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ContinuousWave: Whaler Performance
Propeller Basics: Bow and Stern Lifting
|Author||Topic: Propeller Basics: Bow and Stern Lifting|
posted 04-06-2002 12:11 PM ET (US)
This message thread is for comments on my article Propeller Basics: Bow Lift and Stern Lift which explains the principles which allow "bow lift" and "stern lift" to be incorporated into propeller designs.
posted 04-06-2002 12:22 PM ET (US)
I don't claim to have presented a scientific breakthrough in this analysis, but I don't think this phenomenon is explained anywhere else. Either the entire rest of the boating population has already come to understand how these propellers work as well and as easily as they understand how an electric door bell works, or they have just accepted the bow lifting and stern lifting claims without argument or inquiry.
For me, I need to have a clear grasp of what is going on with the simple physics of something like this. I think I have distilled the underlying principles on which these things operate and presented them to my readers.
If you disagree with my assessment, feel free to respond with comments or criticisms.
posted 04-06-2002 08:57 PM ET (US)
Nice explanation, thanks for presenting it. One question, can anything be inferred about the efficiency of different prop designs? At first glance, it would seem that less of the energy transmitted from "bow lift" props gets wasted in surface blow out so this should be the most efficient. One could conclude that the "stern lift" prop's characteristic of lifting more of the boat out of the water reduces drag to the point that it is more efficient in producing propulsion. What do you think?
posted 04-06-2002 09:15 PM ET (US)
Jim - I do not have experience in the design of props - however, I have worked extensivly in pressure wave propagation, fluid dynamics and thrust vector control. Accordingly, I take some exception to the discussion you presented and, as your topic caught my attention - will do some 'digging' into the subject and continue this thread as possible.
Incidently, I have never heard of the bow lifting or stern lifting props prior to your discussion. Previously, I would put these two concepts in the same context as a left handed monkey wrench. Therefore, I would appreciate any additional information you might be able to refer me to.
As with any flow/thrust device, the thrust generated is dependent on the atmospheric pressure at the exit plane of the device (the prop in this case). There is no thrust acting on the engine/boat other than at this plane. As such, no thrust is lost if the prop wash cone breaks the surface as indicated in your discussion.
In my current thinking, the thrust from a prop should be symmetrical and the thrust vector in the direction of the prop shaft.
However regarding your discussion, the only effect that I can see - and the issue I will study further and discuss with a naval prop design engineer - as the prop wash cone breaks the surface a pressure disturbance wave is generated. Pressure waves are reflected by contacting solid boundaries, however, I strongly doubt that reflection would result from the diffused prop wash cone exiting from the prop. But again, I am not sure and could be wrong.
However, if the pressure wave can be reflected via fluid interfaces and if the pressure disturbance reaches the upper part of the prop discharge plane before it reaches the lower part of the prop plane, the flow rate would be asymmetric which would then potentially explain the bow/stern lifting result.
This subject is a good problem and would be a good candidate for a doctoral dissertation - any fluid dynamics doctoral candidates out there?
posted 04-07-2002 07:01 PM ET (US)
Im not sure if I understand, the " elephant ear or stock aluminum is matched for decent overall performance, with trimming the boat you'll raise the bow but regarding " stern lift" would it just " blowout" ?
The performance props with thier aggresive rakes and alloys I bet do push the water as illustrated in a tighter thrust but are designed to be turning high RPM's close to waters surface, and rooster tails arent they just the result of overtrimming?
Im no prop expert so bear with me if wrong here,
|Tom W Clark||
posted 04-07-2002 08:19 PM ET (US)
I read this article of yours and it makes sense. I am wondering if this is all the result of theorizing on your part or if you relied on objective outside information at all. I see no bibliography.
Even after all the good threads regarding propellers and the one long one where we all speculated about how they work, I have to admit I never was able to visualize how they might bow or stern lift. Your theory here never occurred to me at all. I like it. But surely there is someone out there who really knows if you've got it right.
If the bow-lift/stern-lift is caused by a loss of downward thrust as the thrust cone breaks the surface then in reality all propellers are stern lifting. Some are just more so than others.
posted 04-07-2002 11:34 PM ET (US)
Jerry--With regard to use of the terms "bow lift" and "stern lift", these names seem to be in widespread use among performance oriented boaters. The people that make these props use these terms all the time.
