Re: why boats point well
I write this years ago for another purpose so I don't know whether this will be helpful or not and apologize that its quite long having been written for another purpose.
Good pointing ability is a witch’s caldron full of factors. The way the question is worded I am assuming that you are looking for an overall catalogue of pointing abilities factors.
To begin with when you talk about how well a boat is going to windward there, are a number of components that actually define a boat’s instantaneous windward ability; true wind angle, speed through the water and leeway. The combination of these three factors is what determines VMG or Velocity Made Good. The most obvious component of VMG to a lay person is the angle that the boat is pointing toward the wind. Obviously, the higher that angle the closer towards the wind the boat is heading. Speed through the water is an often-overlooked component of the boats VMG. If you have ever pinched (pointed too close to the wind) on a boat you have probably noticed that the boat’s speed drops off rather quickly. In that state, you are aimed closer to the wind but you are making relatively slow progress in the direction that you want to go. You have a pretty slow VMG.
As you bear away there is a point where speed builds rather quickly and then beyond that point the boats seems to heel more but it does not build speed, if any, more speed. Beyond that point you may be going really fast through the water but you are aimed far from away from upwind. So even though you have a lot of speed you do not have very good VMG. The goal is to balance the two.
Then there is leeway. Leeway is a product of a lot of things but essentially the keel and lifting foils need water flowing over them to generate lift. If you point to high then the water passing over the keel slows and the boat generates more leeway often negating the increased VMG of pinching. By the same token, falling too far off creates an abundance of side force and that causes a greater leeway as well. There is a spot where all of this balances out making for the maximum VMG for the conditions.
In a general sense, the single largest thing that prevents a boat from pointing is anything that causes drag. Drag is the enemy of pointing higher. As drag increases, the need for additional drive increases. Drive comes at the price of more powerful sails. (By more powerful I mean sails with deeper camber-more curvature, not simply larger sails) More powerful sails come with the price of greater side loads, which translates to more heeling and more leeway. Beyond that the deeper camber of a powered up sail also means that the sail cannot point as high without creating back flow (luffing) as a flatter sail.
High aspect ratio sails and overlap:
Upwind, all other things being equal, drive is pretty much proportional to the length of the leading edge of the sail. Higher aspect ratio sails (Aspect ratio is the relationship of the vertical height of the sail to its horizontal dimension. A high aspect ratio means that the sail is relatively tall compared to its horizontal measurement and as compared to a low aspect ratio, which means that the sail is relatively short.) High aspect ratio sails have a higher proportion of the leading edge of the sail producing drive in relationship to the trailing edge of the sail, which, upwind, is simply producing drag. The price of having high aspect ratio sails is that they produce more heel for a given drive on a reach, they have higher stresses in the fabric and they often less forgiving of sail trim.
One of the popular myths is that overlapping sails are good for going to weather. Strictly speaking that is not the case. As mentioned above since the leading edge of the sail generates the bulk of the drive and the rest of the sail is generating proportionately greater drag for minimal drive. Most boats can point higher with a 110% (i.e. minimally or non- overlapping headsail) than they can with their Genoas in almost all conditions. So then, why use Genoas? Well, as I mentioned above, the ability of a boat to go to weather has two components, speed and direction. The greater sail area of the Genoa increases speed but requires the boat to fall off a little bit in order to do so. This results in a greater VMG to windward than would be achieved without the Genoa on boats with rigs designed to use Genoas. Modern high performance rigs are generally not designed to use Genoas. If you look at the current crop of race boats, and many of the cruising boats that are evolving from them, the rig of choice up wind is a fractional rig with a non-overlapping jib. The sails are proportioned to work through a wide range of windspeeds from next to nothing on up to very heavy air. The fractional rig allows depowering without shortening sail though a wider range of windspeeds.
Off the wind they use masthead chutes and reachers but that is another topic.
Efficient hull shape and foils;
If drag is our enemy here, the underbody becomes the place to really do battle. The bulk of a boat’s drag comes from its passage through the water. Boats produce drag in a number of ways but the two biggies are surface drag, which is more or less proportional to surface area and smoothness, and residual drag, which essentially is the energy that is required to overcome drag due to turbulence and other shape generating drag.
