Updated: Nov 21, 2020
A lot of people talk about pipe “engineering “ these days. It’s a word that seems descriptive and comprehensible, but I have some high school friends who actually became engineers, and have informed me that engineering is a specific discipline and body of knowledge, like medicine, and one does not practice it without a license. Professional Engineers can be a little touchy about this.
So, understanding that we are talking about mere pipe design and NOT the E-word, this post is about what goes on inside a bent pipe.
I’ll just say it: in terms of keeping tobacco smoldering evenly, maximizing taste and minimizing gurgling while making efficient use of materials, the straight pipe rules. Everything else is a compromise that introduces complexity and weight for the sake of ergonomics or visual appeal. That’s not to say that there’s anything wrong with compromise. Just ask my wife.
But seriously, folks. Bent pipes look great, they decrease the moment presented to the teeth for a given weight by shortening effective length and they cantilever against the chin, an even better way to hide weight. They are easier to light because you can see what you’re doing. They CAN be made to smoke as well as straight pipes if you make the right choices. Probably fair to admit that straight pipes can be made to smoke poorly, too. I’m a big fan of bent pipes. But if your drill bits are straight, you can achieve only so much bend without introducing angles that degrade the pipe’s performance or construction. Everybody has owned a pipe that would collect condensation somewhere in the airway and gurgle like a still. And, quite likely, would not take a pipe cleaner past the mortise so once the gurgling started, it persisted. That can happen with a straight pipe (airway size has a lot to do with it) but odds are that we’re talking about a bent.
The enemy is moisture from combustion, which, when the tobacco smolders, has to go somewhere. When the airway is more or less horizontal (and big enough), the moisture in the smoke has a chance to distribute along the airway and evaporate. As the airway approaches vertical, the moisture deposited on airway walls has a greater tendency to run back to the bottom and collect.
This, by the way, is my opinion. MANY severely bent pipes have been made that exceed what I regard as sensible parameters and yet have been enjoyed, even revered, by their owners. That’s the nature of compromise, some will give up speed for comfort, others will give up comfort for speed. The comfortable car is fast enough, the fast car is comfortable enough, each owner says, “WHAT compromise?”
If we could drill a curved hole, we could . . . what’s that? We CAN drill a curved hole? Really? Yes, and to address that topic without swerving off into the bushes, here is my first ever blog footnote. 1
There, either you’ve scrolled down and read about drifted shanks, or knew what they were to begin with, or aren’t THAT interested, so we’re on to how you make a bent pipe, starting with a straight pipe, with straight drilling. The easiest thing to do is bend the stem. We generally choose thermoplastic materials for pipe stems for this very reason — hard rubber (also called vulcanite or ebonite) is the most pleasant to work, the easiest on tools, and it bends very smoothly and forgivingly. Plastic resin stems, usually acrylic, also will soften with heat and bend, but there is a very narrow temperature range between deforming and melting and they have zero spring. And they are hell on files and rasps.
Anyway, take a straight pipe and bend the stem. A little. Now you have something like a Prince. Make it a really long stem and you have a Churchwarden. In terms of smoking performance, there is zero cost to bending a stem. Given a smooth tube, at the lengths and bend radius we’re talking about, a curved tube is the equivalent of a straight one.
Ok, but there is a limit to how far you can bend the stem before your tobacco falls out or the pipe just looks dumb.
So, now we turn our attention to the stummel, the briar part of the pipe. First, there is the angle at which the air hole and shank enter the bowl. Let’s decrease the angle between the bowl and the airway and shank, which, for now, are on pretty much the same axis.
Not bad. A shape like an acorn can incorporate an even more acute angle with a straightish shank and a bent stem, but we have introduced our first performance cost. IN MY OPINION. Again, what constitutes a performance cost depends on you. You see motorcyclists on choppers with long, raked-out forks, rigid frames, and super-wide rear tires. I know that motorcycles with those design features handle like road graders, but the riders chose those bikes of their own free will and they LOVE them. So I’m not judging, just discussing our options. I keep saying “in my opinion” so we both don’t forget that I’m only the expert on the pipes I make. You are the expert on the pipes you enjoy.
In my opinion, smoking performance is impacted when the angle between smoke hole and bowl axis becomes much smaller than, oh, 70 degrees. I haven't bothered to come up with an exact number, some of it depends on bowl shape, but I've got a pretty good idea of what looks like too much. Conversely, an angle slightly GREATER than 90 degrees can be a good thing. The vast majority of straight pipes are drilled with a three- or four-degree forward cant to the bowl and many people ask why. I suspect this was a manufacturing convenience that allowed easier turning on a lathe, but the greater-than-90-degree angle also allows the air hole to enter the bowl slightly more at the bottom center. A smoke hole that enters at, say, 60 degrees, from above as it were, enters more at the side and presents a ”slash cut” to the smoldering tobacco. At its most extreme, the hole becomes a slot, and even if the bottom of the airway is perfectly flush to the bottom of the bowl, the top of the airway is high. In addition to being less able to draw air down through the tobacco to the very bottom of the bowl, resulting in a soggy bit of dottle at the heel that never burns, this angle creates a thin “V” of briar at the top of the airway entrance. Which stands a fair chance of charring, eroding and enlarging over time. Is this bad? I mean, look at an Oom Paul, look at a Ramses. Well, it’s all relative. How much bend did you want?
