Seems like a better question than asking what rocker "apex" is.
when I’m drawing in CAD, I usually try to find a confluence between wide point, mid point and rocker apex…my last few boards have been more neutral with wide points generally pretty close to the midpoint.
who knows if this is right or not, but I like how they ride.
Tell me how you define entry and exit rocker…
Are they the same as nose and tail rocker? Of do you have a different definition, hence the different terms?
not sure if you meant to reply to me, but I was assuming he was using the terms interchangeably with nose/tail.
I’m with the need for definitions.
Kind of a loaded question? If I'm doing the same size 7' 10" board one for San Diego, and on for the Islands. The apex or entry point will be different because of the over all nose and tail rocker. For instance: The Hawaii board will have 3" Tail and 6" in nose, while the San Diego board will have 2 1/2 tail and 5 1/2 nose. at 12 inches back on the SD board the rocker will be 2 1/4 and the tail 1 1/4 while the Hawaii board will have quite a bit more kick...subsequently making the Hawaii board more rockery all the way through. The San Diego board will have a bigger...let's call it a "flat spot" for lack of a better term. SD board will have more fin toe cant, and Hawaii will be a lot more straight and parallel fin...but I digress on that topic.
So to answer your question in my mind...it depends on where the board is going to be surfed, and in what conditions. One of my favorite mush busters is 8 ft Speed egg with a tail rocker 2 7/8 with a nose 4 1/2. very gradual nose rocker with a lot of the tail rocker in the last 6-12 inched...set up as a tri fin...hauls ass and turns very very well for a big board.But on bigger days you need to take off on a angle if the waves get steep.
Your question is too loaded...I send my seals over to poop on your lawn.
I find most boards have the high spot, apex, not in the middle.
Its rather a few inches towards the tail.
I've stopped using the middle as a starting point when measuring, as when you eyeball the apex, its not in the middle.
When measuring from the middle both entry and exit rocker have the same running lenght.
Imo the entry rocker should be longer, exit shorter.
I've tried this with good results, you could try it too!
+ Eye ball the high spot. Try look at it from different angles, this helps a bit. Mark it and wright down the location. Put it away
+ Lay down your board on the floor (which is level) and measure nose and tail rocker. 5,5 nose and 2,5 tail for instance.
+ Now back on the stand belly up and a rocker stick on top.
+ Find the numbers 5.5 nose rocker and 2.5 tail rocker on your rocker stick. Measure inbetween.
! Voila! Your rocker stick is most likely to sit on the spot you've marked.
I'd like to attach boardcad files to illustrate the difference between measuring from the middle and shifting it towards the tail. (Is that possible???) Same number in total rocker (give or take a bit) but distributed differently.
Or PP it for those who are interested.
Cheers
Mook
On the profile above i've measured the board, taking the middle as a point of reference. On the picture below the center of mass is more towards the tail. Same board though, but measured differently.
Hope its helpfull
Ya'll realize this is a can of worms, which is why I brought it up. Animal poop is great lawn fertilizer, in moderation of course.
As far as definitions, you dont have to agree with mine, but "nose" rocker is not what I equate to entry rocker. Because entry rocker is everything to the left of the red line "T" in the drawing, and I dont consider the middle section of the board as the nose. "T" is short for Transition. The blue curve represents a neutral buoyancy rocker curve moving right to left as the black arrow indicates. The dashed line is level water.
This rocker curve is symmetrical: the entry is the same as the exit rocker.
I don't care much for the loose terminology used to describe surfboard design elements, but I thought entry is as good as any. IMO, it certainly sounds better than nose.
The original question stands:
How do YOU determine where "T" is?
Thanks Mook.
I will go out on a limb and say your method of "finding it" is as good as anyone else's.
That's the thing about design work...its very difficult to judge who is right and who is wrong. No such thing really.
But the worms are hungry and squirming....
In spite of all the options - it seems to be the industry standard to use the center of the board for “T”. To be sure, it is any easy point to find, work from, talk about, etc., etc.
What you seem to be wanting to talk about is the importance, functions, and perhaps “changes” in the rocker curve near the nose, where the board “enters” the water, and the curve near the tail.
Crafty,
Limited experience, and nervous about weighing in here with all you heavyweights, but here goes…
I work with wide point, thickness and rocker low point in combination. I bring them closer together for a more responsive board and spread them apart in guns and grovellers. The rocker low point has the least flexibility.
For my kneeboards I put the rocker low point directly under my weight centre when I’m leaning forward into turns… This is a set distance in front of where my knees will go (my knees go 35cm from the tips of the rail fins - that’s how long my shins ate!), For other people it depends on their legs/height and where they place their feet in relation to the fins
For standup boards I’ve worked on- the front foot position gives indication of where rocker low point should go.
Since the “running gear” (the 1/3 of the board around the rocker low point) is very similar across all functioning boards, (I’ve compared quite a few computer files), the entry and exit don’t move in realtion to yhe rocker low point.
I use pure single curves nose to tail, so once the tip and tail are determined and the running gear is “fixed”, the rest of the curve can’t move too much without starting to look “wrong”.
I guess guys have to hop boards when the rocker low point is too far forward, until they get the board “on a plane”?? Have to think about this more…
Actually no.
When you consider that lift comes from (Newtonian) pressure generated by an angle of attack along the bottom of the board, the position of T, and thus the relative length of entry vs exit is very important. Particularly on how a board generates speed and wave catching.
While driving, stick your hand out of the window, simulate a little surfboard, and make very subtle entry and exit angles relative to flow.
I understand about the "industry standard", I have nothing against it.
I just want to get an understanding how shapers, expert or not, determine where T is.
I will go out on a limb and say the experts want to know where it is and know how to find it.
STATICALLY.
DYNAMICALLY....T moves.
Can-o-worms....
I figured that you had a different definition of entry/exit rocker, than you did for nose/tail rocker, which is why I asked for clarification.
I would say that in “reality” (i.e. thinking about how surfboards actually work with regard to surfers and waves) there is no such thing as entry and exit rocker. They are meaningless terms (like apex) that don’t provide much understanding about how designs perform in surfing situations (at least in my limited mental model). They are meaningless because the pitch, roll, yaw, and AOA of the board during different types of turns changes how and where the flow “enters and exits” (to borrow the terms) it’s interaction with the hull. So, as was mentioned in the other thread, you have to consider plane shape, bottom contours, and foil to fully understand how the design is functioning, because all of these factors also have an effect on how the flow “enters and exits” it’s interaction with the hull.
But, I do think the concepts of entry and exit rocker can play a roll in the designers understanding of how the board will function, just as nose rocker and tail rocker numbers can help guide a design.
To answer your question, I too think that entry/exit rocker are different than nose/tail rocker, and the difference is illustrated in Bill Barnfields excellent drawing:
To state it simply: entry/exit rocker are illustrated by the blue board, and nose/tail rocker are illustrated by the pink board.
If you were to just plot the curve of the bottom (and I say only the bottom curve, because the deck curve doesn’t always end the same distance from the bottom curve at the nose and the tail on most boards) of a surfboard from nose to tail, and make them level (nose and tail are on the same line), then the apex is the highest point on the bottom curve; the entry rocker is in front of the apex, and the exit rocker is behind the apex.
Disclaimer: I am not an expert like BB, or MikeD, or DeadShaper, just a garage hobbyist, so please take my comments with a grain of salt.
Here’s how I do it… very unscientific…
Usually there’s a little “flat” spot where the apex is - the longer the flat spot, the less curve. Find the longest flat spot, and everything ahead of that is what you call entry, and everything behind that would be exit. You can use this method with the board level, tilted, upside down… it won’t matter, because it’s not in relation to anything else.
Crafty - I think you answered your own question, or pointed out why there is not an answer…
DYNAMICALLY…T moves.
I like that NJ. But what if there's no flat spot?
Taylor, the planshape (planeing area) of the board changes too, but as designers, we still need a planshape, we still need design targets.
So the question stands.
Interesting responses so far.
I’m with you Crafty - But, I still think there are two things going on. The old “apex” question vis-a-vis rocker measurements, and the most common method of working from the center (and kooks like me who want to point out the “true” geometric definition, and, as I take encouragement from Bill B. pointing out - one may want to use that type of apex if one is laying out and making their own stringers and blanks (Simply helps maximize material, minimize material waste.))
Then there is the question of function: And that leads straight to why many of us are here - It is very complicated, as you point out, the part of the board that is the planing surface changes, often quite radically and quickly, the surface being planed on changes, often quite quickly and radically, and rocker, as it is, not withstanding, there, as has been pointed out, are many other things going on at once, and their interactions change too…
There’s my rock in the mud puddle. 
Good way to see it ....I can see it easier botton down....grab the board botton down and put it at my eyes level....much better to find the litle flat.
Important though to know where you will set the center of the litle flat.....I opened this can of worms some time ago and they still around hhahahahaah
litle flats hahahaha
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Back in 78, Geoff McCoy did a 4 page interview in TRACKS and either he or Cheyne said something about a flat spot under the front foot. Thats where I put the least amount of rocker.
"Flat" probably means flat by comparison to the rest of the curve.
I think we should start establishing a Chord Base Line for accurately measuring rocker.