Like you, I was at first confused by how a propeller that rotates symetrically about the prop shaft could produce forces that sum to anthing but in line with the shaft's axis.
Tom--I cannot cite any references because I have not read this anywhere else; I basically deduced this from that I could understand about propellers.
It seems reasonable that the shape of the blades could control the shape of the cone of water that is accellerated aft as a result of their rotation.
I really would like it if someone from Mercury's propeller development program jumped in here and offered to explain more.
posted 04-08-2002 05:58 PM ET (US)
Isn't this explaniation very similiar to the explanition of the difference in handling characteristics between clockwise and counter-clockwise rotating props?
posted 04-08-2002 07:43 PM ET (US)
I can't say about similarity to the "difference in handling of clockwise and counter-clockwise rotating props" because I have not seen or read that material. Do you have a source or pointer?
posted 04-09-2002 12:16 AM ET (US)
What I remember from the CT Safe boating course I took about 10 years ago is that a clockwise rotating prop will tend to push the stern of the boat to the right because a prop blade that is going from 3 o'clock to 9 o'clock has better water to push against than the same blade when it is at the top of its cycle and going from 9, past 12 and to 3. Therefor making the prop push slightly to the right.
This is simply a recollection of what some old coast guard captain talked about at the class when I was 14 years old - I was so excited about gettin gmy liscense the I could have misunderstood what he was talking about or I could have easily made it up.
I remember walking out of the class with a huge smile on my face thinking "now I can drive a boat" - that smile was quickly replaced with a look of dissapointment when I realized that I, nor any of my friends, had a boat to use.
I do for sure remember him talking about how you can make a boat with twin counter-rotating props move sideways - but I don't remember how he said to do that. It might have been to put one slightly in forward and one slightly in reverse.
posted 04-09-2002 12:28 AM ET (US)
My analysis of blade shape and stern lifting/bow lifting effects does not change with LH vs. RH props.
The bow lift comes from the blade angle and not from torque effects related to the direction of rotation.
posted 04-09-2002 12:54 AM ET (US)
A Mercury engineer once told me that bow lifting props do not really lift the bow. They work by pushing down on the stern, causing the bow to lift while planing.
I also understand that the stern lifting cleaver props are only for use above 80 mph, where the action of the propeller counters the areodynamic lift caused by the speed and too much air getting under the hull.
None of these bow lifting props are applicable for boat/engine combinations that won't achieve at least 40 mph. That is why they are not made in the lower pitches.
posted 04-09-2002 08:56 PM ET (US)
I believe Larry is on the right track here. I am no expert on props, however in the early days of the OMC SST bass boat props (Mercury too), the idea was to use a adjustable jack plate that allowed up and down movement of the engine to achieve ideal prop depth under various load conditions, in conjunction with the T&T function.
Under "hole shot" conditions you would literally suck the water out from under the transom and raise the bow. As speed increased the engine would be raised and trimed so the prop would be semi-surface pearcing and the boat would almost be running on the last 12" of the hull. The surface pearcing prop had a clever type look and one of the three blades was out of the water as it passed under the cavatation plate of the engine. The surface pearcing drive was pioneered by the "Arronson Drive" for off shore racers in the mid 80's.
I agree with Larry the technology has little or no use or value to a Whaler owner on a Whaler. Tilt & trim and in some cases trim tabs are of most value.
posted 04-09-2002 10:23 PM ET (US)
The left/ right hand prop performance characteristics are discussed in Chapman's classic text on seamanship in the chapter on power boat handling.
posted 04-09-2002 11:15 PM ET (US)
Walt--I cannot accept the theory that the effects of the blade shape only come into play at 40-MPH. I need more underlying explanation for the phenomenon than "it only works above 40-MPH". I am perfectly happy to accept this statement as an OBSERVATION, but it provides nothing in the way of an EXPLANATION.
If you compare propeller A which has "bow lift" with propeller B which has "stern lift" you will easily observe that the difference between them is the shape of their blades.
It seems to me that the mechanism by which they accomplish these different goal must therefore be related to blade shape and not related to the speed at which they spin.
