Earlier this year, I started building the exhaust risers for my Cummins 6-CTA turbodiesels. Verocious Motorsports had all of the straight and mandrel bent tubing I needed. I cut the tubing and tacked it all together using some new toys for 2016: my Harbor Freight metal-cutting bandsaw and AlphaTIG welder. Learning to weld aluminum and stainless has been one of many great things about this refit. Whoever said old dogs can’t learn new tricks was full of it!
Computer aided design (CAD) software has also made it a lot easier to go from concept to execution than old school pencils, rulers, and blueprints. I’ve used Sketchup Make, a free CAD program, to design the aft stateroom, galley, and V-berth, but for the last few months I’ve been trying to figure out how to use it to make showerhead parts for the exhaust risers. After reading tons of material online, it didn’t take long to make the 2D model in Sketchup. The problem was getting the free version to output in a file type that can be read by CNC, waterjet, or laser jet machines. The native file outputs in Sketchup are proprietary (.skb and .skp) and aren’t compatible with anything, near as I can tell. I found a Sketchup extension that changes the output to a machine readable .dxf file format, but for the life of me couldn’t make it work. All of my drawings turned into straight lines when viewed in LibreCAD, a free 2D CAD app. Sketchup’s user interface is a lot easier for me to use than LibreCAD, and I really don’t want to spend a bunch of time becoming that proficient in LibreCAD. Sketchup Pro — the professional version of Make — exports in all sorts of CAD file types, but at $695 it’s a pretty big investment. But eventually I came up with a workaround, found a fab shop with a laser jet, and now have all the parts I need to finish the exhaust riser.
The exhaust tubing is 4″ OD mill finished stainless. The outer tubing for the showerhead is 6″ polished stainless, with an ID of 5.871″. I’ll run a continuous TIG bead around the showerhead backer and the nozzle end to attach them to the 4″ tubing, then slide the 6″ tube onto the assembly and run a continuous bead around both ends of that, too. I also need to add a water inlet to the showerhead, but that comes later.
This “backer” will seal the gap between the exhaust tube and high end of the showerhead tube. Circles in Sketchup are made up of short sections of straight lines that are all connected. The more segments you specify, the smoother the lines are and the rounder the final part will be. A triangle is the least-round circle you can make using only three straight line segments. A square, with four segments, is a lightly more rounded circle than a triangle. When making the model in Sketchup, I changed the properties of each circle from 26 segments (the default) to 99, which yields nice, smooth circles.
There were a few issues I want to avoid with my showerhead, one of which is that at low RPMs I don’t want to cook the rubber exhaust hose to which it attaches. This can happen with OEM Cummins showerheads because the holes are symmetrical all the way around. If all of the water that’s entering the inlet at low RPMs can exit the showerhead from only the bottom holes, the top of the exhaust pipe and hose don’t get cooled. Retaining more water in the showerhead and having it exit the nozzles higher up will cool more of the pipe and hose at low RPMs.
The raw water intakes are 2″ ID, while the raw water outlet at the heat exchanger, which feeds the showerhead, measures 1-3/4″. The raw water inlet for the showerhead will also be 1-3/4″ OD, with an area of ~2.2 inches². I’ve got six 1/4″ diameter holes starting at ~80° from the bottom, followed by four 3/8″ diameter holes, and then seven 1/2″ diameter holes, plus a 1/2″ half-hole for a drain at the bottom. The total area of the holes is 2.2 inches². So even at idle the water will be in direct contact with most of the exhaust pipe and water will be exiting from nozzles relatively high around the pipe and cooling the exhaust and the hose. When the engines are off, all of the water will drain from the showerhead via the bottom hole. This should eliminate the possibility of crevice corrosion.
The resize line is necessary because I imported these jpg files into LibreCAD, then traced the outlines and deleted the jpg layer, leaving the traced outlines ready to be saved in the .dxf format. But LibreCAD’s standard measure for imported jpgs is pixels, and there is no simple way to instruct LibreCAD to convert pixel dimensions to specific linear dimensions. So if I don’t resize it, the LibreCAD file will see that resize line as being 931 units (ie pixels) long, while the laser jet that makes the part would see it as 931 units (ie inches or mm or ???). To re-dimension from pixels to inches after importing the full-size pic to LibreCAD, I trace a straight vector line over the 6.000 inch “resize line” in the jpg. Then I select that vector line and the jpg and use the resize function, entering 6.000/931 as the conversion factor. This shrinks the whole thing to the proper vector dimension in inches. Then I delete the jpg layer and the resize vector line, leaving just the part I want in the proper dimension, to within .001″, ready to be saved in the .dxf format.
After a bit of hunting around, I decided to have Stainless Steel Specialties, Inc. in Raleigh, North Carolina make the parts. Their prices were quite a bit less than what I was quoted from the laser jet fab shops that come up in internet search ads, they didn’t care that it was a small job, turnaround time was reasonably quick, and the parts came out just like I imagined them. I’ll be using Stainless Steel Specialties for some future laser jet work I need to have done.
The tubing isn’t perfectly round, but this fit is well within tolerance.
I really wanted to fire up my AlphaTIG and start putting the final welds on this thing, but I still have to take all the pieces to the boat and “clock” the parts to make sure everything is oriented properly. When the flanges are mounted on the turbos and the exhaust is pointed at the mufflers, the showerhead drain holes need to be on the bottom. Then I need to mark the places where the raw water inlets need to be attached to the showerhead outer tubes so they’re pointed at the raw water outlet on the heat exchangers.
This all makes perfect sense in my head and when I read it, but I’m not sure it’ll make sense to you readers. All I can say is, stay tuned…the pix will tell the story in a week or so.
Oh, and…update on that .dxf extension for Sketchup that I couldn’t get to work for months…it does work. I always start my Sketchup views like I’m drawing on a wall, with the drawing oriented vertically. But the Sketchup .dxf extension views the object as if the drawing was on a table, laid out horizontally. So my 2D renderings of circles laid out on a virtual wall (when viewed from the Front, which is my default) were rendered into 1D lines when the extension viewed them from above, looking down. I could never understand why all of my converted 2D files changed from the shapes I wanted into single dimension lines. I just assumed it was a poorly designed extension. By rotating each rendering 90° along the X axis, all the sudden it’s a lot easier converting Sketchup renderings to .dxf files. I don’t have to do any of that resizing from pixels to inches anymore. Sure would have been nice if the extension author had put that little tidbit in the instructions! 😉
Next up in our 1969 Chris Craft Roamer 46 Refit: Fitting the Transom Door