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Old 05-03-2007, 20:36   #46
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Hi Ernest. Good questions. OK, when I use the term "dumb" I mean that the blade itself is not inteligently being told where to place its'self. A mechanicaly feathering prop is being "told" where the blade should be placed. The Auto prop blade is simply swinging freely and yes, it's attitude in the water is a result of the water flow over the blade.
Quote:
result of the water pressure on the propeller blade
Not quite. It is not water pressure, it is water flow. That is the key difference. It is important to understand a modern day blade shape. They are infact wings. Not flat blades. The old propellor that gave the common Navel term, especialy on subs, were "srews". This was fromt he days when that was exactly what they were. But designs have dramaticly changed and actually the concept of Screw to actuall wing was found quite by accident over a 100years ago, when during the failure of a propellor, the boat actually went faster. that's another story though.
So a modern day propellor does not push its'self throught the water. It "lifts" its'self through the water, just like a wing does. It is exactly the same principle as a wing, just that it is in water. It is the water flow over the wing that causes the blade to take up the attitude it does. It is the off set of the blade at the hub that causes that blade attitude to transfer its forward lift to the hub and thus shaft. It is the combination of that offset and the shape of the "wing" that makes it all work. Infact, the best way of me describing it and it may make it clearer for you, is it is exactly llike how a Sail and boat works. The wind is coming in from an angle and the sail produces lift. But the offset is provided by the Keel and so the boat is forced to move forward and not sideways. Very clever the fellow that came up with that prop design.
So when I said "dumb" I was meaning the Blade had no way of knowing the Hp of an engine.
example = Lets say a shaft rotation of 800RPM will propel your boat forward at say 4kts. A 10Hp engine spinning at 800RPM will propell the boat at 4kts. A 50hp engine will propel the boat forward at 4kts as well. Just because the engine is more poweful, will not mean the extra 40Hp will make the boat go any faster as long as you continue turning the shaft at 800RPM. That was what I meant by "dumb". It has no power of inteligence to say, oh goody I have an extra 40Hp so I will work the engine harder and make the boat go faster. Infact, if it did make the engine work harder at low RPM, it would be very bad for the engine.
Does that help or make sense???
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Old 06-03-2007, 02:46   #47
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Alan

I dont know if you meant to, but your previous reply seemed to imply that the Autoprop just operated in two positions - forward and reverse - and could swing between the two.

For sure, this is NOT the case. It is genuinely sell pitching: if you are motoring into head winds (ie on normal prop, water flow would be lower for same engine revs) the Autoprop presents a finer pitch. Result: you dont over-load the engine. When motorsailing (ie with a normal prop water flow would be higher with same revs) Autoprop presents a courser pitch. Result: you dont have to spin the engine so fast to 'catch up' with the water flow before it contributes to forward thrust (which would have wasted fuel and could harm engine, as it would be high speed with low load). The same thing happens as the boat takes off from stopped: the pitch gets courser as the waterflow increases.

Agreed in does not have much to do with engine power. That would have a relationship to Autoprop size/blade count, as normal, I guess.
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Old 06-03-2007, 11:39   #48
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Quote:
It is genuinely sell pitching:
Absolutly.
Quote:
if you are motoring into head winds (ie on normal prop, water flow would be lower for same engine revs) the Autoprop presents a finer pitch
Absolutly not
Think about this. If this was the case, then getting away fromt eh dock would be impossible. It would be like try to accelerate a car on ice. The rotational speed into the water is far greater than the water speed onto the prop as a whole as the boat is moving along. The leading edge of the prop at say 500RPM shaft speed, is traveling through the water about 12ft/sec (very ruffly. The leading edge would be traveling through the water double that at 1000RPM shaft speed. The forward momentum of the prop is a tiny fraction of that.
Now looking at a slightly different angle. The prop is literaly "sucking" or lifting its'self forward throught the water. As the RPM is increased, the lifting force increases. It does not "notice" or is not affected by water force flowing into the prop, as to the prop, there is no force. It is "flying" through the water and pushing the boat along with it.
The only advantages of autoprop over a standard fixed prop are,
It self pitches to provide an equally efficient blade angle to Reverse as it does Forward.
The blades fold to neutral when not spining and provide the least resistance to the boat when sailing.
What it does not do,
It does not self pitch or feather to provide the best attack angle dependant on engine Hp.
It does not do the above dependant on engine RPM
The only optimum angle of attack is set in the factory with the design of the blade. It is subject to all the same laws of physics as any conventional fixed propellor is.
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Old 06-03-2007, 22:27   #49
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From the Autoprop website:
"Automatic Variable Pitch
The Autoprps blades are custom designed for the particular power, shaft RPM and speed of your yacht. The components of hydrodynamic force (boat speed), and centrifugal force (SRPM) balance, to set the blades at the correct pitch angle. As the yacht speed or the engine revolutions change, the blades will automatically re-adjust to keep the optimum pitch angle to the waterflow. This gives a high thrust and efficiency over a wide range of conditions."


