Channel and Suction

Please correct me if i am barking up the wrong tree or if this is common knowledge but...

If have read that the use of vee in a surfboard tail can cause suction at the rear of the board (not optimal for speed and lift) and therefore the back few inches of your board should be flat to minimise suction. This seems to fall in line with the Simmons design of the the Concave between the back fins to reduce suction and provide further lift. I understand that there are performance draw backs associated with having a concave between the fins and off the tail so is there any way of reducing this suction (and increasing lift) without impacting on rail to rail performance.

I know channels are used to help re-direct the water under the tail and generate further lift but does this reduce suction as well? Have channels been designed specifically to deal with suction? Would these channels start infront of the fins, in the middle or behind the fins to optimise lift / performance? Which leads into my last question, will channels starting behind the front fins provide any benefit or are full channels up to the middle the best way to go?

I am thinking of having a crack at putting some channels on my next board after christmas and i was curious about the reasons for putting them in. Any feedback or pictures of your work would be appreciated.

Also BB if your still there, can you explain more about how channels make a board stiffer?

Compound curves, like the corrugation in the middle of "card board," make things stiff... Hence the deck side rail channels for stiffness/strength... 

Sweet...Thanks for the info, and yeah that makes sense

Listen to Bill Barnfield about advice. I don't know much about channels, but was making a statment based on other threads and info on the subject. I believe that rails still have to do alot with suction, so you could be having problems with suction because of the rail shape, and not necessarily bottom shape.

Also BB if your still there, can you explain more about how channels make a board stiffer?

Most are unaware of, underestimate, or simply ignore the profound effect channels have on form stiffness.  Just about every ride feeling, commonly suggested to be caused by channels, can also be attributed to the board becoming, in the flexural sense, stiffer.

Regarding your specific question.  If the preliminary conclusion that, vee causes “suction” in the tail, is not accurate.  Then the rest of your design thinking will be skewed if not totally inaccurate. 

I don’t recall the discussions that have led you or others to conclude that vee causes “suction”.  But be very cautious concluding any particular thing is fact, just because a chunk of people on Swaylocks seem to have agreed it is so.

In the same way, your conclusion that concave between the fins or off the tail, has substantial drawbacks may also be inaccurate.  If so, then you will be inclined to head down a path of trying to “solve” all these problems with new designs, when the problems don’t exist.

Rather the problems are often just inflated speculations on a forum, where each contributor wants to participate and have something more significant to say then the previous contributor.  This whole process tends to over exaggerate things and drive them into a rarified significance well beyond appropriate reality.   But hey… is is a lot of fun and is the whole reason these tech forums exist.  Everyone gets to throw in their 2 cents and a community develops from it.  Just be cautious trying to sort your way through all fog, hoping to dock your boat safely next to the fact pier.

Speaking of boats and forum communities, try searching the boating forums for information on which is better… joining electrical wiring on boats via soldering or crimping.  Amazing!

Rails have alot to do with it. Down rails with a super hard edge will cause water to release whereas a round 50/50 will cause alot of water to wrap and suck over the rail.

I don't know how much suction vee causes, but if you're having problems, concave set within the vee may help.

As for channels, I'd think that they would prevent suction, because they direct and keep water underneath the board.

[quote="$1"]   Rather the problems are often just inflated speculations on a forum, where each contributor wants to participate and have something more significant to say then the previous contributor.  This whole process tends to over exaggerate things and drive them into a rarified significance well beyond appropriate reality.   [/quote]

A wise caveat to keep in mind as we peruse the words of the pundits!  And not only that, but my new tagline too! (had to condense it slightly, oh well)

Aloha Yorky

Since the overriding factor is the stiffening of the form, everything else has to be set up to accommodate that.  Since the board will be both stiffer structurally and in the way is surfs you have to add or subtract other features to compensate.  So what I start with bottom wise would be relative to the rider and the waves he will ride.  

The shaped in rocker, would typically be increased a bit due to the stiffer form and less flex.  And also, because the rocker (depending on channel types) will be straighter in the bottom of the channels.  Unless they are the type that carry out the tail and get deeper as they go which would increase rocker in the bottom of the channels.

