Please excuse my feeble attempts at drawing!
First off, we'll look at a straight pipe. The above version of a billiard will do. Forgetting the practical side, in theory, it is quite easy. The airway finishes right in the bottom of the tobacco chamber, and the tenon is exactly the same length as the mortis. One continuous diameter airway from bowl towards the end of the mouthpiece, where it tapers and fans out to accommodate the bit. Keeping the flow even all the way through, without obstruction. The result a pipe that will smoke well and without gurgle. Day to day cleaning can be done without taking the stem out, just pushing a pipecleaner straight through the stem into the bowl.Assuming that the airway finishes right in the bottom of the bowl, we have 2 possible problems. The first is if the tenon is longer than the mortis is deep. Easy to see, as there will be a gap between stem and shank.
The 2nd, is where the fun starts. This is when the tenon is shorter than the mortis. Smoke will move through the airway and suddenly get to this wider part, where it'll start swirling. Physics will tell us that the heavier particles (oil and moisture) will be deposited. Result: gurgle!
But there is also another important thing to consider. Tobacco smoke is basically CO, CO2, water and other bits and pieces. But the reason we smoke pipe, is because of the taste. And a lot of these flavours that we love are soluble! Thus, a wet smoker = loss of flavour.
Now on to 1/4 or 1/2 bents. If you have a look at the above sketch, in theory, you can still drill the airway, bowl and mortise with ease, resulting in perfect flow from bowl to bit. With the same problems that can arise as with the straight pipe. But nothing that practise and a little attention to detail can't overcome.
Now we get to the full bents. Because of the angle of drilling needed, things are not so easy. The above would be a "perfectly" drilled Oom Paul. Note how high the airhole ends in the wall of the bowl and the position towards the back. Ever wondered why a full bent tends to burn unevenly and more towards the stem side? Yip!
And then of course there also is the obvious: there will always be unsmoked tobacco left in the bottom of the bowl. The airflow through this will be minimal, condensation will take place, making this soggy and thus the chances that gurgle will occur, increase.
Lets go to the tenon and mortise. If you have a close look at the drilling lines of the above, you'll notice that by drilling the airhole, the mortise will be touched (nicked) by the drill bit. For me personally, not a problem, as this will be covered by the face of the stem. However, for some people, this is a definite no-no. Can it be solved? Sometimes. Making the diameter of the tenon and mortise bigger, might work. But then the walls of briar around the mortise also gets thinner, which increases the chance of breakage. Catch 22?!
One last note: observe the length (or shortness!) of the tenon. This is done in all perfectly drilled full bents, to be able to get the airway to finish centered and right in the bottom of the mortise, a prerequisite for even flow of air and a "dry smoker".
All of this ramble regarding full bents is the theory, in practise it is not that easy. Somebody one said: the difference between theory and practise; is that in theory practise is theoretical, but in practise, theory is not practical. Anybody ever drilling through wood at angles, will tell you that drill bits always want to follow the path of least resistance. Which is nowhere where you want it to go. Makes you think, doesn't it?
But how many of us have ever smoked or even just held an Oom Paul drilled like this one? Not many, I can guarantee you.
This is more like it.
This airway is towards the top of the mortise. To enable the smoke to pass through to the stem, the tenon is shortened. Swirling of smoke, condensation, gurgle!
What can a pipemaker do to rectify this? Ever heard of the pipecleaner test? Yeah, it passes a pipecleaner right into the bowl!
This is what some pipemakers do: they ramp the airway, thus enabling the tenon to finish flush in the bottom of the mortise. The symptom is cured, but not the cause. You again have a widening in the airway!
Another example: the mortise is made deeper. An oil chamber, sump whatever you want to call it. The fact is, the draw is compromised, along with a loss of taste and quality of smoke.Shall I rest my case, your Honour?
