Ultra-light air performance
Posted: Sun Nov 10, 2013 11:01 pm
I've long maintained that Macs can't keep a heading below about two knots, as the rudders don't have enough hydrodynamic flow below that speed to keep course. That was my experience helming my boat with a wind instrument and GPS to know with certainty what speeds I could keep and what I couldn't. Given the light-air efficiency of about 35% that Mac's have, this meant that you need more than 5 knots of true wind to bother attempting to sail.
Well, I discovered today that not only was I wrong, but I was off by half: I just spent a very lazy day sailing with Stephen on Luna Sea with half the day spent between 1 knot and 2, and we kept course just fine.
It turns out that my previous estimate was based on me at the helm. At very low speeds, any error at all in heading is immediately punished with a loss of speed: Even slight oversteering will cause rudder stall, turning into or off the wind will cause sail flow stall--anything at all taking you off the heading you have your sails set at will immediately overwhelm the minor wind power, and then you're caught trying to get restarted with no forward momentum and truly no rudder bite and the boat drifting in a circle.
At 2 knots, there is so little energy going into forward momentum that it is easy to "overwhelm" the heading with any other kind of energy transfer: A person moving across the cockpit or to the cabin will cause a heading change of 10 degrees almost immediately. Changing the heel angle at all will also cause a "mini-roundup", going over a wake wave, even changing the daggerboard depth, all have exaggerated impacts on the boat's course at 2 knots. It's very much like "dinghy sailing" at these speeds.
And consider that at 1 knot, you've got 1/4 the input energy that you have at 2 knots, which further magnifies all of these impacts.
But, and this is critical, there actually is enough hydrodynamic flow over the rudders to keep course at 1 knot.
The right way to think of ultra-light air performance is that it's completely unforgiving of any cause of heading error. Any mistakes or waves or wind changes, and the boat will change course and if you don't correct it immediately, you're dead in the water. Since you're bound to have one of these causes occur every minute or more, it's nearly impossible to keep a helm below 2 knots.
For a human. But for the autopilot, it was easy. With the autopilot keeping the helm, and never making any of these errors, and compensating for wake waves, movement about the boat, and heel changes instantly and perfectly, we were able to keep course all the way down to about .8 knots reliably. This means sailing in as little as 2 knots of true wind.
I was absolutely astonished. I'd thought we were going to spend the day wallowing around in circles with luffing sheets as there was no wind, but with the autopilot going we kept course just as well as the few gusts that got us up over 4 knots.
Being able to keep course meant that we could point the boat, trim the sheets, and have the same lazy day sailing even though the speed drifted mostly between 1.5 knots and 2.5 knots. On the helm myself, that would have been constant frustration as we'd lose all forward momentum every time I took my eyes off the heading. But with the AP, all it meant was it took an extra hour to do the same course we would have done at 3 knots. And it left me free to keep the sheets in perfect trim for the wind.
Below 1.5 knots, I did need both rudders down to get enough "bite" in the water to maintain speed, and below 0.8 knots the autopilot started working quite a bit and allowing a lot more heading. At 0.5 knots, the autopilot gave up and alarmed as it could not keep course any longer. By my instruments, 0.8 knots defines where the true hydrodynamic limit of the rudders actually is.
Probably some credit goes to the 6oz fully battened Judy B hyde sails, as even at these ultra-light wind speeds there was no luffing or loss of body in the genny or the main, and luffing is a big loss of energy.
For me, this doubles the amount of time I can spend sailing, as winds below 6 knots are my typical weather problem here in San Diego bay.
Well, I discovered today that not only was I wrong, but I was off by half: I just spent a very lazy day sailing with Stephen on Luna Sea with half the day spent between 1 knot and 2, and we kept course just fine.
It turns out that my previous estimate was based on me at the helm. At very low speeds, any error at all in heading is immediately punished with a loss of speed: Even slight oversteering will cause rudder stall, turning into or off the wind will cause sail flow stall--anything at all taking you off the heading you have your sails set at will immediately overwhelm the minor wind power, and then you're caught trying to get restarted with no forward momentum and truly no rudder bite and the boat drifting in a circle.
At 2 knots, there is so little energy going into forward momentum that it is easy to "overwhelm" the heading with any other kind of energy transfer: A person moving across the cockpit or to the cabin will cause a heading change of 10 degrees almost immediately. Changing the heel angle at all will also cause a "mini-roundup", going over a wake wave, even changing the daggerboard depth, all have exaggerated impacts on the boat's course at 2 knots. It's very much like "dinghy sailing" at these speeds.
And consider that at 1 knot, you've got 1/4 the input energy that you have at 2 knots, which further magnifies all of these impacts.
But, and this is critical, there actually is enough hydrodynamic flow over the rudders to keep course at 1 knot.
The right way to think of ultra-light air performance is that it's completely unforgiving of any cause of heading error. Any mistakes or waves or wind changes, and the boat will change course and if you don't correct it immediately, you're dead in the water. Since you're bound to have one of these causes occur every minute or more, it's nearly impossible to keep a helm below 2 knots.
For a human. But for the autopilot, it was easy. With the autopilot keeping the helm, and never making any of these errors, and compensating for wake waves, movement about the boat, and heel changes instantly and perfectly, we were able to keep course all the way down to about .8 knots reliably. This means sailing in as little as 2 knots of true wind.
I was absolutely astonished. I'd thought we were going to spend the day wallowing around in circles with luffing sheets as there was no wind, but with the autopilot going we kept course just as well as the few gusts that got us up over 4 knots.
Being able to keep course meant that we could point the boat, trim the sheets, and have the same lazy day sailing even though the speed drifted mostly between 1.5 knots and 2.5 knots. On the helm myself, that would have been constant frustration as we'd lose all forward momentum every time I took my eyes off the heading. But with the AP, all it meant was it took an extra hour to do the same course we would have done at 3 knots. And it left me free to keep the sheets in perfect trim for the wind.
Below 1.5 knots, I did need both rudders down to get enough "bite" in the water to maintain speed, and below 0.8 knots the autopilot started working quite a bit and allowing a lot more heading. At 0.5 knots, the autopilot gave up and alarmed as it could not keep course any longer. By my instruments, 0.8 knots defines where the true hydrodynamic limit of the rudders actually is.
Probably some credit goes to the 6oz fully battened Judy B hyde sails, as even at these ultra-light wind speeds there was no luffing or loss of body in the genny or the main, and luffing is a big loss of energy.
For me, this doubles the amount of time I can spend sailing, as winds below 6 knots are my typical weather problem here in San Diego bay.
