Model Boat Mayhem
Mess Deck: General Section => Model Boating => Topic started by: tubby tomo on November 26, 2008, 07:10:30 pm
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hi all if a electric fan was put on the stern of a sail boat would the fan blow the boat forward {:-{
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You aren't the first person to ask that question. Back in the 60's Model Maker magazine published a plan of a catamaran hull which used a large airscrew propellor catching the wind to drive a marine screw on the other end of the shaft. I think it was called something like "Will it, won't it?".
I don't know whether it worked or not!
Colin
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its wether the air being sucked through the fan would stop the forward motion of the boat
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I was always led to believe that "to every force, there an equal and opposite force".
The fan would have to be on the quayside.
I should imagine that it wouldn't work, but credit to you for an inventive mind. :-))
ken
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I should think the main problem would be the length of the extension lead you'd need :-)) :-))
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If there was no wind, it would not only make it stop, it would go backwards.
It's the low pressure in front of the sail creating lift, which acts against the keel stooping leeway that creates forward movement.
Perhaps if the fan was in front of the sail creating a vacuum behind the fan, that may work. But there again no, as the fan would blow the yacht backwards. %%
Almost all the above is meant to be ok2 ;) :-)) %)
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The fan would be trying to blow the mast off the boat. Any motion would only be that imparted by the thrust of the fan as deflected by the sail. You'd do better to bin the sail and let the fan blow the boat along - as in a Florida swamp air boat.
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In theory, it should work, if you put the fan near the bow, blowing towards the sails at the correct angle, thus giving two methods of forwards motion.
The first is using the fan itself, just like a prop on an aircraft, sucking the boat along by a low pressure area in front of the prop at the bows (or near to it), and the generated air at the back of the prop pushing the boat along.
The air blowing over the front of the aerofoil shaped sails produces an area in front the sails of low pressure, that pulls the yacht along.
This is all of course dependent on there being no wind blowing, which would soon make it get very confusing.
People are under the misapprehension that the wind blows a yacht along, it does, but only in certain conditions, a lot of the time the yacht is sailing across or actually into the wind, being dragged forwards by the action of the low pressure area being generated in front of the aerofoil shaped sails.
In practice I think you would be struggling. But a very debateable subject as to will it or won't it work.
John
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I am afraid that many of the now discredited theories of how a wing (or sail ) creates a driving force are being perpetuated in this thread.
Whilst a low pressure area is created on the top of an aircraft wing (or in front of a sail - which is really just a wing on end) it is insufficient to produce the amount of lift or drive needed to fly an aircraft or drive a sailing boat.
It is the air which is deflected by the sail as the air passes over the shape of the airfoil which creates the main driving force due to Newtons third law - "To every action there is an equal and opposite reaction."
If the vacuum created in front of the sail was sufficient to suck the boat along then the air behind the sail would be blowing forward from the stern towards the back of the sail.
This simply does not happen - in fact the air deflected rearwards by the sail can be felt. Often on a long sea passage I have seen homing pigeons trying to land on the boat for a rest. Approaching from the stern they are always blown down and back away from the deck by the downdraught (deflected air) from the sail.
Similarly on an aircraft wing if the lift was created by the suction above the wing then the air would be flowing upwards underneath it. Again this simply does not happen.
An observable example is a helicopter. The rotors are simply narrow wings rotating in a horizontal plane. If the suction theory was correct then the air lifting the helicopter must flow upwards. As anyone can see from the disturbance on the ground or water, all the air from the rotors is deflected downwards creating the lift.
To see this deflection of the air over an airfoil shape - hold a table spoon in the flow of water from a tap so that the water runs over the convex side of the spoon . You will see that the water flow is deflected in the same way as the air over a wing (or sail) is.
Don B.
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This sounds like the famous NASA bootlace - bend down, pull VERY hard on your shoe laces, fly upward.
And no it won't work, all you will do is try and blow the mast over. Point the fan in the opposite direction to your desired travel and the boat will move.
Phil
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Hi all,
It is unheard of up till now, for me to disagree publically with any one, but on this occasion I am going too.
OK Welsh_Druid I think you are wrong.
By your own explanation 'Newtons third law' it therefore could be the deflected air is second and the vacuum is third :-))
And whilst were at it, how does a boats rudder work?
And yet another thing, who discredited it?
Please remember, I reserve the right to be wrong on all occasions. ;)
I will not enter into any further communications on this thread. (promise) :((
Moderators, please feel free to remove this post. :((
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Don B,
Similarly on an aircraft wing if the lift was created by the suction above the wing then the air would be flowing upwards underneath it. Again this simply does not happen.
