How to go faster on hydrofoils?

[rnelias]

I've been riding on hydrofoils for about 1 year now and something that intrigued me is how I can easily reach 35-40km/h (even more) on a twin tip and, with a hydrofoil, I've been struggling to reach 40 km/h.

I don't have plans to compete on foil race and nothing on this subject. It's just a curiosity... VMG is easy to see that we go much faster on HF but, here, I'm just trying to understand the physical differences that make a HF much easier to ride on light winds, due to less friction, than TT and why it does not reflect on speed, at least on straight line.

Yesterday I was riding a F-One Mirage 650cm^2 wing with IC6 300cm^2 stabilizer. Thus, a small freeride's plane. The wind was not too strong, about 16 knots and I've tried to push some speed on a Bandit 10m and on a Dice 7m. The fastest speed I've reached was 39km/h according to my Gamin watch. On the first session on my TT (138x42), I've easily reached 37km/h on the same Bandit 10m. What's the secret to make hydrofoils go faster?

I know people will reply about high-end foil kites with small racing wings and masts that cost a kidney and can reach insane speeds on formula kite but almost any newbie rider can easily push 35km/h on a TT without having to develop a refined technique or spend tons of money on special boards, masts and wings...

[bragnouff]

To go faster, essentially, you either need more power, or less drag.
When reaching 20kts on a TT, it probably required more than the 16kts of wind you had when doing the same with foiling. So more ambient energy to tap into. When there is plenty of energy around, the surface of the edge and fin(s) of the TT in contact with the water might become less than the half-mast/fuse/wings surface of the foil underwater, so there could be less drag with a TT (provided there is sufficient power to get it out of its bogged down state). So that's maybe an explanation.
Other explanation could be about the confidence to push really on the foil. As you go faster, there is more lift that is being generated, and it takes some skills to control that lift, and push hard on it to maintain a low AOA (lower drag) and transform that power into forward motion. Since you're higher on the water and you probably had a few spankings in your foiler's life, going fast is more intimidating and more consequential than on a TT. Maybe there is some level of fear that holds you back.
Could be also a trimming issue with that particular foil. Sometimes there's some angle adjustment on the stab, some shimming, that can make a big difference in how a foil behaves. Shimming could improve stability or earlier take off but it can also increase the drag, which becomes an issue at higher speeds. I've never played with that myself though (not an option on my kit) so I can't really comment on that. I'm typically using a GoFoil Kai, which is a 1200cm² super thick low/mid AR profile, and definitely not designed for speed. It's happy to cruise around 16-18kts, but if I push it hard with my 100kgs, I have reached almost 24kts on a Peak 5m, which feels like some terminal velocity for this combination of foil/kite. At 650cm², yours should be able to go faster than that.

[tegirinenashi]

There are many variables in your question. Going fast on beating, beam, or broad reach? Hydrofoils go significantly faster on beam and broad reach, but it is more challenging to do so. For example, you can see the top riders in the competition going downwind with their posture and kite almost the same as going upwind. That's right, if you sail downwind towards your kite you are doing it wrong, the kite must not drift but fly at noticeable angle. It is somewhat counter intuitive, because if your speed is higher than apparent wind the kite shifts into the opposite side of the wind window! This is very unstable setup where it is easy to make a mistake.

Also, unlike twintip (and surfboard) you are controlling the flight in one more dimension - vertical. With speed downwind and less kite pull it is quite a challenge to maintain a level flight over the waves, because one has to react faster to correct the height.

When powered up, a rider at some point in his progression experiences a stage when he feels uncontrollable acceleration on the broad reach. This happens regardless of the specific watercraft used and is valid for for TT, SB, and hydrofoil. If you have passed this stage already, then your question is legitimate. If not, then try bigger kite?

When you are passed the uncontrollable power learning phase, then you start playing with small variations of your riding position. For example, if you are on smallish wing, then the poop stance helps you avoiding the bronco ride. When riding downwind you can experiment with various sheeting positions of your bar. Then, sailing in a "locked" mode is also important because you don't waste the propelling energy bouncing all over the place.

[rnelias]

To go faster, essentially, you either need more power, or less drag.

The way both boards convert power into forward movement might be a good direction to understand the differences. Hydrofoils don't need too much power to keep us going on water. We just need a small peak of power to make the plane take-off on the water start. After that, we basically use, let's say 20-30% of the kite's power to keep the movement. It's usually what 99% of what freeriders do on HF. However, we should also consider that we can "edge" the HF's plane against the water like we do when edging on a twintip. In this case, we should be converting more kite's power into forward movement, like what happens with a TT, but it doesn't seem so efficient on the hydrofoil's case. On my test, it's basically what happened. I've used the same kite on both boards but, on the HF, I had to "edge" the plane in order to reach ~39km/h. It was hard and had to put a lot of pressure on my front leg. On the other hand, with the twintip, I easily cruised on speeds of ~35-37 km/h effortlessly.

