Kites can have higher performance than sails, for a number of reasons. Before we dive into why, first look below to see the state of big air kite boarding and kite foiling, and check out the sailing speed records of kiters and windsurfers.
There are barely even whitecaps in the background and this kid can still boost with a high-aspect foil kite. As of writing this, he currently holds the world record for jump height at 35.3 meters.
No windspeed recorded but it looks to me like less than 10 knots, yet this guy is tacking upwind at about 45 degrees and around 20 knots. Good luck getting up on foil with a windsurf foil in those conditions.
In 2008, with improving technology, kiters overtook windsurfers for top speeds.
However, much greater performance gains are found at lower wind speeds.
Torque
The torques inherent in conventional sailing are important to discuss because they suggest an engineering design spiral of reducing weight when switching to kites. Remember, if we can reduce weight in one part of the boat, we can make the structures supporting that part lighter too, and the parts supporting those parts lighter too, and so on. That is how Harryproas can be so light, and how a kite boat could be even lighter.
To understand the role of torque in this discussion, we can start with aspect ratio. First, to be efficient upwind, both sails and kites need to have a high lift to drag ratio, and therefore need to have a high aspect ratio.
To be high aspect, sails need to be very tall, which lifts the center of effort of the sails high up, creating a torque towards one side of the boat or the other, depending on the tack. The boat must have sufficient port/starboard righting moment to resist this torque, and if the sails are properly trimmed, the interaction between these torques creates a third torque, a forward torque.
This is the torque which drives the sail and mast forward, but for that to be translated into force that pushes the boat forward, the forward torque must be resisted by fore/aft righting moment. All sailboats are designed to have enough righting moment in either axis to resist these forces, that is, until the wind and wave conditions reach a certain point, past which the captain must reduce sail area for safety.
If the captain pushes too hard for the conditions, the sailboat may capsize port/starboard from not having sufficient righting moment in that axis, or pitchpole forward from not having enough righting moment in that axis (hull not long enough).
So then, just make the boat longer/wider to increase the righting moment!
The issue is that there are downsides to increasing righting moment in either axis, because (unless you are switching from normal boats boat to a Harryproa) it increases weight and wetted surface area, two primary determinants of performance. And structures need to be even stronger still to support the increased sail forces that can make use of that extra righting moment. This creates an upward engineering spiral of weight, and can result in a weight increase of more than the square of the righting moment increase.
How do a boat’s structures resist the torques created by sails?
Most boats use mast stays, which spread/increase the load to various points on the boat, meaning the entire boat needs to be stronger, hence heavier. Some modern boats use free-standing masts, possible with modern carbon composites, which mean only the mast bases need to be strong and heavy. This results in a lighter boat overall, as evidenced by Harryproas.
But a kite base could be even lighter than a freestanding mast base?
While the attachment point for a large kite will still need to be very strong to resist the kite’s pull, it will not need to be built to withstand the torque of sails which multiply the forces the area experiences. Using the Harryproa as an example, the mast base will experience about 6 times as much force (to resist the torque) as a kite base for an equivalent amount of generated power.
Heeling
If one is not in a multihull, which cost exorbitant amounts of money (if not a Harryproa, cough cough), one will be heeling in a monohull as in the mast picture, which I consider a crappy compromise. Neither cargo nor cruise ship companies can abide with heeling. Regarding monohull cruisers, almost all of them are heavier than water too, meaning they can sink, which I consider even worse. A kite boat mono-hull does not require a heavy keel to create righting moment, basically solving those two issues that turn me off from monohulls.
Kite boats need very little righting moment.
Lastly, a kite boat with fixed kite bases on deck still needs some righting moment. But its moment arm is 4 to 6 times shorter. It can fly multiple times more high-aspect sail/kite area than its equivalently-sized sailboat counterpart without capsizing or pitchpoling.
In other words it can use multiple times more power without risk of capsizing or pitchpoling, assuming all that high-aspect kite area can be launched and landed safely.
