Fin drive?

[nalu-sup]

On the forum I have read comments about having enough fin area, or base, on a downwind board to generate "drive". Burchas made a comment along those lines regarding the new Naish board today.
I have a comfortable understanding of using lateral pressure against a fin to generate speed on a windsurfer, and clearly a larger fin can transfer more lateral pressure into forward drive at lower speeds. I have a vision of how lateral leg drive against the fin(s) on a surf board or surf SUP can generate forward drive and it is generally agreed that larger fins or fins with more base area can help in creating more forward drive through a turn. However, I don't have an image of how a fin on a downwind board would generate drive, since in my mind there is little to no lateral pressure that the fin can transfer into forward drive. Could someone give a good explanation of what is meant by "drive" from the fin on a downwind board?

[burchas]

Nalu-sup, if you read my comment again you'll see I'm referring to a downwind situation where
a lot of paddle engagement is necessary like the conditions we had today, hence the need for
a fin with a drive like the Larry Allison GT Moray that I used today:

http://www.paddleboardspecialists.com/Moray/productinfo/LA16MO/

If you have consistent and sufficient wind push no need for that.

[PonoBill]

If you want to identify the direction and magnitude of each force acting on an object, you draw a free-body diagram. For a standup board you can easily identify the force generated by the paddle and it's vectors--one pushing the board directly forward, one smaller force shoving the board sideways because of the offset between the center of gravity and the line the paddle travels. You identify drag, and with wind and swell pushing from behind you can add those forces. When you're all done, you won't find drive anywhere, unless you are a marketer selling fins.

If you did draw a force line called "drive" that appears just because the fin resists sideways movement you'd be hard pressed to find the force that initiates it. If such a force existed, dropping a surfboard in the water would start it accelerating without limits other than relativistic ones. You'd also find drive from any wheel that rolled easily forward, resisting force from the side. Railroad cars wouldn't require engines. You can draw a force line called lift, and it's inevitable brother drag, but for lift to exist you have to have movement through a medium, and that means force needs to be applied to accelerate the foil initially and maintain forward motion against drag.

In other words, "drive" is like "centrifugal force"--pretty much nonsense.

[Area 10]

I think that drive is just a word people use to describe the feeling they get from lift of the fin when sideways on to the direction of travel of the bump, just as in surfing. You can translate that to forward momentum if you know what you are doing, especially when combined with the greater tracking that long fin bases give. The downside is that the board becomes stiff to turn, and can be more prone to broaching/rounding up.

[ukgm]

If you want to identify the direction and magnitude of each force acting on an object, you draw a free-body diagram. For a standup board you can easily identify the force generated by the paddle and it's vectors--one pushing the board directly forward, one smaller force shoving the board sideways because of the offset between the center of gravity and the line the paddle travels. You identify drag, and with wind and swell pushing from behind you can add those forces. When you're all done, you won't find drive anywhere, unless you are a marketer selling fins.

If you did draw a force line called "drive" that appears just because the fin resists sideways movement you'd be hard pressed to find the force that initiates it. If such a force existed, dropping a surfboard in the water would start it accelerating without limits other than relativistic ones. You'd also find drive from any wheel that rolled easily forward, resisting force from the side. Railroad cars wouldn't require engines. You can draw a force line called lift, and it's inevitable brother drag, but for lift to exist you have to have movement through a medium, and that means force needs to be applied to accelerate the foil initially and maintain forward motion against drag.

In other words, "drive" is like "centrifugal force"--pretty much nonsense.

A different context but we went through this in cycling too whereby certain solid disc wheels (used in the rear in time trials) could be designed so that they actually contributed forward thrust when at certain yaw angles. Punters went nuts for free speed. The problem was, I'm yet to see anyone replicate the effect outside of a clinically controlled wind tunnel.

[DavidJohn]

The only time I'd use the word 'drive' re fins is when pushing through a turn when surfing a wave. (e.g. normal surfing not DW surfing)

[PonoBill]

The only time I'd use the word 'drive' re fins is when pushing through a turn when surfing a wave. (e.g. normal surfing not DW surfing)

Even then what you are really talking about is lift.

