propeller between catamaran

[Bemair]

Hello everyone. I recently came across an article: "CFD Study of an Innovative Catamaran with Asymmetrical Hulls". In short, the authors of this article hypothesize that it is advisable to install propellers in the space between the hulls of an asymmetric catamaran (flat outside), namely in the place where the water reaches maximum speed. I have a question for the experts of this forum, do you think it is advisable? And let's imagine that the design issue has been resolved.

 

[TANSL]

Where would you place the engine?

 

[baeckmo]

No, that's a bad idea. The thrust is (mass flow)*(velocity change) and the necessary pumping power is (volume flow)*((exit velocity)^2-(inlet velocity)^2)*(fluid density)/2/(pump hydraulic efficiency). Now, if the inlet velocity is increased, the exit velocity has to increase in order to retain the required velocity change for the specified thrust.

That leads to an increased pumping power requirement and a corresponding efficiency loss, since the effective power, ie thrust*(ship velocity) is produced by increased pumping power. This is why waterjet efficiency increases when the low-speed boundary layers are ingested, and why midship paddlewheels are less efficient than stern wheels.

 

[Bemair]

Where would you place the engine?

In my understanding, it's in the catamaran hull. I have two ideas on how to connect the propulsion system to each other. First idea its install engine in middle of one hull and connect propeller with help propeller strut.Propeller strut will be install on some angle to the central line plane catamaran. Second idea its install diesel engine on middle and connect RIM propeller with help electrical wires, which will be install in bridge

 

[jehardiman]

No, that's a bad idea.

Bemair, welcome to the Forums!
From a pure propeller efficency perspective, baeckmo is correect. However the following statement ....

Curves of speed component Vx, (in the same direction of motion) inside the canal and along domain, in the case A, show the possibility to effect a partial recovery of the provided energy to the mass of water. In fact, hypothesizing the propeller installed inside the tunnel in correspondence of the section where water reaches the maximum speed, the kinetic energy of water can be converted into energy of thrust, increasing the catamaran efficiency. This problem is not treated in this paper and may be object of further research.

...is subject to interpertation based upon where you draw the energy boundries and what "efficiency" you are trying to calculate. The quote is also ambigious WRT Figures 20 and 21. Does "speed component Vx, (in the same direction of motion)" mean the complement velocity of what is shown in the figures? The photos and renderings specificaly show fluid stacking with considerable Vz so any flow in the direction of travel (normally encountered astern of the vessel) between the hulls forward of position 0.5m would need to be re-evaluated with respect to any propulsor placed there. (FWIW, I think someone has made the classic CFD error, i.e. the Bernoulli/d'Alembert issues of moving ship vs moving water....carefully read the Asymmetric Hull discussion on page 2).

However, from a thrust perspective, flow between the hulls would function like any accelerating nozzel (i.e. like a Kort Nozzel, see Figs 20 & 21 when the position is aft of 0.5m as described in the quote). In the specific case where the prop diameter is limited (i.e. you want to keep the vessel very shallow draft as per the papers' model which had T as 7.5% of L), and you need a specific thrust as opposed to maximizing propeller efficiency (i.e. you can throw HP at the problem), then the concept may have some merit...YMMV.

 

[fallguy]

Answering is outside my paygrade, but I have a catamaran, so I’ll add my dime.

Placing the engines inside the hulls means they will be in turbulent water. This is not ideal.

The ideal is water that is less affected by the hulls.

 

[bajansailor]

This is all outside my paygrade as well, but I have a question - shouldn't a propeller ideally have a transom ahead of it (for an outboard engine) or a hull surface above it (for an inboard engine) re possible issues with ventilation / cavitation?

 

[gonzo]

This is all outside my paygrade as well, but I have a question - shouldn't a propeller ideally have a transom ahead of it (for an outboard engine) or a hull surface above it (for an inboard engine) re possible issues with ventilation / cavitation?

You get the possiblity or running a surface piercing propeller

 

[jehardiman]

This is all outside my paygrade as well, but I have a question - shouldn't a propeller ideally have a transom ahead of it (for an outboard engine) or a hull surface above it (for an inboard engine) re possible issues with ventilation / cavitation?

You get the possiblity or running a surface piercing propeller

Yeah, but don't do it this way....
Surface drive with large diameter propeller https://www.boatdesign.net/threads/surface-drive-with-large-diameter-propeller.68788/

 

[Ad Hoc]

This is all outside my paygrade as well, but I have a question - shouldn't a propeller ideally have a transom ahead of it (for an outboard engine) or a hull surface above it (for an inboard engine) re possible issues with ventilation / cavitation?

Make sure you don't fall into the trap of confusing the two.
Ventilation, requires "a path" for air to be dragged down to the surface of whatever "it" may be. With an outboard if the prop is too close to the surface, i.e just about tips touching or very very close to the surface, the torque of the prop can draw air down. Which is why outboards have a ventilation plate...but incorrectly called cavitation plates!

 

[Jimboat]

What is the performance application or expectation of your proposed boat?

 

[philSweet]

I think the main fallacy here is that you will actually be able to find an accelerated flow between the hulls. The bow shock losses, spray, interference drag of the hulls, and seaway motions will make that unlikely. I though convention wisdom was to have a straight interior tunnel to lower interference drag. The answer probably depends on scale since seaway drag terms don't scale like wave drag does.

 

[Bemair]

What is the performance application or expectation of your proposed boat?

I am currently considering this option for my graduate thesis project. The problem is as follows:The picture shows that a "pocket" is formed on the transom, in which it is impossible to place the propeller. The propeller is also not possible under the vessel, as the vessel will be operated in a shallow area. As an option, it was possible to place a water-jet propulsion, but I do not think that it will be effective on the Froude number equal to 0.661. The length of the vessel is 24.64 at the waterline, the speed is 20 knots.

 

[Bemair]

Thank you for your answers and hints

 

[gonzo]

The problem is as follows:The picture shows that a "pocket" is formed on the transom, in which it is impossible to place the propeller.

I think all the high performance catamarans running surface piercing propellers show it is possible.