Fins would generally be smaller.

Side to side bottom contours would vary pretty much like they would in any custom board adapted to the riders and waves.  For example… if it is a narrow board, then it can handle flatter or more concave contours.  If it is wider, then maybe a more vee type channel.  Etc.

I wish I had a more mystical “legendary shaper’s secret” to share with you.  But like most all surfboard design and shaping.  It is just a bunch of delicate compromises all executed with hand skills capable of mixing them together properly in the foam.  The surf media and and guys who want to build their reputations on “inventing” stuff, like to make it appear that there are these singular specific design revelations that change everything.  But mostly it is just a bunch of slow plodding along, mixing and matching things to create the right balance of features in a board that puts the right combo under the feet of the right guy, at just the right moment in his evolution as surfer, when the window of opportunity within the surf scene is ready for it.

TaylorO explained it pretty well.  

Just imagine a simple sheet of paper and how much more rigid it will get if you crease it once.  Then multiple creases like a 6 channel bottom.  Now imagine you do that with not wimpy paper, but do it with 4 oz or 6 oz fiberglass!  That corrugated effect is extremely powerful even when in just a freeform detached sheet.  Back it up with some foam and it will be even stiffer!

Remember, we are talking FORM STIFFNESS.  That is… rigidity from the shape of the form.  Not stiffness in the way the board rides.  Where a lot of confusion comes is that a stiffer form (board) will also likely surf “stiffer” in ride also.  The board that is stiff and doesn’t flex may not be able to bend easily enough to add rocker when needed and it will then be referred to as a “stiff” riding board.  It is very easy for the terms to get all confused and overlapped when discussing them.

hey thanks for the response. I don't want to be that guy who tries to "one up" the next guy. I was simply trying to relay info that I learned from geniuses like you and other pros that help people out on this site. Again, thanks for the explanation, it's always appreciated.

Thanks Huckleberry.  You make me blush!  :-)

Hey Bill,

i have a question for you.

In a shortboard, What bottom design do you put in them before you add your channels.

like - flat into a vee style (with the vee in the channels), or same kind of style, concave into flat (with the flat in the channels)

or something different altogether…just curious to know from an expert who went through the whole channel era.




Thanks for the feedback,

It seems that i may have jumped the gun based on a couple of articles I have read and thanks for pulling me up on a couple of my conclusions. This is why I posted the topic and why i value the forum of swaylocks so much; it helps question thoeries of the less experienced and stops them going down the wrong rabbit hole.

My thinking was drawn to to try and solve problems that it turns out may be unsubstantiated by the more experienced Swaylocks community; I also failed to identify the real issues associated with implementing channels on a board. Thanks to Bill B for putting me straight on a few things and also posting some great advice on altering the characteristics of the board to manage the increased form stiffness; im sure that will help quite a few back yard hacks like myself in the future.

In an effort to gleam some further info whilst there are still a few on board can i pose the following questions:

- How does channel length affect the perfomance of channel bottoms? I see most seem to start just behind mid way.

- What effect does narrowing or widdening the the channel further towards the tail have? I would imagine narrowing towards the tail would cause the water to bunch up and lift the tail? Would this be a bad or a good thing?


As mentioned before, there are 2 things to consider.  The change in form stiffness and the effect on water flow.  But since there are so many ways channels can vary in… amount, length, depth, angle, placement in board, etc.  Many details need to be known to predict or estimate effects.  But basically, Longer = More.  That is, whatever the effect was from a short channel, will be magnified and duplicated as it extends further.  For example, belly channels that were popular once, will stiffen the board slightly in the central area where they sit.  If they are extended out the tail also, then stiffening will exist right out through their length.  In the water flow department, the water will be encouraged to flow more toward the tail and with less “leaking” so to speak out toward the rails.  As you can imagine the amount, length, depth and angle of the channels will have more or less effect on the water and form stiffness, depending on each of these factors.  It is all about finding a balance.