SSCH and tlynch: I am familiar with effects induced by the direction of rotation, but my explanation depends upon blade shape and not upon the direction of rotation. One can make a bow lifting propeller for an engine that has RH rotation and one can make a bow lifting propeller for an engine with LH rotation. The shape of the blades will be identical, just mirror images. Again, it is the shape of the blades and not the LH or RH rotation which induces the effect of bow lift or stern lift.
posted 04-10-2002 01:31 AM ET (US)
Jim - I agree - there cannot be a prop configuration that 'kicks' in at 40 mph - however, there can be props that work better at low rpm/speed or at higher rpm/speeds.
I still am dubious of the bow lifting and stern lifting props that is being kicked around here. I hope to get to talk with a prop expert - however, there is a vast difference between naval props and the designs we are using on our Whalers. As JimH mentioned, hopefully a bona-fide prop expert designer will enter into our discussion.
Regarding the LR / RH props - one effect that will be evident is the boat will 'tend' to lean in the direction of rotation as power is applied. This effect will be more noticeable with larger diameter props. The reason for this is there will be an upward 'thrust' vector on one side and a downward 'thrust' vector on the other side of the prop. These two 'thrust' vectors act to twist the engine and hence the boat. -------Jerry/Idaho
posted 04-10-2002 03:54 PM ET (US)
I agree we are all amateurs here regarding propeller design. Mercury's latest prop cataolg says "What we know about propellers would take a lifetime to learn". I believe them. When it comes to Outboard and Stern Drive props, Mercury controls the technology, with others merely copying their design innovations. Here are a few quotes from Mercury's propeller catalog that could be relevant to this thread:
Regarding the mid-sized hub 75-125 HP engines:
"Bass boats and lightweight fishing boats (this could include Whalers). These will need propellers with higher pitch and may need the higher rake of the Laser II prop for holding and bow lift. Boats in the 45+ MPH speed range and of 90 HP or more can generally use this extra lift to help actually raise more of the boat out of the water, thereby reducing hydrodynamic drag.
Regarding V-6 outboards:
Laser II, High Five, Tempest, Trophy, Mirage & Offshore (props) can be used to increase top speed further by more substantial increase in "X" dimension/transom height on V-6 outboard powered boats. .....these propeller families provide superior holding for even greater bow lift and trim potential. An inherent design feature of the 4 blade Trophy series is the reduction of steering torque to the operator." (JimH - how's that for a new problem to solve!)
"Lighter and faster outboard powered boats requiring very high pitch (25" plus)(often 75mph or faster) can be of the tunnel type.
Here are a few quotes on the actual propellers lines:
Laser II: "Delivering excellent bow lift, the SS Laser II offers better top speeds and holding in turns. Thinner blades minimize drag for optional acceleration..."
Tempest Plus: Their higher rake and bigger cup help deliver great holding and lift."
Trophy Plus: "The larger area of four stainless steel blades delivers tremendous bow lift for quick planning and excellent stern lift to keep you running in rough water. When it comes to the extra torque put out by fuel injected engines, Trophy Plus will harness it."
Offshore series (4 bladed): "....Provides enrichments like trememdous lift for big twin engine boats. Better hook-up, even in rough seas..."
Mirage Plus: "...has the blade area to carry larger sized boats with ease...."
High Five: "...delivers the ultimate in hole shot. ...it also delivers good bow lift and handling, great holding in rough water, and a smooth low vibration ride."
So picking up some of the correct words here gives clues as to what aspect of these props provide certain performance capabilities.
Because of a separate, and interchangeable hub, Mercury props can now be
posted 04-10-2002 04:41 PM ET (US)
It's a little off the original subject, but I can verify Merc's claim that the 4 blade props will reduce steering torque. My 17 Mako is very sensitive to prop torque and heels to port at high speed. A guy at the local Admiral propeller shop told me a 4 blade would reduce it - I tried one and it worked! The heel was almost entirely eliminated. But the $400+ price tag has kept me with my 3 blade anyway.
Why a 4 blade reduces steering torque is as much a mystery to me as the bow lift/ stern lift thing.
The stock Yamaha SS props (the ones they paint black) have that cleaver type shape to them (although not radically so like a surface piercing prop), and I've heard folks say that they don't give any bow lift. So that would tend to support the theory here. But I don't know that from first hand comparisons. Interesting topic.
posted 04-10-2002 07:31 PM ET (US)
Well now I am stumped:
LHG quotes Merc:
"Trophy Plus: 'The larger area of four stainless steel blades delivers tremendous bow lift for quick planning and excellent stern lift to keep you running in rough water."
