Pusher vs Tractor/Puller Propeller

In the thread http://www.boatdesign.net/forums/props/contra-rotating-prop-designer-46298.html BMcF said that he was working on a project involving a tractor configuration propeller and jehardiman responded "Tractor [configuration propeller] is not really smart, 10-15% less efficient and the only reason would be for motor cooling. Being a tractor you will really need to pitch up the root, or make the hub very large, both of which reduce efficiency."

I'm curious about the relative merits and efficiency of the pusher configuration vs tractor configuration for watercraft. The pusher configuration is used by the vast majority of watercraft though the tractor configuration recently has been used with pods. It's interesting to me that the vast majority of propeller driven aircraft are the tractor configuration, though pusher configurations are more common than in watercraft.

I looked through the naval architecture references I have and didn't see much about the tractor configuration. There was a little bit more in some references on aircraft design. An internet search yielded a bit more information but mostly on "enthusiast" forums.

For efficiency I've seen claims that a tractor configuration propeller can be more efficient because the inflow is more uniform without anything in the flow entering the propeller. I've also seen claims that for aircraft there is increased drag due with a tractor configuration due to the flow behind the propeller and over the engine nacelle/fuselage/wing being accelerated in the propeller wake. Another claim for aircraft at least is that a pusher propeller located aft of the fuselage can reduce drag by accelerating the flow over the aft portion of the fuselage and delaying separation.

Noise and vibration is another area where various advantages and disadvantages. The pusher configuration is thought to be likely to vibrate more and radiate more noise due to greater inflow non-uniformity and asymmetry. For aircraft with a propeller mounted ahead of the fuselage the propeller wake interacting with the fuselage can be a source of noise.

Then there are the packaging and operational considerations. For watercraft the pusher configuration may be less likely to suffer damage from debris in the water, which can be important for some watercraft. This is generally not a concern for aircraft.

Responses with comments, thoughts, questions and references will be appreciated.

 

When Volvo Penta came out with the new drive pods, in a tractor configuration, the test boats could attain the same speed with approximately 20-25% less power. I have not seen any claims to the contrary substantiated by any data. The original Regal Marine boats with the setup are some I am personally familiar with, so I know the results were not fixed.

 

I am more used to pusher puller terminology.

First, I have never heard any claims that pusher aircraft have more vibration than pullers. It is usually the other way around.

The reason aircraft prefer puller over pusher is very simple.

Flight control.

With aircraft, pushers behave like jet engines. That leads to a different behavior when flying, and different concepts for pilot behavior. The flight envelope is very different for pusher from puller designs.

NOW, with ships, the impact upon ship behavior would not be as extreme.

HOWEVER, you are not really discussing pusher versus puller.

I think you are DISCUSSING pods. Upon aircraft, pods have been used. However, placing the engine out of line of the main body causes significant engineering problems. The greater off of center, the greater the problems. The greatest immediate problem is fuel efficiency. About the only common aircraft using something similar to pods would be flying boats.

The level of engineering problems in ships is not as severe as it is with aircraft.

 

To the issue of pods.

Pod propulsions are able to move the drive line out of the turbulence of the hull(s). By giving the prop (or jet-drive) less disturbed water, the efficiency of the drive system will be greater.

However, draft is a problem. Since your drive system is dropped well below the hull to achieve this increased efficiency, you place the drive system at a much greater risk of debris, and impact ....

That is the main reason the traditional pusher configuration has dominated ship and boat drives. Naturally, technology has improved. Gears, etc are stronger. So, engineering a non-traditional drive is easier now than it would have been 50 years ago.

IMHO.

 

Could you elaborate on what the differences are and what causes them, or provide a reference?

Actually I asked about pusher versus tractor/puller configurations in general. Pods are one implementation of tractor/puller configurations with the propeller mounted at the front of the pod. But a pod can also be a pusher configuration with the propeller mounted at the rear of the pod. It is possible (though very uncommon) to mount a tractor/puller prop near the bow of a boat.

