lw395
Well-Known Member
Yet you feel compelled to post....
Prop rotation when sailing
- Thread starter oldgitofthewest
- Start date
Yet you feel compelled to post....
Solid object
New Member
I think every body is getting drag mixed up with lift. It’s very easy to do because the difference between the two are so subtle. The reason you should not allow the propeller to spin freely is not because of drag it is because of all the lift it generates opposite the direction of travel of the boat.
Drag is always in the direction of the relative fluid flow that causes it. The relative fluid flow that causes the drag on the spinning prop is not at all the same (direction and speed) as the relative fluid flow that is influencing the rest of the boat hull but most everyone is determining drag as though it was the same. The more the prop rotates the more the relative fluid flow influencing it becomes more circular, and the more the direction of the lift it generates becomes more parallel to the prop shaft and in the rearward direction in relation to the boat and increases in speed. As the prop starts to spin the angle of attack becomes less generating more lift and less drag because the prop is operating well below the stall angle.
A prop that is auto rotating is not unlike a sailboat sailing 90% to the wind. Sailboats cannot sail faster than there power source but they can sail faster than the wind. A sailboat sailing 90% to a 20 mph wind will never lose any of the power from the motion of the air (wind) and will gain power from its motion through the air.
The power source of a sailboat is relative airflow or in other words its powered by the motion of the air as well as its motion through it depending on its point of sail.
Drag is always in the direction of the relative fluid flow that causes it. The relative fluid flow that causes the drag on the spinning prop is not at all the same (direction and speed) as the relative fluid flow that is influencing the rest of the boat hull but most everyone is determining drag as though it was the same. The more the prop rotates the more the relative fluid flow influencing it becomes more circular, and the more the direction of the lift it generates becomes more parallel to the prop shaft and in the rearward direction in relation to the boat and increases in speed. As the prop starts to spin the angle of attack becomes less generating more lift and less drag because the prop is operating well below the stall angle.
A prop that is auto rotating is not unlike a sailboat sailing 90% to the wind. Sailboats cannot sail faster than there power source but they can sail faster than the wind. A sailboat sailing 90% to a 20 mph wind will never lose any of the power from the motion of the air (wind) and will gain power from its motion through the air.
The power source of a sailboat is relative airflow or in other words its powered by the motion of the air as well as its motion through it depending on its point of sail.
lw395
Well-Known Member
You are right in that drag is normally the force on an aerofoil in the direction of flow, but in this discussion it is being taken as the force retarding the boat.
I don't think too many people are confused about that.
I don't think too many people are confused about that.
Blueboatman
Well-Known Member
Not enough time this AM to work through the pages.
Not a fan of moving parts whirring away..
Three big No No s for me would be Wear, Noise, Bigger chance of catching a foul line when Oops we didn't see that one pot marker...
Not a fan of moving parts whirring away..
Three big No No s for me would be Wear, Noise, Bigger chance of catching a foul line when Oops we didn't see that one pot marker...
Laurie
Well-Known Member
Little Rascal
Well-Known Member
As always it becomes difficult to be specific and accurate in this type of discussion without being as dull as ditchwater 
Well it depends on the frame of reference. Drag and lift are technically the resolved parallel and perpendicular components of a single aerodynamic force acting roughly perpendicular to the airfoil. But in terms of the induced drag bit of it I agree with you, although the term 'negative thrust' is maybe better when talking about propeller systems.
We have three different frames of reference here:
1) The boat as an entire system and drag forces acting to slow it down. The prop will add parasitic drag to the boat when locked. The prop will also add negative thrust (experienced as drag by the boat) when allowed to spin. In this frame of reference the total component of drag is measured as a force trying to slow the boat.
2) The propeller system and the forces it creates from spinning. In some senses this is actually 'lift.' It is is called 'negative thrust', but it comes from the prop trying to turn against the induced drag of the blades and the shaft friction. In this frame of reference the drag is measured as a torque working to slow the prop rotation. This drag is opposed by the negative thrust.
3) Each individual blade of the prop. This is induced drag from the production of lift. As indicated this has a lot to do with the advance ratio and the angle attack of each blade which is measured against the helical plane of rotation. As soon as the prop starts to turn the flow to the blade becomes helical. In this frame of reference the drag is measured as a force parallel to the helical plane of rotation, or parallel to the fluid flow.
Bear in mind that a windmilling prop is much less inefficient at absorbing power than transmitting it as thrust, because the camber of the blades is the wrong way up. Like a plane flying uspide down.