I'll leave a very rough math using just the contact surface to let us think on other aspects

a). A small HF (Mirage 650 + IC6 300cm^2): (650+300) x 2 (up+down sides in contact with water) = 1900cm^2

b). A regular tt board (138 x 42) = 5796 cm^2. Considering just 1/3 of the board touching the water would give us 1932cm^2

Someone may find these numbers weird and think that both boards would give the same resistance on water, however, we should remember that drag comes in two components: pressure + (skin) friction and the boards convert the kite''s power into movement in a different way. We know that, on lighter winds, we need a big door board to ride with a TT. It's also true that bigger wings are easier to take off on light wind days but, in the HF's case, it's mostly due to the lift produced. The pressure drag on HF is much smaller after the take off. On the TT, the pressure drag is what makes the ride possible by redirecting the water flux and make the board plane over water (Newton's 3rd law).

Fluid mechanics is really hard

[tegirinenashi]

I don't think there is meaningful wetted area comparison between submerged HF surface and planning board. Here is quite entertaining final leg "race" between the top HF competitor and some random? windsurfer:



Excessive foot pressure on your front leg is easily fixable. 650 cm^2 front wing is a sweet spot for me, and I don't experience this problem at those speeds. Maintaining consistent straight line flight and shifting the kite back into the wind window is. Also note, that the racer's stance is somewhat odd, because they keep most of the their weight over the front leg. On the other hand their wings are significantly smaller than 650 cm, so that front leg pressure is less of a problem.

OK, you started the thread that your goal is not necessarily to become a racer. But if you want to go fast, then you should think and act like one

[rnelias]

OK, you started the thread that your goal is not necessarily to become a racer. But if you want to go fast, then you should think and act like one

let's say that my "need for speed" is to cover a larger area faster.

Hydrofoiling redefined the freeride frontiers for me

[airsail]

I find water turbulence is also a factor. We have a large bay to foil on, winds above 15 knots produce a lot of short sharp chop which requires navigating. Speeds above 25 knots (46kph) are tricky in winds above 15 knots, you can feel the foil being knocked by the turbulent water from breaking whitecaps, it’s quite unnerving.

At lower wind speeds, smoother water there just isn’t enough power in big kites for really high speeds, big kites have big drag.

[tkaraszewski]

As other people have mentioned, if you want to go faster, try and ride like a racer. You don't need special race gear to go improve your technique, you can go faster on any gear if you have the right technique (of course, you will go even faster on race gear).

First: notice how, as freeride foil kites have gotten smaller, racers are still riding kites about the same size as people on twintips. If you're riding in 16 knots on a 7m or even on a 10m, you're underpowered. 16 knots would be perfect for my 13m foil kite. To go fast you need to extract as much energy from the wind as you can, and that means bigger kites. When you're going fast on your twintip, you're riding a bigger kite than you would be in the same wind on your foil, right?

Second: How power is applied. If you want to go faster, the kite needs to be pulling on you. That's obvious, right? But how do you feel that? You feel it as tension in your lines on your harness. If the kite is not pulling hard on you, then you're not going to accelerate. So how to get more line tension? Fly the kite low and sheet in far. You want maximum power generation. If you sail too far downwind, you reduce your apparent wind. You're riding toward your kite instead of forcing it to pull you along. So, you have to point upwind a bit more to keep maximum power in the kite. This is a balancing act, because the more you point upwind, the more you lean the foil over, and the more drag the foil generates, slowing your down. You want to have the foil pretty straight up and down, but sailing an angle that's still generating a lot of pull. And keep the kite low so that all of the force in the kite is pulling you across the water and not up away from the water. And sheeted in, you want max power! (but note it still needs to be sheeted properly, don't back stall the kite or anything, keep it flying where it pulls the hardest). You should have no trim pulled on when going for maximum speed.

Go practice like a racer for a few days and you'll get faster.

[edt]

make sure your mast is long enough. You will be applying a lot of pressure to the wing, and because of this you'll be heeled over. On smaller masts you can't put enough pressure because if you lean over too much the wing breaches. For example the moses kite race 106-670 is 106cm (42 inches) tall with a front wing surface of 570 cm. I think the tallest f-one mast is 95cm? Only $150 so pretty cheap upgrade. I don't remember if f-one has a racing upgrade to the fuselage or not, that can also increase speed by reducing drag.

[rnelias]

make sure your mast is long enough. You will be applying a lot of pressure to the wing, and because of this you'll be heeled over. On smaller masts you can't put enough pressure because if you lean over too much the wing breaches. For example the moses kite race 106-670 is 106cm (42 inches) tall with a front wing surface of 570 cm. I think the tallest f-one mast is 95cm? Only $150 so pretty cheap upgrade. I don't remember if f-one has a racing upgrade to the fuselage or not, that can also increase speed by reducing drag.

My mast is 95cm (alum) and, from what I could feel, it should be mounted as close as possible to the back of the board as it was pretty hard to keep the wing on water when I've pushed some speed. Maybe I should try other board too as I've been using a very light pocket board. It seems it'll break if a push it too hard on my front foot