and

"Maneuvering Characteristics
Due to the self-pitching action of the Autoprop, maneuvering is different to a conventional propeller. Firstly, in most every case, there is noticeably less prop-walk experienced. This is due to the finer pitch setting at low speeds giving a reduced "paddle wheel" effect. Secondly, in reverse the blade shape is exactly the same as in forward. Thirdly, due to the finer pitch, at low speeds there is less "bite" felt when engaging ahead or astern from a standstill. This means that more engine Rpms than normal should be used when moving off from a standstill, or at very low speeds. Once some speed has been attained, the engine Rpms can be reduced.

This unique feature of the Autoprop enables the full power of the engine to be used in situations such as towing, or in emergencies. With conventional or feathering propellers, the pitch is too coarse at very low speeds, the engine cannot achieve its full Rpms, and therefore full thrust is not achieved."

Seems pretty unambiguous. Are you saying these are lies or what?
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Old 06-03-2007, 23:36   #50
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As far as words go, I don't think I have said anything more or less, nor Autoprop said anything more or less than what I have already said. All the above is true to a point. There are only two comments that I would have to say are not correct.
Quote:
Firstly, in most every case, there is noticeably less prop-walk experienced. This is due to the finer pitch setting at low speeds giving a reduced "paddle wheel" effect.
Prop walk has absolutely nothing to do with a "paddle wheel" affect that is often said. Prop walk is about the angle of the shaft/propellor and that angle only. The rotaional force of the blades are cancelled out by opposing forces as the prop turns 360 degrees. Prop walk is about the angle or arc the blade takes on it's path through the water. Complicated and best described by a diagram. I will see if I can find one.

Quote:
With conventional or feathering propellers, the pitch is too coarse at very low speeds, the engine cannot achieve its full Rpms, and therefore full thrust is not achieved."
This is either poorly written to show there true meaning or they are in simple era.

Quote:
Thirdly, due to the finer pitch, at low speeds there is less "bite" felt when engaging ahead or astern from a standstill. This means that more engine Rpms than normal should be used when moving off from a standstill, or at very low speeds. Once some speed has been attained, the engine Rpms can be reduced.
this is ruffly as I said in the last post. It would be like trying to accelerate a car on ice.
Quote:
This unique feature of the Autoprop enables the full power of the engine to be used in situations such as towing, or in emergencies
I don't see the point in having the engine Reving and you going slower. The real importance is getting power to ground as the say. Once again like trying to tow with a car on ice. You may have a big block chevy under the bonnet, but if you are spinning the wheels, whats the point. True towing vessels are about getting "grip" in the water. Large blade surface, slower RPM and it all equates to real pulling grunt.
Quote:
The Autoprps blades are custom designed for the particular power, shaft RPM and speed of your yacht
This is as I said above somewhere, the "true" pitch is designed as the blade shape. The specs are given to autoprop by the vessel owner and autoprop spec a diameter and pitch to suit.
Quote:
The components of hydrodynamic force (boat speed), and centrifugal force (SRPM) balance, to set the blades at the correct pitch angle.
Yes they are correct, but to a point. I also stated this above somewhere. About centrigugal force being the main contributing factor to the blade oreantation. Water force to a small extent, but as I said in the last post, the rotation is far in excess of water speed.

