wetted area and drag

[windfreak74]

which parts have greater wetted área and drag?
what im triying to understand is which part is the most contributing factor in a foil.

1.mast. most of the time half of the mast is under wáter so there is about 1000cm2 of wetted área
2. wings. have about the same wetted área about 900cms of wetted área but has greater frontal area
3.fuselage has 900 cm2 of wetted área in average with the smallest frontal área.

[zfennell]

pedro,
for streamlined shapes, drag is dominated by the component due to skin friction.
skin friction , as you can imagine is function of surface area, velocity and viscosity.
....so surface area is variable you'd be comparing


frontal area is certainly a consideration.
but if you were to look up the drag coefficient of any of the profile shapes you are using they would all be a function of plan area (not frontal or wetted), velocity, viscosity and angle of attack.

-your device should operate at small (1-5 deg) AOA which will keep parasitic pressure drag low
the thicker sections do tend to have slightly higher drag coefficients, but they are all pretty close.
-the drag coefficient for each is probably around .01 +/- .002
(you can look them up in foilsim or xfoil or any NACA book)

-so , my guess is that your strut has the greatest wetted area and will contribute the largest percentage of drag.
it is also the only component to pierce the water surface and create an additional wave drag component.


close enough?
-bill

[windfreak74]

Hello Bill,
Again thanks for replying!
you are very helpful!
i bought a limited amount of carbón so im trying to figure out how i will use it wisely.
mast and Wings are a challenge!
i can make wings strong with Wood and fiberglas but they will be more inefficient due to thickness.
lots of ideas but not enough time and carbón goodies!

[cglazier]

Bill
I have been told to lightly sand all the foil surfaces with very fine sandpaper (800 grit).
How significant is this really?
(I do race on my foil)

CG

[tkettlepoint]

2-5 knots faster , get a rubber sanding pad from car quest or napa . most the time they are black and blue... use it not the bare hand ( you will never see a body shop sand with a bare hand because it will leave finger marks , uneven surface)
800-1000 wet paper will leave micro scratches which will make less retention surface for the water to flow off.

The green house acrylic roof panels have micrio scratches on one side too help the water to shed off of them so the droplets of water will not dope onto the flowers and burn the peddels
Is the building of side of the acrylic panels we put them the other way so snow and water to shed off the top ( self cleaning) and I know for sure that you can not walk on them in the rain because you will slide off the roof it is that slippery.


all my board that I do now are flat /satin surface and you can feel it faster/ loser. not as sticky

just my 2 cents
terrie
www.jellyfishboards.com

[BWD]

re: the original topic, the main thing I think an amateur can easily do short of going to mainly or all carbon build is to go with thinner wood wings stratified with carbon, reduce main wing area to 570cm^2, and reduce sweep, raise AR a bit but not so much you get excessive flex.
If you want to spend the time and money, a thinner carbon mast, <15mm, will increase speed too.

OT: I am "finishing" a board I want to be fast, and between 240 and 320 grit I kept touching glass, so now it has yet another gloss coat, complete with tiny dust marks, etc.
I will end up riding it as is I think this weekend, but eventually want a proper 600 or 800 finish....
Feel silly as I'm pretty good at building and occasionally even manage a clean gloss coat but a really fine sanded finish is still a challenge, without laying on way too much resin.
Tips appreciated...

[tkettlepoint]

BWD I don't know what resin you use but I know in the past thinning out the hot coat help getting a tin glossy coat I have used alcohol but only 2-5% or so just enough to break the retention of the epoxy to flow. Then I block sand it with a rubber preform hand block with 800 wet Gives a nice buff finish..

terrie

[ahmthai]

re: the original topic, the main thing I think an amateur can easily do short of going to mainly or all carbon build is to go with thinner wood wings stratified with carbon, reduce main wing area to 570cm^2, and reduce sweep, raise AR a bit but not so much you get excessive flex.
If you want to spend the time and money, a thinner carbon mast, <15mm, will increase speed too.

OT: I am "finishing" a board I want to be fast, and between 240 and 320 grit I kept touching glass, so now it has yet another gloss coat, complete with tiny dust marks, etc.
I will end up riding it as is I think this weekend, but eventually want a proper 600 or 800 finish....
Feel silly as I'm pretty good at building and occasionally even manage a clean gloss coat but a really fine sanded finish is still a challenge, without laying on way too much resin.
Tips appreciated...

To get a lightweight final coat, add Qcell (micro balloons) to the final top coat. You can add 3-5 times by volume to make a lightweight, easily sanded surface. The amount of Qcell you add depends on how thick a coat you want. You can also add a pigment to the resin at this point so you don't need to paint for color. Unfortunately, you can't have the sexy carbon finish, but it will be lighter and much easier to finish.

[zfennell]

Bill
I have been told to lightly sand all the foil surfaces with very fine sandpaper (800 grit).
How significant is this really?
(I do race on my foil)

CG

hi chris,
micro-grooves or 'riblets' are pretty much accepted as a legit method to reduce skin friction in turbulent boundary layers.

from memory (of someone with alzheimers) i would say that controlled experiments have shown a few percent decrease in skin drag. that would NOT be enough confirm terrie's estimate of 2 kts 'speed' increase. but, still measurable for something with a significant amount of skin friction contributing to the total drag.
one of the 12M campaigns in the Americas Cup thought enough of the process to get 3-M to manufacture film with grooves of specific dimensions for laminating to the entire hull. ( and never seen again)
i think if you searched for 'riblets' or turbulent boundary layer drag reduction techniques you'd find more specifics.

i do endorce terrie's procedure for sanding the surface, it cant hurt.
unless the grooves are not aligned with the flow.....Sand front to back.

there is the small possibility that some of the fancy profiles being used actually have laminar flow over the leading edge of the wings and high pressure side ( places with accelerating flow).
if true, you'd need to polish the wings like performance sail-planes.
..doubtful.

i think the bigger issue is that sanding is a bit like good hygene for your foil.
a methodical method to inspect the entire surface.
fill the weave, remove pits, gouges, goobers, waves ,wrinkles, etc.
a cheap primer coat makes a great sacrifical layer to spray on and sand off.
you'll be amased a what surface imperfections are revealed.
confirn that 'magical' contour in your design ended up in the final product

good luck,
-bill

[cglazier]

Thanks Bill.
A clear explanation.