Which is Faster, Smooth Hulls or Rough?

Jan 2, 2017
765
O'Day & Islander 322 & 37 Scottsdale, AZ & Owls Head, ME
Having just had Bella Zee's hull recoated, I noticed that the finish was rough, gritty really. Dennis Connor in his Sail Like a Champion talks about sanding his hulls when he was young to increase boat speed. Before grabbing some sandpaper, I thought, “Hang on. Golf balls have dimples because they travel further than smooth ones; the theory being that dimples hold onto a cushion of stable air around the ball, reducing the friction between the ball and the (relatively speaking) moving air.”
Checking up on this, I found the following article:
https://www.sciencedaily.com/releases/2014/01/140117153637.htm
It seems that roughness does increase speed, but maybe it has to be the right roughness. (?)
 

jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
I think there's something to that. But what are the dimensions of the roughness, as you ask? Maybe one could scale down the effect on golf balls for fluid viscosity and velocity, and end up with the right number for boats?
 
Apr 5, 2009
2,774
Catalina '88 C30 tr/bs Oak Harbor, WA
The golf ball and the boat are traveling through 2 very different matter states. The ball through a gas and the boat through a liquid. While both gases and liquids are classified as fluid, they behave in very different ways. The gas that the ball is passing through is highly compressible whereas the liquid that the boat passes through is non-compressible. I suspect that this difference will negate the dimple issue because the local turbulence that is created is much greater in the liquid than what happens in the gas.
 
Oct 19, 2017
7,732
O'Day 19 Littleton, NH
I remember reading an add for Maharaja water skis that illustrated what you are talking about. It suggested that the back side should be rough while the forward and side surfaces should be smooth. The idea was that the roughness caused the water flow to follow the surface smoothly around to the tail where it then let go without a back eddy of reverse flowing water trying to pull the hull (ski) back into the turbulence. It sounded good, but all they had to demonstrate with was an artist's concept, no actual dye testing of laminar flow.

- Will (Dragonfly)
 
Oct 22, 2014
20,993
CAL 35 Cruiser #21 moored EVERETT WA
There is the theory then their is the pragmatic.
Take it out and sail it in a given wind. See if the speed is reasonable/enjoyable. If not sand it and see what happens. I am going out on a limb here, But I suspect. You will not see any perceived difference.
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
All boat bottoms, whether smooth or rough, will have a layer of turbulent water next to the skin surrounded by laminar flow. The thinner that turbulent layer, the less drag and faster it is. While some people (like DC on Stars&Stripes with the ’sharkskin’) have experimented with surfaces that decrease that turbulent boundary area, general consensus (and tons of testing) say that smoother is faster.
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
if i recall correctly, dennis conner did sand his hulls before races. it was with 2,000 grit wet sand paper. which by most sailboat bottoms means extremely smooth. the theories behind dennis's sanding i agree with. surface tension of water , water being the slipperiest substance in the world, etc .....
to the naked eye of the yachtsman that means smooth as can be


these same theories that dennis used suggest that you should never wax the hull of a boat, because of the surface tensions on between the water and wax. interesting considerations for the speed/performance guys.
 
Sep 25, 2008
7,076
Alden 50 Sarasota, Florida
, “Hang on. Golf balls have dimples because they travel further than smooth ones; the theory being that dimples hold onto a cushion of stable air around the ball, re)
Actually, that isn't the reason for dimples on balls. Golf clubs impart spin on the ball. Certain spin direction promotes longer flight as well as direction and the dimples enhance that effect which is why good golfers can "flight" the ball controlling height, direction and length.

There is no relationship to hull bottoms. Smooth is good; rough is bad.
 
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Apr 16, 2017
841
Federation NCC-1701 Riverside
Theres some interesting research on this. It goes like this.
1) chaos theory: every change results in more change later in time. Change = drag

2) The shorter,the less duration the change of molecules, the less chaos later.

The articles ive seen suggest the first 1/3 to first half of a hull should be polished. After that, there is so much chaos, that the flow wont stick no matter what.

Appendages are short duration surfaces so every effort should be made to keep them super polished since it is possible to keep laminar flow over the entire surface.

The leading edge of everything is super critical. Clean up those bows from beach landings and trailer loading, rudders and centerboards and props.

The slower you go the more of the hull that should be polished.

400 grit is good enough for normal hulls, no polish is good enough for appendages.

Rc boats are so small that the entire hull should by hyper polished.