An observable example is a helicopter. The rotors are simply narrow wings rotating in a horizontal plane. If the suction theory was correct then the air lifting the helicopter must flow upwards. As anyone can see from the disturbance on the ground or water, all the air from the rotors is deflected downwards creating the lift.
I would love for you to be able to explain to my instructors of theory of flight, they tried to teach it me for two years, now it seems that they have got it all wrong, what a total waste of my time, and having worked on helicopters for nearly ten years, thank goodness the pilots had never heard about your theory, it would have completely unbalanced them.
John
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I would love for you to be able to explain to my instructors of theory of flight, they tried to teach it me for two years, now it seems that they have got it all wrong, what a total waste of my time, and having worked on helicopters for nearly ten years, thank goodness the pilots had never heard about your theory, it would have completely unbalanced them.
John
Try this then http://www.eskimo.com/~billb/wing/airfoil.html ok2
or this http://www.avweb.com/news/airman/183261-1.html
Don B.
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Similarly on an aircraft wing if the lift was created by the suction above the wing then the air would be flowing upwards underneath it. Again this simply does not happen.
An observable example is a helicopter. The rotors are simply narrow wings rotating in a horizontal plane. If the suction theory was correct then the air lifting the helicopter must flow upwards.
Incorrect
The theory you describe neglects to take in to account the forward motion of the wing / sail and its angle of attack when presented to the airflow.
Again incorrect.
A helicopter's rotors have to have an angle of attack towards the airflow (deflecting air downwards) to produce lift.
Any wing or sail has to have an angle of attack towards the airflow and be moving forwards in the air to enable the differential in air pressure, low pressure above, normal pressure below... to enable it to 'fly'.
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With regards to the first one, if you want to join the flat earth society, then you are welcome.
The second one I am sure, has never studied aerodynamics in a wind tunnel, also he isn't showing the whole picture of the way airflow goes around an airfoil section, his is missing in his ramblings the easily seen area of high pressure on the leading edge of the aerofoil. He is showing only what he wants you to see, and are in my mind, his own assumptions. Maybe he has joined the above society, and just spreading the same drivel about.
Put it this way, if you believe that stuff, then don't buy any sort of vehicle that uses a carburettor, because for you, it would never run, as it works on the same principle as an aerofoil, increased speed, producing a reduced pressure thru a venturi (a back to back aerofoil) that sucks the fuel thru the jet. Also don't ever fit a Korts nozzle, because that works on the same principle, speeding up waterflow thru a venturi, hence more thrust at the back because of increased velocity. If the above are to be believed, you would in fact stand still or even go backwards by using a Korts, by sucking water from the rear, around the outside of the nozzle and moving it forwards to the Korts intake.
There is definitely something wrong with their way of thinking, and methinks I will stick with the normal proven scientific conclusions from the last 100 years.
A standard flat bottomed aerofoil in fact doesn't need an angle of attack to produce lift, nor an increased pressure area underneath. The airflow will automatically speed up over the top surface and produce lift, not a lot, but it will make a low pressure area.
Helicopter aerofoils are usually symmetrical for most of their length, so when they are sitting on the ground and turning, they are in a state of neutral balance, only when an angle of attack is introduced do they start to produce their lift above and cushion below. The weird shapes you see on modern helicopter blades is to do with the control of air flowing from high pressure below to the low pressure above, by way of the blade tip ends, if that happens, the tips of the blades can stall, and on a fully articulated head, cause the blade to drop dramatically, to such an extent the blade can drop and hit the tail section or groundcrew at the front or side. You will also see the same sort of control on modern aircraft, with the winglets sticking up on the tips, and little air dams (fins) over the top and bottom surfaces, all designed to keep the airflow straight, to stop them meeting at the wing tips. At high angle of attacks, and usually slow speeds, you will most probably see vortex produced at the wing tip, where the high pressure from underneath is attempting to get to the low pressure above, by way of the wing tip ends, NOT over the leading edge.
Here endeth the lesson. I am sure I have forgotten a lot, but I also remembered a lot of the basics, plus a lot has happened in the design side from 20 odd years ago. But not on how they work.
John
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How do aircraft flaps work? If they increase lift by increasing the vacuum then as they look like they are going down at a sharp angle the vacuum is largely behind them so must be pulling the plane backwards more than up.
Is there an equivalent of flaps on a sail?
How do leeboards work as they are so small relative to the slab side of the barge?
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[Incorrect
Again incorrect.
A helicopter's rotors have to have an angle of attack towards the airflow (deflecting air downwards) to produce lift.
