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Author Topic:   Propellers: Bow Lift -vs- Stern Lift
jimh posted 09-28-2001 12:22 AM ET (US)   Profile for jimh   Send Email to jimh  
Over the past several months there have been some threads discussing propellers and their characteristics. One aspect which has not been adequately explained (to my mind) is the ability of various propeller designs to exert "bow lift" while other designs can exert "stern lift."

I summarized my incredulity on this question by asking "How do it know?"

Today, I have had some new insight on this problem, and I will present it here for comments.

The difference in a propeller's "lift" characteristics has to do with the direction in which the blades focus their thrust. In propellers which are termed "bow lift" props, the design of the blades is such that the thrust is produced in a vector that is mainly directed straight aft of the prop. Very little thrust is produced radially around the prop--it all goes straight back. This causes the boat to tend to pivot upward around its center of gravity/center of resistance, which in turn raises the bow.

This explanation seems rather straightforward and is pretty easily understood.

Now for the case of the "stern lift" propeller...

To create stern lift, the propeller blades are designed in such a way that they also produce thrust that radiates from the prop in a direction at an angle to straight aft. Since the prop is rotating, there is an equal amount of this thrust directed upward as there is directed downward. Initially you would think that these cancel out and there is not any cumulative effect.

However, you have to consider that the thrust going upward acts on water that is much "thinner" than the water below the propeller. And the water above the propeller can be aerated with bubbles. The thrust coming off the prop which goes upward therefore does not produce much of a resultant moment on the boat. Most of the upward thrust might even go right through the thin water above the prop and blow out into the air.

The thrust that goes downward acts against solid (and deep) water, and therefore it can produce a resultant moment acting on the boat, which creates a tendency for the stern of the boat to rise.

This effect reinforces itself, because as the stern rises, the upper thrust vector coming off the prop sees thinner water and more aerated water, reducing its power. This leads to more stern lift.

In addition, there is the action of the cavitation plate which would tend to deflect any upward thrust back 180-degrees and downward, again tending to create a rise in the stern.

In extreme cases, the stern lifting prop can lift the boat so high that the upper blades of the propeller are entirely out of the water, a situation in which they cannot produce any thrust acting upward and hence making the stern go down. These are known as "surface piercing" props. This type of propeller is used on highspeed hydroplanes which are designed to run with the stern of the boat entirely out of the water.

Comments and corrections are welcomed.

--jimh

Dr T posted 09-28-2001 02:00 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Jim,

Your arguement seems to hold together reasonably well.

This line of reasoning also suggests why there are mixed results with cavitation plate hydrofoils. Since the bow lift props will have a force vector in line with the axis of prop rotation, the wing of the Doel Fin will generate the lift that will tend to bring the bow down. Thus the user of one of these props will get the most efficient forward motive force together with popping the boat up more quickly onto plane.

If a user has a stern lift prop, the results with the hydrofiol would tend to be a bit more complex. The force directed normal (at right angles) to the axis of prop rotation will be scattered by the hydrofoil, which will act as a big cavitation plate. through part of the arc of rotation (while the prop blade is traveling towards the surface) the force will be directed back toward the prop blade which will place an additional load on the engine. Thus, Jim's arguement would imply that hydofoils will not work as well with this type of prop.

Make sense?

Peter posted 09-28-2001 12:09 PM ET (US)     Profile for Peter  Send Email to Peter     
This theory has much to like about it. Now all we need to do to test the theory is to make and mount a zero pitch propeller, wind up the engine to WOT and observe whether the stern lifts up. Any volunteers?
Dr T posted 09-28-2001 01:22 PM ET (US)     Profile for Dr T  Send Email to Dr T     
If anyone does this, Peter, I want to see the picture. I will put next to the one of Tom Clark in his flying Outrage.
Flipper posted 10-01-2001 01:58 AM ET (US)     Profile for Flipper  Send Email to Flipper     
Jim, I like your analysis alot, and this is the only thing I think I can add to it:
Given that the drivelines of boats that run surface piercing props (hydros and v-hulls)are usually so high that the prop is running half out of the water most of the time; this is my take on 'transom lift':
To achieve transom lift, the 'cleaver' is usually used (among other variants)
because the straight trailing edges of the blades,as they re-enter the water parallel to
the surface, create the most lift while they
'impact'with the water.
Hope this is close!