Looking at the whole boat: The parasite drag of the locked prop reduces as it starts to spin. The parasite drag is very high with a large blade area so for that type of prop letting it free wheel is a significant improvement.
But the negative thrust (drag) increases as its starts to spin. The negative thrust will depend on the shaft load, the Pitch/Diameter ratio of the prop and boat speed. If the shaft load is high then the increase in drag from negative thrust will be high. In theory at least, it's possible for it to be more overall drag on the boat than the locked prop.
Well it depends on the frame of reference. Drag and lift are technically the resolved parallel and perpendicular components of a single aerodynamic force acting roughly perpendicular to the airfoil. But in terms of the induced drag bit of it I agree with you, although the term 'negative thrust' is maybe better when talking about propeller systems.
We have three different frames of reference here:
1) The boat as an entire system and drag forces acting to slow it down. The prop will add parasitic drag to the boat when locked. The prop will also add negative thrust (experienced as drag by the boat) when allowed to spin. In this frame of reference the total component of drag is measured as a force trying to slow the boat.
2) The propeller system and the forces it creates from spinning. In some senses this is actually 'lift.' It is is called 'negative thrust', but it comes from the prop trying to turn against the induced drag of the blades and the shaft friction. In this frame of reference the drag is measured as a torque working to slow the prop rotation. This drag is opposed by the negative thrust.
3) Each individual blade of the prop. This is induced drag from the production of lift. As indicated this has a lot to do with the advance ratio and the angle attack of each blade which is measured against the helical plane of rotation. As soon as the prop starts to turn the flow to the blade becomes helical. In this frame of reference the drag is measured as a force parallel to the helical plane of rotation, or parallel to the fluid flow.
Bear in mind that a windmilling prop is much less inefficient at absorbing power than transmitting it as thrust, because the camber of the blades is the wrong way up. Like a plane flying uspide down.
Looking at the whole boat: The parasite drag of the locked prop reduces as it starts to spin. The parasite drag is very high with a large blade area so for that type of prop letting it free wheel is a significant improvement.
But the negative thrust (drag) increases as its starts to spin. The negative thrust will depend on the shaft load, the Pitch/Diameter ratio of the prop and boat speed. If the shaft load is high then the increase in drag from negative thrust will be high. In theory at least, it's possible for it to be more overall drag on the boat than the locked prop.
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Solid object
New Member
As always it becomes difficult to be specific and accurate in this type of discussion without being as dull as ditchwater
Well it depends on the frame of reference. Drag and lift are technically the resolved parallel and perpendicular components of a single aerodynamic force acting roughly perpendicular to the airfoil. But in terms of the induced drag bit of it I agree with you, although the term 'negative thrust' is maybe better when talking about propeller systems.
We have three different frames of reference here:
1) The boat as an entire system and drag forces acting to slow it down. The prop will add parasitic drag to the boat when locked. The prop will also add negative thrust (experienced as drag by the boat) when allowed to spin. In this frame of reference the total component of drag is measured as a force trying to slow the boat.
2) The propeller system and the forces it creates from spinning. In some senses this is actually 'lift.' It is is called 'negative thrust', but it comes from the prop trying to turn against the induced drag of the blades and the shaft friction. In this frame of reference the drag is measured as a torque working to slow the prop rotation. This drag is opposed by the negative thrust.
3) Each individual blade of the prop. This is induced drag from the production of lift. As indicated this has a lot to do with the advance ratio and the angle attack of each blade which is measured against the helical plane of rotation. As soon as the prop starts to turn the flow to the blade becomes helical. In this frame of reference the drag is measured as a force parallel to the helical plane of rotation, or parallel to the fluid flow.
Bear in mind that a windmilling prop is much less inefficient at absorbing power than transmitting it as thrust, because the camber of the blades is the wrong way up. Like a plane flying uspide down.
Looking at the whole boat: The parasite drag of the locked prop reduces as it starts to spin. The parasite drag is very high with a large blade area so for that type of prop letting it free wheel is a significant improvement.
But the negative thrust (drag) increases as its starts to spin. The negative thrust will depend on the shaft load, the Pitch/Diameter ratio of the prop and boat speed. If the shaft load is high then the increase in drag from negative thrust will be high. In theory at least, it's possible for it to be more overall drag on the boat than the locked prop.
When accuratly determining aerodynamic force the frame of reference is always the relative airflow that causes it. When determining relative airflow the frame of reference is always the solid object being influenced aerodynamicly. Thrust is the force generated by an object as a result of an onboard power source. Thrust can be the lift from a propeller or the drag from paddle wheel but only if they are spun by an onboard power source.