Exactly - thats what I said - the airflow directed downwards produces the lift NOT suction from the top of the wing (or the front of a sail)
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John -A lot of what you said has no relevance - all that stuff about carburettors, venturis and kort nozzles, because wing and sail aerofoils do not operate in close proximity to other surfaces. ( Granted if aircraft are flying in "ground effect" it may be of significance but not normally - and certainly not for sails)
"The airflow will automatically speed up over the top surface and produce lift, not a lot, but it will make a low pressure area." Agreed - I said so - but not enough lift to be significant.
Much of what you said about helicopter blades is interesting and certainly true when considering the spanwise flow on all wings, especially high aspect ratio wings (and particularly so on the ultra high aspect ratio helicopter blades) . But will you please explain why, if the lift is simply caused by air flowing from the underside to the top side of an aerofoil, there is such a very great downflow of air from the helicopter blades which increases in proportion to the amount of lift created.?
I am sorry that you should seek to make this discussion rather personal - "if you want to join the flat earth society, then you are welcome." - I have no intention of doing so but when you refer to sticking with the scientific conclusions from the last 100 years I am reminded of my old physics teacher who, in the late 1940's, insisted on teaching " the smallest indivisible portion of matter is the atom" - this at a time when not only had the atom been split but a couple of bombs had been exploded based on splitting that self same indivisible portion of matter.
But of course his teaching was based on the proven scientific conclusions from the previous 100 years. He, like many other people I'm afraid, seem to have closed minds and are not prepared to discard their earlier teaching at any cost.
Don
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Back to fans not sails (spanner in the works time). How about Lord Brabazon`s autogyro type sailing boat. (note to Toes , autogyros have negative angle on blades) he he . Now slinking off into the heather for another week or three :D
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I think aircraft flaps increase the effective camber of the wing surface and hence develop more lift - but if you combine it with a Handley Page slot on the front of the wing, you may be making a wing-shaped body of turbulence that acts as a more effective wing. cf the Flettner rotor which makes an air wing/sail to drive the ship. I've long wanted to develop a sail with flaps ahead of the mast to do the HP slot thing.
The wing generates lift because the air going over the curved top surface of the wing has further to go before it rejoins the air going under the wing. The air on top has therefore got to stretch itself out so it becomes thinner and therefore is at less pressure so the wing gets sucked upwards. But to develop lift the air does have to be passing over the wing at some velocity - just as the barge has to develop some speed before the leeboard becomes effective. You have to get a flow over it to make it work. This is why you can point higher as you pick up speed on the new tack.
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I think aircraft flaps increase the effective camber of the wing surface and hence develop more lift - but if you combine it with a Handley Page slot on the front of the wing, you may be making a wing-shaped body of turbulence that acts as a more effective wing. cf the Flettner rotor which makes an air wing/sail to drive the ship. I've long wanted to develop a sail with flaps ahead of the mast to do the HP slot thing.
The wing generates lift because the air going over the curved top surface of the wing has further to go before it rejoins the air going under the wing. The air on top has therefore got to stretch itself out so it becomes thinner and therefore is at less pressure so the wing gets sucked upwards. But to develop lift the air does have to be passing over the wing at some velocity - just as the barge has to develop some speed before the leeboard becomes effective. You have to get a flow over it to make it work. This is why you can point higher as you pick up speed on the new tack.
Sorry Tobyker but some of what you say is rather contradictory. You say, quite correctly, that flaps increase the camber of a wing and thus develops more lift. But then you say that the wing develops lift because the air has further to travel on the top surface.
But when plain flaps are deployed the distance the air travels on the top surface does not change.
What does change of course is the direction of the flow at the trailing edge - more downwards - supporting the theory that the main amount of lift from a wing is created by the deflection of air and the application of Newtons law.
Speaking from personal experience during my 1000+ hours as a glider pilot, lowering the plain flaps on my Kestrel 19 sailplane had a dramatic effect, resulting in the glider shooting vertically upwards as the lift increased momentarily.
Of course I am talking here about the observable effects of plain flaps - Fowler flaps are somewhat different - not only increasing the camber but also the wing area.
Leading edge slots delay the onset of the stall as they act somewhat like Bogstandard's venturi , accelerating the flow of air over the wing surface and delaying the turbulent breakaway of the airflow over the wing.
The problem with flaps and slots though is that at higher speeds they create a lot of drag so that they are most useful at low speeds.
Don B.
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..so no more about fans then, just wings and aerodynamics ? The fastest sail powered craft ,with the exception of sailboards have wing sails I think. Dunno what that proves but the thread was about FAN power. Anybody done one ? Perpetual motion experimenters step forward please,bring your patented "petrol from water pills" with you ;D
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I've long wanted to develop a sail with flaps ahead of the mast to do the HP slot thing.
