We don't modify the C bowl. That's one of those performance per dollar spent things. We do make the ring spacers, but have not done so in the last four years. The one pictured above is not tapped, but clearance drilled to slip over fasteners threaded into the bowl.
If I was approaching this project, I would base the length primarily on what material I could find. A Dominator bowl diameter is 7.900 on the droop side. The inside diameter is 6.250. A Dominator bowl is about 9-1/2 long and a Berkeley is about 8-1/4 long. An Aggressor is the longest at about an inch longer than a Dominator, but I don't have one right now to measure. With that being said, an 8" diameter x 3" of 6061 is a healthy chunk and a substantial investment at $2.99/lb. I think making it unnecessarily long is not beneficial and cost prohibited.
I would focus first on retaining as much length on the vanes as possible. This provides water control. After the impeller pisses off the water, the vanes are there to straighten out the turbulence into laminar flow that can be useful to thrust. A lot of open volume in between the vanes and droop can promote turbulence and back-flow. The C bowl has a pretty good transition from the bowl into the nozzle which will be lost when altered. This transition will now be an effect of whatever droop you install.
Next, I would focus on the the transition on the inside from the bowl into the ring you are making. If you are feeling ambitious, you might consider lengthening the vanes.
My final opinion on the topic is that the work you are doing should improve performance, otherwise, you are just adding weight. Foil the trailing edges to smooth the water transition and reduce turbulence.
Of course, the benifits of all of this is diminished if you don't detail the the leading edges of the vanes.
Good luck,
Joe
