One of the common tips people give for effective pumping is to “fly the foil high” on the mast.
Can someone give some tips on this specific point? I feel like any time I try to bring it higher out of the water I’m losing forward speed (by trying to tilt the foil upwards by a fraction), then I just stall my pumping by not being able to get my weight forward enough…
It’s a bit of a circular problem in my head. Either I’m:
trying to get the mast high and I bleed forward speed and can’t get my weight forward.
or I try to keep forward speed and weight forward, and end up gradually bogging or the nose just dives.
use energy to get high, but then you’re more efficient for the rest of your run and its a net gain
use the lift from a wave you’re pumping over to gain altitude. that’s free energy too.
The stiffer the mast the more comfortable and efficient you’ll be higher on the mast. That was a big a-ha moment in testing so many different masts when we did the 800. There’s more flex on a mast when it’s out of the water from my feels.
Another benefit of being high as when you get to about 8 inches from the water surface you start to get free lift. I call it reverse ground effect as its the same feel. The foil will start to feel firmer in the water. Same feeling as when you’re pumping in super shallow water and in ground effect.
This is cool…so the surface of the water stops the wingtips loosing energy through vortices like a solid bottom?.
Is this effect accepted/studied by hydrodynamics boffins or just rider’s feel?.
From all the research and thinking I’ve done on the topic I think we have two effects compounding:
staying high in the water means the least amount of mast drag
once you get to within about 1-2 chord lengths from the surface of the water, the flow over the top of the wing can easily go up and over by making a bump or wake in the water surface. This means we get nice low pressure zone along the top of the wing → much less drag for the same lift. When the wing is deeper down in the water, the pressure on the top of the wing is higher. Instead of the water going up and over the wing, it has to speed up to get over the wing.
I think effect 2 is the more major factor. You’ll notice immediately how much easier it is to maintain pump if you can keep the wing just below the surface.
I’ve come to the conclusion that the feeling of extra lift when the foil is close to the surface is caused by a change in overall pitching moment.
The vertical offset between the center of drag (at the foil) and center of gravity (rider) causes a downward (negative) pitching moment that is counteracted by the upward (positive) pitching moment from the downforce on the stabilizer. When you go high on the mast, the total drag is reduced, so the negative moment is also reduced while the positive moment stays the same since speed is the same. This leads to increased front foot pressure and the feeling of extra lift.
It is clear that the free surface degrades the performance of the foil, but the effect is small in our speed range and modern high aspect foils will feel it even less than older foils with long chord lengths
JL, I dont see in the results of the paper you linked to that foil performance is degraded with proximity to free surface. The 0.84 span case, as far as I can see, shows similar drag to the deeply immersed case, while the lift at a given speed and angle of attack is higher. Didn’t go through the paper rigorously so please correct me if I missed it.
The conventional theory, as I am familiar with it, is that is that within the range of 1 to 0.5 of a span (note span is the key dimension, not chord length) within the free surface, performance of a lifting body is improved due to an increase in lift at the same geometric angle of attack, meaning lift to drag improves as drag is largely unchanged. This isn’t kind of like ground effect, it is exactly ground effect - the free surface is acting like a mirror plane, exactly like the ground does in the case of an airplane. However, unlike the ground, the free surface can and does deform when subject to a pressure field like that generated around a lifting body. This deformation influences the lift and drag of the foil by changing the flow primarily over the top of the foil, as well as creating a wave system on the free surface, which solely increases drag. Somewhere quite near the surface, but usually before ventilation for most designs, the net effect of the free surface deformation is to dramatically reduce the lift to drag ratio - wave making drag gets quite significant, friction drag from the accelerated flow over the top of the foil is getting significant, and pressure over the top of the foil doesnt have much room to drop further as ventilation is right around the corner. The crossover point is not exactly at 0.5 span, that is just a rule of thumb for where you should be quite confident of getting benefit with a foil alone. The actual value varies on a lot based on the particulars of not just the foil but the whole system. My own experience with a conventional foil plus stab, mast, and fuse system suggests it is quite a bit closer to the surface than that.
A result of the increased lift at the same angle of attack is that as you near the surface, you need to flatten out the main foil in order to keep the system balanced in heave (vertical direction). This means the stab is running at an increased angle of attack compared to the more deeply immersed case, and increased front foot pressure is required to keep the system stable in pitch.
