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Fastest Foil Combo (To beat AC 75)

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Mossy 757
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Re: Fastest Foil Combo (To beat AC 75)

Postby Mossy 757 » Wed Sep 18, 2019 7:39 pm

edt wrote:
Wed Sep 18, 2019 6:05 pm
There's a certain speed for those boats...
Yeah that's the thread in a nutshell. Race on their terms and you'll certainly lose; propose to race on kitefoil terms and they won't show up (too low wind speed) or will be outside their safe-envelope (too high).
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Re: Fastest Foil Combo (To beat AC 75)

Postby Iridian » Wed Sep 18, 2019 7:40 pm

Lets say open course. Gotta go for best all around since the courses are pretty varied.

If we can't beat the AC 75, what will get us the closest?

From a pure physics approach, kites should have more speed than AC boats, we just aren't quite there yet..

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Re: Fastest Foil Combo (To beat AC 75)

Postby Mossy 757 » Thu Sep 19, 2019 1:57 pm

Iridian wrote:
Wed Sep 18, 2019 7:40 pm
Lets say open course. Gotta go for best all around since the courses are pretty varied.

If we can't beat the AC 75, what will get us the closest?

From a pure physics approach, kites should have more speed than AC boats, we just aren't quite there yet..
I think if we had the kinds of sails they have we could get really close, the thing holding kite hydrofoilers back speed-wise is the kite, not the foil.

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Re: Fastest Foil Combo (To beat AC 75)

Postby juanpasala » Thu Sep 19, 2019 2:11 pm

Iridian wrote:
Wed Sep 18, 2019 7:40 pm
Lets say open course. Gotta go for best all around since the courses are pretty varied.

If we can't beat the AC 75, what will get us the closest?

From a pure physics approach, kites should have more speed than AC boats, we just aren't quite there yet..
Nope, Kites will never have more speed than AC Boats.
Speed comes to 3 factors:

Power Generation: for us it s the kites, for AC it s their Sails, in this case AC sails are completelly rigid, so they can be more aerodynamic, have less flutter, so there is an increased efficiency, specually in stronger winds.
Power Transfer: In this case when we kite it is US that transfer the power, so Muscle and Bones, compared to Carbon fiber masts and composited boat structure that are completelly rigid for AC Boats. In this case Kitesurfing will loose again in power transfer efficiency.
Power Expenditure: Here we might have the only advantage in comparisson with AC boats, as the hydrofoils we use dont have to stand the same loads as AC Boats (was checking the testing on the foils for the new boats and they are talking about expected race loads up to 12 tons, and maximum structural loads up to 24 tons before failure), they can be smaller and more eficient than the AC Counterparts (we have Main wing and stabilized with one Mast, they need 2 masts, one for main wing and one for the stabilizer), as they will genrate more drag, but ultimatelly as the power they can generate and they can hold is higher, they will be able to reach higher speeds than we would in our discipline.

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edt
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Re: Fastest Foil Combo (To beat AC 75)

Postby edt » Thu Sep 19, 2019 2:28 pm

Mossy 757 wrote:
Wed Sep 18, 2019 7:39 pm

Yeah that's the thread in a nutshell. Race on their terms and you'll certainly lose; propose to race on kitefoil terms and they won't show up (too low wind speed) or will be outside their safe-envelope (too high).
You said it way better.

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Re: Fastest Foil Combo (To beat AC 75)

Postby Iridian » Fri Sep 20, 2019 12:55 am

juanpasala wrote:
Thu Sep 19, 2019 2:11 pm
Iridian wrote:
Wed Sep 18, 2019 7:40 pm
Lets say open course. Gotta go for best all around since the courses are pretty varied.

If we can't beat the AC 75, what will get us the closest?