Drag, lift and thrust being infinite in direction can make a boat move in any direction as well as oppose its motion. Sailboats use drag to move down wind. Any boat can use the drag from moveing water to move in the direction of the water. Drag forces do not always act to slow it down and lift does not always act to make it move and thrust does not always act to move it forward.
The angle of attack of an individual prop blade is not measured against the helical plane of rotation although its degree rating may be. A 10 degree prop will have roughly have a 10 degree angle of attack when spun under power but this angle will change the more forward motion of the spinning prop. If this 10 degree pitch prop were kept from rotating while being made to move through the water it will have an angle of attack of 80 degrees and no helical plane of rotation with witch to measure it by because it doesn’t need it. The more the prop is allowed to spin the less angle its cord will have to the relative airflow (angle of attack) more lift less drag
If you use your frame of reference (The boat as an entire system) and your definition of drag would not the force the prop makes when spun by the engine be drag also because it is a force parallel to the relative airflow that is influencing the boat as an entire system?
Ludd
Well-Known Member
I lock my prop by putting g/box in reverse. Boatspeed I really don't care that much about,but I reason that a spinning prop is more likely to wind in a pot line or similar.
VO5
Well-Known Member
The solution to all this is to get your chequebook, bite the bullet, and write two fat cheques; one for a Brunton's propellor and one for a shaft lok.
Thenyou can look forward to silent, vibration free sailing like I do. Simple.
Thenyou can look forward to silent, vibration free sailing like I do. Simple.
Laurie
Well-Known Member
LUDD and VO5 plus one! Good ol' sycamore seeds! They've had it sorted for years now!!
Must admit I just go for locking my 2 "blader" by putting gearbox astern, but Brunton props are beautiful.....
Must admit I just go for locking my 2 "blader" by putting gearbox astern, but Brunton props are beautiful.....
VO5
Well-Known Member
The thing is...most of the time, problems can be worked around ingeniously to arrive at solutions at minimum or no cost. But sometimes, there is no alernative. Reluctant as we all are to throw money at our beloved tubs, this is one exception in which biting the bullet and forking out properly sorts it.
VO5
Well-Known Member
I forgot to add...
I remember being in a fleet of boats in the early days of Bruntons ownership.
All under sail in a steady breeze. I had forgotten about the Bruntons for a moment and was fleetingly puzzled as to why we seemed to be overhauling everyone.
I remember being in a fleet of boats in the early days of Bruntons ownership.
All under sail in a steady breeze. I had forgotten about the Bruntons for a moment and was fleetingly puzzled as to why we seemed to be overhauling everyone.
Little Rascal
Well-Known Member
Solid Object - I'm in agreement with most of that. The frames of reference were just a way of breaking down what's actually happening in the context of this discussion.
It's just nomenclature. The term negative thrust is valid - it's what the Stratclyde paper uses. Especially as the freewheeling prop is operating at negative CL in the context of a normal lift slope/drag polar. And in the context of the propulsion system where the power transfer has been reversed.
Quite agree.
This is where the advance ratio is helpful as a rough guide to blade CL. The true blade angle of attack will vary across the blade (even with helical pitch) as will the lift coefficient.
Yes. I was using the term negative thrust too differentiate the two components of drag present. But in this frame of reference both forces are'drag'. But in the context of a single blade it's lift. (I had thought that was your original point?)
It's just nomenclature. The term negative thrust is valid - it's what the Stratclyde paper uses. Especially as the freewheeling prop is operating at negative CL in the context of a normal lift slope/drag polar. And in the context of the propulsion system where the power transfer has been reversed.
Quite agree.
This is where the advance ratio is helpful as a rough guide to blade CL. The true blade angle of attack will vary across the blade (even with helical pitch) as will the lift coefficient.
Yes. I was using the term negative thrust too differentiate the two components of drag present. But in this frame of reference both forces are'drag'. But in the context of a single blade it's lift. (I had thought that was your original point?)
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snooks
Well-Known Member
Really?
Have you read the article in Yachting Monthly?
Don't let the fact that propellers designed to work in different mediums are usually different shapes bother you. Or that the cross sectional shape of a sycamore seed bears no similarity to that of a bronze propeller effect your reasoning. Don't let the fact that the sycamore seeds evolved to slow the passage of the seed downwards to take it out of the shadow of the parent tree and a yachts propeller is designed for propulsion ruin your day. Don't forget that a propeller has a gearbox on one end and a sycamore seed doesn't. Let's not bring to the table that a sycamore seed has 1 blade and the propellers used in the YM test had 3 blades eh?
