Articles: Upwind Angles

Article by Geoff Moore

"How close, on average, can a J-24 sail to the wind in certain wind-conditions? I know this is a pretty variable topic, considering sails, waves, experience of helmsman, etc. I'm new to competitive helming and just trying to get a feel of what I should be expecting, and striving for."

As usually there are no simple answers.

Because we don't sail with instrumentation I don't have a good grasp for what angles we actually sail. But, I am not sure how helpful that would be.

The skipper drives the boat according to the "apparent wind angle". That is the wind we feel when we are sailing, and that is the wind that affects the tell tails. The faster we go the more the wind appears to come from in front of us, so the lower we have to head.

Apparent wind is a fairly simple concept, but it gets more interesting than that. If we could sail upwind at a boat speed of say15 knots, and the true wind was only 10 knots we wouldn't be able to point very high at all because the apparent wind would be so far forward. But most, practically all, boats can't sail at 15 knots up wind because they are limited by "hull speed". To be brief hull speed is like a natural speed limit. The hull speed of any boat is affected by many things, but most profoundly by the length of it's waterline. Boats create a wave as they move. Since longer wavelengths tend to travel at higher speeds longer waterlines create a higher hull speed because they make waves with longer wavelengths.

It sounds complicated and it is, but I visualize it as though I were sailing around in the trough of a big wave. If I want to go faster I would have to start sailing up hill to get over the wave in front of me. Sailing up hill is hard…, especially upwind. Occasionally downwind we gain enough power to actually sail up hill and break through the wave. When this happens we skip across the surface tension of the water and we call it planning or surfing.

So what does this have to do with sailing angles? Well, I like to think of a boat as traveling in one of three different modes. The first one is the one we just dealt with, "planning". Keelboats don't plan up wind so this mode doesn't affect our upwind angles. The second mode is when we are sailing in the trough of that big wave. There is enough power to move faster, but we can't because we are starting to sail up hill. This is what I call hull speed mode, or displacement mode. We just aren't going to go any faster no matter how much more wind we see. But, since we don't go any faster the boat doesn't drag the apparent wind forward. So we point closer to the true wind. This is the mode where we sail the very highest angles. The third mode is what I call "below hull speed mode", or "light air". In this mode we aren't limited by that wave in front of us. We are almost sailing downhill! It doesn't take much of a push to accelerate us. In 5 knots of wind we can generate maybe 2.5 knots of boat speed. That's 50% the speed of the true wind. But since we are accelerating so easily we drag the apparent wind way forward and we don't point very high. The transition between the two hull speed modes is usually around 8 knots of true wind.

The other interesting thing about racing in "below hull speed mode" is that there are often huge discrepancies in relative boat speeds across the racecourse. This is because the minor wind variations have a much bigger impact when boats are about to sail "downhill". As a result the optimal sail shapes are also very different between modes, but I will save that for another time.