I really don't see anything that Autoprop have stated as being lies. Just some licence with explanation. But it is complicated and how do you describe how a product like this works to the normal joe bloggs that maybe a cutomer. Hell, I have not succeded in explaining it very well to you and I have used a hundred times more words than they did.
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Old 07-03-2007, 00:03   #51
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To explain the theory of operation a little more, Alan:

There are two forces trying to balance each other.

1) Centrifugal force (I know it is not the correct term, but we know what it means) is trying to set the blades with maximum drag = "100% pitch" (which is the same position as zero lift). Ie the blades are parallel to the shaft. Note THIS drag is drag on the rotation of the prop - drag for sailing purposes is minimised. The same position is adopted when there is NO rotation - the water flow forces the blades to be parallel to the shaft. So if you spin the prop in vacuum, or pull a non-rotating prop through the water, the blades adopt the same position.

2) Rotational drag. When power is applied to the prop shaft, the prop tries to rotate and sees huge drag (due to being at '100% pitch" as above). This drag force causes the blades to rotate in their sockets, reducing the drag and increasing the lift. At some point, centrifugal force balances drag and you get a certain pitch - and hence lift - for the given conditions.

The rotational speed of the prop is irrelevant (other than that is what provides the centrifugal force, above). With zero pitch (and ignoring viscous coupling) the prop would see zero rotational drag whether going at 500rpm or 50,000 rpm. The lift/drag it sees is due entirely to the prop pitch (in static water): and hence increased water flow (due to motor sailing) or reduced water flow (due to headwind), and also the change in water flow from being a stopped boat to one at crusing speed has a big impact on the prop rotational drag. This means the prop will adopt a different position (and hence pitch) for each case, as the centrifugal force/drag balance changes.

I see no violation in physics here.
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Old 07-03-2007, 00:19   #52
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"I don't see the point in having the engine Reving and you going slower"

To use your truck anology, Alan: it is not like spinning wheels on ice. It is like engaging lower gear. Higher revs but more 'pull. Once underway, you automatically engage a "higher gear" and and can reduce revs.

(dont bother on prop walk issue, I agree!)
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Old 07-03-2007, 01:51   #53
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The propeller shaft angle (the angle between the surface of the water and the shaft) causes the thrust of the up-going and down-going blades to differ, so producing asymmetric thrust, or prop walk.
The amount of tip to hull clearance also affects prop walk.
The more the shaft is angled, and the less clearance you have, the more walk you experience, regardless of the prop.
Some boatbuilders try to minimize prop walk, by offsetting the engine shaft alignment off centre.
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Old 07-03-2007, 01:55   #54
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it is not like spinning wheels on ice. It is like engaging lower gear.
Absolutly true.
The difference with "pitch" over a wheel is that the pitch equals the wheel daimeter. So reducing pitch is like putting on a smaller wheel. So the engine is indeed gear such as if it was in a lower gear. The difference with a wheel, is that the diameter helps put more "footprint" on the ground and thus gives more grip. With a prop, it is not the pitch that gives grip, it is the Diameter/blade surface area. All props have a "slippage" in the water. It doesn't matter if the prop has a low pitch or high pitch, the slippage of effiecency is ruffly the same. Although it is true to say that efficiency is increased with a faster RPM and thus having a small pitch and spinning the prop faster woudl increase efficiency. This is what is kinda taking place with the autoprop. Once flow comes into the equation, the pitch alters just enough to create a little more bite, which you have said yourself, so don't think that I am totaly disagreeing with you.
Personly, I am not sure I would like the idea of having to rev the engine to get moving and then back off once the prop bites. My boat does that to an extent now. I want and like the fact that I can idle in or out of my birth or manouver around a mooring bouy with idle.
I do however, Like the idea of the prop blades presenting themselves in the most efficient manner in Fwd and Rev and I would love the idea of the blades settling to a neutral position when under sail. I see many benifits to the autoprop. I don't know enough about Autoprop or any other self pitching prop to make and assumption on which would be better. If I had the money, a feathering prop would be really fantastic, but I think overkill or a sailboat.
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Old 07-03-2007, 05:04   #55
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One thing I have noticed on my Autoprop that hasn't been mentioned is that I notice NO cavitation. Maybe that will change with more experience.