Clark Roberts posted 10-01-2001 07:22 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Jim, best explanation I've heard and it works for me.... Happy Whalin'.. clark.. Spruce Creek Navy
Hank posted 10-02-2001 12:37 AM ET (US)     Profile for Hank  Send Email to Hank     
Jim
In your analysis you are relying on the effect of "thinner water " on the upper part
of the prop's "radial thrust" to raise the stern. Consider what happens to the axial thrust at the same time. The thinner water at the top means greater axial thrust at the bottom at the same time your scenario is taking place. This results in a moment which causes the bow to be pushed up ! I wonder if you can really get so much "radial thrust" that it would overcome the effect of the axial thrust.

Hank

Scanner Boat posted 11-06-2001 11:52 PM ET (US)     Profile for Scanner Boat  Send Email to Scanner Boat     
I do not like even using the term Stern Lifting at all unless you are talking about trim tabs. I think that the standard language when talking about props should be to say that a prop is more directional thrust pattern (bow lifting) or less directional (neutral) prop design. I can prove this by removing some variables in a theoretical test. Take any prop you want and mount it up to a submarine with a fixed propshaft. Once fully submerged with basically equal flow above and below the centerline of the prop the bow of the sub should not raise or lower, however if we could measure the cone of thrust it would be very narrow or almost cylindrical behind a so called bow lifting design and very wide behind a neutral (stern lifting) prop more like the cone we eat ice cream out of. In my opinion on a boat any prop can lift bow or stern depending on the trim angle just that some props can lift it more than others. Scanner
jimh posted 11-07-2001 08:49 AM ET (US)     Profile for jimh  Send Email to jimh     
Scanner Boat--Thanks for your comments; they are really quite insightful.

First, I think you have introduced an excellent concept in observing that trim tabs should be called "stern lifters." That is a good way to think about what they are actually doing. This may also account for some of the improvement in performance that is often cited in their use. The tabs help to raise the stern of the boat, reducing wetted surface and decreasing the amount of engine lower unit submerged, all of which reduces drag.

Second, your suggestion to consider the effect of a propellor when used on a submarine is a classic. Yes, it becomes clear that there cannot be any element of the propeller that would cause the submarine to suddenly assume a bow-up trim, since the effects of rotating the propeller would cause any thrust vector to be distributed equally around the shaft axis. An excellent example!

--jimh

Flipper posted 11-08-2001 12:31 AM ET (US)     Profile for Flipper  Send Email to Flipper     
What about P-factor? In aviation,we encounter this when in a climb: Air not striking the spinning propeller at right angles causes more thrust,and more lift, on the side of the downgoing blade. This is because the downgoing blade is at a greater angle of attack to the relative airflow.
This force may act on a sub's prop in the same way, on a 'cruise climb' underwater.
-But does this mean straight-inboards have a stern-lifting quality built right in?
Tom W Clark posted 11-08-2001 01:05 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
flipper, we have talked about P-factor before in threads pertaining to propellers and torque steer. You are correct in observing that a propeller's thrust will not be equally distributed when the axis of rotation of the prop is not the same as the direction of travel of the prop. But the manifestation of this phenomena is torque steer.

Anyone with a pt&t motor and steering that allows feedback can feel the effect of P-factor by trimming the motor all the way in and out. Assuming the boat and motor is trimmed for neutral steering when the motor is in a normal cruise attitude, the boat will steer to starboard when the motor is trimmed out and to port when it is trimmed in.

Inboards are not stern lifting due to P-factor, but rather due to the downward angle of thrust. P-factor does, however, tend to steer an inboard to one side.

lhg posted 11-08-2001 12:30 PM ET (US)     Profile for lhg    
Jimh's comment about trim tabs actually being "stern lifters" is interesting. I would agree with his observation.