Here is a propeller that has both!
posted 04-10-2002 07:43 PM ET (US)
Well, Jim, just remember, it's a lifetime to catch up!
But for some reason, they advertized that both of the 4 bladed designs, Offshore (for boats like Whalers) and Trophy (mostly for bass boats) give TOTAL hull lift. Actually, I think all of the bow lifting props lift the whole hull a little. I remember switching from my "elephant ear" SS props to the Lasers, and the boat DOES feel like it rides higher out of the water.
posted 04-12-2002 01:48 AM ET (US)
I've eavesdropped on many posts over past few months and commend your forum for the wealth of info provided. I think this is the best boating site on the web. But I cannot buy into the theory of "thrust lost to surface breakthrough."
My Physics is a little rusty but here is the way I remember it. According to Newton's 2nd Law, F=ma; F(thrust in this case) is equal to the mass of water impelled aft times the acceleration given to that mass of water. Note that the thrust occurs at the instant the water leaves the prop and as far as the boat and motor are concerned it matters not what happens to the water after it exits the prop. Newton's 3rd Law would also be another way to look at it: forces occur in equal but opposite pairs so as to sum out to zero in a closed system. The thrust given to the prop is equal but opposite that given to the water at the instant that the acceleration to the water occurs. Therefore, I have a hard time accepting a theory in which thrust varies with propwash breaking the surface.
Never heard of bow raising or stern raising props but could this be explained by the trim of the motor and balance of the boat?
posted 04-13-2002 08:50 AM ET (US)
JimCha has raised a good point.
I respond with this explanation:
Consider for a moment the water being accellerated by the propeller to be like water coming out of a nozzle from a hose. Think of it like the boat has a big tank of water onboard, a large pump, and an exhaust nozzle that can be aimed.
The pump is run by electricity, and we have a volt-meter and current-meter connected to it. By measuring the voltage and current, and multiplying them together, we can calculate the energy being applied to the system.
I aim the nozzle skyward so all the exhaust from the pump goes into the air. I turn on the pump. The motor draws current. Water is accellerated. Work is being done. But the boat does not move forward much at all, if any! The water is sent into the air and makes a big fountain. The boat does not move.
Now I aim the nozzle so the flow of accellerated water is directed into the lake. I look at the readings on the current and voltage meters. They don't change a bit. The same amount of work is being done. The same amount of water in being accellerated and pumped aft. But now the stream of water produces THRUST. The boat moves forward.
What's wrong with this picture? Anything?
posted 04-13-2002 12:16 PM ET (US)
Jimh - as you and I discussed earlier - the thrust is in the direction of the prop shaft. And as I mentioned earlier, the thrust of the prop is that force (mass of water flowing through the prop times the acceleration of that water across the plane of the prop) acting at the exit plane of the device (the prop in this case). That is the principal reason that the thrust from a prop is not influenced by the prop wash diffusion cone breaking the surface. That effect is history - the prop plane is well forward of that occurrance.
And as I mentioned earlier - the only way that the diffusion cone breaking the surface can affect the thrust is for a pressure wave resulting from that occurrence to reach the plane of the prop - which I doubt would happen.
Now, regarding the two examples you brought up - with the electric motor powered prop angled upward and you mentioned the boat will not move - the developed thrust force is angled downward and forward. The boat would tend to dig a hole and not move forward all that fast - because the boat then has to move more water out of the way - which requires a lot of energy - basically the entire developed thrust.
With the electric motor prop angled downward - the developed thrust force is angled upward and forward. The stern of the boat will tend to be raised out of the water and the boat will move forward much faster than before - because much less water has to be moved out of the way and the bow will be lifted by the hydrodynamics of the water flow under the hull.
Of course, here we are only discussing the thrust force vector - but I think that everyone is in agreement that the thrust vector is in the axis of the prop shaft.
