Ultra-light air performance

[mastreb]

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.

[Ixneigh]

I dont know about windspeed, but if there is enough wind to fill the jib, I can keep a course. If I cant keep the jib filled, even by sitting on the leeward rail, I hang it up, in any sailing craft.
Ix

[bscott]

I dont know about windspeed, but if there is enough wind to fill the jib, I can keep a course. If I cant keep the jib filled, even by sitting on the leeward rail, I hang it up, in any sailing craft.
Ix

X2

Bob

[bartmac]

Second the use of a autopilot....its a lot more reactive than me...all the time and as previously stated leaves me to tend sails....an aside is the sensitivity adjustment on my auto pilot (raymarine sportpilot) allows me to tune down how fast it reacts

[NiceAft]

I always was able to sail in light winds. I owe that to the Muscle-head mainsail. What I discovered early on was that in order to come about, I had to have forward movement of at least 2 mph( 1.736 knots). Any less than that, I could not do it.

Being far up on the Delaware River means that I do a lot of coming about. Some times the iron Genny is needed, but don't tell anyone

Ray

[dlandersson]

Ditto.

I always was able to sail in light winds. I owe that to the Muscle-head mainsail. What I discovered early on was that in order to come about, I had to have forward movement of at least 2 mph( 1.736 knots). Any less than that, I could not do it.

Being far up on the Delaware River means that I do a lot of coming about. Some times the iron Genny is needed, but don't tell anyone

Ray

[mastreb]

The other thing I noticed about light air is that we were moving faster below 2 knots that all of the sailboats on the same tack, even a 35 footer that spent about a half hour trying to overtake us. And these are boats with a more efficient hull shape than the Mac, higher hull speeds, and more sail aloft. It seems that the lightness and lack of keel inertia to overcome makes Macs more efficient than keelboats in the <2 knot regime.

However, as soon as the winds increased and we got up to 2.5 knots, it seemed the keelboats took off and easily bested us.

It is hard to come around at this speed--we failed to tack on our attempt and wound up gybing instead. Gybing is the way to go in very light winds. Not enough force to make it dangerous, and there's never a point when you're going through irons.

By the way, "Keel" is the first english word recorded in history, in 435 a.d. It was was the Anglo-Saxons called their clinker boats, which were build with keels deep enough to help keep course and allowed them to sail off the wind more than Roman triremes.

[yukonbob]

the four feet of free board helps while running as well.

[Newell]

Many of us have light air weather periods, I hate to nix sailing just because of them. I learned to sail on the Great Salt Lake, where due to the salinity many times it's glassy even with some wind present. I have went from the advise to 'bag the main for light air' to 'keep the main tight' in light air, mostly I spent my time watching the telltales and playing with the outhaul. I have always been able to keep the boat moving if there is any air, enough to point the windex, if I don't have ballast. If you can fill the jib, you can move the boat. Many times I throw crumbs or dead brine flies overboard to see if I'm moving. However, I think I am a much more patient sailor than some.

[Wind Chime]

Newell,

Why the hull do you keep dead brine flies on board?
What do you do with them? ... lunch mix

by Newell » Wed Dec 04, 2013 12:03 pm
Many times I throw crumbs or dead brine flies overboard to see if I'm moving.

[mastreb]

I don't think I have the sac to sail without ballast in. There's no such thing as an unexpected gust in San Diego, but I don't think I weigh enough to pull a MacGregor back up from a knockdown.

[dlandersson]

Ditto. When I sail - water ballast is a given.

I don't think I have the sac to sail without ballast in. There's no such thing as an unexpected gust in San Diego, but I don't think I weigh enough to pull a MacGregor back up from a knockdown.

[Newell]

Newell,

Why the hull do you keep dead brine flies on board?
What do you do with them? ... lunch mix

Sometimes brine flies look something like a sand bar they are so numerous floating on the water. If they decide to land on the boat there is no recourse. They could become lunch if you open your mouth and say 'ouch'. Yeh, the GSL isn't always a great sail.

[Boblee]

Ultra light breezes usually mean dropping the motor but the spinnaker certainly helps if plenty of time is available.

[Seapup]

If you can fill the jib, you can move the boat

Sometimes you just have to pop the ol motor in reverse to fill the spinnaker up on the dead days, the steering gets tricky though...