By coincidence I found a sycamore seed on the deck of our boat, under a locker lid, so I threw it over the side , it spun a few times and it floated, I would do the same with our old prop, but I know it wouldn't spin in air and would sink in water, goes to show they must work in exactly the same way.....
All that was before you take into account the university findings or the test done by Oldgitofthewest on their own boat which agree with the findings of Yachting Monthly.
When I replace my prop with three sycamore seeds stuck together your reasoning might hold water. At the moment I'm sorry to say, I don't think it does.
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Laurie
Well-Known Member
Really?
Have you read the article in Yachting Monthly?
Don't let the fact that propellers designed to work in different mediums are usually different shapes bother you. Or that the cross sectional shape of a sycamore seed bears no similarity to that of a bronze propeller effect your reasoning. Don't let the fact that the sycamore seeds evolved to slow the passage of the seed downwards to take it out of the shadow of the parent tree and a yachts propeller is designed for propulsion ruin your day. Don't forget that a propeller has a gearbox on one end and a sycamore seed doesn't. Let's not bring to the table that a sycamore seed has 1 blade and the propellers used in the YM test had 3 blades eh?
By coincidence I found a sycamore seed on the deck of our boat, under a locker lid, so I threw it over the side , it spun a few times and it floated, I would do the same with our old prop, but I know it wouldn't spin in air and would sink in water, goes to show they must work in exactly the same way.....
All that was before you take into account the university findings or the test done by Oldgitofthewest on their own boat which agree with the findings of Yachting Monthly.
When I replace my prop with three sycamore seeds stuck together your reasoning might hold water. At the moment I'm sorry to say, I don't think it does.
Aw....go on........
Greenheart
Well-Known Member
Gents, you’re aware of the extent to which most of this thread reeks of geeks? I’m embarrassed to be enjoying it so much.
As I gave up physics at thirteen, I won’t pretend to know much; and while I’ve tried to follow the thread carefully, I may have missed most of its points…but…are certain skippers claiming a locked sailing yacht propeller causes less drag? Isn’t that completely illogical? Did YM honestly spend time and money looking into it?
If a prop which is left free to turn, is motionless when no water passes over it, why would it begin to spin as the boat moves ahead, if indeed the hydrodynamics of the scenario were less disturbed by its remaining motionless?
Doesn’t the fact that it wants to spin in a current, demonstrate its being subject to pressure, which must be diminished/lessened by the blades’ freedom to rotate? If there were less drag from a motionless prop, it wouldn’t turn in the current!
Then again, the theory might explain all those non-turning wind-turbines, in high winds – perhaps they produce even more power when they’re not turning!
Did I read, or only dream, of an ocean-racer whose crew was equipped and trained to remove and replace their prop, at the ends of long lonely trans-ocean legs? That’s commitment to performance…
For zero-drag, has any sailing yacht yet been fitted with a water-jet auxiliary? It could comprise some nifty bow & stern thrusters, too. And it might free-up accommodation space, by allowing the engine to sit elsewhere onboard...
As I gave up physics at thirteen, I won’t pretend to know much; and while I’ve tried to follow the thread carefully, I may have missed most of its points…but…are certain skippers claiming a locked sailing yacht propeller causes less drag? Isn’t that completely illogical? Did YM honestly spend time and money looking into it?
If a prop which is left free to turn, is motionless when no water passes over it, why would it begin to spin as the boat moves ahead, if indeed the hydrodynamics of the scenario were less disturbed by its remaining motionless?
Doesn’t the fact that it wants to spin in a current, demonstrate its being subject to pressure, which must be diminished/lessened by the blades’ freedom to rotate? If there were less drag from a motionless prop, it wouldn’t turn in the current!
Then again, the theory might explain all those non-turning wind-turbines, in high winds – perhaps they produce even more power when they’re not turning!
Did I read, or only dream, of an ocean-racer whose crew was equipped and trained to remove and replace their prop, at the ends of long lonely trans-ocean legs? That’s commitment to performance…
For zero-drag, has any sailing yacht yet been fitted with a water-jet auxiliary? It could comprise some nifty bow & stern thrusters, too. And it might free-up accommodation space, by allowing the engine to sit elsewhere onboard...
charles_reed
Well-Known Member
Unfortunately, for such a discussion as this, available published evidence supports both theories, fixed prop = less drag, rotating prop = less drag.Gents, you’re aware of the extent to which most of this thread reeks of geeks? I’m embarrassed to be enjoying it so much.
