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Old 29-06-2020, 13:09   #106
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by dgmultimedia View Post
There is a valid reason for the difference between aircraft props! On an aircraft engine the propeller does not have a gearbox that can be disengaged so the prop is extracting energy from the air flow in order to push the motor through each cylinder compression - hence significant drag! Even on turbines there is a planetary gearbox in which the reduction ratio when driven by the prop requires a large amount of energy hence drag... on a boat the prop has very little resistance to rotation when in neutral....

Exactly! There is literally "windmilling" of aircraft un-feathered props, as they are still doing drag-inducing "work" against resistance, as is turning grinding stones or driving wind turbines.

Couple a stout generator to your free-wheeling prop shaft and you've got an apt comparison with aircraft, and it will definitely slow your sailing speed a bit!
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Old 29-06-2020, 13:10   #107
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Re: Captain/Owner Sleeping at the Helm

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Transmission position is on topic though - jus saying....
No it isn’t. Topic is “Captain/owner sleeping at helm”.

Transmission position = thread drift.
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Old 29-06-2020, 13:23   #108
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by CassidyNZ View Post
No it isn’t. Topic is “Captain/owner sleeping at helm”.

Transmission position = thread drift.

But Science is so much more fun than ruminating about an incompetent, pompous, self-indulgent, Science-denying Captain at the helm!
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Old 29-06-2020, 16:00   #109
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by CassidyNZ View Post
No it isn’t. Topic is “Captain/owner sleeping at helm”.

Transmission position = thread drift.
Nope, you are confusing the thread title with the thread topic.

Here, let me quote a significant part of the opening post -

Quote:
Originally Posted by Phisher View Post
........... One being the terrible vibration undersail. I questioned why he did not engege the transmissions to the reverse position to stop the vibration. He said that the boat has had that vibration since new (2016 Lagoon 45) and several captains had noted in there logs.(Charter Boat) The vibration was all to obvious that the props were the cause. So I engaged the trannys into reverse BOOM! vibrations gone. ...............
So methinks transmission position is part of the OP's topic unless he didn't mean it....


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Old 29-06-2020, 16:17   #110
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Re: Captain/Owner Sleeping at the Helm

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We've been through this discussion on CF a number of times. I've pretty well given up. I remember in Fluid Dynamics under Prof Jacques Hadler (a leading light in propeller design) doing the calculations for this very question. Then we trooped down to the water flow tunnel for testing. Less drag with the prop fixed. It seemed counter intuitive so a bunch of us trooped down to Prof Hadler's office. He pointed out that our calculations showed less drag with the prop fixed. We agreed. He pointed that the water tunnel tests showed less drag with the prop fixed. We agreed. He suggested we needed to work on our intuition.



.

Hey, don't give up, post a copy of an authoritative test or two, preferably with video, that refutes those that demonstrate the opposite. Please, I am always willing to learn and be shown just where I am wrong.
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Old 29-06-2020, 17:22   #111
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by Cpt Pat View Post
The practice procedure is to chop power to idle (done as a surprise when with an examiner), pitch up slightly if in forward flight to covert the forward kinetic energy into stored potential energy until forward velocity is lowered to the recommended speed for an autorotation (which gives a better glide ratio), set collective pitch for maximum rated rotor RPM (NOT maximum possible RPM unless you want the blades to fly off the hub!)

And don't forget the pedals. The first time my instructor cut power, I went the wrong way with the anti-torque pedals and suddenly we were going sideways at a great rate of knots! I never made that mistake again

(It took me a while to teach my reflexes that the rudder pedals work the opposite way to a bike's handlebars or a go-carts front wheel steering)
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Old 29-06-2020, 17:30   #112
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by CassidyNZ View Post
No it isn’t. Topic is “Captain/owner sleeping at helm”.

Transmission position = thread drift.
Thread Headingis not necessarily the Topic. We see many threads where the Thread heading says little or nothng about the actual topic.


The content of the first post is the Topic.
First post: ... So I engaged the trannys into reverse ..


What's the topic of the current threads: "What do you think?" and "GP upended"
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Old 29-06-2020, 17:58   #113
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Re: Captain/Owner Sleeping at the Helm

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Hey, don't give up, post a copy of an authoritative test or two, preferably with video, that refutes those that demonstrate the opposite. Please, I am always willing to learn and be shown just where I am wrong.

Unca' Bob, you've got your own prop test bed in your dink and its little outboard! Just tow it with engine down and in neutral to free-wheel, and observe if the engine tilts up towed at 5knts or so. Then put it in Fwd (not reverse, as anti-tilt is engaged) to lock the prop and do the same. Dollars to donuts the engine tilts up with the much greater drag of the fixed prop.

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!

Ol' Granpa Pete
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Old 29-06-2020, 21:46   #114
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by Auspicious View Post
.....


