depth of crack Swallow tail formula


i was reading greenlight design guide website about tail shape. If i understood what they say, to calculate the deep of crack for a swallow tail they multiply tip to tip distance of the tail by 0,49.

I took a template (5’4 Steve Lis) from blending curve, tip to tip distance is about 29,4cm (11,56") and deep of crack is about 5" (12,5cm).

if i calculate 11.56"x0.49= 5.66" i am far from 5"

How you guys calculate your deep of crack?



My formula: What looks good to my eye. The narrower the tail, the shallower the depth of the swallowtail cut, and the less curve (or none at all, like on the bat tail). If the tail’s a little wider, you can go with a deeper cut, and a little curve, especially if you like that old school look.

(Disclaimer: I am not an expert)

Sorry, but you are asking 2 different questions.   A swallow tail is not a fish tail.  A Steve Lis fish tail or any fish tail is based on the assumption that you are bisecting the shape to make a pin tail on the forehand and the backhand.  A swallow tail is a variety of of a standard square tail on a thruster type of surfboard.  Please search the archives before posting so you are asking a question most of us old farts can answer with a bit of experience… Just my 2c…

Hi soulsurfer51. The general guidline for a fish is the crack depth is one half the distance of the distance between the tips. 10 inches between tips, 5 inch crack. Mine are usually around 6.5 inches deep. Anything less than 10 inches between tips I call a swallow tail. Well, tiny swallows we called dove tails back in the day. Mike


Kinstle, J.F.  1977.  Surfboard Design and Construction, p. 61.  Published by Natural High Express Publishing, Long Beach, CA.

6.5/13 = 0.5

6.5/20.96 = 0.31

13/20.96 = 0.62

Formulas are useful as a rule of thumb, and I think the info Rooster and Stoneburner posted is good.  I’m just leery of getting too hung up on “formulas”, because there are too many variables for a formula to factor all of them in.  There is a lot of variation in what people are calling a “fish” today, and what a person might want from that design.  Thats why I trust my eye more than a formula.  What some people call a fish might not be a fish at all to my eye, so a formula that works for my fish might not apply to theirs.  

Looking back at the original post, I see he’s describing a Lis fish with an 11" tail and a 5" deep cutaway vs. a 5.66" cutaway - instead of splitting hairs, or saying blending curves got it wrong, I say allow for some flexibility and pick the depth that looks right to your eye.  And BTW, I don’t see a half inch difference in depth of cutaway on an old school fish like that as being all that significant to performance or aesthetics, just sayin’.

Just to confuse things , I seem to remember someone posting a formular of 14% for the area that is removed from the fish crack as being optimum , I dont know how you work that out and I dont know they arrived at that number .

I agree 100 percent Huck. Just a ‘general guidline.’ Almost all my fishes have a 6.5 inch crack for that reason. It looks correct to my eye. Doubt it plays any significant role in performance. Bottom contours, fins, and there placement do though. 

I will say I’d call pretty much all the tails you showed swallowtails, and not fish. Just my opinion growing up and learning to surf where the fish all started and what we called what back then. Anyone can call their boards anything they want. Not judging. Just saying. Mike

  Rare insight, and understanding, enjoyed by few.    Kudos.

Swallowtail is the title of the thread. There was only 1 board that I call a fish, but the term has wider use among today’s surfers.

True and your wooden board is a true “Fish”.  The last couple of pics are what people these days are calling a “Modern Fish”.   Although neither look anything like your wooden board.

If you want to know all things fishy there is a thread called fishy buttcrack porn , # 44 talks about the 14% buttcrack removel

…fish tail is one thing and swallow tail another.

The secret of the swallow tail is to “put closer” the rear fin to the tail. I mean; is that crack permits to see the fin; so permits to engage the fin better in the pocket for hot doggin surfin. If you set up a right rear fin or change fins is a killer set up for a thruster.

Aloha everybody,

Here the final result. I did also a fabric inlay. First attempt with some imperfection but I am happy with the final shape. Thanks to everybody for the help.

Your board looks great soulsurfer. Mike

Turned out great Mr. Soul.  Just to add fuel to the fire;  I am sure any of the"regulars" that frequent this site can remember somewhere, sometime when somebody didn’t know any better and posted an outline of a design that they intended to shape that had NO Butt Crack whatsoever and yet the Poster called it a Fish or Fish design.  I myself have tried in the past to correct a couple of these misinformed num nuts to no avail.  In fact; took flack about the crack when I insisted that a big wide Squash Tail or Rounded Pin without a Butt Crack was in no way shape or form a “Fish”.  There is also a complete and obvious difference between Fish and Swallowtails.  But there is also a lot of variation and blending of the two.  I gave up.  Call it a Parrot Beak.  I don’t care.  Lowel

This board is just a memory, I bought it around 1978. 6’6" shaped by Dennis Pang. I think Craig sold it to me for $175, brand new with fin. I only rode swallow tails back then. The Brewer fin is not the fin that came with the board. The fin that came with the board was similar to a GG 4A, second image has the original fin and you can see where there are 2 strange channels that are near the fin on each side.


I like that shark.  Nice clean board.