As far has how to get there, I try to imagine trying to suck both my knees up into my chest to exaggerate unweighting the board so that I can let the foil lift the board up underneath me without having to increase pitch (same angle of attack means same lift while unweighting means lift briefly greater than weight and system has to accelerate upwards). Find it feels fairly similar to doing a set of box jumps, actually.
You are right, drag is the same for a given AOA, lift is reduced. And as you said, L/D is reduced due to wave-making at shallow depths.
This paper talks entirely in Chords, not spans, so the shallow immersion case is .84 chords
Any idea what the magnitude of the free surface “ground effect” is? I haven’t come across any literature that describes it.
Figure 5 shows that an AR =10 foil, 1 deg AOA at 20 fps (12 knots, so a normal foiling speed) submerged to 3.84 chords develops 600 lbs lift and 23 lbs drag. The same parameters at .84 chords produces 550 lbs lift - both have the same drag as far as I can tell. To maintain 600lbs lift at .84 chords looks like it takes ~0.75 degrees more AOA and creates ~27lbs drag.
Fig.7 shows the reduction in CL, about .38 at 2 deg in the .84 case and .43 in the 3.84 case.
Speed has a big effect as well, sub-critical speeds are worse at shallow immersions than supercritical. (the critical speed is defined as when waves are created). That’s why chord length is important since a longer chord will generate a longer wave that moves faster, making the critical speed higher.
Some interesting knock-on effects to all this:
Very high aspect ratio foils with short chord lengths have a lower critical speed, and so can run shallower/slower with less efficiency loss.
If you buy the theory of why we feel extra lift when high on the mast, a shorter mast will be easier to fly high since there’s less leverage (drag vs COG), and therefore less change in pitching moment as drag is reduced.
A non-tapered bottom half of the mast will feel better since drag reduction is linear vs a tapered mast that will be slightly non-linear making the instability more pronounced.
My experience suggests that it is NOT about some multiple of span-length to the surface. I’m often riding very high aspect wings that are a meter in span - quite a bit longer than the mast even. I have to be right up by the surface to get the super low drag effect. I’m talking about just barely below where we start hearing slurping noises. From my experience on a few different foils, I estimate it is much closer to .2m of water above the foil.
I know you say that wave drag increases when we’re up high, but that’s not my experience.
With regards to “ground effect” on a plane - I suggest that this effect is exactly OPPOSITE of ground effect. On plane wing flying close to the ground, the ground helps increase the pressure on the BOTTOM of the wing by trapping the air below (or as you say, acting as a mirror plane). That increased the pressure delta bottom to top for extra lift at lower angle of attack = less drag. For the foil high in the water column, that allows the pressure on the TOP of the wing to be relieved, resulting in a high delta pressure bottom to top. The pressure is relieved not because of a mirror effect, but because of the air/water boundary allowing water some freedom. Same effect of high lift/low drag, but completely opposite reason.
I haven’t seen a good study on this exact effect yet, its a very very interesting topic that needs more research. If I were better at computational flow dynamics, I would simulate it. Maybe I’ll ask someone I know to look at it.
Is this feeling specific to pump foiling. Something definitely happens when the foil is near the surface but I don’t think I’ve ever heard kite foilers talk about it.
I’m not saying that drag increases when we are high on the mast, in fact I think that it decreases dramatically. What I’m saying is that the feeling of increased lift is not the product of an effect on the wing, it’s a product of the decrease in drag due to less mast immersion.
Wave drag (actually lift reduction) with a shallow foil is real, but since we are in the supercritical speed regime (not making waves) most of the time (particularly with HA/short chord foils) it isn’t a significant source of drag practically speaking.
Does " ground effect" only for airplanes and compressible air?
Water is not compressible.
On the other hand water is very heavy to lift, maybe that’s what we’re looking at.
I see the reason for the increased efficiency a bit differently but ultimately the same as aero “ground effect” like you mentioned. Riding close to the surface mitigates the induced drag of the foil through disrupting the wingtip vortices. It also makes the lift more efficient by reducing the downwash angle:
Increasing aspect ratio is another way to make a significant reduction in wingtip vortices/induced drag. When you run over someone else’s path foiling you can “feel” their wingtip vortices. A more efficient foil will generally be less harsh to ride over their path. A fun way to covertly assess someone’s gear!
