. Remove some of the kinetic energy from the boat means there is less to move the boat i.e. it slows down.

Surely that is now clear (unless you want to argue against fundamental physics and the conservation of energy and I'm sure you don't)!

[Wotname]

Originally Posted by gah949

.....
Sailboats without folding props will have a shorter gearbox life and fixed props spinning constantly cause a huge drag and reduce the speed of the boat.
...........

Good to a see lurking member making a first post .

However just to clear, if the fixed (boat) prop is forced to be stationary (i.e. locked), the drag will be much greater and the boat will slow down more than if it was left to spin freely. Physics tells so and so does real life experience.

[Wotname]

Originally Posted by Skipper John

I'm not sure what Yanmar's reasoning is for not having the gear in reverse when sailing. ........

I believe it only applies to transmissions which are fitted with cone clutches (but I could be wrong).

AFAIK, unlike a plate (or disc) clutch a cone clutch requires a higher torque at the input end compared to the output engine to work properly. If the transmission is engaged whilst sailing and the engine is off, the torque provided by the prop (trying to rotate) is higher than the input end. Put another way, the cone clutch is now being driven by the wrong end and long term damage will occur.

But don't quote me, I may be off base!

[Wotname]

Originally Posted by aeronautic1

............
Treat them no different than know-it-all internet posters? It’s a known FACT that a free-wheeling spinning AIRPLANE propeller creates more DRAG than a stationary AIRPLANE propeller. Ask any pilot or flight instructor.

There you go, I fixed it for you!

All good pilots and flight instructors know about airplane props and drag but you apparently don't know much about boat props nor have you read any of the references upthread otherwise you would realise your statement was incorrect.

Read this one http://www.plaisance-pratique.com/IM..._sdarticle.pdf

If that is TL;DR, here is part of the conclusion (shamelessly stolen from StuM's post 121) The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds).

If you disagree with the paper, please do tell us why but please don't reference anything from your flight instructor unless (s)he is knowledgeable about sail boat props


Props ain't props.

[wingssail]

Originally Posted by capn_billl

Folding propellers do not auto rotate.

That reminds me, I need to shop for folding props.

Our Martec 2-blade folding propellers (we have had three of them over 34 years) definitely do autorotate if left in neutral after have been used to power the boat in forward.

We have always moved the shift lever to reverse to force the blades to fold and stop spinning.

[StuM]

Originally Posted by wingssail

Our Martec 2-blade folding propellers (we have had three of them over 34 years) definitely do autorotate if left in neutral after have been used to power the boat in forward.

We have always moved the shift lever to reverse to force the blades to fold and stop spinning.

I suspect he meant:
Folding propellers do not autorotate when folded.
or
Folded propellers do not autorotate

[waterman46]

Originally Posted by StuM

To prevent the prop from rotating requires a force. As we should all know, for a body in equilibrium every force requires an equal and opposite force. In this case, that equal and opposite force rotates the hull away from it's natural angle of inclination. The locked prop is in effect having to work against gravity.
Of course a rotating prop stores energy. Every rotating object possesses kinetic energy. But that has absolutely nothng to do with comparative drag of a rotating object in a medium such as air or water.


The autorotation argument has been debunked multiple times. It's an entirely different issue which relies on changing the pitch of the blades to generate maximum allowable rotational speed from the movement of air upwards through the rotors when descending and then changing pitch when close to the ground to convert that speed to lift.
That's a total non sequitur. A stalled airplane wing has a tremendous amount of drag. If you'd ever stalled an aircraft you'd know that. (It's done all the time in initial flight training and in aerobatics)

You really should take the time to read some of the technical literature and empirical research findings previously quoted rather than relying on misapplication of half understood concepts which have led you to the wrong conclusion.

1) The point of autorotation was mentioned to show that really significant energy is stored in a prop dragged through the medium of air or water. Enough to nearly stop a descending helicopter with no engine. That energy continues to be constantly created, by dragging a rotating prop. That energy must be dissipated somehow. Not you, but some other posters seem to have missed this point.

2) The drag of a stalled wing that you are referring to, is when the stall occurs with still significant forward motion. I understand stall. I piloted a hang-glider and have some experience. But that's not my point at all. My use of "stalled" is misleading if not taken in context. What I really meant was the condition of zero forward velocity, only a vertical flow, airplane sinking directly downward, not the classic aircraft wing stall with forward velocity and tremendous drag that you may have assumed I meant.

We don't really know how well a boat propeller compares to an airplane wing, but I think the concept of lift with forward airflow vs. no lift if no forward airflow of any wing, can be applied in some way to a propeller blade being dragged through the water.

3) Torque has zero effect in the direction of boat travel, by itself. You say it may cause some loss of boat speed since it forces the boat off her lines a bit. Could even cause boat to heel less on one tack, more on the other, balancing out to no real effect.

4) I did my research before commenting. Most of the literature on this subject can only be applied indirectly to our situation. It's mostly concerned with aircraft propellers, and we have to be careful because they mostly deal with adjustable pitch props that are directly coupled to the engine.