For the sake of discussion, let’s say that we’ve decided to limit our bowl/airway angle to 70 degrees or so, but we still want more bend, so we want the shank to curve upward more. We turn our attention to the mortise. Let’s angle the mortise upward relative to the airway. To avoid excessive performance cost, the air hole drilling must — ok, SHOULD — avoid two things: the wall of the mortise on the inside, and the face of the mortise on the outside. Given a very slight bend, it is possible to angle the drill through exactly the center of the mortise floor without hitting the edge of the mortise face. For what it’s worth, I regard this as zero cost to pipe performance. As the angle increases, though, something must give. First, the tip of the drill leaves the center of the mortise and angles toward the wall. A little of this is not a huge problem (although some smokers object to it and insist that holes meet center-to-center), the misalignment can be massaged out with a thin curved rasp so that a pipe cleaner passes and the draw is good. Also, as the angle increases further, the drill nicks the edge of the mortise face. Not a big deal, a bit of chamfer will take care of that. But as we increase the angle, we are actually drilling either THROUGH the mortise face or the wall. The hole in the mortise face is easy, we’ll cover that up with a floc.
Uh, but now when the airway gets caked we can’t ream it with a drill bit because the floc is in the way. OK, we’ll drill through the mortise wall instead. That‘s a time-honored solution, but now we have to shorten the tenon and introduce a gap. If we’re looking at a Peterson System pipe that’s not a flaw, it’s a feature.
You see the limitations. You can build a lot of your bend into the mortise if you are willing to accept the cost of drilling through the mortise face (no airway reaming), or the cost (air gap, condensation, not passing a pipe cleaner) of drilling through the mortise wall. The wider and shorter the mortise, the more angle we can achieve, but the thicker (and therefore heavier) the shank must be, and the more we must ramp the airway.
Let’s say we have decided to drill so that we just nick the edge of the mortise face and the edge of the mortise wall. It’s an acceptable compromise; the airway can be massaged into passing a pipe cleaner. Are we done? That’s all the bend we can get?
Actually, no, there is one more trick; we can angle the bowl forward relative to the shape. The bowl angles forward, the rim is tilted backward, the stem is curved a little more, increasing the effective bend in the shape without increasing our other angles.
Is there a performance cost? As always, it’s the dose that makes a poison. A little bit of forward cant is just fine, I think. Most of us introduce a little cant with whatever pipe we smoke by letting it dangle a bit. Angling the rim back so that it is level with the horizon is fine so long as the inside of the back of the rim doesn’t get sharp enough to burn, and angling the bowl inside the shape is only a problem to the extent that it adds weight and makes the walls unevenly thick. If your pipes must weigh less than 50g a full bent tomato probably isn’t your shape anyway.
There is one last factor to consider: shank length. You've probably noticed that, as the shank has curved and the airway has stayed straight, the airway has started to accumulate extra briar underneath and lose briar above. This can't go on forever; the longer and more curved the shank is, the thicker it must be to keep the airway covered. Some makers are quite daring in shaping very close to the airway; I’m not. It’s a common new-carver mistake for the desired bend to write a check that the shank thickness can’t cash. At one Chicago show I saw a guy smoking a deeply bent pipe, a piece of plastic tubing spanning the inside of the shank like a viaduct. Why, yes, he had made it himself. Clearly he considered it a feature rather than a problem but I don’t think it’s marketable.
You see why I like the Acorn so much as a shape for a bent pipe: by moving the shank attachment up a little ways on the bowl, it gets rid of a little junk in the trunk and permits a slimmer shank for a given bend.
Also, maybe you’ll see why my spec pipes usually are either A) straight, B) gently bent, or C) Acorns. I’d rather have someone say that my pipe is a great smoker but the shape is a little conservative than have someone say that it’s a fantastic looking pipe that doesn’t smoke all that well. I’ll make a pipe to the customer’s shape requirements, but at some point we both have to agree that he is proceeding against advice. Kind of like the scene in Dirty Harry where the villain hires a guy to beat him up. “Are you sure you want your money’s worth?” asks the mercenary.
As an illustration, I made, more or less, the pipe from the last drawing. It represents my personal limit in each parameter of internal design compromise. The mortise is wider and shorter than usual, the drilling through the mortise nicks both the face and wall, which required as much massaging of the airway as I care to do to bring it in line with the stem so there is no tenon/mortise gap and a pipe cleaner will pass. The shank is chubby, which keeps the airway safely buried. The drilling angle into the bowl is fine, I'll regularly go this far on a bent pipe but no farther. And the bowl is slightly canted forward. The result is clearly a bent pipe. Maybe not a FULL bent like an Oom Paul, but it is relatively much more bent than my usual offering. Since I'm selling it on spec I wanted to stay inside the lines and make a pipe I thought would smoke well.
So, if you were wondering what the most deeply-bent pipe I would make without requiring a waiver might look like, there you go.
1. It is possible to drill a curved hole using a burr driven by a flexible shaft in a curved housing. In pipe making this is called a drifted shank. I’ve seen some breathtaking examples by Viprati and See Jacopo, among others, and I used to own a couple of pipes that had drifted shanks because, well, they had drifted shanks.
Here’s the problem: drifted shanks tend to have a rough internal surface, because they are, necessarily, nibbled. There is not a clear path to the surface for the chips/dust, so the bit has to be withdrawn and reapplied over and over. The hole tends to be large, and the surface tends to be rough. That, again in my opinion, is a performance inhibitor all by itself. Then, when the pipe is completed and has been smoked enough for the airway to be caked and clogged beyond what a pipe cleaner can remove, well, then what? A pipe that has been drilled with a straight drill bit can be reamed with the same drill bit and we’re back to original condition. A drifted shank, not so much.
My drifted shank pipes went the way of eBay because, for all of their shaping possibilities, they didn’t smoke very well. Which is not to dispute anyone who loves drifted shanks, I’m just explaining why I do what I do. Which is, drill pipes using straight drill bits, which leave a nice, smooth finish and a nice, straight path for any future reaming that might be needed.