the golfball effect

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J Drew
Professional
Posts: 104
Joined: September 9th, 2013, 12:39 am
Boat Make/Model: SC 20
Location: n. florida

the golfball effect

Post by J Drew »

I was looking at a golf ball the other day, wondering about all those dimples. I'd love to see the machine that makes them.
Then I started thinking about the use of the dimples. It has something to do with creating turbulence to reduce drag. I guess it works, you don't see any smooth golf balls?

So then I'm looking at my hulls. When I painted them, I went all the way around. Why not, it's a slick surface, it should slide real nice. I wouldn't dare slide it up on a sandy beach. Yeah, I know, beach cat, but those scuffs take forever to repair.

Well, maybe that's why they put bottom paint on them?
Now, I have a big boat, and I understand about bottom paint for protection, so I always figured they put that stuff on because they're to lazy to take it out of the water over night? But bottom paint isn't smooth, it's kinda dimply.

Anybody play golf?
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: the golfball effect

Post by Bill Roberts »

The golf ball moving through the air presents a unique problem. The ball is going 100 mph plus and the curvature around the ball is quick. The air cannot stay attacked to this short radius object at the speeds it normally travels. The dimples are on a golf ball to keep the airflow attached to the ball further around the surface of the ball than the air would have remained attached if the ball were smooth. Therefore the effective aerodynamic diameter of the ball is smaller with a dimpled surface. The drag on the ball is related to its aerodynamic diameter. The smallest aerodynamic diameter of a golf ball is with dimples, therefore minimum drag and greatest distance driven. Dimples only help reduce drag when you have a special problem, usually a separated flow high drag problem and sometimes dimples can help. We do not have a sharp turn separated flow problem on our hulls or centerboards and rudders.
SJBrit
Novice
Posts: 9
Joined: March 4th, 2016, 11:59 am
Boat Make/Model: SuperCat 17

Re: the golfball effect

Post by SJBrit »

Generally speaking a rough surface helps break surface tension which causes drag, so a textured surface is faster. You can feel this very strongly if you are a slalom skier: a slightly rippled water surface is much better for acceleration than an absolute flat calm which feels like it's sucking at your ski (because it is!) Slalom skis have a textured bottom to help defeat this effect.

However, I'm with Bill on this: I don't think sailing speeds are fast enough to notice much of a difference. You can read all kinds of online discussions about the pros and cons of textured hulls, but I think any effect would be totally swamped by other much bigger speed factors. A lot of people will drag their jet ski hulls across the sand to simultaneously true it and texture the bottom, but again, they are going MUCH faster than we are.
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