Best Equation to determine thruster force?

[oliebol]

Hello all,

What is the best equation to determine the reaction force for a tunnel thruster (bow or stern)?

I heard about involving windarea and even transverse current.

Please help me out!

Greetz,

Martin

Unigraphics NX4,NX3,NX2,R17
Inventor 2008,11,10,9,8,7
Solid Edge 20,10,9,8
Solid Works 2000,98
Mechanical Desktop 4,3
Autocad 2004DX,2000,R14, R12
Teamcenter Express 3, 9

 

[chief]

Tie your vessel to a dock wall and install a graduated gauge in your wire or rope spring. Ramp up the thruster and take your pull readings at various thruster speeds and angles.

Offshore Engineering&Design

 

[oliebol]

That would be a nice way to check the equation, but i need a proper equation first bcs the yacht is still in the design phase.



Unigraphics NX4,NX3,NX2,R17
Inventor 2009, 2008,11,10,9,8,7
Solid Edge 20,10,9,8
Solid Works 2000,98
Mechanical Desktop 4,3
Autocad 2004DX,2000,R14, R12
Teamcenter Express 3, 9

 

[GregLocock]

What do you really want to calculate? The force on the hull from the thrust bearing?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rob768]

I got some rough formulas for an estimate of thrust for propellers (both open water and tunnel). As we do bollard pulls as well, we know them to be a fairly good approximation. I'll look them up for you tomorrow

greetz
rob

http://www.technofysica.nl

 

[rob768]

Been searching thru my cabinet, but can't find what i was looking for. Only thing I have come across are some very rough rules of thumb. I also asked the guys who do the bollard pull, but they can give only rough estimates. Sorry.

 

[oliebol]

Ok,

Let me rephrase my question.

What are the inputs to determine a thruster in general?

The result is in kN. Thats a given. But what should I put in for the rest of the equation and what does the equation look like?

I have seen some calculations before, involving windarea and a pivot point.

Unigraphics NX4,NX3,NX2,R17
Inventor 2009, 2008,11,10,9,8,7
Solid Edge 20,10,9,8
Solid Works 2000,98
Mechanical Desktop 4,3
Autocad 2004DX,2000,R14, R12
Teamcenter Express 3, 9

 

[GregLocock]

Grabbed from an old thread

T = rho*pi*(D^2/4)*lambda*ve^2

where

rho = density of air
pi = 3.14159
D = diameter of propeller disk
lambda = ratio between velocity on propeller disk and exit velocity
ve = exit velocity
T=thrust

for a diverging duct lambda > 1
for a parallel duct lambda = 1
for a free propeller disk lambda = 0.5

the power absorbed is:

P = rho*pi*(D^2/4)*lambda*(ve^3/2)

so you can calculate the thrust by power (at fixed point):

T = (4*lamba*rho*(pi*D^2/4)*P^2)^(1/3)

this is tha maximum thrust because remebering that an equivalent form of thrust is

T = mp*(ve-v);
and at fixed point v = 0;

That assumes no friction and other losses, in practice you'll need anything up to twice that power to create the thrust.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JRMacGregor]

There are two questions;

1) given the dimensions of your vessel, what thrust do you need

2) given the thrust needed, what thruster power do you need to develop it

As to the former, it sepends on the size of vessel of course. But also on the preferences and policy of the owner and operator. If you look at ferries in Northern Europe, thirty years ago they might have had one bow thruster, then it became 2, and now it is three. The weather has not changed.

One way of asking the owner or operator to clarify their wishes is to specify the wind (or current) against which you would like the boat to move bodily sideways (or at least hold its position). Once that is defined it is easy to work out the thruster force required (it is the same as the applied wind and/or current force - equation given by Greg Locock - all you need are the drag coefficients).

When you have worked out the required force , a rough rule of thumb (for LARGE thrusters) is 100 hp per tonne of thrust. This does vary depending on size and type of thruster but should get you in the ball park.

Larger diamater and slower thrusters are more efficient and will give more thrust per square meter of blade area. But dont make the diamater too large or the thruster will ventilate.

Keep the thruster tunnel short (put it in a NARROW part of the boat).