fin position

[rasdpm]

Given the same sail, mast foot position, foot strap position and harness line position, what effect does changing the fin position have? If you slide the fin forward in the track, what effects would you expect with all other variables held constant?

[spyder]

i had to think about this one for a bit. perhaps someone else has a better explaination.

as we know, the fin is basically the force that resists against the lateral force generated by the sail pressure. which also provides lift to get the board planing.

given that, your position on the board as a sailor, uses the rear foot to maintain downward pressure on the fin. this gives the sailor control of the board from this pressure point, used in conjunction with the sail pressure about the mast base to drive the boards direction and force forward.

as for placement, the leading edge of the fin should be placed to be underneath the rear edge of the back footstrap. i usually put the fins leading edge inline with the rear footstrap screw, or slightly forward.

moving the fin more forward makes the board more manuverable, but more chance of the fin breaking loose, resulting in spin out. moving it more back, makes the board less responsive, can also spinout easier.

a way to think about is that the fin is a resistive force when its active, and your foot pressure makes a pivot point on the board. if the fin is too far forward of the back footstrap, the fin can break loose. too far back from the footstrap makes it harder to control the boards direction. also when in jibes, the fin can skip out.

[rasdpm]

That makes sense. But, specifically how would your body position change with the fin further forward in the track? In order to counterbalance the center of effort, would your body be further back or forward, when the fin is moved forward? I replaced my fin last weekend, sailed the Hatchery, and found myself in a very different body position while planing. It was awkward and frustrating, even though i was still in the harness, lines just right with little hand pressure on the boom. It took me all day to realize that the only thing changed was the fin position, or at least this was my conclusion. The fin was further forward in the track, and i found myself always leaning further back toward the tail of the board. Why?

[spyder]

what was the problem if you didnt lean further back? would the board become unstable? what was happening?

as for a guess for what was happening, assuming the sail was rigged ok (balanced..etc). it seems like the fin force is pretty critical with respect to position with the sailors back foot pressure.

i think the fin works properly when the leading edge is under the back part of the rear footstrap. the control is with the back foot just in front of the fins leading edge, or just right above it. in your case, it sounds like the leading edge was further forward, making your foot behind the leading edge. so, in this case, you had to compensate by pushing behind the fin to keep it under control and on course, which corrected the abnormal fin position. your normal front foot pressure was too much, since the fin was forward. kind of like a see-saw effect.

_xF____F__b______>

normal setup

_Fx____F__b______>

your setup

F = Foot
x = fin
b = mast base
> = front of board

hopefully this makes sense, i havent really thought much about this, since i just set things up that have worked well in the past.

[mitchgingrich]

move the fin back = better turns; youll be standing on top of the board. Move the fin back = better tracking, more speed, better upwind ability; youll be leaning away from your board. If you want to turn better, move the fin forward, and the universal back. For speed, do the opposite.

[Windlover]

In response to your original ?, moving the fin a little forward will help you point, a little back will give you better speed control offwind.

Spyder is on the right track for fin to foot positioning for adjustable postion fins. Although, that goes out the window with the newer fixed postion fins. Work with his advice to find what works best for you. Different fins will require different adjustments, but basically he has the theory down.

There is also a sweet spot where sail/mast postion and fin postion are in harmony. As harness lines need to be postioned to the CE of the sail, postioning the mast is important to fin position. It will vary with different sail sizes but basically, if the main force of the sail is forward or aft in relation to the fin position, it will affect body postion and ease of sailing. Not being a physicist, Im unable to explain the actual theory. It takes a little fine tuning to find what works best for you.

Something else to remember, your body is moveable ballast and the rig is immoveable ballast. Changing your body postion forward for pointing or back for offwind speed will cause the board/fin to feel different. Just try to get sail/fin postion dialed in for a straight out and back reach, then it should work well for the other points of sail.

As for your frustration at the Hatchery, Spyders theory is sound. The other possibility is the mast postion to the fin postion. For example, if I rig a 5.0 w/slalom board & pointer fin and find myself a bit overpowered. I move my mast back about 1-2 and adjust my fin about 1/2-1. It is all relative to the force of sail being transmitted to the fin via the body. If you move one, move the other accordingly. We are constantly moving our body in relationship the direction we are headed. So, the sail/fin postion need to be dialed so the body will be comfortable in all sailing postions. Some of this is theory but mostly experience. Keep asking ?s and making minor adjustments and you will get it dialed in.

Windlover