technical question

[kickingbug1]

here's my question-----i have decided to remove the abative coat from the bottom of my boat.(its old, rough and looks awful) i have done a small area and it reveals light gray barrier coat (very smooth). now, i intend to only sail the boat off the trailer and i think that the light gray barrier would be fine. i was told however. that a smooth bottom would slow the boat. it was explained to me that the rough surface holds the water to the hull and creates a " water on water" situation which has less resistance---meaning more speed. he also said never to wax the hull beneath the waterline for the same reason. seems to me that a slick hull would mean more speed.  who is right-----??

[Craig]

Not a racer but I do know many racers take great pains to buff the bottom before a race. That being said, unless a tenth of a knot or less is important to you, I wouldn't lose any sleep over the issue. I would recommend that you consider a "hard" bottom paint(does not have to be anti fouling) to protect the barrier coat. It was not meant to be a first line bottom paint.

[rbh1515]

You definitely want a smooth surface.  Many of the racing boats at our local club that race and dry sail their boat, don't even have any bottom paint.  BTW, there are some alative waxes on the market that you can use.
Rob

[Bob23]

Steve:
   My son builds surfboards and unless his customer requests it, the bottoms are not glossy but rather a flatish finish for exactly the reason you state. Not a big deal for us non racers.
Bob23

[brackish]

Wow, I've never heard of that.  I used to race and everyone who was serious about it always cleaned the bottom as slick as possible before a race.  Surfers wax their boards.  Snow skiers (I'm one) wax their skis.  All of this in the name of speed.  I'm building an epoxy composite skiff right now and the designer emphasizes that the bottom be faired and slicked out as much as possible because rough means more HP used to get it to plane.

I do know that if you are not planning to carefully pressure wash every inch when you pull it out, it will dry with some residue on that rough surface and that will slow you down. 

While not exactly relevant because I have an ablative on the boat, last summer I motored out to my bottom cleaning place, thoroughly cleaned the bottom to remove all slime, then motored back in.  With the throttle set at exactly the same position, the boat ran a full knot faster coming in with a clean bottom.

[rbh1515]

brackish,
1+
Rob

[NateD]

smooth=less friction=faster

Does a sheet of notebook paper slide easier on the floor, or a piece of sand paper? Are car bodies smooth or rough?

A rough surface is going to disturb the flow of water over it and increase friction. A completely smooth surface is going to move more easily through the water (or air).

[tmw]

If you want to see the scientific testing, download this:
http://www.sponbergyachtdesign.com/keel%20and%20rudder%20design.pdf
and look at figure 10 (page 13).  It basically says that if you have an efficient hull shape, having bottom paint can more than double the amount of drag, while it's less critical (but still a significant factor, like 30% less drag) with less efficient shapes (comparing right side to left side).

[kickingbug1]

   well, i always tend to go with the common sense answer----smooth means less friction.  i will probably paint over the barrier coat for looks if nothing else. just for laughs i will post the previous info that i found here

[Floridagent]

"Surfers wax their boards."

That is true.  But, we rub parafin wax on the TOP of the board to create a rough surface so our feet, hands, and butts don't slide right off the slick fiberglass.  The bottom of the board should be shiny and slick.

Ahh... I miss those days!
Bob

[rbh1515]

http://www.practical-sailor.com/news/Smooth-Bottom-Paint-Surface-10475-1.html

Talks about the benefit of a smooth surface.
Rob

[Craig Weis]

Skip wax's his water base house paint [latex over factory barrier coat ] that is below waterline with at least three or more coats of Mother's Wax right from the shelves of most auto supply stores and any Wal-Mart.

The very last coat is never removed. Slap it on and let it dry.

I don't think the house paint ever sees the water. LOL

skip.

[kickingbug1]

 i have read that many kinds of paint have been used but i have to say that latex house paint isnt high on the list. i just cant imagine how it would last and further how it would re-act with automotive wax. i thought of using something like oil based enamel on mine or possibly having my brother spray paint the bottom with automotive enamel. i guess whatever works. i would kind of like it to be slick and look pretty good as i sail off the trailer

[mattman]

I am looking for the link on this as I type. There are several aspects here. Back in the day I think that they put riblets on Stars and Stripes that resulted in about a quarter knot increase in speed.  Ah here is one

http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html

and

http://www.sailingscuttlebutt.com/2014/01/17/return-riblets/

also
also I think that mythbusters had an episode on this with a car covered like a golf ball

http://www.landracing.com/forum/index.php?topic=6955.0

sooooo....not everything that is intuitive is the case. (kind of like stalling the top tell on the main...)

I would take her down to the epoxy and keep it as smooth as possible because the look is important to you (me I would wet sand) . The guys I race with don't wax the hull.

a few more links

http://www.epickayaks.com/article/article/to-wax-or-not-to-wax

and

http://forums.offshoreelectrics.com/showthread.php?46582-Does-Waxing-a-boat-hull-increase-speed  (third post down)

http://www.landracing.com/forum/index.php?topic=6955.0

[tmw]

These are some really interesting articles, thank you for sharing.  The one thing to keep in context is the speed condition, if the flow is laminar or turbulent. These applications are mainly high speed, turbulent flow (high Reynolds number) conditions.  I'm not sure a com-pac hull typically gets into high enough velocity for there to be a benefit.