Ventilation is a very complex phenomena, and while I have addressed the concept many times directly with clients via email, or made postings on instagram, I think it’s time to write blog entry about it because there is still a lot of confusion and general lack of understanding in regards to foil ventilation.
Simply put, ventilation occurs when a pocket of trapped air between the mast and water through which it is traveling builds to a point where it can no longer stay attached to the mast, and releases. When this giant bubble releases, it can cause loss of control due to the immediate changes in pressure distribution around the mast (which is effectively a wing, too). Now what actually causes the air to get there in the first place, and how quickly it builds, and the nature of the release, are all very complex and to be honest not entirely understood topics, even by professional engineers, CFD analysts, and designers. I wrote about ventilation in 2018 when I worked with my partner Tom, who has designed foils for BMW Oracle racing (America’s cup) for nearly a decade. Even he can’t accurately predict how and when a foil section will ventilate, but he does have the ability to predict flow separation, or stalling, which has shown to have some correlation to ventilation characteristics. Here’s the brief blog entry from 2018 where I touched on ventilation, and below is a video showing ventilation occuring on a Project Cedrus mast when being ridden aggressively by @FoilTheGreats.
Within the first few seconds, you can see flow separation on the low pressure side of the mast as he angles upwind. Could be chop on the water surface, could be his angle of attack, or something else. But you can see the flow separate, and the bubble grow, and propogate all the way down the mast to the foil. This results in loss of lift from the mast, and creates a catastrophic disturbance to the foil wings. It’s really fascinating, and incredibly complex. There are a few key contributors to ventilation, and other random things that also seem to have an influence:
- Riding style
- Water surface
- Foil section shape
- Other: temp, salinity, FOD, surface finish
Not trying to take blame off of equipment here, but it’s a reality: at aggressive angles of attack upwind and/or high speeds, foils will be more prone to ventilation. I like to equate ventilation to a car skidding around the corner of a race track. Of course, some cars can turn tighter and faster, but ALL cars can and will lose traction if pushed beyond a limit, even F1. If you can learn to sense the limits of your foil, you can eliminate the most catastrophic effects of ventilation. That being said, if you like to ride powered and aggressively, there are certainly things we can do from the design side to improve performance at these more aggressive riding conditions, and change the point of ventilation onset.
Water surface also has significant impact. Again, foil design is important, but wind chop and waves will trap air against the mast regardless of size and shape. Furthermore, on windy days, there is simply more air in the top of the water column due to the wave action and shop. These bubbles can get trapped agains the mast, and build with time, eventually releasing.
When it comes to foil section shape, in general, a longer chord length and thinner section will be less prone to ventilation. Now a longer chord will increase wetted surface area, and subsequent drag. A thinner section will have less stiffness, both bending and torsion, which result in other undesirable characteristics. Furthermore, a foil section designed for high speeds can actually have lift-induced drag at low speeds, resulting in undesirable performance at the low end. Again, think of cars here. Sure a Porsche 911 is fun to drive around a track or windy mountain road, but it is not so fun to drive in a city with steep hills, potholes, and lots of traffic, especially with a manual transmission. In this case, a little electric city car will be much more pleasurable to drive. A mast optimized for racing is not the right mast for low-speed prone foiling, just like a mountain bike is no fun on the road, or powder skis chatter on groomers, etc.
Structural performance can also have an impact on ventilation characteristics, and arguably the most important variable is torsional stiffness. If the mast is twisting under load, this will effectively change angle of attack at different points along the length and can lead to flow separation. So a thin mast can be just as prone to ventilation as a thick mast, if it lacks torsional stiffness where it’s needed. This is one of the many reasons I chose not to taper Project Cedrus.Finally, there are a bunch of other little things that seem to have an impact as well. Water temperature, salinity (anything to change density of water). Foreign object debris (FOD), things like sea grass, surface algae. And finally, foil surface finish. We have found that even the tiniest of pinholes can make a mast more prone to ventilation, and have multiple layers of paint after sanding for this reason. As for the non-structural edges, the joint between the PVC and carbon is impossible to feel with your finger if your eyes are closed, it is seamless. The very tip of the trailing edge may deflect under aggressive angles of attack, which can actually lower the angle of incidence, and reduce onset of flow operation. But the current iteration of non-structural edges are actually quite rigid, especially in cooler waters, and I do not think this design choice impacts ventilation characteristics.
In closing, and as stated many times throughout the site and blog, Project Cedrus was optimized for stiffness, strength, weight, and finally drag. I believe there is a healthy balance of performance characteristics for *most* riders, especially those who prefer to ride between 10 and 20kts. Above this speed, any mast will be prone to ventilation and it is simply impossible to ask designers to eliminate it given the impact of riding style and water surface conditions. I believe in honesty and transparency, and at this point with hundreds of masts in the water, we are bound to get reports of ventilation. No mast, even Project Cedrus, can be completely immune to ventilation, but we will continue to learn and help reduce the likelihood however we can through good design, engineering, and manufacturing.
October 1, 2022 Update:
I had a potential client email and say he was interested in a mast, but that with the release of the new fences was concerned that there was a design issue with Project Cedrus. I directed him to the above blog post, and told him that I was always looking for ways to improve my riders’ experience with my products, whether that’s new adapters, fuselages, or accessories. If a $10 stick-on part can improve performance, I see that as a really awesome thing. Why redesign the mast (that may not even help, see above) and obsolete my clients’ investment, if a simple 3D printed part can eliminate ventilation for those few who experience it? Seems like a win for everyone.
The video below shows the power of the $10 3D printed fence. You can see the same bubble building on the low pressure side of the mast, however it cannot propogate down the fence or develop to a size that causes irrecoverable loss of lift. Pretty cool!