Captain/Owner Sleeping at the Helm

[StuM]

Originally Posted by fivecapes

Didn't anytbody read this???

Yes, and I even quoted from it back in Post #23 - to wit:
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).

[Jim Cate]

Originally Posted by fivecapes

Didn't anytbody read this???

for those unwilling to plow through this rather turgid treatise, here are the conclusions summarized at the end:

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). Furthermore, for the freewheeling case, the
magnitude of the hydrodynamic resistance is significantly
affected by the amount of frictional torque on the shaft,
low torque being accompanied by low drag.
Finally, a simple model of sailboat hull resistance has
been used to illustrate the likely scale of the drag penalty
due to various arrangements of trailing propeller. This
shows that, especially for the case of craft having moderate
or low DLR combined with powerful mechanical installations,
the impact on sailing performance of a trailing
propeller is very significant indeed. By combining the
present findings with other more detailed techniques which
exist for modelling hull drag, the influence of propeller
drag on sailing performance should be substantially
predictable



For those unwilling to read even this brief bit, it says conclusively that fixed props have lots more drag than freewheeling, and the more torque applied to the shaft of the rotating prop, the greater the drag, so having good bearings helps. BUT having a folder is way better yet! It also points out that prop drag becomes more of a factor in boats with lower DLR, ie lower displacement to length rations, ie more modern, lighter cruising designs.

This work seems like damn good science to me and I would hope that it would settle the amateur pundits with helicopter analogies and such arguments.


Jim

Edit: Seems like StuM types faster than I do!

[Uncle Bob]

Originally Posted by Jim Cate

for those unwilling to plow through this rather turgid treatise, here are the conclusions summarized at the end:

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). Furthermore, for the freewheeling case, the
magnitude of the hydrodynamic resistance is significantly
affected by the amount of frictional torque on the shaft,
low torque being accompanied by low drag.
Finally, a simple model of sailboat hull resistance has
been used to illustrate the likely scale of the drag penalty
due to various arrangements of trailing propeller. This
shows that, especially for the case of craft having moderate
or low DLR combined with powerful mechanical installations,
the impact on sailing performance of a trailing
propeller is very significant indeed. By combining the
present findings with other more detailed techniques which
exist for modelling hull drag, the influence of propeller
drag on sailing performance should be substantially
predictable



For those unwilling to read even this brief bit, it says conclusively that fixed props have lots more drag than freewheeling, and the more torque applied to the shaft of the rotating prop, the greater the drag, so having good bearings helps. BUT having a folder is way better yet! It also points out that prop drag becomes more of a factor in boats with lower DLR, ie lower displacement to length rations, ie more modern, lighter cruising designs.

This work seems like damn good science to me and I would hope that it would settle the amateur pundits with helicopter analogies and such arguments.


Jim

Edit: Seems like StuM types faster than I do!

Come on Jim, we all know that the aviation "experts"will disagree with your's and my conclusions.
And so it should be, heaven knows we should accept the knowledge of our esteemed aviation experts.

[Auspicious]

Originally Posted by PineyWoodsPete

I'm going to consult with my 21 year old grandson - a rising senior at Southhampton-Solent U.'s school of Naval Arch./Yacht and Powercraft Design - to critique Auspicious's Prof. Handler's data and counter-intuitive conclusions. He speaks in co-efficients and equations, but hoping for some plain English!

That's Hadler. https://www.webb.edu/academics/facul...ques-b-hadler/ .

Would you send along the coefficients and equations please.

College was 40 years ago for me and I don't have my notes or those old textbooks anymore so I'm working from memory.

[Wotname]

Originally Posted by Kelkara

Careful there wottie ... your intuition has made an assumption which your thought experiment alone cannot support. The whole debate centres around proving that assumption.

The graph I posted earlier suggests that it is possible to design a prop for which by slowing the rotation down you are actually decreasing the energy lost to turbulence behind the prop, and indeed it would appear that most aviation props are designed that way. Empirical tests show that boat props are not designed like that. However your thought experiment does not, because it assumed the answer from the beginning.

Feel free to delete any reference to turbulence in the thought experiment if that makes you happier. As far as the thought experiment is concerned, the reasons why it takes effort to slow the prop shaft is not important. The important thing is that is does take energy to slow the shaft (and even more energy to stop it) and that energy must come from somewhere.

Where does it come from? There is no external input as the external elements have not changed so that only leaves a change in the internal elements.

I suggest the only source for the energy required to slow / lock the shaft is for it to be extracted from the boat's momentum. Momentum is the product of mass and velocity. The mass of the boat doesn't change so that only leaves the boat's velocity to decrease.

