Do you need some rocker at tail of board

[slowboat]

I am seeing some smaller boards with a fair amount of rocker at tail of board. Does this make a significant difference in some way?

[RalfsB]

I do not think you need a tail rocker for hydrofoil boards. Totally flat works fine. I have a 140cm long Alpine board, there is no tail rocker.

[BWD]

For takeoffs, flat tail should probably be most efficient for small to medium foils.
For freeride/freestyle, or waves, a little tail rocker will reduce drag or tripping from momentary touchdowns in carving and transitions.
You can also argue rocker with a big hydrofoil wing facilitates higher angle of attack and max lift for easier straight-to-foil takeoffs. Although with pocket boards it probably doesnt matter.
For race wings I would think flat tail would be significantly better to help on water planing acceleration to the required higher takeoff speed....

[TomW]

I started on. Moses T40. A monster i Never got up on.
Then a homemade stubby 4-10 surfboard conversion I spent 50 hours on. 25mm tail rocker. Worked good.
Then another home built that I folded in half quickly. Had 25mm tail rocker.
In a panic i bought a slingshot 4-6 dwarfcraft. It had negative tail rocker and very little nose rocker. I could barely ride it. It was terrible. More a torpedo than a board.
Rushed to make a 135 foilboard. It had same rocker as the one I folded. Strong and worked great with 20mm tail rocker and beveled rails.
Made a 120cm with 20mm tail rocker works great.
Made 90 and 105 with tail rocker. All work really good.

Basically tail rocker has zero influence on the board behaviour in the air or water, per se. What it does is decide the board AA in relation to the water surface, complementing the nose rocker. This influences board behaviour on surface touch down at speed and starts. It can be significant.

One thing I noticed on the slingshot dwarfcraft( several foilers on them here) is the that the slingshot foils seem to ride with much more nose-up than with my Moses foils attached. This would explain why they made a board that has so little rocker: the foil AA to the water means the boards nose and bottom are pointing upwards despite " neutral riding AA". What I'm trying to say is the foil influences the boards AA and how it behaves in touch down and starts.

[slowboat]

I started on. Moses T40. A monster i Never got up on.
Then a homemade stubby 4-10 surfboard conversion I spent 50 hours on. 25mm tail rocker. Worked good.
Then another home built that I folded in half quickly. Had 25mm tail rocker.
In a panic i bought a slingshot 4-6 dwarfcraft. It had negative tail rocker and very little nose rocker. I could barely ride it. It was terrible. More a torpedo than a board.
Rushed to make a 135 foilboard. It had same rocker as the one I folded. Strong and worked great with 20mm tail rocker and beveled rails.
Made a 120cm with 20mm tail rocker works great.
Made 90 and 105 with tail rocker. All work really good.

Basically tail rocker has zero influence on the board behaviour in the air or water, per se. What it does is decide the board AA in relation to the water surface, complementing the nose rocker. This influences board behaviour on surface touch down at speed and starts. It can be significant.

One thing I noticed on the slingshot dwarfcraft( several foilers on them here) is the that the slingshot foils seem to ride with much more nose-up than with my Moses foils attached. This would explain why they made a board that has so little rocker: the foil AA to the water means the boards nose and bottom are pointing upwards despite " neutral riding AA". What I'm trying to say is the foil influences the boards AA and how it behaves in touch down and starts.

Fascinating information. So you are saying there is really no way to ultimately know until you ride a specific board and foil combination? As a rough approximation, for a 112 x 45 cm board, how much nose and tail rocker would you want to optimize touchdown recoveries and what is the trade off for having the rocker (front and back)?

[TomW]

Slowboat,
Look at the groove skate. Or the Ketos boards. They have public acclaimed good rocker curves for a wide variety of foils ( at least the groove skate). The Ketos been around a awhile and look great.

I've built 5 foilboards and making 2 more now, and this is my own thinking ( hypothesis).

I can only compare my personal experience and observations of 5-6 other riders on slingshot gear.( various ss wings and mast lengths - dwarfcraft, alien air and even that huge log they sell as a learner board).

Irrespective of the effect of various foils on the board AA, an educational exercise is to sketch out a side view of a board rocker. Then sketch a foil mast onto the tail, so that mast is " normal" to the surface of rocker at the attachment point. You'll see that as rocker increases, the whole board will be pointing more and more up in relation to water surface. This is assuming the foil setup is designed to ride with mast vertical in the water, 90 degrees to surface( which may not be the case)

Now, if board has a lot of tail rocker, the board will be pointed "up" ( front foot higher) and as the board at speed comes down at an angle to touch down, the angle is going to help keep the board from nose diving and skim off better. At any given angle of approach to the water, The board will hit the water further back from the nose. Also the board will be less inclined to submarine at low speeds when foil is give a little lift, but you are still on the surface. At speeds where foil is not lifting at all and perhaps you are way underpowered, this angle is not going to help much, if at all.

[windmaker]

Having too much angle (too much rocker) is uncomfortable and almost as bad as not enough. For those using foils with plate mounting shims can be used to adjust the board to foil angle precisely.

[juandesooka]

A buddy and I are DIYing 2 SUP foils. This webpage gives a good discussion of tail options: http://supsurfmachines.com/
including flat, stepped, straight chine, or curved, with explanation of pros/cons of each. He goes with curved.

But that's for sup foil, where you need to paddle to catch a wave and then pop the foil up to release from water surface. For kite foil, where you're being pulled by a big skiboat in the sky, I honestly suspect it doesn't matters much either way. Maybe if you're a racer, where a bunch of tiny advantages may add up to get you to the podium. For recreational kiting, the videos show you can do this on a flat sheet of plywood, a chunk of 2x10, a cafeteria tray, a coffee table, or an oven door. My recommendation: don't over-think it.

[Peter_Frank]

Tail rocker does not change angle of the board at all.

There is quite some space after the mast, where your foot is placed, and sometimes a tad more (there should be, so you can stand wider and ride waves t.ex).
A foot from center of mast to board end, sometimes more even on supershort boards.

Having rocker here behind the mast wont change the board angle at all.

Pretty sure this is what slowboat asks about, and not the board angle nor nose rocker or center rockerline.

Having tailrocker (actually tailkick as we use on surfboards) will give a much smoother transition when waterstarting, up to foiling, especially if choppy, and touchdowns and back up in chop or waves or flat, will feel a lot gentler.

Opposite, a flat tail will start earlier, no matter what.

Another parameter that will give you the same effects, is whether you have a wide or a tapered (narrower) tail.

The tapered one will be smoother up/down when starting or in touchdowns, and another advantage is you can carve REALLY hard turns, without catching with the rear edge of the board, really good thing IMO.

The downside is, same as with tailkick, that they dont start as early if underpowered.

Horses for courses.

IMO it is somewhat like waveboards, a few mm here and there, and the overall context of all small details, can really be felt hugely and not possible to know beforehand - it is the full constellation and how it FEELS, it is all about.

Only difference is, that on a waveboard you use it all the time in the water, so critical, on a hydrofoil it is for starting and touchdowns only - but this is often a major part of hydrofoiling too

Individual likings - but it means A LOT, thats for sure

PF

[TomW]

What I mean by tail rocker is not " tail kick," . It is the rocker from 1/3 up from tail to the tail. The foil is mounted on that curve.