Dimpled hull?

[BHChieftain]

Hi,
Has anyone tried dimpling the bottom of the hull or sponsons to further reduce drag (like a golf ball)? Would that even be a legal modification?

Chief

[Fluid]

It will increase drag and slow the boat down. Dimples work because they modify the airflow over all the ball. Briefly:

Dimples on a golf ball produce a boundary layer that clings to the ball's surface. The smoothly flowing air outside this boundary layer follows the surface of the ball farther around the back side of the ball, reducing drag by decreasing the size of the wake.

We create a similar effect on sponson bottoms by sanding the surface to eliminate the shine. This creates a boundary layer of water which reduces overall drag.

Dimpling the top of a boat won't work either - when was the last time you saw a fast dimpled airplane?




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[T.S.Davis]

Jay, did you see that episode of Mythbusters where they dimpled a whole car? I actually made for better fuel consumption. Crazy. I thought it might translate to our stuff but maybe not. Our boats aren't exactly an aero disaster like cars tend to be.

[wparsons]

The biggest decrease in drag is because the air releases cleaner and doesn't create as much of a low pressure area behind the object. It doesn't really effect how slippery the object is right were the dimples are.

Dimpling the top of a boat might work, but it depends on the shape at the transom. I can't see getting measurable differences from it though, because there is much more drag from the water than air. The rolling resistance of a car is VERY minimal compared to air resistance at speed.

Another side effect is that the dimples make the air/water stick to the surface better. This is great if the whole object is in the liquid/gas (picture a sub with a square transom, dimples would help a lot there). Boats are fast because they have very little contact with the water, increasing that contact would just slow it down further. It might help a displacement hull move a bit quicker too, tough to say for sure though, the water might be too thick to behave like air.

You don't see dimpled airplanes because their shape is finely tuned in a wind tunnel to not create the trailing low pressure zone at all.

If you can make everything a tear drop shape, dimpling won't help you at all, but since a ball or car can't be that shape the dimples help.

Fluid, I think sanding the ride surfaces is slightly different. That reduces the stiction of the water to the surface, but doesn't disrupt flow like the dimples do.

You can see the low pressure zone (cause of drag) here:

http://www.youtube.com/watch?v=vQHXIHpvcvU

[graill]

Jay, did you see that episode of Mythbusters where they dimpled a whole car? I actually made for better fuel consumption. Crazy. I thought it might translate to our stuff but maybe not. Our boats aren't exactly an aero disaster like cars tend to be.

I saw that episode, was blast to watch.

[Fluid]

Fluid, I think sanding the ride surfaces is slightly different. That reduces the stiction of the water to the surface, but doesn't disrupt flow like the dimples do.

Actually it does disrupt the water flow by creating a boundary layer of water which does stick to the sponson. The rest of the water slides past this layer with less total resistance.

I didn't see the MythBusters episode in question, but if it is anything like many of their "scientific proofs" I'll pass. They are 90% Hollywood and 10% science most of the time. I enjoy watching the show for entertainment, but so many of their "scientific proofs" are nothing of the sort.....


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[PBO]

Actually it does disrupt the water flow by creating a boundary layer of water which does stick to the sponson. The rest of the water slides past this layer with less total resistance.

Yep, this is correct

[wparsons]

^^ That might be correct, but it's still not the same way that dimples work. Dimples work by creating turbulent air flow that sticks to the surface longer.

Sanding the surface would work on any shape, no matter how hydrodynamic it is or isn't. This is because it reduces friction from surface tension. Dimples wouldn't work on very clean aerodynamic shapes (like a low drag airfoil, etc), they only work on shapes that create relatively large low pressure (high drag) areas behind them.

[keithbradley]

This is in fact done...they are called phazers and they are meant to create a "bubble" between the water and the bottom of your craft. They are generally used on flat surfaces, such as wakeboards, kneeboards, ect. and many say they dont really do much, others swear by them. They also use chevrons to do the same thing. I cant see it having much of an effect on a model boat hull though.

[PBO]

^^ That might be correct, but it's still not the same way that dimples work. Dimples work by creating turbulent air flow that sticks to the surface longer.

Sanding the surface would work on any shape, no matter how hydrodynamic it is or isn't. This is because it reduces friction from surface tension. Dimples wouldn't work on very clean aerodynamic shapes (like a low drag airfoil, etc), they only work on shapes that create relatively large low pressure (high drag) areas behind them.

No doubt, the golf ball theory is irrelevant when applied to FE boats

It has been tried in 1:1 yachts before & was proven very inefficient because there are three components of (hull) resistance that boats have to deal with (in calm seas):

1) Form drag - due to the shape of the boat, and the pressure field around the hull. The more streamlined the body, the less the form drag.
2) Skin friction drag - due to the friction between the hull and the water next to the hull
3) Wave making drag - due to the fact that boat is operating at the air-water interface.

At very low speeds, item 2 dominates. As the speed increases, 1 and 3 become more important. Eventually, 3 takes over (unless you have a planing hull)

A golf ball (and a car), only see items 1 and 2. For a golf ball (and a blunt car), form drag is the dominant component.

Dimples can help reduce form drag (at the expense of increased frictional drag). Since for the golf ball, form drag is the dominant component, the decrease in this component is noticeable on the overall drag reduction of the ball (or other bluff bodies).

This is not the case for boats, in general.

Bulbous bows are used on large commercial vessels to help reduce wave drag (at the expense of increased skin friction drag), yet you don't see them on sailboats (or golf balls for that matter).

In a seaway, then there is an additional resistance component due to the vessel moving in the waves. IF the vessel has poor motion characteristics, this component can contribute significantly

Again, FE boats aren't in the golfball scenario as such

[BHChieftain]

Enough of this endless speculation. I'll just try it out myself.
Chief

[keithbradley]

Enough of this endless speculation. I'll just try it out myself.
Chief

Looks fast!
Might want to add a turn fin...

[Jeff Wohlt]

I was shocked to see it on one of the big cats at the lake offshore races...I will try and find the video...might have been team CRC but can't remember. I asked him about it and it was on one the steps but only partially.

[blizard05]

Enough of this endless speculation. I'll just try it out myself.
Chief

DO it,the easy way. Spray it with Rust-oleum hammered, I did my zipp kits gas C boxes with it,I think it works

[BHChieftain]

Looks fast!
Might want to add a turn fin...

No, this is a SAW hull.

Chief