We've been through this discussion on CF a number of times. I've pretty well given up. I remember in Fluid Dynamics under Prof Jacques Hadler (a leading light in propeller design) doing the calculations for this very question. Then we trooped down to the water flow tunnel for testing. Less drag with the prop fixed. It seemed counter intuitive so a bunch of us trooped down to Prof Hadler's office. He pointed out that our calculations showed less drag with the prop fixed. We agreed. He pointed that the water tunnel tests showed less drag with the prop fixed. We agreed. He suggested we needed to work on our intuition.
.........





There are no dumb questions, only stupid answers.
Prof Jacques Hadler did you (and science in general) a disservice. First of all he likely considered your questions as dumb and gave you a stupid answer. Perhaps he was uncomfortable about being challenged.

A better answer might have been "OK, challenge the calculations and see why they don't agree up your intuition. Make an hypothesis that matches your intuition and test it". Science works best when challenged and many great scientists have been proved wrong later on. There is nothing wrong with that, in fact it is a good thing; it is how we learn about the world. Now I don't know what the professor was trying to prove until I read his work but I do know it doesn't match other findings in the case of a fixed pitch three bladed sail boat either being locked or allowed to freely rotate with the transmission in neutral.

There have been several considered studies mentioned in various CF threads that disagree with what you have posted about Jacques Hadler's conclusion. Good science needs others to test the findings and if they can't be consistently repeated then at least one of the findings is incorrect.

Try this for a thought experiment -

You are sailing along nicely on a light day with flat water doing say 4 knots in your 35' sailing boat with your three blade fixed pitch prop freely rotating (transmission in neutral). The prop is say doing 100 rpm and everything (boat speed, wind, seas, prop rpm etc) is stable. Energy input (from the wind in the sails) equals the energy lost by heeling, prop drag and hull drag etc and the boat speed is constant.

Now start the engine and engage forward and bring the prop rpm up to 1000. Intuition would suggest the boat speed would increase and science will tell you why. You have added energy into the system by way of the diesel being converted by the engine into the increased shaft rotation. The prop now delivering additional thrust causing the increase in boat speed. Straightforward really.

OK, now shut the engine down and put the transmission back into neutral and wait until the system stabilises and the boat speed drops back to 4 knots and the prop rpm returns to 100.

Now apply some braking force on the shaft until the prop rpm reduces to 10 rpm. Does the boat speed up or slow down?

Intuition will suggest you are extracting energy from the system. There is increased turbulence behind the prop and there is heat being generated by the shaft brake. You know that it takes energy to create the additional turbulence and to slow the shaft down. Where does that energy come from? The only source available was the wind in the sails and as that hasn't changed, the amount of energy to move the boat along must have decreased at the rate as the amount of energy consumed by the shaft brake and additional turbulence. It follows the boat speed must decrease. If there is less energy now available to move the boat, the boat speed must decrease.

Take this to the logical conclusion which is when the prop is locked stationary, the energy required to keep the prop stationary must be subtracted from the total energy in the system. Boat speed must decrease.

Some argue that once the prop is locked there is no energy required to keep it locked but clearly that is a fallacy. If it were so, then holding the prop shaft stationary could be done with a finger and a thumb. Clearly not so!

Now the same outcome can be demonstrated by a vector analysis of the forces involved but I will leave that for another day!!!!!!
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Old 29-06-2020, 23:02   #115
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Re: Captain/Owner Sleeping at the Helm

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Originally Posted by Wotname View Post
Now apply some braking force on the shaft until the prop rpm reduces to 10 rpm. Does the boat speed up or slow down?

Intuition will suggest you are extracting energy from the system. There is increased turbulence behind the prop and there is heat being generated by the shaft brake.
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.
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Old 29-06-2020, 23:58   #116
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Re: Captain/Owner Sleeping at the Helm

Sheesh....

https://www.yachtingmonthly.com/gear...let-spin-29526
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Old 30-06-2020, 00:22   #117
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Re: Captain/Owner Sleeping at the Helm

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Yep, this and at least three others and still the aviation experts here refuse to accept that air and water are fundamentally different.
As I have asked both on this thread and a previous thread, show me the proof if you disagree with the results of those tests that indicate that a spinning prop produces less drag, with test results and video of the outcome.
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Old 30-06-2020, 00:32   #118
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Re: Captain/Owner Sleeping at the Helm

What a crazy thing to argue over. If you have a boat, just try it!

Go sailing. Not too fast so you don’t cause transmission damage. Just a few knots, but a steady speed.

Sail in neutral with free spinning prop.

Note speed.

While still sailing at the same speed, put the transmission in reverse to stop the prop from spinning.

Note the speed.

I’ve done this. It took about 20% of the boat speed off in my case on a particular boat I used to own. There is no doubt a freewheeling prop has less resistance. Actually try it. It’s easier than debating it. I tried it because I was reading debates on it and wanted to check my own hypothesis.
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Old 30-06-2020, 00:34   #119
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Re: Captain/Owner Sleeping at the Helm

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Didn't anytbody read this???
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Old 30-06-2020, 00:49   #120
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Re: Captain/Owner Sleeping at the Helm

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Didn't anytbody read this???

Yes of course I and I suspect others have, but our esteemed brothers that refuse to accept the results continue to argue otherwise. Once again I say to them, show us the evidence.
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