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Lift from fin?
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U2U2U2



Joined: 06 Jul 2001
Posts: 4977
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Mon Mar 25, 2019 10:33 am    Post subject: Reply with quote

From Fagg ?
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bred2shred



Joined: 02 May 2000
Posts: 959
Location: Jersey Shore

PostPosted: Mon Mar 25, 2019 2:27 pm    Post subject: Reply with quote

isobars wrote:
You guys seem to have missed one vital factor: As discussed often here and in windsurfing magazines over the years, lift, in this context, refers to lateral/upwind, not vertical, force.


Bingo.

I think the OP was considering “lift” to mean vertical elevation of the tail of the board caused by the fin (based on his reference to the vertical stabilizer on a plane).

In the context of airfoils and hydrofoils, “lift” simply refers to a force generated normal (perpendicular) to the flow as opposed to “drag” which is force generated parallel to the flow. So when windsurfers talk about a fin generating lift, they are generally talking about the upwind component of the fin’s force vector. This lift is what allows the board to track in any direction other than dead downwind.

There can also be an upward (vertical) force generated by the fin but that would only occur if you tilt the board (rail it up) or if the fin is designed to flex under load to such an extent that a portion of it’s surface generates an upward component (as is the case with very long Formula fins). Even so, this component force is still much less than the horizontal (upwind) lift component. The only way to get a significant vertical lift component from the fin would be to install a foil.

sm
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akrausz



Joined: 19 Sep 2008
Posts: 125
Location: Sarasota, FL

PostPosted: Mon Mar 25, 2019 2:30 pm    Post subject: Reply with quote

isobars wrote:
You guys seem to have missed one vital factor: As discussed often here and in windsurfing magazines over the years, lift, in this context, refers to lateral/upwind, not vertical, force.

Exactly. I often wonder how many people get confused by this. Lift that does not go up.
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rigitrite



Joined: 19 Sep 2007
Posts: 476
Location: Kansas City

PostPosted: Mon Mar 25, 2019 3:21 pm    Post subject: Reply with quote

Quote:
I whole heartedly disagree as I venture would every shaper, fin designer and naval architect.


Yeah....whatever. Viscous forces govern and fin lift opposing side slip:I don't buy it.

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grantmac017



Joined: 04 Aug 2016
Posts: 691

PostPosted: Mon Mar 25, 2019 5:11 pm    Post subject: Reply with quote

Fin lift creates a leeward rolling motion which enables us to get out on the rails (on slalom/formula gear). Excess fin will also cause tailwalking.
Neither of which is "lift" but it can certainly feel that way at the time.
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boardsurfr



Joined: 23 Aug 2001
Posts: 1106

PostPosted: Mon Mar 25, 2019 5:39 pm    Post subject: Reply with quote

Can of worms indeed ...

Windsurfing fin theory is quite interesting and complex, and most of the answers given so far contain some truth - that include the "it doesn't matter". But note that this answer came from a freestyler! Freestylers are well known to sail on tiny little fins that most "normal" windsurfers fins near-impossible to use. They also often use cut-down, beat-up tinsy fins that have very little capacity to create lift - hence some of them don't believe fins give lift. "Viscous forces" rather than lift can be essential for freestylers to get upwind - although those are dominated by the windward edge of the board that is pushed into the water to create sideways resistance.

Dan Weiss is coming from the other end of the spectrum, and has used large fins on formula gear (and, quite similar, large centerboards on raceboards). For these, the lift forces can have a rather large vertical component, as Dan has explained. The same is true for the somewhat shorter fins on slalom boards. If you watch slalom racers, they often push the leeward edge down, which serves two purposes: it increases sideways drag resistance (very similar to what rigrite and co. are doing on freestyle boards), and it increases the vertical lift, thereby helping to reduce the wetted surface. If you see top slalom sailors skipping over the water with just the fin in the water, you see someone expertly using the vertical component of fin lift.

To confuse things a little more, there is actually another way that some fins can create vertical lift. In a normal (pointer) fin, the water hitting the front of the fin simply creates drag. But if a fin has a high rake angle, for example 50 degrees or more as seen in delta fins, the water hits the front of the fin at an angle, and therefore creates a vertical, upward-directed force. So while fast pointer fins tend to be very narrow, with a sharp front edge, to limit drag, some delta fins are designed with a much thicker front edge specifically to generate more lift. We sailed such fins (the "Fangy fins") almost exclusively during our 10-week trip to Australia, and they work beautifully.

For most "regular" windsurfing, though, the vertical lift from a fin is so small that it's often unnoticeable. The hydrodynamic "lift" of the fin is oriented sideways, and counteracts the sideways force on the sail. In Jim Drake's words:

Jim Drake wrote:
Hydrodynamic side force acting sideways almost entirely on the fin ... in reaction to sideways component of the
Aerodynamic wind force acting on the sail


Does lift have a practical relevance in typical windsurfing? Well, if you're trying to go fast, tilting the board slightly so the leeward edge is in the water may help. That's especially try if you are already on freerace or slalom gear, but it can even make a noticeable difference on slower gear if you have a stiff and wide fin. But the bigger relevance is probably seen if you ding your fin a lot: more dings means less lift and more spinouts. So sand it or get a new fin!
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isobars



Joined: 12 Dec 1999
Posts: 18972

PostPosted: Mon Mar 25, 2019 6:31 pm    Post subject: Reply with quote

bred2shred wrote:
I think the OP was considering “lift” to mean vertical elevation of the tail of the board caused by the fin (based on his reference to the vertical stabilizer on a plane).

I suspect he simply made the same error most WSers make, that the term "lift" means vertical thrust.

I'd like to see a longitudinal video from a GoPro mounted beneath a board looking back at a long blade fin being pushed hard. (The camera may have to be above the water, cantilevered, viewing the fin from above and behind, to minimize drag.) Logic and other posts suggest that it's bent laterally quite a ways when generating enough vertical lift to ride it. (I was too busy to look down and back at my fin when I was pushing my first generation Malibu to the point my waterline was behind my back foot when I had both feet in the rear straps decades ago.)
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rtz



Joined: 31 Oct 2010
Posts: 246
Location: Oklahoma City

PostPosted: Mon Mar 25, 2019 6:48 pm    Post subject: Reply with quote

I'm trying to get this beast to plane easier. For easiest planing; run the mast base all the way back? On a 9m sail; what is the ideal fin size?

Viper91:

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DanWeiss



Joined: 24 Jun 2008
Posts: 2272
Location: Connecticut, USA

PostPosted: Mon Mar 25, 2019 7:09 pm    Post subject: Reply with quote

rigitrite wrote:
Quote:
I whole heartedly disagree as I venture would every shaper, fin designer and naval architect.


Yeah....whatever. Viscous forces govern and fin lift opposing side slip:I don't buy it.


Viscous forces impact every foil in every fluid, from fins to airplane wings. It's a matter of semantics since a fin pushing nearly sideways in water still feels water flowing around it, as opposed to the example of pushing a fin into a brick wall. The former is lift, the latter is an example of opposing forces.

The flow is there, however turbulent, even when the fin washes out at low speeds.

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DanWeiss



Joined: 24 Jun 2008
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Location: Connecticut, USA

PostPosted: Mon Mar 25, 2019 7:12 pm    Post subject: Reply with quote

[quote="isobars"]
bred2shred wrote:
SNIP

I'd like to see a longitudinal video from a GoPro mounted beneath a board looking back at a long blade fin being pushed hard. SNIP


Here you go: https://www.youtube.com/watch?v=OEhYC9IYOSU

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