Incidently, the prop expert I wanted to contact is on his boat somewhere in the Pacific. I will eventually make contact with him and see what he has to offer - but it will not be in the time frame I had hoped. ------ Jerry/Idaho
|Tom W Clark||
posted 04-13-2002 02:09 PM ET (US)
I want to point out that a major assumption being made here is incorrect. The thrust vector is not necessarily in line with the prop shaft. The only time this would be true is when taxis of the prop shaft is in line with the direction of the boat, or more precisely with the flow of water under the boat. and past the prop.
This situation rarely occurs. Typically the axis of the prop shaft is considerably out of alignment with the direction of the boat and the thrust vector is thus altered. This is exactly why we have power trim on our outboards.
If the flow of water is not in alignment with the prop shaft the effective pitch of the blades changes as the blades rotate through their arc. This why we get P-factor or steering torque (not to be confused with simple propeller torque that causes the boat to list as the rotation reaction opposite to the spinning of the prop.)
Anybody with a steering system that does provide feedback knows the effect of this as the motor will want to steer to port when trimmed out and steer to starboard when trimmed in.
posted 04-13-2002 04:40 PM ET (US)
Excessive bow lift creates poor efficiencies and increases the wake --- in other words your running on your back side displacing a lot of water ....... Having the stern lifted and a practical level boat also know as on plane is the best performance with minimal displacement.......... this is what you want to achieve hopefully at low speed ranges so your able to keep that bow slicing in chop and not floundering around ........... you end up stern steering and control is questionable at best in all but relatively calm conditions with that bow up ........... some boats have a hell of a time achieving the proper attitude at low speeds thus we can counter this problem in three ways prop design and the use of trim tabs to force the bow down --- but the real need is the ability to run enough negative trim enough to accomplish the bow down stern "up" level position which is optimum .......read the referenced articles ............
Quit a few of the photos depicting boats at plane aren't really on plane at all in this sites photo gallery...... if you think the bow up in the air is on plane best think again ....... the boat as mentioned above should almost be level ........... by the way want to store all them cases of Canadian beer forward don't yeah know fellows up there in Michigana or you'll be walling around like a bath tub on the way home ..................
posted 04-13-2002 11:09 PM ET (US)
Tom - strictly speaking you are right - the angle of the prop shaft relative to the axis of motion of the boat controls the relative water flow angle entering the prop - however, as the water flow direction is being incrementally changed during the entire length of the prop, the thrust vector should be pretty close to being in the axis of the prop shaft at the exit plane. But I have not designed a prop or studied in detail the detailed fluid dynamics of a prop.
Bugsy - thanks - those links are quite informative and present an explaination addressing the stern lifting and bow lifting concepts. Good food for thought. ----- Jerry/Idaho
posted 04-13-2002 11:59 PM ET (US)
Moving engine back will give it more leverage and allow more bow lift. It also moves the cg of the boat aft which also allows higher speed. Unfortunately it give more bow up trim at lower speed and that can start a boat porpoising.
posted 04-14-2002 08:54 AM ET (US)
Bugsy--sorry to shoot you down, but your cited reference explain nothing about propellers. The just describe the results, not explain the mechanism.
There was some interesting observations on setback, but the people that sell "bow lift" propellers don't demand you move the engine aft to get the bow lift.
We need an explanation of how propellers create bow lift, not homilies on engine brackets.
You also introduce a divergent discussion--whether a particular photograph shows a boat on plane or not--but that is not the topic of this thread. You can start a new thread if you want to discuss the definitions of on-plane or not; let's just stick to an explanation of the simple physics of bow-lifting propellers for this thread.
posted 04-15-2002 12:22 AM ET (US)
Re the water hose analogy as similar to thrust of prop seems almost right to me but I would differ a little with interpretation.
If the water jet is directed straight up the resultant thrust vector is directed straight down but of course the boat does not move as this force is cancelled by the lift of the boat (resulting in no thrust to the boat). No work is done in this case upon the boat as here again we get back to basic physics, work equals force times distance moved. No movement, no work. If the water hose is directed straight aft then equal but opposite resultant foward thrust is applied to boat and it accelerates foward until drag equals thrust and boat then the boat no loger accelerates. work is being done in this case as force is applied and resultant movement occurs.
