When aeronautical engineering students first learn about lift, they often start with a strange example: a spinning cylinder.
At first, that feels backwards. A cylinder is not a wing. But the point is to introduce circulation, which becomes one of the foundations for understanding how lift develops around an airfoil.
The next lesson is just as important: not all shapes create drag equally.
A well-shaped airfoil does not just cut through the flow because it is thin. It works because it guides the flow around the body and allows it to close back down cleanly behind it. A round cylinder does the opposite. The flow separates early and leaves behind a large turbulent wake. That wake is lost energy.
This is why the numbers are so dramatic. An efficient airfoil section can have a drag coefficient around 0.045, while a round cylinder can be around 1.2. In practical terms, that means a 19 mm thick Axis mast can have roughly the same drag as something close to a 0.9 mm pencil lead.
That is the part that is hard to visualize until you see it drawn.
A full-size mast section and a tiny wire can be in the same drag conversation because shape, separation, and wake size dominate the result. The water does not care that a bad shape looks small. It cares how much flow it separates and how much wake it leaves behind.
That is the lesson:
A tiny bad shape can create the same drag as a much larger good shape.
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Here is a real-world example of two objects that can be in the same drag conversation: a 19 mm Axis mast and a 0.9 mm pencil lead.
That sounds ridiculous at first, but it is the whole point. I was running the numbers on how much drag a plate might add to the bottom of a mast, starting from first principles, and this is where the math led.
A round cylinder is incredibly draggy. An efficient airfoil is much, much less draggy. The difference is not just the size of the object. It is the size of the wake it leaves behind.
The mast is large, but it is shaped to keep flow attached and close the water back down cleanly behind it. The pencil lead is tiny, but as a round cylinder it separates flow early and creates a surprisingly large wake relative to its size.
This is also why old biplane wires turned out to be such a big deal. They looked small, but they were long, round, and draggy. Once aircraft designers understood how much those wires added to total airplane drag, cleaner cantilever wings became the obvious direction.
Same idea in the water. A small weed, a little line, a bad edge, or a protruding piece of hardware can add much more drag than intuition suggests. The water does not judge by how small something looks in your hand. It judges by shape, separation, and wake.
Crazy, but true: a tiny bad shape can “out suck” a much larger good shape.
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If I thread my car’s antenna into an aluminum mast my truck will get better gas mileage right?
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That’s insane! I know you’re right, but that spins me out 
Can you explain where aerodynamics and hydrodynamics diverge? From my understanding not all aerodynamic principals are replicated in water - but maybe I’m incorrect here. For example, the kamm tail seems to work from an aerodynamic standpoint (for example cars and bikes are using this shape for efficiency, weight and stiffness) but I’m not sure that it works in hydrodynamics. If it did work it could make for a very light, stiff and efficient mast, but there’s a reason we don’t see that. Is that just due to surface piercing and ventilation, or are there other factors at play?
I appreciate all of your summaries which help me understand these principals.
anyone who has any experience fishing knows that even a short length of fishing line has enormous drag going sideways through the water (or gets pulled sideways in the current). Imagine taking a loop of fishing line and trying to pull it through the water - you think it would be super easy but the drag is much more than intuitive. Because of exactly what this post is about - a round shape has 20x more drag than a foiled shape.
I’ve often wondered how a tiny bit of sea-grass can have such a big effect, but its this again. I can easily feel a single blade of grass stuck around my mast when I’m paddling up on a wave - and even this tiny thing can be enough to ruin the paddle up.
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There are scaling and speed corrections you can make to find equivalencies between air and water. For those to work the air flow equivalent needs to be subsonic, and the water flow non-cavitating. Once you’re looking at those regimes the fluids are behaving fundamentally differently.
A rule of thumb though, is that you would be comparing something moving though water to something much smaller and faster moving though the air. E.g. a hydrofoil mast to landing gear struts on an RC plane.
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Since foils interact with both mediums, theres also drag considerations from the spray drag at the water - air interface, as well as any drag introduced by ventilation.
Zeroing in on spray drag - to my knowledge this decreases with both thickness and chord but is more affected by thickness. Surface roughness also increases this.
It only occurs are sufficiently high Froude numbers (which relates the intertia of a fluid to gravatational effects), but foils pretty much always operate at speeds where this occurs (the sheets of water that shed off a mast are evidence of this).
I never hear people talking about water density/viscosity but sometimes it plays a huge part…usually for the worse.
In lakes or sea when algae or microrganisms bloom the drag of the foil and mast increases to sometimes very frustrating levels.
Not talking about weed draping your mast…just soupy water.
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I was interested in this question too, and did a quick computation using XFoil, the software that I use to evaluate sections.
I made a circular section and evaluated it using settings I typically use for hydrofoils. When evaluating with laminar settings, XFoil does not converge at at which makes sense as the flow is unstable. When forcing turbulent flow, it comes out with CD = 0.25. Much better than the 1.2 value predicted above; this is the “golf ball effect”. In practice it’s easy to force turbulent flow by roughing up the surface.
On the other side, I can evaluate the 19mm Axis mast of which I have the section. The CD value of 0.045 mentioned above is extremely unrealistic for this section, it can achieve that only at very high reynolds numbers (25 knots+ speed) in optimal laminar flow conditions (perfectly sanded, no scratches). With more realistic settings for speed and laminarity (15 knots, N_CRIT=2) CD = 0.065.
This makes the difference between these two shapes a lot less dramatic. Not a factor of 26 but just 4…
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On about tuttle box again? 
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Is there any rider or brand that would take the other side of this argument? (Ie “shape doesn’t matter, i want only the absolute stiffest, or absolute thinnest mast”).
A lot of the chatter in this space has a bias conscious or not. Eg: if you build a modular mast, you need minimum thickness for adapters.
Smaller outfits without the capacity to test may also lean more on simplified models, but I personally think its silly to think that major players arent doing modelling+testing.
Hmmm, seems like a certain industry rebel already wrote an extremely detailed white paper on this very subject. He actually goes into some great industry myths real world examples and good engineering. Unlike some, he actually looks at materials and costs to come up with cost effective, performance engineering. It’s kinda like Boeing vs Spacex. One spends a lot of money to get off the ground, while the other spends a lot of time putting things in space. If this is truly the progression forum then a big part is delivering best performance for a reasonable price. That is true engineering.
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