As to Alan's comment on "reving" I don't find it any more different then engaging a clutch in a car, you give the engine a little power and let out the clutch. Same basic idea with the AutoProp. Once one gets a little use to it and we're still working on getting use to it, I don't think it will be any different.
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Old 07-03-2007, 11:42   #56
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Dave,
I dont know if there is a formula on available on the Web.
It is difficult in this form to write down a formula so I will attach a picture.
I havent the slightest how it will present itself in the forum but lets hope for the best.
I have stolen it from Dave Gerr's Propeller Handbook.
There are probabably a lot of assumptions behind the formulas like propeller efficiency and such.
Gerr says that displacement boats reach their maximum hull speed at SL-ratios of 1.3-1.4. The higher value for an easily driven hull.
I have also attached an Excel-file. It is handy if you have Excel on your PC.
Just type in the WL and displacement and you get hull speed.
How did you arrive at your maximum hull speed? The accuracy with two decimals also indicates some kind of formula?
Anyway, we have fairly good correlation since you seem to have a very heavy boat which relates to a SpeedLength-ratio on the low side
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Old 07-03-2007, 11:45   #57
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Dave,
I wonder where the Excel-file went. Somewhere into Cyberspace I guess.
I can mail you one if you are interested.
Ernest
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Old 07-03-2007, 12:43   #58
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Alan,
here I am again. I have taken a little time out but I can see that the discussion really continued.

My comment is to an example you provided about a 10hp engine and a 40 hp engine driving a propeller att 800 rpm. I totally agree that with the same propeller you will have the same speed.
In this case "same" means fixed blades.
But,
the Autoprop is self-pitching and the blades should swivel to a pitch that is determined by the forces from the power/torque provided by the engine
and the waterflow once the boat starts to move.

I guess Moby Dick thinks in the same direction.

Hence the 10 hp engine is only able to drive the Autoprop blade to a small pitch but the 40 hp is able to drive it to a larger pitch.
What I mean is that they dont see the "same" propeller.

This would result in a higher speed through the water for the 40 hp case.

I am still very curious if this phenomenen is observed in the real world by Autoprop users. It probably only occurs if you have a powerful engine.
I would be very happy to know since I dont want to invest USD 4000 to prove my thesis.

Ernest
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Old 07-03-2007, 12:56   #59
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Moby Dick,

I dont think the starting pitch is zero. What I mean is that when the propeller shaft starts turning, the propeller blades will swing to a position almost perpendicular to the shaft but not quite.
I guess it is the small distance between the swing center and the root of the propeller blade ( and possibly some small static pitch between the blade and the swinging arm) that forces the propeller blade to start to "bite" and pull the boat forward or backwards.

Is that right?
Ernest
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Old 07-03-2007, 18:24   #60
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Sigh, I wish I could explain this far better. This is what I struggle with, getting my brain to paper.
Quote:
Hence the 10 hp engine is only able to drive the Autoprop blade to a small pitch but the 40 hp is able to drive it to a larger pitch.
What I mean is that they dont see the "same" propeller.

This would result in a higher speed through the water for the 40 hp case.
No it wouldn't, nor it shouldn't. 800RPM is 800RPM or whatever the revs will be. Hp has nothing to do with that. If it was a fact that the 10Hp couldn't twist those blades further, the 10Hp must be under excessive load at only 800RPM.
If your above thought were correct, then the boat would be reaching hull speed a long way before the 40Hp engine was at full RPM. This is very bad for an engine to be reaching Hull speed when not a full RPM or within coo'ee of it and very bad for the underpowered engine to be reaching a very high load while at a low RPM. So no, it is the revs and the resulting action of water flow that are twisting the blades, not the Hp.
Now here is where I think we are getting confused. We are assuming with the above explanation has the same prop on both engine powers. In reality this will not be the case. The propeller has to be matched to engine power and Shaft RPM. So a 10hp engine is going to have a very different prop than a 40hp engine will. A different ration gear box will also be in the equation of determining the prop.
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