Select to expand quotesailquik said..
I don't know about the boards surface, but while re-reading the old thread this post from GAZ stood out for me:
"looks like this post has been stagnant for a bit but I recently came across an interesting finology article on vmgblades.com (excerpt below) and couldn't help but wonder whether principles of laminar and turbulent flow apply somewhat the same to the bottom of boards as they do to fins? Finish & Flow (of fins) Finish is how the surface of the fin feels and has a large part in how the water flows over the foil. This is why looking after your fin is very important. Nicks, scratches or dents all affect the flow and can create drag or separation bubbles. If you have a tendency to spin out unexpectedly, then this is the section for you to read. Laminar flow is where the water stays attached to the fin and follows the foil shape closely. Theoretically this creates the least amount of drag (always a good thing as we want to minimise drag). However, laminar flow can stall dramatically and you find it much easier to spin out. This is because large separation bubbles happen in the boundary layer of the water flow around the foil. The separation bubbles separates the fin from the water, you suddenly have no lift and you spin out. Turbulent flow is like little swirls of turbulence along the fin. More energy is created in the boundary layer and the separation bubbles are tiny. As the bubbles are tiny, stalling is less likely to occur and a rider is able to push against the fin as it is more stall resistant. This gives you more control and allows you to be more daring in your tactics. There is slightly more drag but at least you don’t have to worry about spinning out in the water. A rougher finish forces a fin into making turbulent flow and a smoother fin is more likely to become laminar flow. Sailors often experience different sensations when sailing new fins. This is because of their style and experience. It can also take a little while to get used to a new fin and adapt your style. Different finishes could be recommended to different sailors, this is so that you get the best possible ride for your style."
Around 2008 - 2010 Mal Wright and I were experimenting with very fine fin finishing. Our usual routine the evening before a big day was to spray our fins with 3d spray paint and sit around for hours very carefully wet bock sanding with fine W&D, and then polishing the fins with wax/cutting compound to a mirror finish.
I don't know if it gave us a speed advantage, and if it did, whether the benefit was due to immaculately smooth fins from the prep or from the polishing. I do know we were consistently doing some pretty good speeds in less than manic wind conditions back then.
But the paragraph highlighted above is very interesting. One thing I definitely noticed was that the highly polished fins had a tendency to be a bit "squirrelly" ("technical" term 
that means they sometimes kinda half let go when accelerating under pressure) for the first few runs! I wonder if that was linked to the info in the above statement?
Often the first few runs were also the fastest. Could it be that we were getting a benefit from 'laminar flow' over the fins resulting in less drag?
The highly polished finish seemed to dull after a few runs, probably due to sand abrasion in my case! 
and those funny little brief 'squirrels' would disappear.
These days I don't pay anywhere near as much attention to fine tuning my fins, but after LG this year I have to say that Sammy the Snail has inspired me again!
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
Select to expand quoteflyingcab said..
Mark _australia said..
sick_em_rex said..
Mark _australia said..
Fastest - always matte
rub back to about 600 or 800 wet.
had a look at the bottom of any new Mistral Speed or Slalom board lately Mark? Shiny as.....
some degree of matte-ness is always preferable until certain speed is reached (dunno what that is though)
You can see smooth has a higher co-efficient of drag early on. As to whether this makes a 0.001kn difference, or a more useful 0.5kn difference, I dunno.
If a certain board is faster, it is more likely to be design / shape. Especially as the coefficient of drag changes so greatly under different conditions. It is a pointless argument really.
I like how you posted this table without knowing what it actually means lol.
<snip>
Tl;dr: a smooth finish is better but the design has a much larger influence on the total drag.
I like how u kinda agree with me.
Yes smooth is faster....
Painted finishes are not smooth, they have orange peel effect. A glossy board is very likely to not be smooth as they don't rub back then polish, it is straight off the gun.
BTW golf balls are not a valid analogy as they operate in a totally different environment and at different speed to a wetted surface. (Reynolds number is waaaay off)
But I have no idea what that means
Select to expand quoteflyingcab said..