With regard to bow lift and stern lift, if it does exist, it could only be explained by the thrust vector being in an up or down attitude with regard to the axis of the boat. since this subject came up i did look at some prop claims in marine catalogues and though some of it looks like marketing, it seems feasable to design props so that in the hole shot they would have more up or down vector-eg., more cupping might give more upward lift as wheel spins up.
posted 04-15-2002 12:54 AM ET (US)
I was wondering if and where prop cavitation might play into all of this. Could you have slight cavitation, enough to effect performance but not enough to overspin?
|Tom W Clark||
posted 04-17-2002 11:33 AM ET (US)
I think you're missing the point. This is a Boston Whaler Forum where we talk mostly about Boston Whalers which are small planing craft. This discussion of propellers has nothing to do with displacement hulls.
Lifting the bow of a fast planing craft does not increase drag but rather reduces it by reducing the wetted surface of the hull and introducing air under the hull. The effect is substantial. Try running a Whaler with the motor at one throttle setting and cycle the motor from full trim-in to full trim-out and watch what happens to your speed.
posted 05-05-2002 10:26 AM ET (US)
I am not sure I myself believe what I wrote in this article. I am leaving the article as-is, but I have prepended a qualifier to it.
I'd like to see a good explanation of bow-lift, but I don't think we've got it yet.
posted 05-05-2002 12:17 PM ET (US)
Guru of the Whalerites,
Did Pit Bull have anything to do with your change of heart?
Even if his answer was not complete yeh know if your only considering the prop and not the other factors then it did hit the nail on the head.
Here Jimmy is another explanation which might prove interesting http://www.geocities.com/tsboatdesign/ctrim.html
Har d'har har the black arts of prop design.
Not getting technical which ain't my bag, them Whaler tri-hulls and the modified V hulls are a world a part when defining 'bow lift' what you want is 'hull lift' ah ha maybe I just coined a new term!
posted 05-09-2002 08:20 PM ET (US)
Jimh & others,
I just went through this thread, and I have a problem with the idea that thrust is lost if/when water breaks the surface.
If you take the analogy of a fireman holding a large hose with a high velocity nozzle, the force he has to use to hold the hose is the same whether he aims the hose at "open air" or at a brick wall.
I guess my theory is the stern lifting propellers provide additional lift from the radial wash hitting the cavitation plate and this causes a net upward force on the engine. The partial uncovering of the top half of the prop would also contribute to a net upward force.
posted 05-09-2002 09:46 PM ET (US)
Next time you're down at the dock, turn your hose on & hold it underwater. Now direct the water flow so it breaks the surface. You should be able to feel the difference in force.
posted 05-09-2002 11:34 PM ET (US)
As the originator of the theory of thrust being lost by breaking out of the water, I have to ask this question:
How does a rocket motor work in space?
posted 05-10-2002 01:27 AM ET (US)
the answer to the rocket motor in space question is that the rocket motor works the same at sea level as it does in space.
the thrust in either case is determined solely by the mass of expelled exhaust times the acceleration imparted to the exhaust the instant it leaves the exhaust cone. i think many people are under the impression that when a rocket blasts off it is the exhaust pushing on the air that gives it the thrust. this is not true. again, newton's 2nd and 3rd laws apply. rockets work the same regardless of
a boat's propulsion follows the same laws. hence, i too cannot accept the theory of force disapation at water's surface. as another poster noted, a firehose directed at a brick wall feels no different than one shot into the air. by the time you see the wake, you're looking at ancient history as far as the prop and boat is concerned.
posted 05-13-2002 11:06 PM ET (US)
even if the nozzle of the fire hose is an inch away from the brick wall?!?
posted 05-14-2002 03:30 PM ET (US)
Being a former rocket scientist, I will second JimCha's answer. The thrust of a rocket is not dependent on the atmosphere it is operating in. The movement of a rocket is caused by the ejection of mass (i.e. burned fuel) from the rocket in the opposite direction. Although the mass of the exhaust is much less than the mass of the rocket, it is traveling at a much higher velocity than the rocket so the energy of the exhaust and the rocket are identical, just in opposite directions.
energy of rocket = energy of exhaust
The rocket's thrust is not dependent on the what atmosphere it is in. The same principle applies to propeller thrust (in general). I do not claim to know anything about propeller characteristics, though. Intuitively, the ability of a prop to lift the bow or stern seems unlikely. The only way it could seemingly work (again, just conjecture) is if the thrust were altered to a level where the boat could not plane as easily.