Sail Wins
Simplicity and reliability.
The kite robot will use electrical components which will need to be carefully waterproofed, handled, and maintained. So, a sail system that is made up of fewer components and fewer moving parts will inherently have fewer points of failure and be more reliable than the robotic kite system. Especially if that sail system does not require anything outside of human power to be raised and controlled. To me, most sailboats are complicated enough that they don’t win because they don’t require electricity to function. Most sailboats use stays to hold up the mast. As Mike on the Harryproa forum points out, there are risks of a “bad clevis pin, cracked chainplate, bad turnbuckle T, bad turnbuckle threads, bad/corroded swage, fatigue/crevice corrosion wire rigging, UV damage in synthetic rigging, and that’s just the bottom end of one shroud. If you count just the two main shrouds and the forestay, there might be over 50 points of failure. If you add in the other shrouds, it gets worse.”
To me, only boats with freestanding masts are a clear winner against the kite system in terms of simplicity and reliability.
Other Kite Wins
Self trimming.
Sails need to be trimmed (angle of attack adjusted) properly to eke out maximum performance. Kites trim themselves, settling into the spot in the wind window which is the optimum angle of attack for any given wind strength and point of sail.
Maneuverability and acceleration.
Gybing when boat speed is equal to windspeed is one scenario in which I see a multi-kite system on a non-proa boat colliding or falling out of the sky unless it flys some creative patterns involving kite loops. But maneuverability for non-proa kite boats holds up in my mind in all other scenarios, and perhaps even lowers the odds of a failed tack.
For all boats, it will greatly increase acceleration through kite movement. The amount of power generated by a kite or sail increases exponentially with apparent wind speed. If AWS doubles, the power quadruples. Therefore, a kite that is making aggressive turns is generating significantly more power than an equivalent size sail or kite sitting still.
Upwind performance.
Once accelerated, for a good upwind angle, the kite(s) need to be still, so the performance gains then come from:
- Reduced weight and wetted surface area for a given sail/kite area. Accentuating this effect, the kite force has an upward vertical component to it, lifting the boat out of the water, whereas most conventional sails (other than spinnakers) have a negative vertical component pushing the boat down into the water. As I understand it, this even more so increases performance potential for boats with hydrofoils as described in this other kite project’s design manifesto.
- Modern kites have efficient airfoil profiles compared to conventional sails. Wing sails are addressing this for racing leagues, but they are beaten by kites in the other aspects discussed.
- The ability to place sail/kite area at higher altitudes where the wind is stronger, without raising the center of effort on the boat.
Kite braking.
A kite boat can stop on demand on any point of sail — just bring the kites to 12 o’clock. Sail boats with freestanding masts can already do this, but it astounds me that it is accepted as the status quo that the vast majority have stayed masts, which can’t simply depower and stop on most points of sail — they have to make a turn to get the boat head-to-wind, which is sometimes impossible if they are prevented from turning by other boat traffic, big storm swell, or other obstructions.
For proas, however, there is an additional envisioned performance gain. Proas have to stop and go the other way to shunt rather than conserving their momentum in a tack or a gybe — this is what keeps them from winning on a short triangle course. Certainly, proas will benefit from enhanced acceleration with kites, like the other boats. But they will also benefit from enhanced deceleration, with kites making aggressive turns on the opposite side of the wind window. Regarding maneuverability for proas, kites may be a game changer.
Kites boats are remarkable if we can make them work.
It’s worth noting that to design and implement a kite-powered boat system requires careful engineering, considerations for safety, wind conditions, and effective control mechanisms. Real-world testing and refinement are necessary to optimize the system’s performance and address potential challenges.
If you haven’t already, read my other foundational post about the challenges kite boats face, and how I plan to solve them.
Talk to Me
Have questions? I am happy to consider your ideas.
Eager to support the project? You are more important than your money. But if you want to. . .