Values for force on object too big

Hi everybody,
I’m trying to compute the resistance on an object (a ship) with Palabos. I based my code on the OffLatticeExternalFlow, in the case of the Mutligrid3D. Everything works quite fine, the Octree-Multigrid allows me finally to use domains, which are “big enough” for my cases . The forces on the object are given in my code, as usual for almost each external flows, through the “GetForceOnObject” property:

T factor = param.rho * (param.dxFinest * param.dxFinest * param.dxFinest * param.dxFinest) /
(param.dtFinest * param.dtFinest);
Array<T,3> force = factor*boundaryCondition->getForceOnObject();
forces << (double) (iter * param.dtCoarsest) << " " << force[0] << " " << force[1] << " " << force[2] << std::endl;

My problem are now the values of the forces. They are quite big in comparison with the references (approximately 3x or 4x). I still have no free surface implemented in my code and i would therefore expect smaller values at least for the resistance. I also tried with different geometries, the problem remains the same. 

Could the problem come from the conversion from lattice to physical units? Or, since i use a constant pressure as outflow condition, from the pressure reference at the outlet boundary? Would you suggest to use another method to compute the forces in my case? 

Thanks in advance for your help!