Hydra-foiling surfski ?

By now anyone with an interest in downwind paddling has seen the video of Kai Lenny hydra-foiling on a SUP in Hawaii. Why haven't we seen this with a surfski?

Hi Steve,

There have been hydrofoiling kayaks around for years and they work quite well in controlled environments, - even if they do take a fair bit of power and the right paddle to get them up on the 'foil.

Surf Skis operate in mixed conditions and don't always race purely downwind. The difficulty here is that you may have a race which is 90% DW and 10% cross or even up-wind.

Under those circumstances the foil's extra drag may more than offset it's advantages (unless you're Hank, Cory, Clint, et al and powerful enough to keep it on the foil into a wind!).

The second issue is that unlike a SUP, you can't easily move your weight distribution on a Ski in order to maintain the optimum angle of attack for the foil. As I understand it, this is critical to getting the hydrodynamic surfaces to work correctly (ie not stall) when you're in a lumpy environment.

Having said all that, I'm no CFD engineer and perhaps there is a set-up that would make a foiled ski practical for even us mortals.

I suspect at the very least, the foil(s) would need to be removable and would require significant structural reinforcement within the ski to be sturdy enough for practical use.

These are just my guesses and I'm keen to hear others' more learned opinions.

Steve Hansen wrote: Why haven't we seen this with a surfski?

Three main reasons:

1. In those sports where hydrofoiling has been going on for years (sailing, kitesurfing, windsurfing) it is still limited to fairly flat water. Though the 'model aeroplane' style foils on the SUP (pretty much the same as those used on kite and windsurf boards for quite a while) are probably more suited to foiling in waves, when you are standing on a board and can transfer weight almost instantly to control the angle of the foils.

2. Have a look round any of the top surf-skiers (or K1 paddlers) blogs and you'll see lots about types of training but very little about the technical side of the sport. Kayaking seems to have this belief that technical development is something handed down from 'The Manufacturers' to 'The Great Unwashed' in terms of new products. The idea that you could get on and do it yourself is quite rare. Question racing kayakers on aspects of boat design and mostly they'll stare at their feet and mumble "that's just the way it is"...

3. That the rules for sprint K1s (which being an Olympic discipline will have the biggest focus on finding a competitive advantage) pretty much preclude foils by limiting rudder width and number.

It's unlikely that the 'model aeroplane' foils would work on a ski as you can't move your weight around quick enough from a sitting position. That leaves a few other options. However first you're going to need the following - a lot of patience, quite a lot of money, an expert composite builder and an elite surfskier who is open-minded enough to accept that they might have to completely relearn all the skills they take for granted!

- Simplest option would be two sets of surface piercing foils (or maybe 1 and a t-foil rudder - see 2000 in the link above). The angled foils control flying height without the need for moving parts. If you reinforced the deck of a ski you could just bolt them straight on without too much complexity.


- The present International Moth system with two t-foils with ride height controlled by a wand. This still proves a challenge to the moths in waves though (but they are moving at up to 60kmh in lots of different directions which makes it particularly challenging). For a ski you'd want to bring the foils as close together as practical (so they are in similar bits of the wave you're on). Maybe one at either end of the cockpit?! You'd probably want a hull just long enough, and with enough freeboard, to achieve takeoff speed - once it's up and flying the hull is just aero drag and weight. And how far forward (and with what gearing) you'd mount the control foil would be a definite puzzle... You's probably want the foils to be somewhat dihedral (sloping upwards to the tips as this would aid stability).


- The more subtle option - a single rudder mounted foil. It won't fly the boat, but is proven(?) in several sailing classes (International 14, National 12) to reduce drag by interference with the stern wave. The gain is small but it only has to be 0.1kt to add up over a 1-2 hour race. Whether it would work the same on a kayak is not clear. You'd probably need to mover the rudder further back and also the whole cockpit as the lift from the rudder would make the boat nosedive otherwise.

So there are a few random thoughts to run with. In one sense foiling should be easier in kayaks as the speed range is much narrower that for wind powered craft. On the other hand things like paddle design would also need to be developed to deal with having to paddle from an elevated position. There is also the question of whether it is a good route to go down - foils are expensive, delicate, awkward to store and a bit of a black-art!

They are fun, but only slowly becoming user friendly for the non-elite pilot! Whether they are the future (as every sailing magazine and website seems to claim) is still rather unclear.

Good example of how long the development path can be...and how long other watersports have been foiling for (i.e. half a century...)

www.internationalmoth.co.uk/the-moth/foiling-history/

I've also been fascinated by Kai Lenny's feat and it's been plaguing my mind ever since. There's a lot going on in how he rides that SUP and I'm still not sure I've got all the key factors figured out in my head for a proper thought experiment but I'll chuck a few ideas at the forum anyway even if they prove to be wrong.

What I think is unique and truly beautiful about the SUP foil is that the paddler is able to generate thrust through the vertical and pitching motion of the craft. Once flying on a wave face the foil maintains vertical lift using gravity but the subtle pumping of the legs and pitching of the board is, I assume, generating small amounts of thrust by rotating the direction of lift forward. This component of thrust is used to keep the SUP's speed up on the wave without the need for paddle input. And, as Spacehopper has said, one of the possible advantages in foil design for this application is the narrow speed range compared to a kite board or a yacht.

If anyone needs convincing about this then check out some videos of human powered hydrofoils surfing like



But as Spacehopper has said controlling this pitching moment is problematic in a surfski because how do you shift your weight to do it? But his thoughts on Moth foil design, the clever mechanism used to control pitch angle on the Aquaskipper/Pumpabike and my old, unloved double spec ski with its twin rudders gave me a crazy idea. Please forgive the lack of specifics that follow.

Using a double ski, take one of the ideas of the Flyak and mount a steerable foil on a fixed mast near the nose. This should be designed to stabilise pitch of the craft and provide yaw control to the front paddler. Next, mount a foil on a mast through the hull behind the stoker in something like a centreboard box so that it's height and pitch can be adjusted. The mast would protrude through the deck behind the stoker and could slide up and down over a cam to change the angle of attack of the foil. Then, design a control mechanism for the stoker so that he can use his footboard to pump the foil mast up and down in the water column to generate forward thrust. Could this enable two people to paddle a double up on foil on a runner and maintain longer flight without paddle input? If it worked you might be able to adapt the system to a single by using a tiller bar.

I've been working on designing and fabricating a pair of removable hydrofoils for a sprint boat. It's been an interesting project, albeit an expensive one. Though I'm roughly halfway there, as the front foil is constructed and working as designed, I just need to design/build the rear one.

Good to see some experimentation. How are you controlling ride height / angle of attack?

@spacehopper

"good example of how long the development path can be."
or Google Forlanini hydrofoil > more than a century back ...

besides those K1s ... e.g. flyak as mentioned in posts above...