Isn't that what the jib does when close hauled?
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I've long wanted to develop a sail with flaps ahead of the mast to do the HP slot thing.
Isn't that what the jib does when close hauled?
Yes that is what I thought too, and pre-Wright Bros, or Handley-Page ;D
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Thought so.
And the foresail can do the same for the main in a fore/aft rigged twin master.
Each sail is set about 5 degrees further out than the one behind it for good effect.
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Probably, but it is now a biplane (or triplane) so there may be "interference" between "main planes" . Having said that there have been "slotted" main bi-winged aeroplanes with main wings fore and aft such as the Flying Flea. (Please dont start the old saw about them being killers etc >:-o )
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I wrote a very long treatise on aerofoil shapes, flaps, slats and slots, but eventually decided to stop preaching to people who have their own views on how things work, and duly deleted it all.
So I thought I would show you MY way of explaining lift on aerofoils, yacht sails, lower pressures in venturis etc.
It is a very expensive exercise, but it does show that lift can be easily generated without the higher pressure underneath.
Take the sheet of paper holding the ends of the narrow side and pull it tight. Take a sharp intake of breath, and blow onto the top of the curve that the drooping paper produces. You should find that the paper at the back lifts up. This is caused by your blow travelling over the top curved surface and producing an area of lift, without any help from high pressure underneath. So in my mind, the area of high pressure isn't required to produce the forwards motion of a yachts sails, it only assists.
John
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Very wise John , succinct and proof in laymans terms :-)) now can we get back to fans >>:-( and the autogyro boat. :D %% :o
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No more fan power then here is one for tobyker and the druid to debate. Cant put a date on the pics but from the activity on the dockside looks like Canary Wharf tower isnt quite finished. The "triplane wing-sailed" PLANESAIL seems to have camber adjustable wings. What ever happened to this experiment ?
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I.
It is a very expensive exercise, but it does show that lift can be easily generated without the higher pressure underneath.
Take the sheet of paper holding the ends of the narrow side and pull it tight. Take a sharp intake of breath, and blow onto the top of the curve that the drooping paper produces. You should find that the paper at the back lifts up. This is caused by your blow travelling over the top curved surface and producing an area of lift, without any help from high pressure underneath. So in my mind, the area of high pressure isn't required to produce the forwards motion of a yachts sails, it only assists.
John
John,
Thats an interesting way of looking at, and interpreting it. But another interpretation is that the air blown over the top curved surface is then deflected downwards at the trailing edge (remember my suggested experiment of the spoon in the tap water flow ?) and the applicationof Newtons law to this downward flow causes the back to lift up !! :-) Obviously we can both adopt the interpretation that suits us !!
BTW - what happened to your explanation of what causes the downdraft from the helicopter blades which I suggested you gave us ?
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Arrow5 -
Is there anymore to be said about the fan power - other than , as has been pointed out, it won't work ??
I must say though that this thread has led me to look at a lot of the research and development which has been carried out on wing sails - fascinating stuff isn't it ?
Don
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Just pulling your leg Druid...hmmm , that was covered in the boot-strap explanation. :embarrassed: OK ,what about the autogyro blades at negative angle of attack creating "lift" enough to power a boat, Brabazon`s "Redwing" and others ? That is for John I think. ;D Yes I agree it is a fascinating subject. Lots of research for the better mouse-trap , all sorts of acedemic theories and learned technical testing in wind tunnels etc etc and yet the sailor still opts for the "tried and tested" cloth and rope devices that havent changed much in a thousand years. Apart from modern materials such as Kevlar, Mylar, carbon fibre and metal alloys etc the only change I can see is the squaring-off the top of the triangular shapes of sails (tip-stalling?). Many millions of quid spent on America`s Cup development and they still are not much removed from Thomas Lipton`s era. My son lives in Germany and crews for some very sophisticated international racing teams and yet his own boat is a vintage Dragon class woodie ! {:-{ Is it nostalga that make them hang on to old ways ? I dunno :embarrassed: thank goodness I`m not a sailor. Too many days hanging onto a wet rope, sorry LINE, getting very cold wet, near lighning strikes, getting bruised and abused....and and well I suppose a good party after the event sort of made up for that O0
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Just found this whilst hunting for something else (as usual):
http://www.rexresearch.com/boats/1boat.htm#ford
Some other daft ideas, too.
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Indeed but the jib is a big heavy thing that needs a stay to set it. I was thinking of really narrow chord flaps hung off the fwd edge of the mast with a fixed amount of float to make them self-tacking. of course to make them work properly you'd need an aerofoil mast with a luff groove.