Sometime ago I did a tutorial on converting aircraft (props only) for catapult launch.   While the subject of the tutorial deals specifically with setting-up an aircraft to launch from a standard aircraft carrier, the system can be used:

- To launch any aircraft (props only) from any ship by simply modifying the takeoff position for the aircraft on the particular ship you are using.....of course, the ship must have a 'landable surface' defined in the ship.cfg file, something that must be added to ships other than regular aircraft carriers. 

- To launch a seaplane (props only) from the water surface by using only those instructions applicable to the plane itself, as there is no ship involved.   

The file is available here at SimV:

http://www.simviation.com/simviation/?type=item&ID=301&page=8

Should anyone have any questions/problems with the system, I'm happy to assist.   Just give me a shout....my email is in the download, and I'm usually hanging around here. 

*** Jet aircraft cannot be cat-launched using this system, but they can be using the 'afterburner' function, but that's another story. 

In the example SHIP.CFG file below, the 'Takeoff Start Position' is defined L/R of the centerline of the ship, height above and fore/aft of the centerpoint of the ship.

The vertices are points that define the hard surface on which the aircraft sits on the ship.  Here there are four vertices that define a rectangular plane....and each vertex is defined in position L/R, Height and Fore/Aft....

[fltsim.0]
title=GC_CAMship
sim=GC_CAMship
model=
panel=
sound=
texture=

[flight_deck_section]
; this section defines the control points the
; AI (and LSO) use to steer towards the flight deck
width=20
length=30

takeoff_start=0.0,13.900,50.40

takeoff_stop=0.0,13.900,81.55
landing_start=0,0,0
landing_touch_down=0,0,0
landing_stop=0,0,0

[platform.0]
; platforms are the actual surface that the planes
; land on.  You can define as many platforms as you
; want by adding subsequent sections with increasing
; numbers.  e.g. [platform.1], [platform.2] etc.
;
; vertices should be defined in model coordinates,
; and counter-clockwise order when viewed from above.
: REMEMBER, positive z is out the TAIL of the ship
; the shape may be concave or convex
; there may be from 3 to 20 vertices (limitation
; may go away later) and they must all be on the same
; plane.

vertex.0=5.450,10.150,81.90
vertex.1=5.450,10.150,45.80
vertex.2=-5.450,10.150,45.80
vertex.3=-5.450,10.150,81.90

[Dynamics]
pitch_wave_moment_scale = 1
bank_wave_moment_scale  = 1
pitch_damp_moment_scale = 1
bank_damp_moment_scale  = 1

While the catapult system is great for launching your floatplanes and CAM aircraft from warships and merchant ships, it really comes into it's own when you use it on standard aircraft carriers.   No more problems getting your loaded aircraft off the deck...

What a blast!  I've been flying TR's modified DB_F4U off of Collin's CVE Sicily (Cat version) and the realism is fantastic.

Plane here:

http://www.simviation.com/simviation/?type=item&ID=301&page=6

Carrier here:

http://www.simviation.com/lair/cfs2shipyard.htm

Using TR's instructions I've also setup one of Collin's Essex class carriers and my Hellcat and Wildcat to use the catapult.

TR's Tutorial here:

http://www.simviation.com/simviation/?type=item&ID=301&page=8