Dimpled speed

[John Vigor]

It has worried me for a long time now that I'm wasting my time and energy trying to make my bottom paint nice and smooth. I have always filled and sanded, and sanded again, because all the experts insist you have to have a smooth bottom to sail fast.

And every time I'm in the middle of sanding I think of the golf ball.

Many years ago, professional golfers discovered that a golf ball went farther and faster if the exterior skin were made dimpled and rough. Rough-skinned golf balls were found be amazingly superior to smooth-skinned ones.

I don't know the exact theory--something about the layer of air right next to the skin being dragged along more beneficially, I expect--but I have always wondered if this would apply to boats, too.

I have certainly been passed by sister boats whose bottoms, I know for sure, were as rough as a goat's backside.

Both air and water are fluids, so why shouldn't dimples in your bottom paint be just as efficient as dimples on a golf ball?

I'm not thinking of the drag caused by barnacles, visiting crabs, and various species of slimy vegetation, of course. Those must definitely be removed. But a purposely rough paint job, something like 80- or 100-grit sandpaper--wouldn't that work? Are we wasting our efforts, trying to make our bottoms smooth?

Fore!

John Vigor

[David van den Burgh]

John,

By George, I think I've got it!

Your upcoming title must be: Nautical Myths and the Sailors Who Will Die Defending Them.

Yes?

[Stan W.]

The fluid dynamics of a golfball and a sailboat hull are fundamentally different, but a shark and its rough skin presents a very interesting question.

[bill2]

Well I certainly admire your propensity for intriguing questions - at least to me a non-engineer.

As to my own ability to reply in a knowledgeable fashion - time will tell ( as Dad used to say " Better to be thought a fool than to open one's mouth and remove all doubt "

So I'd think that a rational person would liken a smooth hull to a smooth ball that in flight behaves much like a locked prop ( ) - free to skid around through the air . Whereas a dimpled surface such as a golf ball would impart a certain spinning motion - think like a free wheeling prop ( ) - from air catching the dimple that would serve to stabilize the flight of the golf ball like a gyroscope effect.

So now you know that I know neither golf nor aero/fluid dynamics. And I won't even venture to start on sharkskin.

However this could become much fun . . .

[Oswego John]

Can't remember where or when I either read or heard someone mention it (it figures) but the idea of the dimpled surface supposedly had something to do with true, straight flight.

Supposedly, the dimples lessened the degree of hooking or slicing when the shot is shanked.

I utterly disagree with this theory. The dimples don't do a thing for me at all, or for the amount of golf balls I lose each year.

Think spring,
O J

[Parfait's Provider]

John,

Thanks for the clue about wicking the standing water away from the port lights. Bought some lamp wick, put some 4200 on one side and taped it in place until the 4200 cured. Condensation now wicks away before I get up in the morning.

As to the other question, I suppose that if battleships have rivets standing proud, then maybe we should add some to the CD. But maybe it would be easier to use some of that slippery paint, or maybe an additive such as negative ion emitting compound powder, such as monazite powder, and tourmaline powder.

What do you think?

[Ed Haley]

I always thought that the dimples in golf balls provided an increase in lift rather than an increase in speed.

Who knows for sure on a boat. Maybe the dimples would provide an increase of length and therefore speed. Eddies behind ridges instead of attached laminar flow close to the surface. Ripples anyone? Waves?

We know waxing a hull actually slows down a boat so John may be right. Leave your hull rough.

[D Rush]

a Golf ball is a sphere and hopefully your sailboat is not a sphere.

There are two types of drag experienced by a Golf ball or sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a golf ball. The majority of the drag comes from the separation of the flow behind the ball, I believe it's known as pressure drag due to separation.
I won't go into greater detail (Reynolds numbers & Lift generated by a spinning sphere) thus avoiding display of my truly basic & properly flawed understand of the subject.
I would think since our boats are not spheres the theory of dimples on a surface does not apply.

[tartansailor]

D Rush hit the nail on the head.

Dick

[Oswego John]

The majority of the drag comes from the separation of the flow behind the ball, I believe it's known as pressure drag due to separation.
I won't go into greater detail (Reynolds numbers & Lift generated by a spinning sphere)

I would be remiss if I didn't touch on laminar surface flow.

I KNEW THAT.
O J

[John Vigor]

Here is a good explanation of why dimples help a golf ball go farther by reducing drag.

http://wings.avkids.com/Book/Sports/ins ... lf-01.html

I can see no reason why this wouldn't apply to boats, too, as it does to sharks.

Furthermore, you'll note that this very erudite explanation of the effect of dimples categorically states that a golf ball also gains lift from the dimples.

So now, if we roughened up our keels a bit, wouldn't we go to windward better? Lift is good, isn't it? Specially as we're already going faster, like the shark.

Incidentally, I rather like the explanation above, that a rough bottom coat adds to the waterline length and therefore makes possible a higher hull speed. That sounds very reasonable, doesn't it?

Cheers,

John Vigor

[Warren Kaplan]

It has worried me for a long time now that I'm wasting my time and energy trying to make my bottom paint nice and smooth. I have always filled and sanded, and sanded again, because all the experts insist you have to have a smooth bottom to sail fast.

And every time I'm in the middle of sanding I think of the golf ball.

John Vigor

BLESS YOU JOHN VIGOR!! BLESS YOU!!!!

May the gods of fluid dynamics prove you correct. The bottom of my CD27 is hardly as smooth as the proverbial baby's bottom, and the thought of what I'd have to do to make it that way puts me right off my feed!!

I have to say that many of my sailing compatriots have told me that they are quite impressed by the turn of speed I manage to coax out of Sine Qua Non. Enough to give a cap'n a swelled head!! Alas, little do these chaps know that my bottom paint is pocked with imperfection. And so it is destined to stay lest a fortuitous wager on the New York State lottery provides the necessary scratch to finance the removal of years of bottom paint and replace it all anew! 'T is a consummation devoutly to be wished!

[Neil Gordon]

And every time I'm in the middle of sanding I think of the golf ball.

It only works if you can get the barnacles to stick to the bottom in a perfectly aligned dimple pattern.

[Warren S]

aside from flow separation at the aft end of the body (one reason for turbulent flow) there has been a lot done to maintain laminar flow along the sides by controlling the "boundary layer" on the surfaces. Here is an example of *what I suspect* is a design feature to control flow at the intake of a jet engine: http://upload.wikimedia.org/wikipedia/c ... CAM)_3.jpg

you can barely see 'em, but there are groups of tiny holes right at the intake.

Oh wait, 1000's of holes in the hull? Naaawwww.

[Parfait's Provider]

How about if we sell advertising space on our hulls? We could get someone to pay us to wrap the hull with an advert for (insert your not-so-fave here) and with all those little holes, we would fly right over that bow wave. And if it were composed of the right stuff the barnacles probably would hate it. Maybe a little jalepeno in the ink? We could have opposing views on each side; who would know? Rent per sq ft would be less the lower it was. I think I have a business plan.... Well, as good as most.