Interestingly this problem does not always happen but did so this morning. I checked the part of the boat bottom with diagnostics, rebuilt all the edges, exploded the polysurface and rejoined, checked for naked edges, etc. Only if I render at a very tight setting e.g. angle at 1.5 degrees and distance from surface at .002 does the jagged render become acceptable.

At the same time the same model without the tower and other demanding details renders perfectly at much lower settings.

s the model a long way away from the origin? Is the tolerance small enough compared to the size of the strakes?

Or is the polysurface Valid? This looks like the normals are flipped on those faces that are transparent. Duplicate faces? Bowtie, self-intersection?

The cutwater is at 0,0,0 so the aft end of the boat is 50’ aft of 0,0,0. That’s not the problem. All the direction arrow point outwards as I want them to be for use with ORCA. The polysurface appears to be valid as I checked under diagnostics and there are no duplicate surfaces anywhere in the model. No self-intersection. I have been toying with the idea of making the strakes larger but am waiting to run the hull through ORCA/Semeric’s CFD which I did for a small 25 footer that led to a) using strakes and b) sizing the strakes to the largest tested. After changing the strakes my hope is that I won’t have this problem. I did changes the Nxt setting from my usual of distance from surface to maximum edge length which tightened up the rendering to an acceptable level and created a liveable file size of around 225500 megabytes.

Sometimes to keep meshing sane, I might increase the Initial Grid quads to a higher value. This gives a good initial griding of a surface and then refinement by distance to surface will be more localized and Maximum edge length may not be needed. Also the Initial Grid quad calculation is quite fast.

Just guessing here.

Many thanks for following up, Scott. One would think that after all these years designing boats with glee in Rhino I would have all the answers! NOT!