Hull Bottoms

[rayray9]

Question, a sanded hull has less drag than a waxed / sheen one?
Please explain

[fweasel]

It does initially seem counter intuitive, but the rougher surface breaks water tension. A smooth surface actually causes a suction effect. It's been 20 years since I last took a fluid dynamics class so I'll keep my answer low tech.

[rayray9]

Thinking now

[rayray9]

Okay ready now:

Me thinks as soon as the hull hits the water, water tension has been compromised, it's just a cut forward?

[Fluid]

Wax wax wax.......who knew it was slower? Me, as an engineer who works with fluids daily, I go with science, not opinion.


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[Beaux]

The slightly rougher surface helps break the tension. A friend of mine who use to race full size boats said the effect of the water was similar to ball bearings.

[T.S.Davis]

Wax wax wax.......who knew it was slower? Me, as an engineer who works with fluids daily, I go with science, not opinion.


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I wish you were wrong Jay. Hahaha. All my boats are scuffed.

[rayray9]

Thanks for the replies folks, learn something everyday.
3M scotchbrite pad or wet sand / what abrasive grade?

[Erroneous]

Ive been using 3m 320.

[Charlietuna1]

Ok. So we have a Reynolds factor in reverse, kinda.
What about hardware? Mirror finish?
And.......I believe Jay Turner mentioned in his book about powdered graphite coatings.
This past weekend, when I was spending an absurd amount on epoxy I noticed West Systems has a powdered graphite filler for "race boat applications ".
Anybody do this? If so, Amy observations?

[ade-rc]

It all gets very complicated very quickly. Rough surfaces wet out and form a boundary layer of water which can reduce friction. They can also form vortexes which can also reduce friction. Highly polished surfaces can form a vacuum layer on the water which can increase drag. Tests have been done with micro bubble systems onto hull surfaces on cruise ships to save fuel.

You see small advantages on full size boats with surfaces areas of few 100x that of an RC sized boat.

[CraigP]

We used to use textured primers on the aluminum plating on the sponson bottom. We called them "wear plates", because we always had to re-do them! Having super slick bottoms in Unlimited's could hook the boat, just starting to make a turn, or chining in, as flatbottom guys would say. I said "wreck the boat, and work all night to fix it"!

[hughb]

The way I was told the scuffed bottom creates micro air bubbles as the hull moves through the water. This in effect means the hull is riding on a microscopic air cushion which creates less drag than water. Not very scientific but seems to make sense...maybe?

[CraigP]

CharlieTune, that would work well. I believe that may have been the stuff we tried. I'm building a 34 Vortex Hydro right now. Very unhappy with sponsons... I'm reworking them soon and will have 0.032" T6061-T6 runner plates on the bottom. I think I'm going to try the graphite coating as a finishing application to the runner plates..