First Principles of Sailing

“First principles” is a buzz word in the tech industry right now, which describes breaking complex problems down into the most fundamental building blocks possible. This article is a bit of writing that I did several years ago to help roll some boat handling ideas around in my head, but it gets down into the nitty gritty of why the best maneuvers work the way that they do.

Boat Handling Philosophy

In any type of sailboat, speed is about maximizing lift on the foils at all times.  Once maximum lift is being generated – in other words, all crew members are hiking as hard as possible against un-stalled sails and foils – any extra power becomes unnecessary drag, and needs to be reduced by flattening sails, or reducing surface area of the foils.  Boat handling techniques discussed here have one goal; to maximize lift as soon as possible after maneuvers.  

An important thing to keep in mind when you are experimenting with techniques on the water, is that lift is not the same as heeling moment.  Just because you can hike harder against the sails does not necessarily mean that the foils are generating more lift – they might actually just be generating more drag.  The concepts presented here should help you determine if your techniques are succeeding at generating more lift, and help you refine your boat handling

Flow Over Foils

The mission to understand how to maximize lift at all times, starts with the concept of “flow,” which is a term that you hear a lot in sailing, but which few sailors really understand as well as they should.  To understand flow better, consider this question: during a roll tack, would you rather execute a hard, fast flatten, or a slow, smooth one?  What might be the performance difference in each? 

Look to any class of boat, and you’ll see that the top teams answer this question with a range of techniques, and in many classes, the accepted “best” technique goes through cycles, with the top teams favoring one, then the other, and then back to the first one.  When you’re looking for a tiny edge on the competition, it’s important to understand the underpinnings of the battle being fought here, and if you’re not yet in that top echelon of competitors, understanding those same principles is even more crucial to seeing the big picture.

In any maneuver around the race course, there are a few goals which should dictate your technique.  First, we’d like to get as much flow over the foils (the ones in the air AND the ones in the water) as possible.  This means that we want our flatten to be as hard and fast as possible – more weight moving through more distance in a shorter period of time.  There is however a limiting factor, that maybe you’ve experienced before: cavitation on the foils.  Imagine the extreme example for a second; a 1000 pound person flattens with all their might, accelerating the top of the mast from a standstill to 50 miles per hour in a fraction of a second.  As the sail is jolted through the air, and the centerboard is jolted through the water, the fluids surrounding the foils (air and water) can’t change shape fast enough to stay attached to the foils, so a vacuum is created behind each, in what we refer to as “stall”.

Often time, as sailors, we spend a lot of time talking about sails but neglect to talk about the other half of the equation; load on the blades.  To understand when stall happens, it’s important to understand that what happens in the water must balance what is happening in the air. Sails generate lift when airflow creates pressure differences from the windward to leeward side of the cloth, and blades do the same thing under the water.  If the blades are generating enough lift to balance the forces in the sails, the boat will go forward, but if not, the blades will stall and the boat will slide sideways.

Chances are good that if you’ve ever practiced acceleration maneuvers, you’ve felt a moment at the beginning of a flatten, when the flow around your rudder separated, the boat tried to round up slightly, and you ended up pulling the tiller to windward with little effect until the boat started moving through the water.  Although you usually hear about dramatic cavitation at high speeds, when boats “lose their rudders” on a reach, and round up uncontrollably, cavitation at any speed is a super common issue, when the loading in the sails is not balanced by the flow over the foils.

In the acceleration scenario, trimming the sails without any flow over the blades, creates sideways load in excess of the lift that the foils can balance, and therefore the blades stall.  Get a little bit of forward speed on first to create lift on the blades, the forces will balance, and the boat will accelerate increasing flow over the blades even more, and allowing you to load the sails more.  In windy conditions, the cavitation on a reach occurs because the apparent wind angle gets too far abeam, sails load up more than the blades can support, rudder is used to fight against the tendency of the boat to want to turn up causing more drag in the water, slowing the boat thereby reducing flow over the foils, and causing the load in the sails to overpower the lift being generated on the blades. 

The bottom line is that more flow over the blades will allow you to flatten harder without cavitating.  As such, a good flatten in underpowered conditions typically increases exponentially as the boat speed increases up to the 50% mark of the flatten, and then decreases exponentially beyond that 50% mark, allowing the boat to settle into is new speed while maintaining flow.  There are exceptions to this idea, but the beauty of understanding the underpinnings at work here, is that you should be able to identify them as they arise.

Watch this vidoe of flow over a wing to better understand flow and stall.

Apparent Wind

The second extremely important concept to understand when talking about boat handling, is the idea of apparent wind.

Stick your head out of the window on the freeway, and how much wind do you feel?  If your car is moving 60 miles per hour, through a true wind speed (TWS) of 0 miles per hour (that is, 0 miles per hour over the ground).  You’ll feel a 60 mile per hour wind blowing against your face.  If however, the TWS is blowing 10 miles per hour in the same direction that the car is travelling, then you’ll feel the 60 mph minus the 10 mph tailwind, for a total of a 50 mile per hour apparent wind speed.

It’s important to think about apparent wind as the combination of true wind speed – the wind over the ground (or in our case, over the water) – and an inverse component of velocity – that wind in your face if you are moving in 0 knots of TWS.  Using some math, you could calculate the apparent wind speed (AWS) and angle (AWA) for any combination of velocity, and TWS.  Whether or not you are a “numbers person,” the important takeaway here is that increased TWS without a change in a boat’s velocity increases the apparent wind speed, and shifts AWA forward.  Conversely, a decrease in TWS shifts AWA aft.  If velocity of the boat accelerates without a chance in TWS, apparent wind shifts forwards, while a decrease in velocity shifts AWA aft.

