Change in drag due to center fin placement

[ConcreteVitamin]

How much change in drag is expected by moving center fin up and down? Is it normal to feel a non-negligible difference in paddling speed (and wave make rate)?

Last Wed I had a great session. 8" 4A slightly forward of center of the box in a 7'4 egg. Made slightly overhead waves and flying down. Board felt snappy. I noticed that on a bottom turn when I was "sitting", I slid out. It's probably user error as my backfoot probably wasn't over the fin -- but I thought it could be the fin being too far forward.

Yesterday I went out with the fin placed dead center in the box --- we are talking about a few millimeters difference from before. The waves were similar (head high on set waves). However, I kind of felt the paddling speed is slower, and I made much fewer waves when paddling.

It could totally be in my head. For example didn't get too much sleep the night before so naturally my body was tired. Just want to ask the experienced folks here whether a drastic change in drag is expected.

[ConcreteVitamin]

I probably fell on my butt on that bottom turn for reasons other than back foot not directly over the fin.

https://youtu.be/JLlW0Y30Gjs?t=238

At 3:58, Joel Tudor executes a bottom turn fully engaging the rail, after he intentionally moved the back foot forward of the center fin. Amazing.

[Naeco78]

Hey CV, I'm not as familiar with longboard fins but I've heard that moving the fin forward in the box with give the board more pivot/maneuverability and moving in back with make more rake or longer drawn out arcing turns. The increased Pivot would be my guess for why you slid out on your bottom turn. The board probably wont have as much hold or grip in Pivot settings. But the benefit is that it can make for shorter turns/arc when you don't have as much wave area to deal with, but you still have to balance that with good use of the rail, foot placement etc to make the pivot work well especially when placed in the max position of the fin box.
Just a guess without seeing what actually happened, but you could've had too much speed and not enough hold with the fin all the way forward like that.

[Naeco78]

Just noticed you said only slightly forward of center.. the concept would probably be the same. I would probably put the fin further back as the waves get bigger. Usually increased rake is better as the waves get bigger.. i think it makes it easier to turn at faster speeds.. more under control

[oldmansurfer]

One would expect the board to turn easier but not hold as much with the fin forward and expect it to track more and hold better with the fin back. I doubt it makes much difference paddling as it has the same drag. Back when I was surfing a longboard I had a kind of breakthrough in performance where suddenly I was doing pretty radical cutbacks and off the lips and all kinds of things. Shortly after that I noticed a loud thumping that was occurring when I turned. Come to find out the screw that was supposed to hold the fin in place had fallen out and the fin was rotating and sliding around randomly. I was having such a good time I didn't want to change things but I went ahead and found a replacement screw and put it in but as near as I could tell that didn't make any difference either with the fin forward or back or loose. It just made a lot of noise when it was completely loose.

[Naeco78]

Back when I was surfing a longboard I had a kind of breakthrough in performance where suddenly I was doing pretty radical cutbacks and off the lips and all kinds of things. Shortly after that I noticed a loud thumping that was occurring when I turned. Come to find out the screw that was supposed to hold the fin in place had fallen out and the fin was rotating and sliding around randomly. I was having such a good time I didn't want to change things but I went ahead and found a replacement screw and put it in but as near as I could tell that didn't make any difference either with the fin forward or back or loose. It just made a lot of noise when it was completely loose.

Lol sounds like you were early to the fin-less revolution. You mustve had the stars aligned just to have the fin still attached after all that.

[oldmansurfer]

I was surprised it didn’t fall off. I checked it from time to time before I bought a new screw and it migrated around but never fell out. I was so sure it was the reason I was surfing so well but apparently not

[waikikikichan]

See what a difference it makes adjusting the single fin to the conditions of the wave.

Chris Ward's back foot gets shoved off the deck as the board slides "butt" so hard. But after adjusting, he makes sections easily.

But in you case, you were probably to far forward or not on rail and the the fin was "flying", not engage into the face of the wave.

[oldmansurfer]

My longboard with the loose fin had side bites and a huge tail rocker. I wasn't aware of boards with much tail rocker. Mostly it seemed you had to place a straight edge on the bottom near the tail to tell. But this longboard had more than an inch of tail rocker and it was quite obvious once I looked. Tail rocker helps the board turn easier where ever you place the fin and the side bites helped to keep the board from sliding out and it also had hard rails on the tail which helped to hold the board from sliding out. Foot placement is important too as waikikichan said. I used to slide out the fins on my quad by turning hard with my foot placement too forward

[ConcreteVitamin]

Interesting notes, everyone - thanks.

I contemplated on that particular bottom turn. (That day I did some other bottom turns which were OK.). It did seem to occur not quite at the bottom of the wave, but more like 4/5 down the wave face. And the rail definitely wasn't engaged. I think these made me slide out, not because of the placement of the center fin.

The consensus about drag seems to be...just try it out. I'll move it by 1/4" and see.

[oldmansurfer]

What forces will change that will alter your ability to paddle a board with the fin a few inches forward or backward? Seems to me the profile is exactly the same, forward or backward it still pushes the same amount of water. Maybe the drag will change due to the fin box causing differing amounts of turbulence from the unused space? You can fix that one by filling in the finbox with something. I have tried this and I know others have but for me at least it's not worth the hassle I imagine it might matter in the top speed the board can go but rarely do I push it that far.