So ideally you want a boat with the minimum surface area. In an idealized world that means a boat that had cylindrical sections (a circle has the smallest surface for a given cross sectional area). Of course, like everything else in yacht design this too is a compromise because hemispherical hull sections produce no form stability. Without some form stability a boat requires more ballast (weight) and will be initially tender. This means greater heel and once heeled the sails and keel will operate at an inherently less efficient angle of attack and in the down wash of the hull producing more of our age nemesis, namely drag in relation to the lift generated. So it is that most boats are designed with more surface area than would be ideal if they were not powered by the wind.
Of course the rougher the surface of the interface between the hull and the water, the more frictional drag that a given area will produce. So it is that racers go to great lengths to keep their boat’s bottoms fair and smooth. This especially is important upwind and downwind.
The other component of drag is turbulence. All boats produce some turbulence. The more you produce the more drag that you have. There is simple surface turbulence that occurs at some point along the hull as the laminar flow of water becomes unstable and begins to tumble. Gradual transitions in the hull, smooth surfaces and small frontal areas reduce the production of surface turbulence.
Of course one of the big drag generators and often the biggest on cruising boats are the foils. As mentioned above, just like sails, in an ideal sense high aspect ratio foil have more lift for a given drag for the same reason, namely that lift is proportionate to the leading edge length of foil. A high aspect ratio foil simply has more lift area for the area of the foil simply producing drag.
There are also number of other factors at work in high aspect ratio keels. The area adjacent to the hull is really pretty turbulent by the time that the water reaches the keel. The upper portion of the keel is operating in this turbulent zone. That means that the upper portion of the keel is producing proportionately less lift than the deeper portion of the keel operating in clear water.
Probably one of the single biggest factors in keel drag is tip vortex drag. As water flows from the high-pressure side of the foil to the low-pressure side of the foil (across the bottom of the foil) it does two things. It reduces the lift of the keel in the immediate zone and it produces a vortex that is dragged behind the boat. The longer the bottom edge of the keel, the bigger the zone of reduced lift and the greater the drag created by producing and dragging this vortex behind the boat.
Keel profiles and sections can help a little with this. To some extent, the use of winglets or endplates can also help some. But in the end there is no way around this proportional relationship between keel bottom length and the production of drag. It is this greater drag due to keel vortex that puts a full-length keel at a disadvantage to a more modern keel when it comes to pointing ability.
The other issue with long keels is the rudder and its contribution to drag. In the case of a keel hung rudder, the rudder operates in a highly turbulent zone at the aft end of the keel. It needs to impart more energy against the water flow to produce the same amount of turning. This greater energy of course translates to greater drag. In the case of a boat with higher aspect ratio foils the rudder is often operating in clear water and by its very shape has a higher lift to drag ratio.
Of course this efficiency does not come without a price. High aspect ratio foils are more prone to stalling. Given too steep an angle of attack the flow breaks down and the foil looses lift. This is particularly true in rudders where their placement close to the surface means that air can be sucked down the low-pressure side of the rudder resulting in a wipe out. Much is made about fin keels stalling out as well. While this can occur (If you doubt that watch a Melges 24 get under way and slide about half as far sideward as it does forward in the brief instant before it reaches sufficient speed for the keel to hook up.) but on most cruising boats even with high efficiency foils, the foil design is such that a modern fin keel rarely stalls. That said, even in extreme going, where the keel may be in a partially stalled condition and not pointing as high as it could in flat water, high aspect ratio fin keels still tend make less leeway and point higher than lower aspect ratio keels.
Of course high aspect ratio foils come with their own price such as deeper draft, more difficult engineering, inherently less directional stability, and, in the case of rudders, the stalling issue.
Beamy boats and exceptionally narrow boats generally do not point as well as more moderate beam boats. Beamy boats have more wetted surface and require more energy to push the water out of the way. extremely narrow boats do not have the stability to stay up on their feet.
In recent years a lot of focus has been paid to bow shapes. If you looked at the drag on objects shaped like a wing, you would think that a hull with a full bow would have less resistance than one with a fine one. In other words if you take a tear drop shaped object and tow it pointy end first, it has substantially more drag than the same object towed rounded end first. So then why are pointy bows fast upwind? Well, bows operate in the zone between air and water. Its interactions with the water are more about impact than it is about laminar flow. The finer bow allows the water to have lower impact on the boat and so produce less resistance. It used to be that you needed weight to ‘punch through a chop' but as boats have evolved there has been an increased interest in the other side of the equation, reducing impact, and so today very light boats can get through a chop without being stopped because the impact has been reduced to the point that it just plain takes less mass to punch through. A side result is a more comfortable motion as well. A fine bow also means that the water is being asked to change shape at a slower rate and so is less likely to become turbulent.
