# Dimples on a Surfboard

There has been some discussion about whether dimples have an effect on a surfboard.  So I posed the question to mybrother who has his Masters from MIT in Mechanical Engineering.  He posed the idea to his former professor.  Here is the reply from a MIT professor,

"Dimples on a surf board is a non starter in my opinion.

For a golf ball, dimples reduce the drag by a massive 50%.  This is because the drag coefficient is reduced by 50%.  This is because for a smooth sphere, the separation point for the streamlines is just aft of the midpoint of the ball and as such, there is a massive low pressure aft side.  Dimples on a golf ball move the separation point significantly backwards which causes the surface area exposed to the low pressure air to be dramatically reduced.   This separation of the dividing streamline is associated with flow past bluff bodies.

A surfboard in not a bluff body.  It is a streamlines body and as such, there is not a dividing streamline and thus not a low pressure side.  For a streamlined shape like a surf board, the drag coefficient is very low to start with and there is very little form drag, which is what happens on bluff bodies.  For a surfboard, 90% of the very little drag there is is due to surface friction.  So to reduce drag, you reduce fouling and surface area.

You could play with changing the surface drag by using very small dimples or a shark skin shape, but you are really only going to see only a very small incremental change at the Reynolds number a surfboard operates.

See any undergraduates fluid mechanics text for discussion of bluff vs streamlined shapes and impact on drag.  For dimpled vs smooth spheres, see http://www.efluids.com/efluids/bicycle/bicycle_pages/sportsballs.jsp

I am sure you could make some hay from a marketing perspective because the general public has been sensitized to the importance of dimples on golf balls, but from a purely technical perspective, I don’t the incremental advantage would be worth the effort".

My position has always been, that dimples on a surfboard bottom was pure BS, and in fact detrimental.      Good on you for posting this.     I’m sure this will upset a few ‘‘true believers’’, but another glass of Kool-Ade, will calm them right down.

Bill

Great explanation, and thanks for the link.

maybe I missed it…

I haven’t seen anyone seriously trying to market this in surfboards

Coil Kick.

Josh Kerr Fins.

I picked up a used 8’0 Willis bros semi for use at Sunset during one winter visit in the mid 90’s,  it had their golf ball bottom.  Rode it a half dozen times then traded it towards a used 8’0 Eric Arakawa.  The EA was both faster and a much better overall board…

regardless of the implied physics, surfboards and golf balls just don’t have that much in common…

I would say that the dimples on the coils do not affect the board in a negative way, and may enhance the overall performance, but most likely very slightly. The professor states that. I have been riding boards with sanded bottoms for several decades. I’ve made several boards with bottoms that have texture and I have not seen adverse affects. My last 4 or 5 boards all have a proprietary rough surface on the bottom, feels like sandpaper or shark skin. I keep the rails smooth to keep my arms and legs from getting scratched.

Why do feel the need to attack Coil so much? The process they use to make boards allows them to create a textured surface, so there’s no extra effort. It saves them work to leave the texture there. Libtechs have a textured surface on the bottoms that mimic the golf ball surface, check their website. Coils are different.

What the Willis Brothers were doing is another different thing. The dimples were large, like the diameter of a tennis ball. I think Eric had been shaping boards for Hawaii much longer than the Willis Brothers. Back in the '90s, my pro friends used to swear they would only ride his boards.

Hi Sharkcountry,

It is a misunderstanding to think I am attacking Coil.  I wish them well and think they are far better than an board made with fiberglass in a hand lamination.  But it is curious to me how some hold them Holy, where they are above analysis or critical thinking.  Critical analysis says that any surfboard with a textured bottom will go slower than if it was smooth.  From aeronautics to boat hulls, smoother is faster.

Its ok to review a Ford in a auto forum, isn’t it?

Mark, don’t you ever get tired of casting pearls?

Bill

Hi Bill,

Nah, its good.  I get enough positive responses to keep me going.  Most of them are on p.m. to avoid the drama I seem to cause.

Sometimes I feel like I have aspergers.  Really I do.  I have no idea at all what pisses some of you off.  Some guys, (not you Sharkcountry) just go nuts if you critically discuss any belief they hold dear.  I mean has anybody seen the Josh Kerr thread over in the industry section?  Deadshaper posted about some fins with the texture on them.  And they weren’t even Dead’s fins.  I posted that the texture on those fins was just marketing, for the same reason as I did on this thread.

The following post from Dead was the most hatefilled thing I have ever seen on this forum.  Followed by one from Huie.  Dead then edited his to make it seem polite.  Huie deleted his altogether.  Dead still has some really ugly stuff up on that thread.  A p.m. he sent me was even worse.

I have absolutely no idea why some completely come unglued.

But again, I’m way ahead in the positive stuff on the p.m. side, so its good.

On a related note, Has anyone noticed how many of the old posters have up and left?  Read through the archives sometime.  Most of what I post was discussed here ten years ago.  It does get to the point where it isn’t worth it anymore.

you have a way of comming across as a total asshole…     really…  regardless of who you are…slow down…step back…most people I know don’t like you…

Ray…

Coil dimples are more the result of building process than everything else. They use an infusion filet design to stay laminate on surface. No more finish. Harder to have good results each times infusing without some sort of drain filet.

Ray, you know me, at least on a Hi how you doing basis.      I, for one, think Mark has his head screwed on better than 90% of the posters on the forum.    He is one of only a few here on Sway’s, that has impressed me with his depth and breadth of knowledge, of the board building process.

Bill Thrailkill

you may not be an asshole, you may be brilliant, you are probably a very nice guy in person, but…you write like an asshole so you come across as an asshole. HTH

Wow,

It went from “Great explanation, and thanks for the link.” and “Good on you for posting” to you are an asshole and nobody likes you pretty quick.

I’m always open to constructive criticism, but this one really has me spinning.

I have been using the textured finish on my bagged boards since late 2013, and not have noticed any speed issue from using the mesh. I use it to add a tough outer layer that wont crack the underlying glass. and it also hides troubles relayed to vacuum bagging. I no longer have hotcoat problems or have to spend hours sanding multiple hotcoats.

Yes indeed.  Have seen them over the years being used by a couple of shapers.  Nobody really bought it. (ie drank the Kool-Aid as Bill says).

lakewakes,

those things are sweet. Love the gloss and textured combo.

Is the texture from a breather cloth? Or maybe a membrane or core material?

Angus-

Mark you are not the Asshole.  I am and damned proud of it.  Lowel

same material as Coil