 

A true puller configuration would be very inefficient in shipping.

IMHO.

 

That is true only when comparing to traditional propeller shaft installation at rather high speed. Pods are not more efficient than stern drives. It's not about pulling vs. pushing. It's about the added drag of propeller installation and rudders, better propeller shaft angle, less disturbed inflow to the propeller and less disturbed hull form.

 

The following paper shows gives a good summary on the different advantages/disadvantages of tractor vs puller propellers in chapter 3.2.2:

Dynamic positioning conference sept 2004
Hydrodynamic Aspects of steerable thrusters
by J. Dang & H. Laheij

Reference is also made to Heinke et. al 2003 who has apparently done some research into both modes, the results of which is illustrated in below graph:

Visible is that at low thrust load coefficients the pulling arrangement is more efficient than the pushing unit, however this is switched in the higher thrust loads (above Ct = 0,5)!

 

..
good paper that is Tackwise and what reasoning and solutions are there, fascinating
why didnt volvo opt for trimable tracktor legs in the first video at 50 secs, drivetrain losses?
and as DCockey mentioned could a pod (or even a prop) be mounted at the bow of a ship?
azimuts have props in front and rear of the pod, is that push and pull?
some planes do and a little cessna with such configuration seems rather hot
boatdesign member DMacPherson must have more on this than i find on his site
i better catch some sunshine before staring props in the dark yet its all to beautifull

 

Ok, David, since I am quoted I will respond.

Most of this I examined when doing my senior study on a SWATH, and like many who have not studied the problem, I thought about tractor props. There are a whole lot of things going on in each individual installation. And the standard answer of "it depends" always applies, so...

1) Better over all efficency is achived by running the propeller in the vessels wake. This is because of the hull efficiency trade off between wake fraction (w) and thrust deduction (t). Hull efficency = (1-t)/(1-w). Thrust deduction is the increase in drag (seen as lost thrust) due to the suction of the propeller over the hull and wake fraction is the reduction in speed of the inflow to the prop. For well designed prop clearences, hull efficiency is > 100%, but the maximum for a bow tractor would only be 100%. Note that is may not be significant for small boat planing i/o units which run out of the wake anyway, but if you notice large ship Azipods, they are generally tucked up under the hull to take advantage of the wake and reduce draft.

2) Skin friction is lower for a pusher because the speed of flow over the hull is lower. For a bow tractor, there really isn't significant a reduction in Cr by a small increase in Rn, but there is a significant increase in V^2 due to the wake. Again this in not an issue for small small boat planing i/o units or most podded MPUs that are dead aft.

3) For a tractor, the momentum disk area is smaller because of the driving machinery and/or hull blocks the disk. How important this issue is depends on how much of the disk is blocked and the included angle of the thrust cause by the flow around the machinery/hull. On small stern mounted shaft driven units, it is not much of an issue, especially if the drag of rudders and shafts is despensed with (i.e. as Joakim points out the increase in efficency comes from the removal of other equipment, not it being a tractor). On big EM driven stern mounted MPUs, there is a fair loss, but the flow over/through the units are used as cooling. For a bow mounted tractor, this is a huge issue.

So overall, it depends on the actual installation, with a pusher winning out in efficiency for most hull forms by about 10%. And there there are the specific applications where efficency is secondary to maneuvering or other requirements such as double-ended ferries with in-line wheels or 4 pods, dynamic positioning for semi-subs/drill ships, berthing tugs, etc.

 

jehardiman, thanks for the explaination.

Tackwise, thanks for the reference and graph. Section 3.2.2 discusses the factors affecting efficiency of the two configurations with a pod
drive. From the reference Tackwise linked discussing the Figure 19 he included:

"Heinke et. al. (2003) have carried out systematic research over the influend of the pod hosing on the total propulsion efficiency, by using both pushing and pulling arrangement. Their results are illustrated in Figure 19.

It is concluded by Heinke et. al. (2003) that a pushing unit is more efficient than pulling unit when the propeller thrust load is high."