To put this to use on a race course, you need to really understand WHY all of this happens.  DO NOT READ ON until the previous paragraph makes sense.  It is not enough to memorize the relationships; you should be able to explain them from scratch.

Here are a few important examples of how we apply apparent wind on a race course:

  • Chop: Sailing upwind, your sails are trimmed for a close hauled course in flat water at 5 knots of boat speed and 7 knots of TWS. You hit a rouge piece of chop, and your boat speed falls to 3 knots. TWS is still 7 knots, and relative to the inverse component of velocity, it is now a more significant factor in your apparent wind, so apparent shifts aft. If your sailing angle stays the same, what do your sails need to do to compensate for the change?

  • Surfing Wave: Sailing downwind in 12 knots of TWS, your speed through the water is 13 knots. You are sailing faster than the waves around you, and as you get to the peak of a wave, your crew steps forward to tip the boat onto the face of the wave. Speed accelerates to 16 knots, while TWS stays the same, making the inverse velocity component of apparent wind more significant relative to the TWS. Apparent wind angle therefore shifts towards the inverse velocity component. If you like your sail trim still, how could you compensate for this change by steering the boat?

  • Puff: In 3 knots of TWS, you are moving 2 knots. A 6 knot puff hits. What is the initial change in AWA for the moment that the boat is still going 2 knots through the water? The boats slowly accelerates in the new pressure building speed to 4 knots eventually. During the period of acceleration where TWS is 6 knots, and speed builds from 2 to 4, what happens to AWA?

  • Advanced: Bad lull response: You are sailing upwind at 3 knots, in 5 knots of TWS, a lull hits, and the apparent wind angle shifts forward. In response your helm pulls on the tiller, to put the bow on a close hauled angle to the new AWA. What effect does the turn down have on AWA? What should lull response be instead?

A Grand Unified Theory Of Sailing: The 100% Rule

In sailing we can talk about the power in the main as a function of how close to stalling the foils we are.  If we are at 100%, any more trim on the sails, deceleration in the boat, lifting pressure, etc. will send us over the edge (i.e.  101%+) and either the sails will stall, the blades will stall, or both.  This accounts for the “crashing” feeling of losing all power when you hit chop.

In slow boats with big, inefficient blades (think Etchells, 420s, etc.), the stall we’re most often worried about is stall in the sails.  We click in the mainsheet, watching the leach tell tales until they begin to flick to the far side of the sail, indicating the presence of flow separation beginning.  This is the point when we are generating maximum lift over the sails, and because we have so much surface area on the blades under the water generating tons of lift (and also tons of drag), we don’t have to worry too much about stalling the blades as long as the boat keeps truckin’ along.

On the other hand, in a skiff, sport boat, or other boats with low drag hulls and high aspect (long and skinny) blades, avoiding stall on the rudder and centerboard is a massive part of the battle.  With the more slender blades in the water, we play a delicate game, trying to convert speed through the water into lift on the foils so that we can squeeze the mainsheet a little bit tighter without losing balance between the forces in the sails and the forces on the blades.  This is where the 100% rule becomes critical.

100% refers to the amount of lift that the foils are generating, but in a boat like a skiff, the faster the boat moves through the water, the more lift gets generated.  This means that as speed increases, we can put more load into the sails without worrying about stalling the blades.  As such 100% is always a moving target, and it’s important to understand when and why that target moves so that we can be as close as possible without going over.

When sailing into a piece of chop, we can anticipate a deceleration in boat speed, which will cause apparent wind to shift aft.  With apparent wind now more abeam, the sails load more, putting us closer to 100% than we started, so the correct way to manage chop, is to ease the sails before we get there, backing off from 95% to 80%, so that when the chop hits and the threshold of force from the sails that our blades can balance drops, the amount of force being generated by the sails is already unde3r that threshold, meaning that we don’t go over 100%.  When we hit the chop, the percentage will rise because the threshold drops, but if we eased in time, it will only go back to 95% instead of jumping to 110%, the boat stalling, power crashing to 50%, and then having to rebuild everything that we lost.  As soon as the chop has passed, the boat will accelerate again and as it does, the force threshold moves higher, bringing the percentage back down, and allowing us to trim back in and load the sails up.

As a rule of thumb, I usually talk about the ideal mode being a 95% power mode.  This means that you are trimming the sails hard, and pulling on the boat, but you have an extra 5% margin of error in the bank in case a puff hits before you have time to react, or a small piece of chop slows the boat slightly.  Feeling the speed is essential here because you need to know how fast you are going through the water to know how hard you can trim the sails without passing 100%.  If you can train your sense of feel to be very sensitive to these small accelerations and decelerations, then sailing at 95% buys you a little bit of time to feel the boat decelerating, and make an adjustment before the boat crashes, whereas, teams who sail super tight and close to 100% all the time will stall before they have a chance to feel the boat getting slow.  

The big question left unanswered here is how you know when you’re at 95%.  In this arena, there is no substitute for time in the boat, pushing the limits of what the boat can take before it stalls, and learning to feel when trimming the sails in causes the boat to accelerate, when trimming has no perceptible gain (this is the moment that you have nearly reached 100%, and should back off to 95%!), and when trimming actually causes the boat to bind up and decelerate (too late!).  

Many sailors train themselves in the opposite direction – trim until they feel stall and then ease, rather than easing the moment before they know the stall will occur – and at the top level, when both of these techniques are honed to near perfection, this is often the difference between the team who hangs for 90% of the beat, and then loses it in the last little bit by crashing to windward, and the team who catches their boat just before the speed crashes.