I look forward to hearing more about the subject.
posted 05-15-2002 12:25 AM ET (US)
Thanks for the rocket science. It was thinking about rocket motors that made me abandon the lost-prop-thrust theory, which is why I asked (rhetorically) the question about the rocket motor in space. It pretty well demonstrates the problem with othe other analysis of the prop thrust.
posted 05-15-2002 02:07 PM ET (US)
I'm not sure how to reconcile the rocket science explanation that it does not matter what medium you are operating in to the empirical observation of the hose underwater that Ready2Rip mentioned. If the hose is under water, it sits pretty still, if it is out of the water it flies around like a, well, like a rocket. What's up with that?
You can try this in your bathtub with the hand held shower. No force under water, lots of force out of the water.
Could there be some term in the equation that looks at the differnce in density of the media?
posted 05-15-2002 06:59 PM ET (US)
I'm no rocket scientist, but I did stay at a Holiday Inn Express last night ; )
I don't think the spaceship example applies, as it does not take hydrodynamics into account. If you put your engine in gear & hold the throttle wide open while you're tied to the dock, someone in the water 10 feet behind you should be able to feel the thrust. As you get closer to the prop, the force will grow proportionally.
How is this possible? Because the water that is 1 foot from the prop is pushing against the water that is 2 feet from the prop (and so on). The bow lift & stern lift issue seems to come into play when the force coming off the prop is cone-shaped, and the force breaks the surface & doesn't have the water to push against anymore.
posted 05-15-2002 11:26 PM ET (US)
I think we're back in business again!
posted 05-16-2002 01:00 AM ET (US)
crashq and i are on same page.
would others accept the proposition that newton's laws of mechanics are the same in the heavens as on earth (or in the Holiday Inn Express, RTR)? Thrust (force) is the product of mass times acceleration imparted to that mass.
hydrodynamics is an interesting issue but the example offered is talking about pressure not force behind the exhaust cone. the pressure diminishes as the inverse square of the distance behind the prop-eg., double the distance and one fourth the pressure (lbs/square inch). Other forms of radiated energy follow the same inverse square rule--sound, light, electromagnetic radiation, etc. so if one were to stand in the wake of a tethered boat certainly the pressure wave increases fourfold each time the distance halves.
finally, i do not buy that a hose held under water exerts more thrust than one shot through the surface. if a difference exists i would bet the reverse is true: greater thrust from the nozzle discharged into the air. why? because the pressure under the water could diminish the velocity of the ejected stream and therefore the thrust.
still a skeptic.
posted 05-16-2002 11:27 AM ET (US)
In space, there are no molecules to push around, so space is infinitely less dense than water. I still don't think that the thrust from a spaceship nozzle can be equated to the forces on a propeller, but I may have unknowingly been wrong before.
posted 05-16-2002 11:19 PM ET (US)
well there is a reason why a propeller designed for underwater use is smaller than one designed for air. because the thrust of a propeller IS dependent on the atmosphere it is operating in. "crashq" how did you come to the conclusion that the sorounding atmosphere dosn't matter? if you had a firehose laying on the ground running, but not high enough to make it move i bet you could make it move by placing a piece of plywood close enough to the nossle thus changing the atmosphere it is operating in
posted 05-21-2002 02:34 AM ET (US)
R2R & Outrage 22,
irt rocket in space, absence of molecules therein, precisely the point: it doesn't matter as far as rocket engine is concerned. the thrust is the same at sea level as on the moon. a hose would do the same thing in space as on the ground should for some reason you would want to try it.
i concede a prop whether boat or airplane wouldn't work in space. but a prop still relies on the same principle as the rocket engine insofar as it must impart acceleration to a mass of molecules. The rocket of course ejects a stream of burned exhaust at high velocity. F=ma (mass of matter accelerated X acceleration imparted). An airplane prop is of different design than a boat prop as it must move a higher volume of less dense and more compressable air at higher acceleration to achieve comparable thrust. F=ma.
