posted 04-06-2002 12:22 PM ET (US)            

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 09:15 PM ET (US)            

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.

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.

posted 04-07-2002 11:34 PM ET (US)            

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.

posted 04-09-2002 12:16 AM ET (US)            

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 12:54 AM ET (US)         

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.

posted 04-09-2002 08:56 PM ET (US)            

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 11:15 PM ET (US)            

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.

posted 04-10-2002 01:31 AM ET (US)            

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.

posted 04-10-2002 03:54 PM ET (US)         

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.
The 5 blade High Five, 4 blade Trophy props are designed for a wide variety of engines and boats. They offer better bow lift, acceleration (planing ability) and holding (for turns)."

Regarding V-6 outboards:

"..Offshore fishing and cruising boats.
...with standard "X" dimension/transom height
(25" generally) can use mid-pitch standard aluminum propellers for good all around performance. Comparably pitched Vengeance SS propellers offer additional durability, slightly better holding ability and increased top speed due to thinner blade sections. The Vengeance propeller will also allow a modest increase in "X" dimension/transom height.

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.
This type of hull generates bow lift areodynamically and does not require the propeller to assist in this function. Cleaver propellers, which provide stern lift, are most effective on this type of boat."

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.

posted 04-10-2002 04:41 PM ET (US)            

Why a 4 blade reduces steering torque is as much a mystery to me as the bow lift/ stern lift thing.

posted 04-10-2002 07:31 PM ET (US)            

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."

posted 04-12-2002 01:48 AM ET (US)            

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)            

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.

posted 04-13-2002 12:16 PM ET (US)            

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.

posted 04-13-2002 02:09 PM ET (US)            

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.)

posted 04-13-2002 04:40 PM ET (US)            

http://www.bogerprops.com/PitchandRake.html

http://www.scaryfast.com/prop.htm

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 ............

posted 04-14-2002 08:54 AM ET (US)            

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.

--jimh

posted 04-15-2002 12:22 AM ET (US)            

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 04-17-2002 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.

posted 05-05-2002 12:17 PM ET (US)            

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.

posted 05-09-2002 08:20 PM ET (US)            

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.

Comments?

posted 05-10-2002 01:27 AM ET (US)            

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

posted 05-14-2002 03:30 PM ET (US)            

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.

posted 05-15-2002 02:07 PM ET (US)            

You can try this in your bathtub with the hand held shower. No force under water, lots of force out of the water.

posted 05-15-2002 06:59 PM ET (US)            

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 05-16-2002 01:00 AM ET (US)            

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.

JimC

posted 05-21-2002 02:34 AM ET (US)            

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 05-22-2002 01:34 AM ET (US)            

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.

posted 02-05-2007 10:36 AM ET (US)            

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.
Is it possible the additional blade has 25% more grab at the same RPM/engine torque and thus moves the "pulling point" 25% further back on the wagon?

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.

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.

posted 02-05-2007 05:11 PM ET (US)            

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.