[Kelkara]

Originally Posted by Wotname

Feel free to delete any reference to turbulence in the thought experiment if that makes you happier.

I'm not trying to argue against a sailboat slowing down when you stop the shaft ... but you asked for good science, I'd just rather that you were right for the right reason. You can't ignore the energy in the water, it's the most fundamental part of hydrodynamics.

As far as the thought experiment is concerned, the reasons why it takes effort to slow the prop shaft is not important. The important thing is that is does take energy to slow the shaft (and even more energy to stop it) and that energy must come from somewhere.

Where does it come from? There is no external input as the external elements have not changed so that only leaves a change in the internal elements.

When you drag a prop through the water it leaves swirly water behind ... this takes energy ... if the prop is allowed to spin at a different speed it will leave a different pattern of swirly water ... a different amount of energy ... so the external elements have indeed changed.

I suggest the only source for the energy required to slow / lock the shaft is for it to be extracted from the boat's momentum. Momentum is the product of mass and velocity. The mass of the boat doesn't change so that only leaves the boat's velocity to decrease.

The mass of the water around the boat doesn't change either but it's momentum does. If the momentum in the turbulent water increases it must have come from the boat, but if it decreases it must go to the boat.

So how do we tell which it is? The easiest way is to go to the boat and try it out for real ... and the pilot will go to his plane and try it, but come to a different conclusion.

It appears that the only reason that this argument even exists is because aeroplane propellers and boat propellers behave differently and so the "intuition" of those who learned by boating and flying is different.

here is another graph showing the drag from four props all identical apart from a single parameter (this time model aircraft props) showing that some of them have a higher drag when spinning and some while locked. Boat props and aircraft props have very different geometries and air and water very different properties, so we shouldn't be surprised that they behave differently. Unless someone wants to post the actual equations for hydrodynamic drag of spinning foils (and explain it simply enough for us) the only good science we're going to get here are the empirical tests (already posted by others) that conclusively show that sailboat props fall on the side of less drag while spinning ... and although it is certainly interesting that plane props behave differently, as boaters it doesn't really affect us.

[MicHughV]

I can see the prop issue as a factor in slow speeds, but once a boat gets up there to it's hull speed in a good breeze, it would appear that the boat would have ample power to spare to overcome prop resistance whether fixed or rotating....yes.no ???

[mglonnro]

Originally Posted by StuM

So you think it was a good idea to slow the boat and risked potential damage to the engines by locking the props contrary to the skippers wishes and the manufacturer's instructions?



Now how do we deal with know it all crew?


Advisory Number: MSA08-003:

DATE February 8, 2008 Dealers and OEMs
TO: All Marine Distributors
SUBJECT: Gear in Neutral While Sailing All MODELS:

All Sailboat Engines

We continue to get questions regarding the correct gear position while sailing with the engine OFF. This advisory is issued as a reminder; Yanmar requires that if sailing with the engine OFF (not running) the transmission shifter must be in the neutral position or internal damage to the gear or sail-drive will result. This damage will not be covered by Yanmar’s Limited Warranty. Please instruct customers and dealers who deliver the sailboat to the customer, of the correct (Neutral) position for the marine gear while sailing.

There seems to be a newer bulletin that replaces that general rule (see attached documents).

There are different instructions for different clutches and here's the one for mine (don't know what they had):

Folding propeller:
When sailing under sail with engine stopped put control lever into reverse position, this enables the folding propeller to fold close blade and output-shaft stops from rotating. After this operation put the control lever back into neutral or remain reverse position as you like

[wingssail]

Originally Posted by mglonnro

There seems to be a newer bulletin that replaces that general rule (see attached documents).

There are different instructions for different clutches and here's the one for mine (don't know what they had):

Yahoo! That means I've been doing it right all these years with my KBW10 transmission.

[StuM]

Originally Posted by Uncle Bob

Come on Jim, we all know that the aviation "experts"will disagree with your's and my conclusions.
And so it should be, heaven knows we should accept the knowledge of our esteemed aviation experts.

Actually,, the aviation experts don't disagree.
It's only the armchair pilots who don't understand about props being linked directly to engines that keep getting it wrong.