Finally, irt the hose on the ground possibly moving if an obstruction is placed close enough to nozzle. i concede this might happen but only if close enough to create hydrodynamic back pressure significant enough to impede the flow of water.
still a skeptic. JimC.
posted 05-21-2002 01:20 PM ET (US)
Hydrodynamic backpressure? If you're willing to accept the concept of hydrodynamic backpressure, then you're no longer a dissident. Welcome to the other side - jump in, the water's warm ; )
posted 05-22-2002 01:34 AM ET (US)
R2R, nice try. water seems cold to me.
my concession is only that if you put a hose up against a brick wall this might produce a back pressure against the hose. this would be based upon the scenario that you have introduced a decelaration to the stream of water-a negative force 180 degrees in oppostion to the thrust vector. another way to look at it would be the venturi effect in which pressure increases as velocity decreases. in any case i am not sure this would be a significant effect unless the wall is close enough to impede the flow of water from the nozzle.
the original propostion that thrust is lost as the wake breaks the surface seems far removed to me from the brick wall scenario.
still a skeptic with dry feet. JimC.
posted 02-02-2007 01:03 AM ET (US)
There is no such thing as Bow Lifting Props. The only correct term is Stern lifting Prop. The only exception would be generalizing the bow characteristics of bow thrusters on larger vessels. The thrust of any propeller and its inherit lifting properties is as so far from the stern of 99.9% of craft. The bow will lift or fall depending on how the prop in relation to trim is offered. The prop can not lift the bow unless it is under the bow.The cantilever effect from the stern is what dictates the running attitude of a power boat.Extreamly common misconception. James
posted 02-03-2007 04:08 PM ET (US)
Congradulations wipper. It took fives years for someone else to figure this out. The friction and density of the water lifts the bow. duh.
posted 02-03-2007 07:16 PM ET (US)
"...for someone else.."
Who was the first?
posted 02-05-2007 10:36 AM ET (US)
Trying to comprehend all this great thread, thx for reviving it.
actually, Ive been looking for the conclusive summary.
maybe I missed it.
I am in the process of going from twin 3 blade to 4 blade wheels. The prop vender mentions the 4 blade with a little bit of pitch removed will have less vibration (my primary target) and should raise the bow when running light (alrighty)...compared to the 3 blade.
The way I understand it, the 4 blade wheel with the same shaft angle as the 3's, will "reach and pull" more water, (less slipage) which, lifts the bow.
I know if I tie a rope to the back of a kids wagon and pull it forward, this pull will lift the forward end, possibly flip it.
Lets say the 3 blade equates to pulling the wagon from midships.
I dont expect the bow lift to be substantial, but certainly, if 4 blade wheels pulled the bow DOWN as a rule of physics, I might reconsider things.
|Tom W Clark||
posted 02-05-2007 11:25 AM ET (US)
No, no, no. I am not sure why this five year old thread was dredged back up. It would probably be best if we just started a new thread to discuss your situation.
Let me just point out that it is rather pointless to talk about the performance of propellers as if they are generic items that only depend on pitch and number of blades. It is not that easy. We have to consider the particular model of propeller(s) in question.
And yes, there certainly are "bow lifting" propellers, though one could argue endlessly about the semantics of that phrase.
posted 02-05-2007 05:11 PM ET (US)
I better hire YOU to dial in specs for my new Tug wheels.
I just run em & probably know just enough about how and why to get myself in trouble. Tug wheels quote $16k.
tho I have a 4 blade single on my BW 18 and considering twins also.
My best friend (DAD) isnt doing well. His time is short. Im not sleeping well, sorry bout the spaced out rambles lately.
posted 04-23-2007 08:38 AM ET (US)
I have a problem with bow lift in a following sea.
I feel that the problem can be rectifed by useing the correct propeller.
I read your info and would like to ask how four blade props would help bow lift, because I have been searching around the web looking for info for props and notice some are described as giving good bow lift.
My boat is an open design centre consol 36ft with twin 300HP Johnsons. I dont know how heavy it is but i believe it should be quite heavy because it was built the old school way, the fibre glass towards the rear I know for a fact is an inch thick.
Im not interested in top speed, but i am interested in maximum control at cruiseing speeds off shore. At the moment the bow runs low especially with big swells from behind.
Can anyone suggest a four blade prop and the reccomend a pitch, brand.
posted 04-24-2007 05:58 PM ET (US)
Contact Ron Hill Propellers, Santa Ana, Ca. on Google. He can build you a pair, or give you the info you need to know. He knows more about high speed props than anyone. He`s been building props for racing and high speed boats like yours for many years.
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