LCB correction for water jets

[Daz925]

Hi guys

Was just wondering what is the best way of calculating the LCB shift for waterjets, the manufacture provides the intrained water weight for the cog correction. Im trying to stay away from putting the hull into maxsurf and drawing in a waterjet to see what the results are as i dont think it will be 100% accurate and would rather do it with pen and paper.

Cheers

[baeckmo]

First, assuming you have the hull and jet CG available, you add the trim moments from the two, using the transom as reference:

(mh*CGh+mj*CGj)/(mh+mj);

where mh=hull mass (kg), mj=jet total mass, CGh=hull CG, CGj=jet CG (note: this often has a negative value, since aft of transom).

Then you have to compensate for the jet thrust line, which normally generates a nose-down (=positive value in this context) trim moment. To this end, you have to calculate the required horizontal thrust at the operating speed, and the trim angle at that point. From there, you find the lever arm connecting the mean pressure center (~resistance center) and the thrust line. The resulting trim moment is then the thrust along the shaft (or nozzle) direction times the lever.

Finally, you add this trim moment to your hull trim moment, and you're done. (To be quite correct, this leads to a trim angle correction, that should lead to a new calc spiral, but none of the factors you have at hand have the precision that makes iteration reasonable).

In addition, at some speeds, the pressure distribution around the jet inlet area will create a change in vertical force in that region, but compensating for that is probably serious overkill!

[DMacPherson]

Baeckmo's response is spot on for the hydrodynamic aspects of jets with planing craft, but I wonder if you're actually after the hydrostatic treatment of the entrained water? If so, then you can look at it either from an "added weight" (change in G) or "lost buoyancy" (change in B) perspective. If you assume that a jet with entrained water is nothing more than a static mass, then the "added weight" approach is far easier. You model the boat with the hull completely enclosing the jet (i.e., no inlets or discharges, meaning no alteration in LCB or any other hydrostatic parameter), and then use the added wight for calculation of LCG and VCG.