Source:
Paul Grimes
A smooth bottom is a fast bottom
07/11/2012
https://www.gp14.org/a-smooth-bottom-is-a-fast-bottom/
 
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Apr 16, 2017
841
Federation NCC-1701 Riverside
I wonder outloud if this also applies to sails. The leading edge of some sails look terrible with ragged uv shields, masts with gadgets and lines all along the length, and sails with reefing ties and telltales dangling.
 
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Apr 16, 2017
841
Federation NCC-1701 Riverside
Actually, that isn't the reason for dimples on balls. Golf clubs impart spin on the ball. Certain spin direction promotes longer flight as well as direction and the dimples enhance that effect which is why good golfers can "flight" the ball controlling height, direction and length.

There is no relationship to hull bottoms. Smooth is good; rough is bad.
This is where riblets get incorporated into the thread. They worked great in standing water, but not so great in the wild.
 
Jul 29, 2017
169
Catalina 380 Los Angeles
With aircraft there have been experiments with adding a layer of warm boundary air to reduce turbulence on top of the wing to *speed up* the air flow. I do wonder if a layer of bubbling air injected across the surface of the hull would increase speed but I doubt it would make much of an effect at the relatively slow speed of a boat under sail. Interesting that with all the "wings" that are applied to cars..it is a proven fact that until the car is traveling at a high rate of speed (120mph+) that spoiler on the back of the trunk has a null effect...just sayin:hijack:
 
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Feb 14, 2014
7,399
Hunter 430 Waveland, MS
The Science is clear, tested and verified.
Your least drag [friction energy loss] is obtained by Streamline flow in the Laminar Flow region.;)

Say What?:confused::yikes::confused:

Simply put for a Sail boat hull designer...

You want a Smooth hull and Slow water flow along the hull, when you are in your ideal Heel for your boat.

The minute you introduce Hull Turbulence you are increasing drag, as long is it water to hull and not air to hull.

So...
Keep your hull smooth, which is most likely the way your boat designer intended.
Jim...

PS: Here is the Science..
http://www.solitaryroad.com/c1021.html
 
Jan 2, 2017
765
O'Day & Islander 322 & 37 Scottsdale, AZ & Owls Head, ME
This is where riblets get incorporated into the thread. They worked great in standing water, but not so great in the wild.
Well, that’s what I’ve got - standing water. (Took me awhile to figure out you weren’t talking about baby barbecue ribs.)
 
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Nov 9, 2012
2,500
Oday 192 Lake Nockamixon
It's better to have a very smooth hull, faired to its lines. There are companies that specialize in fairing racing hulls to their lines. I think as we get to the point where mold plugs are CAD designed and CNC machined, the resulting fiberglass hulls are more true than hand building wood plugs from plans.

Interesting in planing power boats with stepped hulls, like old Gar Wood racers. Nowadays, even larger pleasure oriented power boats have a step or two, and a ridge that extends to the chine of the hull. Really, it kinda looks like channels cut in to a tire tread, but with tires the idea is to evacuate water from between the tire and the road. The purpose of the hull features is to suck air into the boundary between the hull and water, allowing the boat to plane with less drag. But, of course, that's a fast planing powerboat.
 
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jviss

.
Feb 5, 2004
6,745
Tartan 3800 20 Westport, MA
Actually, that isn't the reason for dimples on balls. Golf clubs impart spin on the ball. Certain spin direction promotes longer flight as well as direction and the dimples enhance that effect which is why good golfers can "flight" the ball controlling height, direction and length.

There is no relationship to hull bottoms. Smooth is good; rough is bad.
"Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball's surface. This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake. A dimpled ball thus has about half the drag of a smooth ball."
https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/
 
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Apr 16, 2017
841
Federation NCC-1701 Riverside
It's better to have a very smooth hull, faired to its lines. There are companies that specialize in fairing racing hulls to their lines. I think as we get to the point where mold plugs are CAD designed and CNC machined, the resulting fiberglass hulls are more true than hand building wood plugs from plans.

Interesting in planing power boats with stepped hulls, like old Gar Wood racers. Nowadays, even larger pleasure oriented power boats have a step or two, and a ridge that extends to the chine of the hull. Really, it kinda looks like channels cut in to a tire tread, but with tires the idea is to evacuate water from between the tire and the road. The purpose of the hull features is to suck air into the boundary between the hull and water, allowing the boat to plane with less drag. But, of course, that's a fast planing powerboat.
Might not even need to plane.
https://www.marineinsight.com/green-shipping/how-air-lubrication-system-for-ships-work/

Article on air lubrication. Almost like reducing wetted surface. Works best on flat bottoms. Im noticing they focus on the area that would already be turbulent. The bubbles probably act like dynamic polish.