As I gave up physics at thirteen, I won’t pretend to know much; and while I’ve tried to follow the thread carefully, I may have missed most of its points…but…are certain skippers claiming a locked sailing yacht propeller causes less drag? Isn’t that completely illogical? Did YM honestly spend time and money looking into it?
If a prop which is left free to turn, is motionless when no water passes over it, why would it begin to spin as the boat moves ahead, if indeed the hydrodynamics of the scenario were less disturbed by its remaining motionless?
Doesn’t the fact that it wants to spin in a current, demonstrate its being subject to pressure, which must be diminished/lessened by the blades’ freedom to rotate? If there were less drag from a motionless prop, it wouldn’t turn in the current!
Then again, the theory might explain all those non-turning wind-turbines, in high winds – perhaps they produce even more power when they’re not turning!
Did I read, or only dream, of an ocean-racer whose crew was equipped and trained to remove and replace their prop, at the ends of long lonely trans-ocean legs? That’s commitment to performance…
For zero-drag, has any sailing yacht yet been fitted with a water-jet auxiliary? It could comprise some nifty bow & stern thrusters, too. And it might free-up accommodation space, by allowing the engine to sit elsewhere onboard...
Papers on the aerodynamic behaviour of airscrews are of doubtful validity, the shapes of blades and the behaviour of the element being sufficiently dissimilar to make any but approximate parallels misleading.
The work done at Wolfson suggests that fixed props do produce less drag at low speeds, probably to the point at which laminar drag becomes a smaller component in the overall drag, to be replaced by turbulence.
A further complication is that provided by feathering and folding props.
By practical experiment, I've found that an unlocked prop starts rotating @ 2.5-3.0 knots. Being an Autoprop mine won't feather unless braked and both drag and noise increase geometrically as speed increases.
As my boat surfs quite easily, I've never experimented at high speeds, but fear that the drag of an unbraked propeller would prevent that happening and the vibration from the rotating assembly would shake it to pieces.
So I for one am quite happy with the hypothesis that an unbraked propeller increases drag.
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C
Chrusty 1
Guest
I offer this as it may be of interest to some, I must say that both these posts, made on the Seal & Parker forum, were referring to drag caused (or not) by outboard propellors, it seems that inboard arrangements differ?
Little Rascal
Well-Known Member
A bit harsh Graham? It's very convenient to emphasize the difference between your test (which I think was good btw) and aircraft propellers.
But the point that a lot of people are missing is that they both use the same aerodynamics. Yes the fluid density is different which leads to different prop diameters, blade numbers and blade areas, but the fundamental physics is the same. Even for a Sycamore seed Otherwise it would seem that prop engineers have been wasting an awful lot of their time studying wing sections, and using NACA airfoils on prop blades...
I suggest a look at John Carlton's Marine Propellers and Propulsion if you really care enough to try and understand what is going on under your stern...
So aircraft comparisons are valid if properly understood.
My final say (as a geek) on this subject:
The YM, MIT and Strathclyde tests have shown that for most standard marine propellers the least drag condition for the boat as a whole is to permit the prop to freewheel. This is a fact ...
But there could theoretically be props and boats outside the norm where the oposite is the case. It is not proven that letting the prop freewheel will always be less drag - that is a conclusion too far, not supported by the research or the physics.
But the point that a lot of people are missing is that they both use the same aerodynamics. Yes the fluid density is different which leads to different prop diameters, blade numbers and blade areas, but the fundamental physics is the same. Even for a Sycamore seed
Otherwise it would seem that prop engineers have been wasting an awful lot of their time studying wing sections, and using NACA airfoils on prop blades...I suggest a look at John Carlton's Marine Propellers and Propulsion if you really care enough to try and understand what is going on under your stern...
So aircraft comparisons are valid if properly understood.
My final say (as a geek) on this subject:
The YM, MIT and Strathclyde tests have shown that for most standard marine propellers the least drag condition for the boat as a whole is to permit the prop to freewheel. This is a fact ...
But there could theoretically be props and boats outside the norm where the oposite is the case. It is not proven that letting the prop freewheel will always be less drag - that is a conclusion too far, not supported by the research or the physics.
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lw395
Well-Known Member
I don't think they have even proved that, because they have not been very systematic about measuring the shaft torque and replicating it. But it's probably true all the same.
But it's a bit like proving that square wheels are better than triangular.
Measuring the drag on a prop makes it very obvious to me that folding props are worth the cost and maintenance, IMHO.