I'm not really grumping about the general use of the term, we don't have to be scientifically accurate in talking about fun. I'm not even bitching at Larry. It's residual irritation about a fin company whose name I've forgotten who made a big deal about being scientists and then drew absurd force diagrams where "drive" came out of thin air.

[DavidJohn]

The only time I'd use the word 'lift' re fins is when pushing the opposite rail to turn the board when surfing a wave. (e.g. DW surfing not normal surfing).. 

[burchas]

In stand-up paddling with "Paddling" being the key word, drive usually describe what Area10
called forward momentum or as Larry Allison calls it "Projection".

For me, the most simple way to understand was by a simple exercise, I took my stock inflatable
board fin, a flexible plastic 9inch deep with 4inch base or so, and paddled it against my stiff
FiberGlass Stinger fin, a 7in deep with 7.75" base.

I put both in turns on the same board and went for a flatwater paddle. I immediately noticed
the difference how the board behave. Running with the plastic fin I felt I was barely accelerating
and had to keep paddling hard to keep moving forward.

Changed it to the Stinger fin, the board became responsive and accelerated fast and kept the glide.

In a mixed downwind situation where I can't rely only on wind push, I use a fin that will give me
this extra boost to get on a bump. The Moray fin I was using yesterday is a perfect fin for
situations like this without the mentioned downsides as it still very maneuverable and broaching
was never an issue.

[yugi]

^ have you ever tried your observations with "blinded" (not knowing which) fins?

[burchas]

^ have you ever tried your observations with "blinded" (not knowing which) fins?

^ Didn't need to, had a GPS to corroborate my feeling 

[PonoBill]

As I said, we don't need to be scientific in describing fun, but when people pretend to be, there's a different level of level of expectation. Unquestionably a better fin, or at least a more suitable fin, will make a board behave the way we want it to. Fins do a lot, but they don't generate forward motion on their own. They can make the conversion of the force applied more efficient. They can be more or less draggy, they can deliver more or less stability, rotate more or less easily. Lots of marvelous stuff, but they don't generate any force on their own, they just help direct it. "Projection" is the same as saying "drive". Fine for describing what you feel, but nonsense when it's presented as some mysterious force. Larry knows more about fins than I ever will, but his ideas about physics leave a bit to be desired.

DJ, ALL fins generate lift--it's a big part of how they work. Symmetrical fins generate lift (and drag) when the angle of attack is positive in relation to motion--which (along with increased drag aft of the center of mass) is how the tail gets pulled back straight. Asymmetrical fins, like most thrusters, always generate lift. The lift force vectors cancel when the motion is straight but pulls the tail into a wave when the outer fin is less buried, and also generates some unbalanced lift in turns because the outer fin in a turn is moving faster than the inner fin (larger radius)

[Luc Benac]

^ Didn't need to, had a GPS to corroborate my feeling 

Usually the GPS hurts my feelings rather than corroborate them :-)
It keeps me honest but also slow......

[digger71]

^ have you ever tried your observations with "blinded" (not knowing which) fins?

^ Didn't need to, had a GPS to corroborate my feeling 

Not trying to speak for Yugi, but I think his point is you knew when the "better" fin was installed and may have paddled accordingly.  If you had someone else install the fins and you paddled not knowing which fin you were using the results would be more relevant. 

[Eagle]

This is something I did after getting the 23 -> tested GPS speed and fin tracking.  Sprinted max from a dead stop and turned around and did again right away.  Did this with a number of fins over a course of a few days.  Conditions were similar enough so have reasonable comparables.  These are the 2 extremes.

Oddly really liked how the Elite felt as it tracked so easily.  The 7.0 felt very light and fast but was impossible to paddle straight full out.  The smaller fin had less drag but was very hard to control direction. So was going faster but not necessarily straight where you wanted to go.

Since then have used both fins a lot and really like how differently they perform.  The Elite resists turning so kinda makes you go more forward -> like that.  While the 7.0 does not -> but like that too.  So for speed straight like the Elite and for DW turning like the 7.0 fin.

If "drive" means go more in a straight line -> then the Elite has more "drive".  Basically have found that any fin works but expect certain compromises and limitations.  On a DW board a good fin for me allows quick turns and minimal rounding up.  So a small fin like the 7.0 with M14 or 8.3 with Bullet are what works best for me up in Squamish.