From a pure physics approach, kites should have more speed than AC boats, we just aren't quite there yet..
Nope, Kites will never have more speed than AC Boats.
Speed comes to 3 factors:

Power Generation: for us it s the kites, for AC it s their Sails, in this case AC sails are completelly rigid, so they can be more aerodynamic, have less flutter, so there is an increased efficiency, specually in stronger winds.
Power Transfer: In this case when we kite it is US that transfer the power, so Muscle and Bones, compared to Carbon fiber masts and composited boat structure that are completelly rigid for AC Boats. In this case Kitesurfing will loose again in power transfer efficiency.
Power Expenditure: Here we might have the only advantage in comparisson with AC boats, as the hydrofoils we use dont have to stand the same loads as AC Boats (was checking the testing on the foils for the new boats and they are talking about expected race loads up to 12 tons, and maximum structural loads up to 24 tons before failure), they can be smaller and more eficient than the AC Counterparts (we have Main wing and stabilized with one Mast, they need 2 masts, one for main wing and one for the stabilizer), as they will genrate more drag, but ultimatelly as the power they can generate and they can hold is higher, they will be able to reach higher speeds than we would in our discipline.
Kite wings don't generate heeling force, meaning you can carry a larger kite then a boat. Kitefoils don't need to sustain that kind of force because we don't have that sort of mass. Because our entire bodies can be used as ballast, we can exert even more control then a AC boat.

Our kites aren't as efficient yet, but I think that will come. Foil development is still ongoing as well.

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Re: Fastest Foil Combo (To beat AC 75)

Postby tegirinenashi » Fri Sep 20, 2019 4:43 pm

There is one more factor: boat vs. kite sailor aerodynamic efficiency. Witnessing human body hovering above the kiteboard in somewhat awkward "poop" position, it occurred to me that among the other things they try to minimize the drag. Indeed, when riding at 40 knots the drag due to apparent wind exceeds the one experienced by racer bicyclists. As human exerts about 100 watts of energy, while sustaining the biking speed of meager 40 kph, it looks like the power needed to overcome the aerodynamic drag when sailing at the speed around 40 knots needs to be even larger than that.

For AC boats the vessel shape is unquestionably more aerodynamic, and sailors on the desk are just a footnote.

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Re: Fastest Foil Combo (To beat AC 75)

Postby gbrungra » Fri Sep 20, 2019 5:04 pm

For current kite racers, is top speed really limited by the kite and drag of the human body? I thought it was limited by drag/cavitation/ventilation of the foil?

Top speed for the AC boats seems to be limited by forces from 20+ knot winds? Because the boat would explode?

If the difference is really rigid wingsails vs soft foil kites, how about a rigid kite? Would be destroyed in a crash, and would be dangerous for others, but for a pure speed run it might be worth it?

If the difference is human body aerodynamic drag, why haven't we seen teardrop shaped helmets, like in bicycle racing? Maybe with the rear teardrop piece on a swivel, so you can turn your head and the teardrop piece rotates to stay downwind, like a weathervane?
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Re: Fastest Foil Combo (To beat AC 75)

Postby TomW » Fri Sep 20, 2019 10:16 pm

Average Joe cyclists output 200w. Pros sustain 350w over 4-5 hours with peaks of 800+w. This is info from strava during Vuelta la Spania.
My ride to work I have average of 150-200w. And I'm almost 60 yrs, and not above average.
Have no idea what the relevance of this is. It's probably 80% wind resistance on a bicycle.

Anyway it must be the power available in relation to total water and air resistance. Somehow the AC boats must have more power to resistant drag ratio. The new AC boats will most likely be faster than the catamarans.

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tegirinenashi
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Re: Fastest Foil Combo (To beat AC 75)

Postby tegirinenashi » Sat Sep 21, 2019 12:29 am

Certainly, the total drag is the sum of 3 factors: kite, sailor, and hydrofoil. Any idea what are they in proportion to each other?

For hydrofoil component, how is the drag split between the front wing and the mast? (I assume fuselage and stab impact is negligible). If the mast drag is significant, then perhaps a stainless steel mast would be good engineering solution? I can imagine that stainless steel with half of the typical HF dimensions at the bottom (about 7 mm thick by 70 mm chord) would have the same strength&rigidity as traditional alum/carbon but with noticeably less drag?

Here is little more detailed hydrofoil mast proposal from "armchair designer". Take the two stainless steel 6-7 mm pipes spanning the entire mast length. Connect them by welding steel ribs. This is the skeleton of the mast, which can be reinforced by making triangulated frame with tensioned bicycle spokes. Fill the voids with prepreg carbon.

The mast can be tapered the same way as CF foils currently are, but the key part of this design is to have thin 7mm mast profile at the bottom. Then, perhaps having stainless skin akin to aluminum mast profile would be preferable to carbon filling, at least at the bottom half of the mast.
Last edited by tegirinenashi on Sat Sep 21, 2019 1:02 am, edited 2 times in total.


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