The “speculations” in this area are among the most “contested” as to how these changes work or effect the “ride” of a board.  Not sure I want to jump into that fight.  But… one can certainly “feel” differences from the changes you mention.  Here area some thoughts.  Considering water flow only and not form stiffness, you can “lift” the board somewhat by constricting the water and directing its flow.  But it is a delicate balance between gaining some advantage from this and creating drag or quirky handling issues.  

The often mentioned “Venturi effect” where the board is supposedly speeded up because the flow is forced into a smaller space, doesn’t really exist on surfboards.  Constricting the same volume of flow into a smaller space will increase the flow’s velocity as long as there is enough pressure behind it or suction in front of it to maintain the same volume of flow.  And there is nowhere else for the flow to go.

But this can’t be done on a surfboard bottom and won’t speed up the board because of it.  Mostly because you can’t really create the smaller space needed as the water can too easily leak out of that space.  And any effect gained would be minimal and offset by drag and quirky handling issues.  Consider this, just because a river’s flow will speed up as it passes through a narrow gorge, doesn’t mean the river banks or the earth speeds up going the opposite direction.  And water sprayed into and through a freely hanging pipe, that narrows in its center, will not cause the pipe to speed up in the opposite direction of the flow.  In fact, the drag from the constriction will cause the pipe to move faster and more forcefully in the direction of the flow, not opposite it.  So forget the Venturi arguments and speculations.

Lift is a more viable effect. But It is all about balancing the effects.  If the channels narrow, but the rocker increases.  One will balance out the other.  If the channels widen and the rocker increases, there will be a multiplying effect as both are “release” factors in the control of the water.  If the channels narrow and the rocker decreases, there will be a multiplying effect and more lift will exist.  How that lift or any of these results, is interpreted by the rider is where all the contention comes into the discussions and speculations run wild.

There are often easy ways to confirm effects and their “feel”, but few are willing to take the time to do the testing so that they actually KNOW rather then speculate.  Channels are one of the easiest design features to test.  They can pretty easily be installed in existing boards that don’t have channels.  You will now know the difference between the with and without.  

If a board already has channels, you can simply fill them in with wax.  Fill some or all of them.  Fill them at different depths or lengths.  You will instantly know how they effect the “feel” of the board.  This test won’t change the flex, but the previous one will and you will then know 99% more then most surfers and probably 90% more then most shapers regarding the general effects of channels.  

I have been challenging people to test their theories in these way for decades.  Yet I don’t think I know even one person, other then myself, who has actually went to the trouble.  It is my conclusion that most people don’t really want to KNOW things for sure.  They don’t want the responsibility that comes with absolute knowledge.  It would make the job task all fussy, detailed and hard.  For must, surfboard making is a casual creative process, not a science.  It is way more fun when it is not constricted by absolutes and everyone can have their own opinions, with none being more valid then any others.



The humble truth and, what’s more, coming from someone who could very well be far from humble. Notwithstanding the fact that he could very well keep his vast knowledge for himself instead of sharing it here. Thanks for all that, Bill.

yeah, (wow) i agree with balsa that was an awesome response, just what i was looking for. I think its going to take a few days to digest it all. Thanks to all for taking the time out to respond, my mind has been expanded.


This is the best explanation of no venturi in concave of surfboards i ever read. I try to say that one time but i wasn’t very lucky !

you are a credit to this community BB... thanks for sharing, again...

your posts are always so eye opening

What they said Bill - 

That writing is a work of art and science in balance it self…

They don’t want the responsibility that comes with absolute knowledge.

Thanks for the nice compliments guys.

There are over 14,000 members on Swaylocks.  Presumably must make boards or want to make boards.  And one would presume they want to make good boards… or maybe even the best boards possible??

So how many of these shaper/designer hopefuls, will do this simple test to learn the effects of channels?  Let alone any other one tests, like using ProBox or 4way fin Systems to learn what different fin positions will do.

If this is the reality, then how good are the boards being made.  Are they well researched?  Are the alternatives well researched.  Do these shaper/designer hopefuls become highly motivated to do these tests later when they have tons of orders and are busy running their businesses and have the fame they desired?  NOT!

It is way less complicated to just copy what others do or follow the trends, then to do independent research.