Telltales, Tell Tales

telltales2

I've been working hard to find the best mainsheet tension upwind. Trim the mainsheet tight, and you point higher, but go slower. Ease the mainsheet, point lower and you go faster, but you travel further distance. The Aero has a high boom, so it is hard to know exactly where to trim the main. What we need is a rule of thumb to guide us.

The standard answer is to look at the telltales, especially the leach telltales. They are meant to stream when all is good, and to stall behind the sail when the main is overtrimmed. Unfortunately, the telltales are often lying.

Lying Leaches

In light air, there is no easy way to gauge how hard to trim the main. In some other boats, when you overtrim the main, the leach telltales immediately get sucked into the stalled air behind the sail. In these boats, the telltales provide a clear and easy indication that the main is overtrimmed. But in the Aero, the telltales often stream, even when you are clearly over trimmed and quite slow. [Update: 5/25/15] The telltales can also stall when you are going fast with great VMG. So the telltales are not the answer we'd hoped for.

When I overtrim the Aero main, the leach telltales will happily stream even though the boat is slow, very slow compared to another boat that is sailing more freely. The other telltales are equally ignorant to the bad trim — and will merrily stream. Why are they not stalling?

A possible answer could be that the Aero and its rig are quite efficient as are the foils. So over trimming the main increases the drag / lift ratio before stalling takes effect. It may be that this increase in drag happens much earlier than on other boats. If so, this means that boat speed drops, without the telltales indicating a stalled sail.

In heavy air in the Aero, it is actually quite easy. You play the main to keep the boat balanced on its feet and moving through the waves. Here is a video clip that demonstrates heavy air trim: Heavy Wind Trim.

Two Boat Testing

I did a direct comparison against another Aero. Side-by-side two boat testing is invaluable to sort these issues out. We setup on a parallel course. The other Aero was the baseline, I was experimenting with trim and point mode.

In the first picture below, the main is being overtrimmed in 7 knots of wind and flat water. You can see the leach is hooking to weather at the boom, and it was doing this all the way up to the top batten. At this moment, all the telltales are streaming, but I'm slow.

Tight Main / Eased Main

tight-main tight-main

In the second picture, a few seconds later, the breeze picked up a little and I eased the main to keep the boat flat. I then kept the main a little eased and adopted a slightly lower course. The boat was markedly faster. Once speed was up, I was able to point a little higher without losing speed. My actual VMG was significantly better with the main eased.

Rinse and Repeat

To make sure, I repeated the sequence 3 times. Now there is nothing surprising about going faster when pointing lower and going slower when pinching a little. What was different with the Aero is how much you accelerate when you go low and how quickly and how much I slowed when I over trimmed the main while trying to go high.

I started going fractionally low to build speed. I gradually gained on the other boat and actually lost very little height to weather. Then I trimmed in the main and went into "point mode". I immediately slowed, gained a bit of height and the other boat started to roll me. Ease the main, back to a slightly lower mode, accelerate and move ahead again.

Update 5/25/15

When doing the same test in 10-12 knots of breeze, you can trim much, much harder without the boat stalling. What I've observed is that once you are fully hiked, the fastest VMG course is to trim quite hard with the leach telltales stalled some of the time. You need to be very observant of the boat slowing, and when it does, ease the mainsheet, point a little lower, build speed and then head up and trim the mainsheet harder.

When 7-10 knots and not fully hiked, you need to not trim so hard. A lower course is faster.

Mark the Mainsheet

Because the boom is high, it is essential that you mark the mainsheet so you can get accurate feedback about your trim. A good mark will enable you to reproducibly trim the mainsheet and to quickly catch when you are overtrimming. Over time, you will build up a "pattern" memory of the right trim based on your mark.

Conclusion

Aero sails have tight leaches and so it is very easy to hook the main and increase drag. Unfortunately, there is not direct feedback from the telltales. Like your Mother telling you that you are the smartest or most beautiful, the telltales are telling you what you want to hear — all is okay, when it is not. You're going slow. The real guide is to sharpen your sense of whether you are going 100% speed or if you are a little slow. Watch that, rather than slavishly watching telltales.

Aero Tip #5

If slow, ease the main a little and go lower, build speed and then come back up to course.

Footnote

We've been testing the 9 rig vs the 7 rig. This week, in 12 knots of breeze, flat water sailed by 165-170 lbs sailors, we got some interesting data. The 7 rig was slightly, but noticeably faster upwind. In this breeze, the 9 rig was depowering while the 7 rig was fully powered up. When the wind eased to 7-8 knots, the boat were equal. Under 7 knots, the 9 rig had the advantage.

For a 165 lb sailor, use the 9 rig up to 12 knots and use the 7 rig above that.

[Revised: 5/25/15]