You can see smooth has a higher co-efficient of drag early on. As to whether this makes a 0.001kn difference, or a more useful 0.5kn difference, I dunno.
If a certain board is faster, it is more likely to be design / shape. Especially as the coefficient of drag changes so greatly under different conditions. It is a pointless argument really.
I like how you posted this table without knowing what it actually means lol.
What a rougher finish does is induce turbulent flow earlier along the board (the reason golf balls have dimples), but once a smooth surface has developed turbulent flow, the drag coefficient is much smaller (~4x as seen on the right of the graph).
Not sure where you're going Flying cab? I suspect Mark has posted the total drag coefficient for flow past a sphere? 1st April Mark? Not really applicable to the skin friction drag along a flat surface being discussed here. For skin friction drag I'm pretty sure it's the other way around, i.e. it increases with a transition to turbulence. Hence the efforts to delay the transition to turbulence with carefully designed laminar flow foils.
Select to expand quoteflyingcab said..
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
"Reynolds number!" Now there is a term I have not heard used for a while, probably since Mal used to have long conversations on fin design with me. He was all over it but i have to admit that I mostly just smiled and nodded a lot.........
I remember a couple of coats of mr sheen worked wonders on the bottom of
our body boards bak in the day, was easier catching waves & had so much more
speed on the wave, mayb time for mr sheen on the bottom, if u can still buy it.
Select to expand quoteflyingcab said..
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
Hi there flyer,
Since you have calculated the Re to be 767000, can you explain how you got to this?
ie: Re depends on a variety of factors, including the amount of air ( and sand !) in the water...
velocity*length/kinematic viscocity.
Sure I have made some assumptions but it does give you an idea.
I used values of ocean water at 1 atm and 20C...
Unless you are going directly behind someone or in the surf i doubt there will be enough sand/air in the water to have much of an effect.
Select to expand quoteflyingcab said..
velocity*length/kinematic viscocity.
Sure I have made some assumptions but it does give you an idea.
I used values of ocean water at 1 atm and 20C...
Unless you are going directly behind someone or in the surf i doubt there will be enough sand/air in the water to have much of an effect.
Back when GPS-speedsailing took off (say 10yrs ago) when speed fins were being designed using a specific value of Re - then generate foil shapes based on that. We stopped doing that because the fins weren't all that good... We then started modelling foils while varying Re - this resulted in better fins.
For a board, where do you think the air goes, when it gets compressed by the board, at the water surface ?
The fact that thermoclines exist, shows that varying density is common... so we cannot just assume a constant.
The reality is that the air-water boundary is both turbulent and highly entrained with air.
HI Blazing 928 ,
Mines a 88 S4 in Diamond Schwartz Black . I looked at the Tiga when it was on Ebay but realised my collection is enough and I have a Mistral Equipe of the same era which does me . We have same sort of taste by the look of it . Not sure of your take on Saab 900 Turbos and International Scout 800's ( my other cars ) .
Cheers and hope to catch you round in the 928's
Oh by the way I use Motul 6100 in my 928
sharks have a sandpaper skin and dolphins have a rubber like smooth skin.
nature hasnt even decided which is faster.
why doesnt someone drag a smooth board and a sanded board behind a boat at the same time and measure the pull. or drag a smooth piece of plastic and a sanded piece of plastic under water and measure the pull.
my hypothesis is rough has less resistance for lower speeds and smooth is better for faster speeds, but where the threshold is I dont know, for example is that fast threshold at 60 knots or 200 knots?
Select to expand quotemathew said..flyingcab said..
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
Hi there flyer,
Since you have calculated the Re to be 767000, can you explain how you got to this?
ie: Re depends on a variety of factors, including the amount of air ( and sand !) in the water...
......or number and size of hailstones and or temperature of the water, hence why I have a fridge/freezer in my car to store my fin's in so they are ready for the hottest record breaking session's 
. The hail drop's the water temp at the surface and provides a ball bearing like surface to reduce friction, did it make a difference one time? maybe but so did timing the start of the run in a squall.
