Asymmetric tails

I’ve given up with these and about to explain why!

Conventional asymmetric theory says on a right point wave put the shorter side of the board on the left. Shorter boards turn more easily than longer boards therefore the challenges of cutbacking easily and coming down off the lip will be easier this way while retaining the down the line and bottom turn speed of the longer side.

However I didn’t believe this…



my board can be visualised as half a swallow tail on the heelside (I’m regular foot) and a shorter rounded pin on the toeside. This board made for right pt waves therefore the shorter side faced the wave.

However looking at arrow 1. on the above diagram it can be seen that a swallow puts more tail area behind the rear fin therefore my theory was that this board would be like having the fins far back on the shorter side (and therefore stiffer on the bottom turn) and having the fins more further forward on the longer heelside. Conventional fin position theory says that an increased area behind the fin makes for a looser board if the fins are not moved back to compensate.

However my theory did not work…

What happened was that being a right point wave the board would take off, bottom turn and spend most of its time with the shorter rounded pin side engaged in the wave. What I think happened was that my foot then moved further up the board to make it adjust to the shorter side. Refer to arrow 2. on the above diagram. Ie the sweet spot moved up when going right - i’ve exaggerated the picture but old sweet spot is marked X on the diagram and the new one 5. Thus moving my heel to far forward of the heelside and rear fin.

With the foot further up the board coming off the lip and cutting back was more difficult - I could feel that my heel wanted to be further back over the fins, but there was no time to slide it back mid-turn.

I spent a fair bit of time messing around with my lokboxes to try and correct the problem. I used to leave a screwdriver on top of my car and get out of the water mid-session and move the fins.

The configuration I arrived at was to keep the toeside and centre fin in their originally designed positions but to move the heelside fin forwards. Refer to arrow 3 on the diagram. Looking closely at the rear of this fin there is a small black marker pen line showing a gap of 1/4" from where the fin had been moved from its original position.

Additionally I had read a post from J-Troy who had experimented with 4-way fins and discovered that increasing toe in on the outside fin of the turning arc loosened up the turn. So I replaced the glass Rainbow made fins with hacked plastic future fins and used plastic aeromodelling engine mount wedges to change toe in. This is not a structurally sound way to set fins but the lokboxes seemed to handle he stress of the wedges.

I increased toe in on the toeside to point at the nose. My feeling was that this did not improve the situation very much. Moving the fin forward had the greater effect but all this did was slightly improve the feel of the board but not really overcome my fundamental design error.

I learned to live with this quirky board and surfed it quite a lot. Coming off the lip remained a problem but with practise this board could be sent into a cutback and if I got the timing right and rebounded off the foam it would come flying back round.

My next project was the same shape but symmetrical. This board worked good, but it leaked and I couldn’t be bothered to fix it. However mainly I was interested in getting the asymmetric design to work properly.

I was convinced that if I reversed the asymmetry conventional asymmetric theory would apply and all would be well. So the next one was built this way.

How about asym tail rocker? Flat for frontside drive, more rocker for heelside cutbacks…

However this did not work either! Coming down off the lip was just as bad. I don’t really know why, my guess is that conventional theory applies in certain situations and my original unconventional theory applies in other situations.

What I thought would correct it would be to move the centre box off centre. With the exception of the tail bit my boards were symmetrical, so if the centre fin moved closer to the rail on the right hand side of the board (the wave side when going right) then I would get the area behind the fin theory working together with the shorter board turning theory.

However I never got to trying that as moving the box was a bit of an effort with my limited working environment. Instead what I did was wedge the centre fin at a cant (see arrow on diagram above). This sort of pushes the fin area towards right rail. Banking the board into a bottom turn puts the canted centre fin into a favourable angle, although would increase spin out risk in cutbacks (although the board did not spin out).

Definately this canted centre fin improved things so I think that moving the box towards the rail would have worked.

My conclusion is that an aysmmetric board could probably be made to work by offsetting the centre fin, but there would be so much trial and error getting it right that its just not worth it. Also I’m not sure the result would actually be better than a symmetric board.

However it was a good exercise in understanding fin position.

One thing I forgot to mention in my previous post that moving the toeside fin forward (arrow 3) to get the sweet spot more forward of the foot and loosening things up does not really work. The reason is that the spread between this fin and the rear fin increases and this has a stiffening effect. The back fin can also be moved up to reduce the spread but this over-loosens the bottom turn!

Quote:

How about asym tail rocker? Flat for frontside drive, more rocker for heelside cutbacks…

Yes I think that could be made to work although I didn’t try it. What I had done though is apply John Mellor asymmetric rail theory.

The left heelside rail was chunkier in the rear half than the right rail. The idea would be I would get more lift and less sink when going into a cutback. I think this may have worked but I’m not sure coz the board was so quirky.

hm. seems like you realize there’s a lot going on. The board appears from the photos to be a fairly “weak” assymetrical, nothing very pronounced, just a little more on one side of the tail than the other. Yet is seems to make so much difference… I wonder, did you try this as a “simple” tri-fin first, or just take a blind stab at an assymetrical tri?

In car tuning, it is axiomatic that one only changes one thing at a time.

You got so much going on, and it sounds like you are trying multiple things at once to make it “work” in some fashion that is only to be assessed qualitatively. Hard job.

Y’know, it would be so much easier if you had only one fin on there…

I have long wanted to make an assymetrical but lacked the nerve to do so. Moreso, and maybe this is just me, what would be the point? My round pin single fin still surfs better than I do, seven years plus old, and I don’t even move the fin around. Don’t tell me that maybe I just got it right, tho’ that could be… nah.

Charlie, yes there is only mild asymmetry and the board basically felt ok to take off and carve around on, but when attempting off the lips and major cutbacks there was a big difference. Not many of us have the resources and time to change just one thing at a time but with this one I came close. The starting point was my Pu Pe 6’ 9" custom hybrid made by a local Santa Cruz company - the red board on the right hand side. With a rocker stick I was able to work out which Clark blank the shaper had used - the shaper did not change the rocker from the standard blank - it was actually the Clark hybrid blank. So what I did was plot the rocker points every 1’ onto masonite and construct my own rocker template using a flexible batten and hot wire cut it from EPS. So although I had made my own template the home built hybrid had that very familiar feel as soon as I got on it (this is why I rated rocker as important on the 6 questions thread). With the exception of the tail end planshape was unchanged. The asym rail profile I think had only a subtle effect and if I remember rightly the reverse asym attempt used symmetric rails but it still misbehaved.

All fin changes were an attempt to make it work right and compensate for the underlying design error. The fin changes were made one at a time, first I started with the fins in the usual position. Then started playing with the up and down direction only, then finally putting in asymmetric toe in and on the second attempt asymmetric rear fin cant.

Thrusters are more complicated than singles so I think you would have more chance of succeeding with a single. I only ride thrusters and as mentioned earlier I’ve definately given up with asymmetric boards.