# Conytrapel Propulsion

Discussion in 'Propulsion' started by tom kane, Nov 9, 2015.

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### tom kaneSenior Member

Good maneuverability of a boat is most important so how is this achieved by contrapel.
It takes a lot to beat an outboard or I/O with a driven propeller turning from side to side and rudders and vanes are not very effective.
They say it has revolutionary steering for maximum control.
Steering with the throttle is recommended. So you must have two units.

I would prefer a simpler system, less expense,lighter and more versatile.

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2. Joined: Jun 2009
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### baeckmoHydrodynamics

Just as Gonzo, CDK and others, I experience a slight smell of snake oil from the ”Contrapel” info. The only hard numbers available are the test values with the boat; 2 x 435 hp, displacement 8000 kg, max speed 37 knots and (probably) jet impeller diameter 0,33 m. This performance is not worth writing home to mom about, considering the actual weight/power ratio of 9,2 kg/hp.

To evaluate the statements about the “super performance”, let us take a basic look at the factors that drive the design spiral for a jet. The affinity laws for rotodynamic machines tell us that:

P= C x D^5 x n^3; where P is power, C is a dimensional constant, D is the impeller diameter and n is shaft rotational speed.

In addition there is always the cavitation problem; the local static pressure in the impeller must not be allowed to be reduced below the fluid’s vapor pressure in the operating pump. In real life, the impeller needs a pressure margin above the vapor pressure in order to function. This margin is called the “Net Positive Suction Head”, in short NPSH. The affinity law (slightly simplified) for the cavitation limit is:

NPSH=K x (n^4 x Q^2)^(1/3); where K is a constant, n is shaft rotational speed and Q is volume flow per time unit.

The constant here constitutes a “quality number” that has a fixed value for a certain impeller, so that the cavitation performance can be calculated for different operating conditions.

Now if we take an example, say we compare a one-stage pump for 400 hp at 2600 rpm and a dia of 330 mm with a two-stage pump with the same diameter and the power split 50/50 between the impellers. Tandem or contra-rotating does not matter here
.
From the first equation, we get that n2=n1 x (P1/P2)^(1/3), since D2 = D1. With the actual numbers we get n2 = 0,794 x n1; i.e. 2064 rpm. From this we get two limiting extremes:

A) Keeping the flow constant we can operate the pump at a lower limiting NPSH, due to the lower shaft rpm. The flow for the pump size and power here may roughly be taken as 1 m3/sec, and a “normal” single-stage pump would need a minimum NPSH of ~18 mwc (1,8 bar absolute), and with a pump efficiency of about 80 % the pressure increase would be 24 mwc (2,4 bar) with 400 hp. With “normal” inlet and nozzle losses, the thrust would be about 10,57 kN at a vessel speed of 15 m/s (~30 kn). This speed would be just above the cavitation limit for the single impeller.
With the two-stage configuration the limiting NPSH would be reduced to ~13 mwc , which is a significant improvement for the low- and medium operating speeds. The two-stage pump would operate cavitation-free down to about 19 kn. But note that since the flow is kept the same, the improvement is only seen in the low speed range, where the single-stage pump is loosing thrust due to cavitation. At higher speeds there is no improvement.

B) If the minimum NPSH is kept constant at the limiting value 18 mwc, the flow for the two-stage pump may be increased to 1,586 m3/s, according to the second equation. In this case the total pump pressure would be reduced by the factor 1/1,586 since the power is constant. The increased flow would result in a thrust increase to 10,87 kN at 30 kn, which is a very modest increase of 2,9 %. This increase is easily eaten up by the increased transmission and friction loss in the counterrotating configuration. The situation could be slightly different with another flow, but not enough to change the overall picture shown here.

Somewhere between the two limiting cases there may be an optimum with a slight improvement of the thrust at the cost of higher minimum cavitation speed, but the possible benefit is certainly not compensating for the increased complexity, weight and cost of the counterrotating two-stage pump. The only reason to use a counter rotating configuration would be that it does away with the stator vanes. But a well designed stator for a single impeller does away with transmission losses, so the difference is marginal at best.

A far better way would be to use a single stage pump with a booster impeller; an “inducer”, operating on the common shaft. The inducer could easily produce the pressure margin needed for cavitation-free operation of the main stage with an optimized (read “increased”) flow at low speed. Simple, known technology and even possible as an additional compensating measure in existing units.

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### Mr EfficiencySenior Member

Sounds like they won't be quoting you in the sales brochures, baeckmo !

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### tom kaneSenior Member

baeckmo..I agree..

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