Other drag generators:
Since drag is the enemy of pointing ability, anything which generates added drag needs to be taken into consideration. Items which might fall in this category in the wind resistance category might include items like large amounts of cockpit canvas, furled sails such as a storm sail or light air downwind sail, dinghies hung on davits in the windstream, wind generators and large solar panels. Items which might add to water generated drag might include bow thruster aperatures, fixed blade propellers and especially three bladed props, towable generators and so on. Obviously, these drag producers have virtues which may outweigh their negatives.
Heavy boats are not necessarily only slow because they require more energy to over come the greater inertia of the boat’s heavier mass. They are also slower because it takes more wetted surface to enclose their greater displacement. This also pushes the canoe body (the hull of the boat without appendages) deeper in the water. This affects drag in two ways. First, it creates a form that by its very shape causes the water to bend further out of the water’s natural direction of flow. Bending the water flow by its very nature requires more energy. But the deeper, wider canoe body also means that the boat is more likely to produce greater turbulent flow and creating and dragging this turbulent flow behind the boat also translates to drag.
The last piece of puzzle regarding the effects of weight on windward performance, is the affect of a heavy boat on foil shape and area. As mentioned above, high aspect ratio keels and rudders produce more lift for less drag. When you look at a heavier boat, its deeper canoe body means that for a given draft the foil area of the keel is shorter and therefore of a lower aspect ratio. This means more drag for a given lift. This is a bit of a circular situation. The heavier weight of the boat means that it needs more drive. More drive means greater side force. More side force means a need for more lift. More lift means more keel area for a given depth. More keel area for a given depth means more drag and more drags means that we need more drive starting the cycle over again.
Sail shape is one of the most critical items. As mentioned above, pointing ability comes as the result of the optimization of the whole system. Sails can and should have a particular shape for going up wind. That shape is never constant and so the optimum pointing angle is never constant. As a general rule, the higher the wind and the less drag the boat, and the flatter the water, the flatter the sails can be and the higher you can point. As the wind increases in speed, the sails should be trimmed flatter, which means more halyard, outhaul, and sheet tension. As wind further increases the traveler should be dropped a little to help blade the mainsail. The backstay should be tensioned to tighten the forestay, which further flattens and depowers the jib and on a fractional rig boat, also bends the mast and flattens the mainsail. On a masthead rig with a bendy spar, tensioning the baby stay will do a similar thing albeit at the expense of headstay tension.
In wavier conditions, tensions should be eased a little to give the boat more drive to overcome the increased drag of passing through waves. The boat will point lower but its VMG will be greater.
Of course sail shaping is a science unto itself. Modern sails are really amazing. They are really low in stretch and yet in the hands of a knowledgeable trimmer and be easily altered in shape on the fly. If we talk about sail shape, looking at horizontal slice through a sail, we see a roughly wing shaped section. In a properly shaped sail there is a leading edge with a comparatively tight radius. This is followed by the gently curving section of the mid-sail and then by a comparatively flat trailing edge. This combination of curvatures result in the ‘camber’ of the sail. We talk about camber as having two critical components; its depth and the point of maximum camber. Depth of camber is measured perpendicular to a line running from the luff of the sail to the leech. Deeper camber means more drive but more drag and less pointing ability.
The second factor is the point at which this maximum depth occurs. Ideally the point of maximum camber is approximately 30% to 40% aft of the leading edge of the sail. New, properly made sails, have their camber pretty much sown in at that range, but as sails age, the leech of the sail, which has the greatest loads stretch faster than the luff and as it does the camber migrates aft on the sail. As camber moves aft it produces more drag, and greater side loads. Greater side loads result in greater heeling and more leeway. Also, as the camber moves aft pointing ability drops as the sail becomes less forgiving produces less lift.
A sail trimmer can move the camber around on a sail. Tightening the Cunningham tends to move the camber further forward on the sail. Halyard tension can also be move camber forward. The draft (or depth of camber) can also be altered. Tightening the halyards, outhaul, sheets and increasing mast bend reduces depth of camber. Easing the above increases sail draft which powers up the sail, increasing the drive generated by the sail but also increasing drag and heeling forces.