This shows product marketing at it's best. "Trim Tabs" is such a slick name, bringing up visions of improved performance, speed, aerodynmamics, the ability to trim a boat up OR down, etc. In reality, this is not true. They can only force a bow down, by increased drag, but not raise it for increased performance. Only power trim or a performance prop can do that.

Could you imagine them being marketed as "Stern Lifters"? What a clumsy, heavy handed, non-efficient, performance killing, idea! But that is really all they CAN do - lift the stern to throw more weight to the bow, to keep the bow down in slow going through waves or a chop. And this is not all bad. There is no doubt that for speed, they would decrease efficiency due to the extra drag and slow the boat down. But perhaps, the extra speed in a given wave condition achieved by keeping the bow down, digging less of a hole through the water, more than makes up for the additional drag created. Hence their popularity.

So if you need to address this problem with your hull, get yourself a set of Bennett or Lenco Stern Lifters!

Flipper posted 11-10-2001 10:01 AM ET (US)     Profile for Flipper  Send Email to Flipper     
Tom, please excuse my laziness in not checking the archives on P-factor, and I do agree that the overall effect is torque steer.However, I'd still be very interested to know if there were even a tiny lifting tendency to that downgoing blade when trimmed in.The way I see it, when a propeller blade creates thrust, it also creates drag. When one blade is creating more thrust than another(torque steer), would there not be more drag, and consequently, an upward reaction to that drag when trimmed in and a downward reaction when trimmed out?

Also: My 90, before I got my hydraulics, liked to turn ( violently with a 26" pitch) to starboard when trimmed IN,not out, as you
mentioned.That would mean my prop turns clockwise when viewed from behind. Trimmed in, the greater thrust would be on the starboard side of the prop 'disc', forcing the engine to starboard, resulting in a starboard turn.
One thing's for sure, I do love those hydraulics!

kingfish posted 11-10-2001 11:26 AM ET (US)     Profile for kingfish  Send Email to kingfish     
jimh-

I think what trim tabs do (lhg suggests this in his post) is differentially lift the stern. Since there is no additional or compensating force (other than speed or trim angle of the motor) available to lift the bow, the bow is forced down, relative to the surface plane of the water. Wetted surface is increased, not decreased assuming the boat is on plane (if the boat is not proceeding with enough speed to plane, the effect of the tabs is very limited anyway); more trim (stern lift) brings more of the forward bottom of the hull into contact with the water, but it doesn't lift the stern out of the water. There is no decreased drag from having less lower unit in the water, because at plane the only way to decrease the depth of the lower unit is with a jack plate or by trimming the motor up. To the extent that the deadrise at the bow is sharper than the deadrise at the stern, more trim produces a better ride in choppy seas at cruising speeds by initially "piercing" the wave faces with a sharper rather than blunter object. The other advantage to trimming with tabs that counter acts the negative effects of increased wetted surface is that the hull is forced to run more closely to parallel with the surface; the trade off can be thought of as the difference in drag between the entire stern plowing and fighting the forward propulsion, or the tabs plowing and fighting forward propulsion. The tabs have less surface area than the entire stern and even though their action causes more wetted hull surface, it is at a better angle of attack.

Trim tabs never were able (or designed) to increase top end performance (speed) in smooth water. They are able give a smoother ride at cruising speeds in choppy water, apart from their ability to level a boat from side to side, under most conditions with most boats, and they can bring the bow down to effect better line of sight, all with less loss of propulsion than by trimming the motor(s) down. I'm sure racers have additional perspectives, but I think the dynamics are generally the same.

kingfish

kingfish

Flipper posted 11-10-2001 12:12 PM ET (US)     Profile for Flipper  Send Email to Flipper     
A correction: omit "That would mean" out of the sentence on the fourth line up from the bottom on my last reply- it confuses things. thanks, - F
Tom W Clark posted 11-10-2001 01:39 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Flipper, you are absolutely correct. I misstated my previous post. The boat will steer to starboard when the motor is trimmed in and to port when it is trimmed out.

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