motion constraints for a boat moved by propeller
 

Hello,

i am looking for a way to simulate planing hulls (still water resistance) properly, but i can not find a way to do this with standard motion constraints.

The "usual" way for hull resistance, as in DTCHull tutorial, uses a fixedLine and a fixedAxis constraint to keep the boat (/ship) in position and direction, allowing for dynamic trim and sinkage.
This causes an inaccuracy: using these constraints, the force is assumed to be always parallel to the x-axis, and applied to the hull's center of gravity, whereas in a real ship, the force is applied by the propeller, in the position of the propeller and (approximately) parallel to the propeller axis.

Although this assumption might be reasonable for boats/ships where dynamic trim is small and propeller axis is (almost) horizontal and not too far away from CoG, which applies to most large vessels with moderate Froude Numbers (as DTCHull), the situation is totally different for planing hulls:
-prop is mounted rather deep below the hull (--> big lever)
-prop axis is NOT horizontal
-dynamic trim is significant

the outcome of these are a moment around y-axis and a vertical force component onto the hull, both affecting trim and sinkage and thus resistance- making results useless.

my idea is to start with the "usual" constraints (as in DTCHull) and do the following correction for each time step:
-calculate direction of prop axis in global space (summation)
-calculate propeller force (in this direction) to compensate for constraint force (in x-direction) (-->trigonometry)
-calculate vertical component of propeller force (also trigonometry)
-calculate trimming moment of propeller force (lever is constant..)
-apply both to hull

the calculations themselves are simple, but how can i manage to access the inputs and "feed" the results back to the simulation at runtime? :confused:

please help!
regards
simon