Angle of Attack and apparent wind angle, twist, gradient wind:
For any given wind speed, sea state and vessel there will be an ideal angle of attack for the sail. Angle of attack is the angle between the apparent wind and the sail. If this angle of attach is to flat (either pinching or under trimmed) the wind gets to the low-pressure side of the foil and collapses the sail (luffing). If the sail is over trimmed (stalled) the wind shreds away from the low-pressure surface of the sail too soon creating a lot of turbulence, more heeling and very little drive. In other words, a little bit of a luff beats a little bit of over trim any day (except in light winds) and if the luff comes from pointing a little higher you also have better VMG (as long as your speed has not plummeted and you are not making gobs of leeway).
It would seem like the angle of attack should be uniform up and down the sail but in reality there are good reasons why twist makes sense in some conditions. To start with, as wind speeds vary a boat will feel different apparent wind angles for any given true wind angle. To explain further, few displacement monohulls can exceed their hull speed upwind and most lightweight boats can achieve hullspeed upwind in as little as 8 or 10 knots of true wind. (Obviously heavier boats need more than that). Because of that, the relative affect of the boat’s speed on the apparent wind angle begins to decrease as the boat approaches hull speed. Because of that the apparent wind felt by the boat appears to be fairer as the wind increases and so the boat can point higher in more breeze than lighter breeze. Another reason a boat can point higher in more breeze is the sails can be (and should be depowered) as the breeze builds and a depowered sail can also point higher.
In all winds there is a difference between the wind speed near the water, and the windspeeds as you get above the water. You can visualize that the air right at the surface of the water does move at all relative to the surface of the water being held in place by friction. As you move away from the water surface you reach a point where the friction of the water has little effect on the speed of the wind. In between you have what is referred to a gradient wind affect meaning that there is a spectrum of air speeds between the ambient wind and the water surface. In heavy air, this gradient layer is just about non-existent. But in light air it can easily taller than your boat’s mast. As a result the sail needs to be twisted so that each portion of the sail has a proper angle of attach to the apparent wind that it is sailing in. Twist in a mainsail is induced by easing the vang, moving the traveler to windward, and inducing a little mast bend. Twist in a jib is induced by moving the sheet lead aft.
Mast and rigging turbulence, and air drag and other physical constraints:
Then there is the matter of mast and rigging turbulence. The larger the mast the greater the drag and of course drag is our enemy but also it means that more of the mainsail area is operating in a turbulent zone down wind of the mast and therefore producing drag but little lift. Shrouds and rigging also add drag. Moving spare halyards to the leading or trailing edge of the mast and tensioning them takes them out of the air stream and keeps them from oscillating and creating turbulence. Relocating superfluous rigging like running backstays and topping lifts to a position out of the airstream also reduces drag. Lowering dodgers and keeping crewmembers low to the deck and either in the lee of the cabin or else in the lee of each other helps cut drag as well.
Another issue is the ability to have close enough sheeting angles. On older style long keelboats it is no big deal to have shrouds placed on the hull. Their ability to point is constrained by the keel as much as the sail plan. But as modern fin keelboats began to be developed shrouds moved inboard and spreaders grew shorter so that the angle of attack of the sails could be made flatter. Multi-spreader rigs allow narrower shroud bases and shorter spreaders and so closer sheeted headsails.
One great source of drag is a lot of steering. A boat with a neutral helm has less drag than one with its rudder cranked hard over. That said a very small amount of weather helm is actually a good idea. Even though it induces a bit of drag, it also helps to lift the boat to windward (or more to the point reduce leeway). Balancing the helm involves making sure that you are flying the right amount of sail area and that the sails are trimmed properly in relationship to each other and the windspeen and direction. In heavy air you may actually be fast carrying a pretty big luff in the mainsail but your helm would be neutral and the boat standing on its feet.
Tuning comes into play here as well. As you rake a mast aft you generate more weather helm. As you rake it forward you generate less. Too much of either is not good under any circumstance.
Heeling reduces VMG. As the boat heels it maked more leeway and the sails become less efficient so they have equal drag but less drive which is never ideal when going to windward.
That’s about it.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 04-25-2014 at 09:58 AM.