I haven’t tested this out myself but would assume the more low aspect your foil is the more effective staying high on the mast would be since the induced drag/wingtip vortices make up a larger overall portion of the total drag. That being said it is easier to get “closer” to the surface in terms of ratio to wingspan with a high aspect foil.
Staying high on the mast also reduces the parasitic (and induced if turning/going upwind) by having less mast submerged and reduces the work the tail needs to do in order to provide stability. You can run a bit less tail shim on a shorter mast since being consistently closer to the surface reduces drag which contributes to the downward pitching felt at the board (the other reason being reduction in moment arm).
Staying high on the mast also reduces the parasitic (and induced if turning/going upwind) by having less mast submerged and reduces the work the tail needs to do in order to provide stability. You can run a bit less tail shim on a shorter mast since being consistently closer to the surface reduces drag which contributes to the downward pitching felt at the board (the other reason being reduction in moment arm).
This is exactly what I’m talking about, you can get away with less shim/stab downforce. The flip-side of that is that when you are high, you will feel like you have more shim/stab downforce and more lift.
Check out the link in my comment below, ground effect is due with the change in angle of the fluid leaving the wing and reduction of wingtip vortices so present in both air and water
I agree with you on the less mast submerged part but there is also an interaction between the surface and the wing that leads to more efficiency as well.
I’m curious what it is, everything I’ve read about our CL and Froude number range says there is a small reduction. I think the difference vs ground effect is that when the low pressure field encounters a free surface (a constant pressure boundary), any increase in efficiency is lost due to deformation of the free surface. If the surface of the water were solid (variable pressure boundary), then an increase in low pressure could occur due to tip vortex reduction, but you can’t foil under ice…
That would certainly be an interesting experiment to run! I was thinking for a minute that your theory could be tested by adding a length of mast “extension” under the foil to keep the amount of submerged mast constant. Example: test efficiency of foil at 50cm depth as baseline then a foil at 20cm depth with a 30cm submerged mast extension.
Unfortunately the same phenomena we are testing for (3D flow of vortices around wingtips) would render this test inaccurate since there would be a significant 3D flow around the termination of the mast extension that is unaccounted for. Magnitude could probably be determined by comparing drag from various length masts being dragged at constant speed to determine the drag “constant” of the end but now we are talking a lot of experimental setup without much of an actionable result since we all more or less agree staying high on mast is critical!
That’s a really good point about the free surface deformation @J_L and I agree this could definitely lessen the rate at which the “ground effect” based efficiency increases as you get very close to the surface. At a certain depth though you would be getting the ground effect benefit with little deformation of the free surface since the vortices “reach higher” than the majority of downwash.
If the drag reduction was only due to the reduction in submerged mast is would be a purely linear relationship relative to height, whereas I’d guess it’s a combination of this linear reduction as well as the non-linear reduction due to wingtip vortices reduction and downwash angle change with the latter making a more profound difference at deeper submersion levels and the former taking over much closer to the surface due to your point about free surface deformation.
Thanks for the interesting thought experiment!
To answer @Zarb 's question, one tip I’d have is to focus on unweighting more to increase height and less on pointing the foil “up”. If you point the foil up and are still “heavy” then you will end up stealing a lot of speed in order to lift your weight. Additionally the more you are able to unweight the less drag there will be during this gliding phase.
At the end of the loading phase of your pump try to bring the board flat instead of up and be as light as you can for as long as you can. Proper track position makes it easier to do this too. Too far forward in the tracks and your board wants to steer up aggressively when you unweight, and too far back and your board wants to pull nose down.
@Erik 's point about staying high on the mast when leaving the wave is super key too. This was what was holding me back when learning to foil surf, I’d turn hard off the wave to pump out and have to stay low on the mast to prevent wingtip breech since for my foil at the time this spelled instant wipeout. When I started turning more gradually and focused on staying high I kept way more speed even though my path out was less direct.
I have a leash that is a bit too long and it catches and flops in the water unless I am high on mast. This is a great motivator to stay high. Happy accident.