Perhaps someone could actually run a test. Only if you have a fixed prop, not folding or feathering. In calm conditions, flat water, get up to a set speed under power, stabilize speed (through water speed, not GPS) carefully, note the speed, then shut down the engine, leaving transmission in forward or reverse. Check prop not rotating. Note time it takes for boat speed to reduce to a fixed value, say 2 or 3 knots. Do the same but put transmission in neutral immediately after shutting down engine. Check for prop rotation. Compare time it takes for boat to reduce its speed to same slower value as in the first case. Any takers?

[Chotu]

Can everyone on this thread just go out on their boats this weekend and do the following?

1) sail at a few knots boat speed in neutral

2) slide it into gear

2a) reverse 1) and 2) if nervous about transmission damage.

Note speed log when doing so.

Report back on Monday with your findings.

This thread is heavy on theory and conjecture. Very light on actual data/experience.

[PineyWoodsPete]

Originally Posted by waterman46

1) The point of autorotation was mentioned to show that really significant energy is stored in a prop dragged through the medium of air or water. Enough to nearly stop a descending helicopter with no engine. That energy continues to be constantly created, by dragging a rotating prop. That energy must be dissipated somehow. Not you, but some other posters seem to have missed this point.

2) The drag of a stalled wing that you are referring to, is when the stall occurs with still significant forward motion. I understand stall. I piloted a hang-glider and have some experience. But that's not my point at all. My use of "stalled" is misleading if not taken in context. What I really meant was the condition of zero forward velocity, only a vertical flow, airplane sinking directly downward, not the classic aircraft wing stall with forward velocity and tremendous drag that you may have assumed I meant.

We don't really know how well a boat propeller compares to an airplane wing, but I think the concept of lift with forward airflow vs. no lift if no forward airflow of any wing, can be applied in some way to a propeller blade being dragged through the water.

3) Torque has zero effect in the direction of boat travel, by itself. You say it may cause some loss of boat speed since it forces the boat off her lines a bit. Could even cause boat to heel less on one tack, more on the other, balancing out to no real effect.

4) I did my research before commenting. Most of the literature on this subject can only be applied indirectly to our situation. It's mostly concerned with aircraft propellers, and we have to be careful because they mostly deal with adjustable pitch props that are directly coupled to the engine.

Perhaps someone could actually run a test. Only if you have a fixed prop, not folding or feathering. In calm conditions, flat water, get up to a set speed under power, stabilize speed (through water speed, not GPS) carefully, note the speed, then shut down the engine, leaving transmission in forward or reverse. Check prop not rotating. Note time it takes for boat speed to reduce to a fixed value, say 2 or 3 knots. Do the same but put transmission in neutral immediately after shutting down engine. Check for prop rotation. Compare time it takes for boat to reduce its speed to same slower value as in the first case. Any takers?

As I suggested before, but apparently no takers, all you need is a towed dink with a small OB with un-tilted engine lower unit in water. Start with shift lever in Neutral, prop free-wheeling, and you'll see that that at 5kts of so towed, the lower unit doesn't kick back due to prop drag.

But when put in Fwd, which stops prop due to engine compression, etc, the engine kicks up. Just as the transom-hung 3HP OB on my O'Day 20 50 years ago did under sail...Who needs a test tank?!

[curtis742]

I'm calling BS on the yanmar bulletin. If none of the parts are moving there is no wear. If any transmission in any boat can be damaged by water pressure on a prop in a sailboat it is not worth selling.

[PineyWoodsPete]

Originally Posted by curtis742

I'm calling BS on the yanmar bulletin. If none of the parts are moving there is no wear. If any transmission in any boat can be damaged by water pressure on a prop in a sailboat it is not worth selling.

Has anyone experienced the engine actually being turned over with fixed prop stopped by to leaving engine in gear?

We were sailing out P-F 37' cat at 8-9kn off Costa Rica years ago, with oversized 16" 3-bladed MaxProps (re-pitched for full RPM's) on the 18 HP Yanmars, tranny in reverse to feather, when we hit a short patch of very rough water at 8-9kts. There was a sudden !rump-rump-rump-rump! sound and vibration from the stbd side aft - just like a diesel turning over against compression. The boat slowed a bit and the sound stopped. Deep water, saw nothing in our wake, and I theorized that the prop suddenly "de-feathered" and turned the diesel over - even against the backward step-up of the 2.7:1 reduction gear to 1:2.7. We de-powered sails, slowed way down, started the engine, stopped it, back to Rev and carried on. Saw nothing unusual diving on it later...

[barnakiel]

Roughly 0.4 kts of speed difference in our boat. A simple 3-blade fixed prop.


The problem is that in light winds a big percentage of speed is consumed. Once we are up to speed, the percentage is smaller but the noise is bigger.


There are only two ways outa here: 1) no prop or, 2) a folding fathering one.



b.