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erik 19th March 2007 01:01 AM

min/max fin size for my isonics question for ian fox
Hi ian,

Can you tell me the min/max fin size for my isonic 105/135 2006?

what about the one foot off rule? I hear differend stories.
Is it -/+ 2 cm or -10 /+2 cm ?
Should i have stiff fins or more soft ones?
what fin range should you advise?

i sail 9.9/7.8/6.8/5.8 warps 2007
i way 90 kg.
i sail at big lake with lot of swell/chop

greetings from holland,


geo 19th March 2007 04:04 PM

RE: min/max fin size for my isonics question for ian fox

no Ian here but I'd like to discuss the "one foot off" rule, and hear from Ian about it too.

It seems to me that the rough guide of fin being more or less as long as the OFO measure was born when board designs where not so variated as today. A medium wind slalom board would be about 9'x21.5"x12.5" back then, with not much variations. And a good size for the fin would be 12" to 13".
The OFO rule probably comes from the fact that the wider the tail, the more leverage you have to keep the board/fin flat at high speed. Today, board design has been revolutionized by wider designs and different shapers have different approaches at it: no more "classic" recipe with variations, but much different recipes. One may find wide boards with wide tails to be used in the same (well, actually wider) wind ranges of older "narrow" designs; so I think that the relationship between tail width and fin length may still be valid, but actually could be useless. I mean that a board could carry a very large fin because of its tail width, but actually be used in such a wind range in which large fins are not the call.
You can get the last issue of PlancheMag and look at the slalom boards they tested; if you look at the measured tail widths and stock fin lengths, you will understand what I mean.

Ian Fox 19th March 2007 04:35 PM

RE: min/max fin size for my isonics question for ian fox
Hi Erik,

Any fin sizing limits or formulae, even defining so called "min" and "max" sizes is always an aproximation because of the large number of significant variables outside our immediate control.

So, while a OFO rule -/+ 2 cm could be good for a waveboard, even a -10 /+2 cm is not totally correct for iSonics, especially the larger ones.

For your weight and choppy conditions, typically you could use

For the iS105 ;

32-34cm will be about the optimal minimum for 5.8/powered 6.7m.
36-38cm for mid-underpowered 6.7m and powered 7.8m
40-42cm for mid to light 7.8m

For iSonic 135 :

36-38cm typical 6.8m conditions(overpowered 6.8m use iS105)
38-40cm powered 7.8m
40-42cm typical 7.8m
42-44cm light 7.8m/powered 9.9m
44-48cm light 9.9m

For the higher wind, higher load fins, you will probably have more success with stiffer fins, whereas good quality fins in medium flex can offer some advantage in the mid to light range. The actual brand/model depends a lot on what is practically available in your local market. If you've the chance to meet with fellow surfers and try some "option" fins before you invest in new ones, that's also a good idea and gives you a better insight into different fins character and performance, which in turn helps you feel out further options and personal tuning as you progress.

Please let us know if you have any further questions.

Cheers ~ Ian

erik 19th March 2007 09:52 PM

RE: min/max fin size for my isonics question for ian fox
Hi ian,

thanks for the info.

im a little bit confused about your answer.

you said that for high wind i should use more stiff fins and for mediate and light conditions medium flex fins.

I always heared the opposite. lightwind stiff and strongwind more flex?

could you explane some more?


greetings from holland,


Ian Fox 20th March 2007 04:03 PM

RE: min/max fin size for my isonics question for ian fox
Hi Erik,

The real issue is really to have a fin that the flex/bend/twist character designed into the fin is matching the load the fin is being used under.
Of course, in the matrix of fin options, many things "work", so it comes down to summarising trends -or massive detail on individual fin / rider weight / technique / sailing mode / board / conditions level.

A few years ago, when FW first started out, there was very much a trend to stiffer light wind and softer hi wind fins, for maybe obvious reasons.. But as fin and foil development went further ahead, it was better to restrict (stiffer) the deflection under higher loads to try and keep the foil working more efficiently (and so better control, less spin out etc)- whilst in lighter conditions, too stiff a fin was found to not be responsive enough (under lighter loads) to reach or maintain best efficiency.

Today, the trend is towards less deflection in higher wind fins, and some medium controlled deflection in light to mid wind fins. So it's the opposite of what could have been said a few years back..

Of course, the real key is to have a fin that that is designed to allow for the right deflection at that given load point, deflects just right for the relevant load being applied, allowing the fin to be at it's best under the expected (tuned) working load. So the finer points again come back to finding the perfect combo, which in turn is why the whole "ideal" fin selection thing remains a bit of a personal (style/weight/technique) and conditions relative equation. Some would say black art, with simple answers for generalisations, but very difficult ones for specifics.

A great way to gain some more experience is to borrow some (suitable size) fins from fellow surfers at your location, take a few combos for spin and see if you can determine a better performance (or not).
Pretty quickly you should start to get the feel, and from there you begin the life long quest for that holy windsurfing grail - the "perfect fin".

(always sought... rarely found) ;)

Cheers ~ Ian

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