Axis has the advance fuse which moves the front wing closer to the mast. As a result you have to move the mast a bit forward to compensate. The advantage is supposedly better turning, maybe losing a bit of pitch stability. Some people say that the advance mast is better for pumping, some people say it isn’t. What are your thoughts on this? Not specific to axis, just in general theory. Also would be nice to know what kind of distances other brands use, is Axis the only brand to give you this option?
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Turns better and feels way more surfy. Much more fun on the wave.
Harder to pump imo but you kinda get used to it.
I went from a crazy short to adv crazy short fwiw.
I think of it as moving a fin forward on a surfboard, making it turn quicker and easier. Plus you’ve got a little less flex in the system as the lever from the foil to the baseplate is a lot shorter on the horizontal plane.
I’ve just moved to Axis, straight onto the advanced red and black fuses. To be honest, I haven’t noticed a difference in pumping the PNG wings from the few goes I’ve had on the old fuses, and the Spitfire pumped so easily compared to my last brand, I’m stroked and frothy 
There’s a podcast called The Generic Foiling Podcast which featured Adrian Roper recently, who explains the initial and recent R&D for choosing the mast to wing fuse length. Worth a listen.
Last point, I’ve heard there’s an Advanced Plus fuse coming out, which moves the mast another 20mm further forward.
Been wondering about this myself. Entirely theory here, no practical experience. Assuming an ideal setup where you have your placement on the board just right, and enough adjustment in the mast track, going to the same length advance fuse from a standard fuse means the mast moves forward and everything else stays exactly the same relative to each other - position of main foil and stab with respect to the board, rider position with respect to the board, etc. Center of gravity and inertia change ever so slightly due to the new mast position, but even with a tremendously heavy mast this change is very small. Imperceptible? I dont know but assuming so seems quite reasonable from a first principles perspective.
In this arrangement, yaw stability is the most obviously impacted degree of freedom, as the mast acts much like the rudder on an airplane to stabilize yaw. Just like a shorter fuse reduces pitch stability, moving the mast forward reduces yaw stability. I think the surfboard fin analogy is pretty good here. However, unlike a surfboard and like a plane, the foil system has a strong roll-yaw coupling so the change in yaw stability also changes the system roll dynamics, and thus the change in how turning feelings on the advance fuse is not just down to yaw like the surfboard fin analogy suggests. Generally these couplings are not symmetric (equal effect of yaw on roll and roll on yaw) and that seems to be the case here as unlocked turning is widely reported with the advanced fuse while significantly reduced roll stability is not.
Where I get puzzled is the pitch degree of freedom. Lots of people reporting easier or worse pumping or pitch stability. Lots of discussion about foil distance in front of mast and fuse length behind the mast. In this ideal case, there is zero change to the system pitch dynamics. For a given foil and stab what matters to pitch dynamics is the distance from foil center of lift to stab center of lift, the arm from foil-stab system to the system center of gravity, and the inertia of the system. Outside of stall, pitch has very limited coupling to roll or yaw. Barring huge changes that actually impact inertia or stiffness, the position of the mast makes absolutely no difference, its role in pitch is structural only so talking about length in front of or behind the mast with respect to pitch is meaningless. Nonetheless, empirical data is emprical data, there is quite clearly a change in dynamics. Is this because people are not keeping the same relative position between rider, board, and foil-stab system when they change to an advanced fuse (not enough mast track length, for example), or that the inertia changes are in fact not negligible? I don’t know, there is a gap between theory and experience here that I’d love to see clarified. Unfortunately, so far I see a lot of clearly wrong explanations for the observations, like the length of stab behind the mast being a relevant factor, which is then leading people to buy shorter fuses when they go from standard to advanced to try and improve pitch stability. I have not seen whether or not people are reporting going shorter to help with pitch instability. This is unquestionably the wrong way to go based conventional flight dynamics theory, but maybe there is something missing from that theory when applied to a hydrofoil as oppoaed to an airplane and the shorter fuse actually works in practice, even if the reasoning that got there is incorrect.
There is a very relevant parallel in the sailing world about the slot effect, which has to to with the headsail-mainsail interaction. It says that the air in the slot between these two sails accelerates (Bernoulli) and so focusing on having a good quality jib that is well trimmed is essential to getting good flow over the mainsail and thus making good progress upwind. It is a completely incorrect application of Bernoulli’s theorem and this notion of the slot effect was completely and resoundingly debunked by Arvel Gentry in a series of numerical, model, and then full scale experiments in the 1970s - the air in fact slows down in the slot and can often lead to backwinding the leading edge of the mainsail. Nonetheless, 50 years later I still regularly here tremendously competent amateur and professional sailors going on about the slot effect at length. Why? Well, it turns out that having a good quality jib, with a good flying shape, that is well trimmed is indeed one of the most critical areas to focus on for good upwind sailing angles and speed. But this is due to the effects of the mainsail on the flow over the jib (upwash and accelerated flow at the jib trailing edge, for anyone dying for the punchline) not the otherway around. So, completely wrong theory that lives on because, in the most common area of application it still leads to the correct actions. Now, try to apply that same incorrect theory to downwind sail trim and disappointment and frustration will ensue. So, hopefully we get the pitch theory-reality gap cracked before an incorrect explanation gets enough momentum to hang around another 50 years after being debunked.
I’m really frustrated about this topic too. I have looked at two diametrically opposed sets of information:
- Adrian’s interview on one of the podcasts he talks at length on the development testing leading up to the release of the “Advanced” fuselage, which moves the mast 40mm forward with respect to the wing. If anyone can recall which podcast it is, please link it!
He states that they tested everything all the way up to 85mm forward. Apparently more than 40mm shift resulted in people having a hard time getting the board on foil. That makes zero sense hydrodynamically.
I have a suspicion that after 40mm shift, the mast was maxed out forward in the boxes so in essence the testers had trouble getting out of the water because they technically needed the boxes further forward. Also, I suspect that for mechanical reasons they didn’t want to go more than 40mm forward and were trying to spin up a good story why they did that.
- https://www.youtube.com/@james_goulet/videos has some amazing DIY videos where he makes what he is calling “radically short” fuselages with the mast pushed way forward. He reports that they work great. He had to put new boxes in his boards.
https://www.youtube.com/watch?v=ycN_mw2ahJ0
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ps. @Erik - take a look at ^^^ this fellow’s videos - I’d love to hear from DIY innovators on your podcast…
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You really think that Adrian and the development team wouldn’t think of slapping an extra mast track into a board when they ran out of adjustment? I’m sure that it occurred to at least one of them.
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Meanwhile Gofoil V2 moved the mast further back and their team riders rip just as hard.
V3 coming out, other than 20% lighter, not sure if they are changing the distance
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