Fin toe - what should I have done?

General Discussion
1

I snapped a side fin bottom turning on a left last week - pop!, slide, eat sand/seaweed sludge.

I didn’t have another back left fin for my thruster set up for next session in decent 5’ beachies right handers. So now the missing fin would be the outside fin in bottom turns and the pivot fin in top turns.

I had a spare rear fin for the set (i.e. same size) and a bigger left fin from another set.

I opted for putting the extra rear fin in as a side fin, figuring that the dual foiling was a better trade-off than the bigger fin, because I found that the bigger outside fin tends to restrict bottom turns  a bit (I think it doesn’t clear the water enough).

I found that I felt I was getting caught in the lip during top turns.

Since I had the option of adjusting my fin toe to compensate for the dual foiled side fin, what should I have done?  Toe-d in a bit or toe-d out a bit?

2

Hi Red
may have missed this elsewhere, but what fin system are you using that lets you adjust toe?

And does said fin system have rake set in the plug/box, or the fin? (I’m assuming your choice of a centre fin means the rake is in the plug?)

I’ve been playing with toe recently and noticed some surprising things…

3

My guess is that dual foil can handle a greater angle of attack and gives less lift, so I would reduce the toe and increase the cant.

 

By the way, can you cut a board for me if I am back in Vic?

4

Tomway,

4wfs system allows toe adjustment.  Cant in the box.  Different (yellow, blue) inserts for 2,4 degree cant

 

Claude

Greater angle of attack?  That’s the thing I was struggling with.

Currently building a CNC.  It’ll be 6-8 months before I’m cutting again.

 

Thanks for the input.

5

What you should have done is made a fin to match.

Barring that, you are not going to get the same performance out of a double foiled rail fin as you were getting from a single foiled rail fin. 

I would try decreasing the toe by 1/16". It may help, but you won’t bottom turn as well going the other way (which may or may not matter).

 

6

Thanks for the input, Blake

I tweaked the toe in a bit and this seemed to really help with Saturday’s session in good 3-4’ waves.  No hanging at the top and nice tight radius top turns.

Rode small rights the next day and don’t know it it was my imagination, but didn’t seem as quick off the mark.  Could have been me, the waves or the fins.

Next I’ll have to try your suggestion of decreasing toe a touch.  To my thinking this will have the effect of decreasing the angle of attack in turns?

Now to get a correct fin if work doesn’t keep getting in the way of fun.

[edit some incorrect stuff out]

7

So is the theory behind this as follows?:

single foiled - lots of lift; double foiled - little lift/no lift

more angle of attack (toe in) = increases lift of a foil

increasing toe on double foiled fin increases lift to something nearer the single foiled fin = surfs more like it should

8

Decreasing toe will increase angle of attack.

A single foiled fin has an asymmetric lift/drag relation. More lift at positive AOAs, less at negative AOAs, as though the AOA were increased slightly. To match the lift at positive AOAs, you will need to increase the angle of attack of the rail fin (ie: decrease the toe-in). This will, however, negatively impact thrust as that depends on the relative angles of the rail and rear fin. For coming off the top it can be done. However you will lose some bottom turn thrust. 

Its a fun game to surf the same board with rail fins toed in at 1/8", 3/16", 1/4", 5/16", and 3/8". The low toe boards can turn tightly but cannot generate speed, the mid-toe boards cannot turn as tightly but can generate thrust, and the highest toe boards will slide through turns. Thrust is really just a controlled slide through a turn anyway, isn’t it?

hth

 

9

thanks blakestah. just realised I had absolutely no idea what effect toe had (after shaping and finning about 70 boards).

I guess I need to get some of those adjustable fin systems…

Just to be clear, even a double foiled fin (which presumably experiences no perpendicular forces at 0 degrees) can create lift with an increased angle of attack?

10

I’m worried I’ve got the terminology backwards here.

The more the side fins point towards (below) the nose, the easier the board will come around.  It this called more toe or less toe?  Judjing from the comment above, this is less toe?

I spent a few days in Bali going too fast and straight because I had my fins pointing about 4" off the nose.  Wound it back to 2" or so and found a sweet spot between speed and getting vert.

11

What is the fore-aft spread of your fin cluster? If the rear is at 4 and the front is at 11, it would be a 7 inch spread. 4 and 12 would be an 8 inch spread. If you reduce toe and still want to turn quickly the spread needs to be closer to 7 than to 8 inches - otherwise it will get a little tracky. 

12

Thanks for the input, Blakestah.

Kneeboarding so fin setting rules abused.  Design rail line for projection, push fins forward for looseness, spread side fins for drive, spread front/rear for control (drive):

9" spread front to back.

Side fins 18" up and 8" off stringer (16-17" apart!).  Rear fin at 9"

Yes, things get tracky when the fins are toed 3-4" off the nose (but it’s fun if you don’t want to turn!)

Here’s results from a standard setting: