Surfing Explained: Ep9 Surfboard Bottom Contours

In this weeks episode of Surf Simply's animated surf science series, 'Surfing Explained', we explore the different bottom contours (Concave, Double Concave, Convex, Flat, Channeled, Vee) and their effect on our speed and maneuverability.
This episode was written and animated by Surf Simply Coach Will Forster.
Transcript:
In the episode we going to be talking about bottom contours.
If you’ve never heard the phrase bottom contours, then its what we call the particular shaping of the underside of the surfboard, the surface that the water flow will obligingly follow thanks to something known as the Coanda Effect - in which a fluid flow attaches itself to a surface and remains attached even when the surface curves away from the initial flow direction.
What this means is that we can control the water flow direction under the board, and with it encourage either more speed, or more maneuverability depending on what we are attempting to achieve from the design of the board.
We can use a combination of Concave, Convex and Flat surfaces to create an almost infinite number of variations depending on what the board is designed for, so lets take a look at some of the most common components.
Concave
We'll start with a concave bottom. A concave, or single concave as its sometimes refereed to, creates a channel that runs along the length of the board between the nose and tail, guiding the water into a clean and direct flow. If you remember some of the physics that have featured in previous episodes, you’ll know the the more water we have travelling towards the tail of the board, or the faster that flow of water, the greater potential for lift and thrust there is.
Surfers have been employing this technique since the 40’s, with Bob Simmons credited with its introduction, along with many of the fundamental hydrodynamic principles of surfboard design.
A concave design typically suits surfboards that have speed in mind, for example small wave boards that need to maximize the little energy available to them, and powerful or barreling waves where down the line speed and grip are essential, and where turning maneuvers are not the priority.
Once up to speed, boards with Concave bottoms have a tendency to continue in that direction, a feature known as “tracking”, which to some is a blessing as it means it will hold its line well, though if the rider wants to change direction, one has to overcome that tracking by transferring more weight into the back foot to turn the board.
Although it’s not specifically known why, a concave also assists in producing grip from the rail when in trim mode, and from it comes down the line speed. It is thought to be from a more favorable angle of attack where the water flow coming up the wave face is the redirected by the rail, and this combined with the lift generated over the length of the board, it is generally accepted as the fastest bottom contour.
Convex
The convex bottom was the first innovation in bottom contour design, moving away from flat bottoms in an effort to make the heavier solid wood surfboards more maneuverable.
A convex is almost entirely the opposite of a concave channel, and resembles somewhat of a belly underneath the board across it’s width. This encourages the board through the rail to rail rolling motion during turn.
Convex bottoms, though more maneuverable, create more form drag as a result of greater frontal area and so are typically slower than an otherwise identical concave board. A convex bottom can reduce the hold and tracking nature of the board down the line, and so is at risk of slipping out of trim and dropping down the wave face if the necessary rail pressure is not maintained.
Flat
A flat bottom isn’t so much a contour, but more so the absence of a contour, and was once the standard before much thought had gone into the underside design of the surfboard.
We can safely say that a flat contour lands somewhere between the concave and convex as some of the water will move out towards the rails and some will travel down the length and out the tail, and so no specific benefit or loss will occur assuming we have no expectations for the design.
Vee
In 1967, famed shaper Bob McTavish created the ‘Plastic Machine model’, a 9ft board with a deep vee-shaped keel through the back of the board and a wide tail. This particular board became one of the first examples of the coming "short board revolution", with designs eventually going from 10ft down to 7ft or less over the course of just a few years. It was during McTavish’s quest for a more radically maneuvering board that he employed the deep vee bottom contour.
Similar to the convex bottom, a vee will direct water out to the rails and so although compromising the lift for down the line speed, it does allow for a lightning fast response when transitioned rail to rail by the rider.
When a surfer is riding in trim, a vee bottom provides only half the board...character limit met.

[@MrJunelly]

As someone who completed a degree in marine engineering it's amazing to hear people start talking about the physics of surfing! I feel its crazy how little people know about whyyy these things work. Keep it up, loving the videos!

[@SurfSimply]

Thanks Jonny!

[@noahlaizer7434]

@johnnysnow it's probably mostly how nobody thinks to think about it. A lot of people just see it works and dont question how. And you learn how and it sounds all complicated until one thing clicks and then everything else seems to follow. I've always loved learning about the physics of why things happen. AND IM NOT EVEN THAT INTO PHYSICS!!!! I just like seeing how things ACTUALLY work. It's pretty cool how our minds have figured all this stuff out

[@juusomiinalainen]

I'm marine engineer student and I just started building my very own surfboard. I can totally agree!

[@davidvelazquez5460]

Perfection! I absolutely love these videos. The strong effort to tie in science-based principles to surfing is such a breath of fresh air. I've always loved that about all the videos and podcasts you've done. Thanks for another amazing video.

[@SurfSimply]

Thank you David, I’m glad you enjoyed this one!

[@bonefishboards]

Flat bottom contours are overly discounted in today's surfing world. They work, are neutral and pretty forgiving. Trackiness of concave bottoms can be reduced with soft transitions leading into the rail. Rails and rocker are very important aspects to consider along with bottom contour. Pro tip: don't try to put every bell and whistle into your (custom) board. Focus on what you want, the kind of waves you usually surf and how strong of a paddler you are. This was a great video depicting how bottom contours work and the effect they have while riding.

[@SurfSimply]

Great advice Bonefish! Thanks for watching and commenting :)

[@paulrooney1581]

Totally agree

[@antoniovandias8653]

20 years of surfing. Never have I been schooled this well. thank you!!!!!

[@SurfSimply]

Great to hear it Antonio!

[@Surfmus]

I've shaped surfboards with all concaves. I've been lucky with the double concave for speed. The single to double to V for various conditions and the single for "lift" and stability. Is a never end thing with this hydrodynamics.

[@doptimist]

This is fantastic. Theres so much hocus pocus when it comes to surfboard design. Clear and informative, well done!

[@SurfSimply]

Thanks for watching Patrick, we’re stoked it’s cleaning some of that hocus pocus for you!

[@howardclemetson640]

Awesome video! Love how your channel is able to fuse surfing and physics. Its much needed for the sport. Keep on making great videos !

[@SurfSimply]

Thanks Howard, happy to hear you’re enjoying the content!

[@a.lame.username.]

An exceptionally appropriate planshape you've chosen to use in the examples👍

[@kelvinbatista9259]

Please do surfboard recommendations (specific models) for levels. Love the vids!!

[@surfsenegal]

another outstanding video. honestly, if the Oscars had a surf theory category...

[@SurfSimply]

Haha thank you Pierre, that’s very kind. Maybe we can partition for an award!

[@freekyvideo]

Amazing work guys! keep it up!

[@barrel1808]

This video is phenomenal thanks so much

[@SurfSimply]

It’s our pleasure! Thanks for watching!

[@TheYurisam]

Great video.very informative.

[@jul1us_tv]

this was simply amazing

[@SurfSimply]

Thank you Julius, I’m stoked you enjoyed it!

[@ezvibe8310]

As you mentioned in your last example, I have a fish type board that has a single to double concave with slight "v" at the tail. These contours are subtle, almost imperceptible, even while using a straight edge for reference. The board is absolute magic.

[@SurfSimply]

It sounds like a magic board EZ!

[@ezvibe8310]

sorry to geek out on your forum...good subject though

[@SurfSimply]

Don’t worry EZ this is a safe space for geeking out! The more the better!

[@ezvibe8310]

Happy to oblige then. ;)

[@gilliamm.5732]

Great video!

[@harrisonabry2280]

Man this video is so good!

[@SurfSimply]

Thanks Harrison!

[@hawaiiansouthshore]

Awesome!! 🤙🤙

[@user-uu1ql5bf4v]

all right in the paper, but in practice things change, concave can be slower, disastrous for the rocker, have a windy effect on the paddle and before gliding, especially on medium-length boards and in dirty and rough conditions. Even channels can ruin an entire board made to perfection, such as generating too much speed. The most important thing is definitely the rocker and then (unfortunately) you need to experiment and modify the board countless times to make it work really and well, it would be too easy to get results from the theory, which still remains a good starting point.

[@johndoe-bu7vt]

the main aspect many people dont understand is the possibility putting more rocker on the rail than in the center by choosing right bottom contour combo.

[@crabby_crafter]

I’m a beginner shaper/upcycler. This video is incredible, thanks so much! I experimented with concaves/channel bottoms on my most recent upcycle project. Watching the vid helped my understanding a lot. Adding the Vee in the tail may have been a detail I missed.

[@SurfSimply]

Glad to hear you’re getting something from it Crabby!

[@StraplessReligion]

thanks for sharing

[@SurfSimply]

And thanks for watching Strapless!

[@robarnoldbik8014]

Hi Will, I love this series! Very informative and makes the science easy to understand. One thing I hear shapers talk about and miss in this video is how convex to single to vee creates a flatter center rocker for paddling and increased rail rocker for turns. You don't mention that in this video, do you think this effect is insignificant compared to the ones you mentioned about weight distribution during speeding up on your front foot and turning off your back foot? Or is it all one and the same?

[@SurfSimply]

Hi Rob, I would agree that the different contours alter and impact other areas of the design. For example having a concave towards the front half reduces the entry rocker into a lesser curve. The impact is difficult to measure but will have some effect, and managing the overall design and having a general understanding is enough to use them to their overall advantage. I think having your big picture view and how they influence other aspects is important at avoiding a narrow view of design. Thanks for commenting Rob :)

[@julesmorrissey]

Nice one!

[@SurfSimply]

Thanks Jules!

[@yellowbug772]

Does fin box filler a necessary for a 5 fin boxes board?

[@davidgough3512]

Ahead of their time, the Campbell Bros introduced a reduced center fin with long- based, low-aspect, very canted side fins aligned to "deepen" its double concaves and channel flow rearward when pressured. Check out cutback master Taylor Knox on 5 fin Bonzer in Mexico for proof.

[@adamcoyne9574]

I personally run a single to a triple then back to single into a quad with slight double v off the tal.

[@gusman8925]

Hey, im thinking of making my own shortboard 6'0 34L what do you think the best bottom contour is?

[@jessejamesb]

what would be a good shape for a river wave? perhaps for someone who hasn't surfed but has skate/snowboarded for 20+ years and would like to be very technical very quickly and should be able to adapt to surfing very naturally. I'm talking about the various waves in the st.Laurent river surrounding the island of Montreal.

[@SurfSimply]

That’s a difficult question to answer Jesse. The advice il give you is to ride something with enough volume to help you get accustomed to the slight differences between the sports. How much that volume is though depends on your weight so have a look at our volume to weight chart on the website for more info! surfsimply.com/volume-to-weight-calculator/

[@calvinkwaak6866]

i shape my own boards and these vids are very informative! (it is a single concave all the way through) for speed in smaller waves.

[@SurfSimply]

Glad you’re enjoying them Calvin!

[@geordiethegreat]

I love bottom contures..

[@SurfSimply]

Was there a spelling mistake I missed? Maybe we invented a new contour that no one knows about yet; the Contures! Haha. Thanks for watching G!

[@workingtitle1455]

I have a board which has flat bottom contour from nose to chest area (When your lying face-down and paddeling) and transitions gradually to a double concave in the middle of the board (No single concave before the double) then it transitions to a Vee at the tail end. I do not know what does that do for the board or the rider or how the board will behave in the water. It's a 6'6" x 21 x 2.75 Egg shaped board. I still haven't tried it because we are still on strict quarantine. It's still wrapped in it's packaging. It's my 1st board. Been riding rentals. Would appreciate it if you could tell me what to expect with that kind of bottom contour. Most boards start with a single concave then gradually transition to double but this one, from flat, goes to double right-away. Please do share if you have tried a board like this. Thank you and best of luck to your channel. :)

[@rodndtube]

I rode on one of Gus Acosta's designs while on a trip to the Big Island about 8 years ago. He speaks of the boards as riding on hulls rather than concaves and indeed that board had 3 hulls and another around 5 hulls (counting the rails as 2 hulls). One take is here: mypaipoboards.org/interviews/GusAcosta/WA_3b.jpg

[@workingtitle1455]

@@rodndtube Woah! that's a pretty unique board. How was the experience?

[@brainbeats7555]

Brilliant, quick dirty...

[@SurfSimply]

I’m going to assume that’s a compliment. Thanks Shane!

[@MrHateNN]

Been following this series, I build my own boards, and also try to search for scientific papers to support my designs, while also being a bit mad and innovating, following my personal theories while there is no scientific research for surfboard design. altough I agree to the theories presented on other videos, I don't agree 100% with this one, you are forgetting a lot of factors on this one, adding a concave just modifies a surfboard so much, you are adding more than removing something. You might be adding wetted surface area and rocker. Also doesn't the redirection of water create drag? Therefore the concave by redirecting water creates drag, giving that the fins are giving the surfboard the directional stability/velocity and preventing from it spinning out?

[@MrRickyrecon]

More knowledge bombs!

[@danielbarbieri8199]

👍 now let's talk about entry belly, to flat, then concave ... I'm stuck on mini simmons boards 😁

[@spinafex5729]

Sorry you've got this concave thing wrong. Put a concave bottom board bottom up on the racks. Lay a straight edge on the centre line at the deepest part of the concave so its running nose to tail. You'll see the straight edge teeters on the apex of the rocker. Now rotate the straight edge, you'll see when the edge is at a diagonal to the stringer it'll be mostly in contact with the bottom. This diagonal path is the fastest way for water to move across a rockered concave. This is how concave creates lift when a board is in trim. Water is being displaced rapidly toward the outside rail. The lift on the outside rail in a turn is how concave creates drive through turns when the inside rail is weighted.

[@SurfSimply]

Hi Spinafex, thanks for your engagement. We’re always looking for more information to grow our knowledge from. I’m going to share your comment with our team and we’ll try your example!

[@brainbeats7555]

Thx spin.. interesting insights..

[@spinafex5729]

@@SurfSimply check these guys for under board flow clips www.cambersurfboards.com/built-from-science

[@jordankeeler7297]

interesting. if concave boards direct water out past the rails, what is the goal of a convex board then? arent those supposed to accomplish the same goal if that is true?

[@spinafex5729]

@@jordankeeler7297 I don't know - I think those designs tend to be high volume, large planning area with flat rockers i.e they don't need more lift. The convex is probably about reducing lift and adding control.

[@Manaviecreations]

🤩🤗💛💛💛🤗🤩

[@soulsurfer3102]

Do some specific models please.grovler.all around....

[@turbolad99]

Rode a really weird board the other day that held a line with a lot of speed but just would not turn. I guess it must have had a deep single concave....

[@doug5372]

Could of been too much volume for you ? Maybe a thick tail that you need to really get your foot back and pivot on? Or maybe it’s just a down the line speed board.

[@oneRella]

Surfboards spend most of their time moving on a rail but you mostly talked about how boards act when moving flat like when going straight away from the wave.

[@kickamatus]

i love EVO shape ;)

[@reidjohnson4281]

Bob Mctavish did not invent the Vee Bottom...…..Midget was months if not years in front of the pack. Bob was documented riding them BUT he did not invent the VEE Bottom.

[@rvrrb]

concave is hard to shape, a machine has to do it. flat bottom is fast, round bottom is slow and flat bottom with tail lift works the best. V bottom spins out and slows the board down..

[@GIF_CANCEL]

Everything they say is backwards from what it really is. A convex bottom surfboard does a high line way better than any other board as long as the rail is not too hard or Sharp. When the waves are larger and bumpier the board should have a convex bottom. You get more speed and control. I'm very flat weak waves concave is better but again only to a certain size. There are so many other variables like thickness in foil that have to do with this but I don't understand how big wave Surfers don't have convex bottoms under big wave boards for paddling they should have convex not only in the middle but toward the rail with downrails that are full and you'll have a lot more maneuverability

[@Brlazer]

can you make the video easy to understand I don't think I understood one thing

[@calex901]

The what? The oonderside. Oh, the oonderside... gotcha

[@SurfSimply]

My accent gets almost as much attention as the surf content! Haha. Thanks for watching Calex!

[@charleshawk6668]

Good video