The facts have been known for at least 90 years.
See for example this document from NASA: "Improving the Performance of Multi-engined Airplanes by Means of Idling Propellers : the "free-wheel" Propeller" https://ntrs.nasa.gov/archive/nasa/c...9930094853.pdf


(lots of coefficients and equations in this one for Auspicious

In order to demonstrate the importance of this device, I shall consider successively in what follows the braking effect of the propeller of a stopped plane when the propeller is rigidly connected with the engine shaft and also when mounted on a free-wheel hub.
...
On this graph we have also plotted the curve obtained in the laboratory with the propeller locked, so as to show the advantage of an idling propeller over a fixed one , that is to say, the uselessness of locking the propeller.
...
[empirical test described - see attachment]
...

This represents a gain, due simply to the use of the free-wheeling propeller, of one kilogram raised or 1300 meters radius of action per horsepower of the stopped engine.
...

Conclusions
The free-wheel propeller increases the radius of action of two engined airplanes by 600-650 km(373-404 mi.) with one engine stopped.

[Nani Kai]

Just because someone owns the boat does not entitle them to endanger the lives of the crew. When an owner takes crew on their boat, paid or unpaid, they are assuming responsibility for the safety and welfare of those souls. Period. If they fail at that responsibility then they fail as a captain. Falling asleep at the helm is a DEFINITE FAIL.

Regardless of your experience, you have the right to question the owner or captain if you are concerned about your personal safety. An owner worthy of being called 'captain' will answer your questions, go over procedures, make sure you understand your role in an emergency and hopefully are confident enough in their leadership that you will stay on board. But if you are not comfortable or the owner is ignorant of their responsibility you have every right to leave the boat.

Falling asleep while on watch is NEVER acceptable on my boat.

[Wotname]

Originally Posted by MicHughV

I can see the prop issue as a factor in slow speeds, but once a boat gets up there to it's hull speed in a good breeze, it would appear that the boat would have ample power to spare to overcome prop resistance whether fixed or rotating....yes.no ???

Yes and doubly so when at hull speed and reefed!

[Wotname]

Originally Posted by Kelkara

I'm not trying to argue against a sailboat slowing down when you stop the shaft ... but you asked for good science, I'd just rather that you were right for the right reason. You can't ignore the energy in the water, it's the most fundamental part of hydrodynamics.

..............
<snip>
...............
It appears that the only reason that this argument even exists is because aeroplane propellers and boat propellers behave differently and so the "intuition" of those who learned by boating and flying is different.

here is another graph showing the drag from four props all identical apart from a single parameter (this time model aircraft props) showing that some of them have a higher drag when spinning and some while locked. Boat props and aircraft props have very different geometries and air and water very different properties, so we shouldn't be surprised that they behave differently. Unless someone wants to post the actual equations for hydrodynamic drag of spinning foils (and explain it simply enough for us) the only good science we're going to get here are the empirical tests (already posted by others) that conclusively show that sailboat props fall on the side of less drag while spinning ... and although it is certainly interesting that plane props behave differently, as boaters it doesn't really affect us.

All good and fair comments

[MicHughV]

nowhere on this thread do I see any talk about the noise a spinning prop makes...a whining, grinding, irritating, teeth grinding, annoying, vexing, exasperating, maddening and irksome noise enuff to make a person want to open yet another beer....

[Wotname]

Originally Posted by Kelkara

.......
When you drag a prop through the water it leaves swirly water behind ... this takes energy ... if the prop is allowed to spin at a different speed it will leave a different pattern of swirly water ... a different amount of energy ... so the external elements have indeed changed.
The mass of the water around the boat doesn't change either but it's momentum does. If the momentum in the turbulent water increases it must have come from the boat, but if it decreases it must go to the boat.

So how do we tell which it is? The easiest way is to go to the boat and try it out for real
...............

Not arguing against the general concept here but expanding the conversation.

The freely rotating prop finds it's own stable rpm for a given stable hull speed and creates a certain amount of swirly water. Put another way, it is allowed to find it's own rpm and the swirly water will be a minimum for the particular prop design. In a free wheeling system, I don't think anyone would argue that it will it would find it's stable rpm where the swirly water was anything but at the minimum for the particular prop design.

If the rpm is forced to change, the energy to enforce the change has to come from somewhere. That additional energy will be proportional to increase in the amount of swirly water. The amount of swirly water will never decrease as it had already found it's stable minimum. Any input of energy can only increase the swirly water.

This is easy to understand if we increase the rpm of the shaft because the only way to do that is to run the engine. Clearly the amount of swirly water increases and clearly the energy to do this comes from the diesel.

The same principle applies if we force the rpm to decrease however it isn't so intuitive. The amount of swirly water increases as the prop is forced to slow down until it reaches a maximum when the prop is stationary. The only source of energy to do this is the boat's momentum which is why the velocity of the boat must decrease.