This is an English version of an article published 120125.
Sails are fascinating. It doesn’t matter how much you learn about how they should look and be trimmed, there’s always areas you don’t understand and need to lear more about.
If you’re a curious person, my job is perfect. I get to meet the world’s top sail designers, those who develop new materials like 3Di, those who write software and those who analyze real world performance like Johan Barne and Aksel Magdahl.
In the case of wind tunnel tests, there is one place on earth where all teams go with their yacht- and sail designers – Twisted Flow Wind Tunnel at the University of Auckland. Especially when it comes to downwind sails, many refer to their expertise and experience.
And often when I meet great sailors they’ve done their master thesis here.
The unit belongs to the University of Auckland, but is located in an anonymous building southeast of the city center. The goal is to fund their operations with commercial assignments, and given how many boat builders, VO70- and large IRC projects that uses the tunnel, it seems to work out pretty well.
They host a number of student projects spanning a wide range of topics. You get a pretty good idea of what you’re doing by looking at this list: Current research projects available to students. Great mix of scientific theories and practical tests. Over the years a large number of Swedish students have been there.
Every two years they organise the High Performance Yacht Design Conference. Next time will be in March. The documents from the old conferences is great bedside reading. Right now I am getting through “The effects of stay sails on yacht performance”.
Here’s the setup. To the left are the big fans. The white “curtains” can be adjusted to get a “twisted flow” as the wind turns with height, to artificially simulate how the wind hits a real yacht.
In the centre is the model of the yacht. It sits in a cradle that allows it to heel, and the black disc on the floor can be turned to adjust the angle against the wind. Sheeting is done by remote control.
The computer to the left performs most of the magic, and contains the designers numbers for boatspeed, heel and everything else that’s used to setup the test run.
The mest important figures in real time; speed, heel angle, driving force, wind strengt and angle.
One spin-off from the wind tunnel is VSPARS, an application that uses cameras on deck to track sail shapes in real time to see slight differences between designed an flying shape.
Here’s the actual sail shape on the model compared to the designed shape. All data is stored to later analysis and reference. Also snapshots of the video is stored to go back and see how it really looked with correct trim.
Some projects use VSPARS in sail testing and even while racing. Then they can adjust trim based on the data they get from the software. Here’s an interesting article from Wally Cross on how VSPARS can be used on board. I also spoke with Johan Barne who worked with Telefonica, and he found it very useful.
They also have a special package called VSPARS Olympic, that’s used by several teams including the British olympic sailors. It allows them to use a smaller camera, like a GoPro, in the boat and then upload all the photos and automatically track the shapes. This can be synced with video from the boat and crew and performance data from Cosworth Pi Garda box. Hightech!
A very common scenario is that a team or project spend 3-4 days in the tunnel. They bring the sails that they want to compare and establish cross-overs between, ie to find out when it’s better to change from one sail to another. To have the whole group with boat- and sail designers, trimmers and performance analysts in the room at the same time is great to rapidly go through a process that would take forever on the real boat. As an example, ETNZ Camper and North Sails did 1400 tests in just a few days. How long would that take on a VO70?
A number of tests mapped by wind strength (TWS) and angle (TWA). Different sails have different colors and it’s easy to see which sail have the most driving force in each condition. It’s still a tough decision how to put together a balanced inventory for a certain race or course, but this is great input.
Naturally we had a discussion on computer simulations vs runs in the wind tunnel (especially interesting after my interview with Michael Richelsen before christmas). Upwind CFD (Computational Fluid Dynamics) have come a long way and works great, but downwind is still more complex so it’s faster and cheaper to do much of the work in the wind tunnel. Really good CFD is an art, and the set-up time is much greater than the actual calvulatiin which makes it hard to run through many scenarios. But both areas are developing.
Downwind in a lot of wind it’s hard to compute the forces and how they affect heel and resistance through the water. On a VO70 with very limited sail inventory it’s important to do the right choices early – to design and produce sails to be left ashore isn’t cheap. There’s a great article from Farr Yacht Design in the latest Seahorse, on the work they did with Abu Dhabi Ocean Racing.
They build models of all the boats they’re testing. Hull shape is very important to get the correct flow around sails and hull, so details are really important.
Here’s a trial with the crew on the rail of a TP52.
Another way to measure the differences in pressure on both sides of a sail is to build thin profiles with lots of holes aling the sail…
…that are connected to lot’s of tubes that goes down through the sail. By taping all holes on a sails except those that should be measured, you can use instruments to get data from the pressure in every little tube. Then you move the tapes to get data from the whole sail.
They’ve also experimented with rigid downwind sails to meassure preassure on a flying sail. Harder than expected.
By mounting small tubes in the hull it’s possible to get data from the hull as well. This is a Dehler 33 and experiments showed that the hull and cabin represented 12% of the side forces on a beat, which is quite a lot?
I really like their “hands on”-approach to what they do. They have experimented with pressure-sensors in real sails, both upwind sails and spinnakers. David sails a Stewart 34 that doubles as experiment platform.
The sensors that measure pressure on both sides of the sail. Thin cables run along the luff to a box for collecting data. This is a great way to verify the theoretical values they gat from a sailmakers CFD but also the values they get in the wind tunnel.
The orange stripes is there for VSPARS to recognize the shape.
They also experimented with sensors in the spinnaker.
When I asked David to look into the future, he’s pretty sure most of the rutin work will be done in computers instead of in the wind tunnel, and that they will spend more time with practical application, experiments and real time scenarios with VSPARS. But there’s always new complex questions, so I’m pretty sure that the wind tunnel will play an important role many years from now.