F1 has tire warmers,... the Pit Crew have Fin Coolers, made from gel/ice packs that keep their fin's cool whilst waiting on the beach for a run. Other teams say it's a conspiracy against them for the Pit Crew to have an unfair advantage over them just like the Pit being on their doorstep which is nonsense.
So other teams don't fall for this conspiracy talk and be seen at your local spot chillin ye old fin before a session as all ya will end up with is a loose fin in ya box and that's not fun
Wind strength, water state, hard balls of ice/steel, technique and good design probably have more of an effect than gloss or sanded finish when your going flat out
I'm pretty sure the top speeds at Luderitz have been done with both. Most have more chance of going faster than before by spending more time on the water trying to be the fastest at their local spot honing their skills.
I imagine you'd want more resistance on the bottom of your board down at Sandy Point so you can maximise the advantage of the outgoing tide..
If you had a slippery board you wouldn't be able to catch that current, what is it on a good day, +4 knots?
Select to expand quotebarn said..
I imagine you'd want more resistance on the bottom of your board down at Sandy Point so you can maximise the advantage of the outgoing tide..
If you had a slippery board you wouldn't be able to catch that current, what is it on a good day, +4 knots?
That's correct Barn, why do you think Slowboat tried multiple step bottom boards, it was to take advantage of the current. Don't forget you get eddy current's close to the bank going in the opposite direction where the water is smoother/faster, so slippery better on outgoing tide. Of course if you have an Eddy Current Suppression Ring fitted to your board Barn and left turned on it does not matter.
Gloss or Matt, Low maintenace or High maintenance.... here's a song/vid' with water, sand and skiffle boards for those having trouble "Which way to Go"
Stepped hulls, so obvious! This way you can gain maximum push from the downwind current..
Yes, same tastes DB. Great Mate of mine has two Saabs, I've spent many miles in early EMS & ex Rainsford rally car - massive respect, but I was an Alfa boy - now I have two Alfasuds awaiting restoration.... Also Haflinger & Ducati 907 & Lambretta ...I looked for Mistral for years, always for sale interstate, I recently missed a local one, so bought the Tiga. Will still hunt for the elusive Mistral panam .....the 928 is slowly finishing a 4+ year restoration, some mechanical fixes & completely new leather interior. Ciao Nigel
Select to expand quoteTroppo said..
your attention to detail is second to none barny! absolute classic
.....though I don't recall Slowie doing a trip to Sandy over summer
.
As for the Tiga Nigel I'd go matte first, try it out.... if you get tired of keeping it clean wet n drying it from time to time, polish it to gloss, by then all the dips and bumps will be gone, flat is fast. The gloss will seal it up and it will keep better, that's what I did with my 36 knot 12"6" SUP, as the paint got thinned out from wet n drying it so much it would rub off on your hands after being out in the weather.
Would be good to see the Tiga down at Sandy Pt to see how fast it can go in 30 knot's, post some pics when you finish restoring it.
Alfa Boy Hey , Say hello to Mum for me , I have had 13, yep one three, 105 coupes in various engine sizes in the last 20 years . Probably wouldnt have much to talk about if we catch up some time , bet your not 120Kgs though
Select to expand quote
I think you nailed it here Mark. I don't have the patience to polish, so if I do anything at all, it's 800 for meSelect to expand quote
And there is a good point.
For the "gloss is faster" camp: How many imperfections, or how much orange peel effect is present in your glossy board?
Block sand it briefly with say 1200 wet n dry and you will see circles all over it cos it aint flat. Anyone who has painted a car will be well and truly familiar with that. It is hard to see orange peel in white, especially in the sun.
Sanding will make it all even, and even is better (less turbulence)
So if you feel gloss is better you should rub back you factory board with 600 - 800 - 1200 - 2000 - 2500 (wet) then polish well.
If you feel matte is better you do the above and stop at 800.
Personally, like I said I feel matte is better, but the best research shows it is negligible so who cares.
Select to expand quoteflyingcab said..
For laminar flow on a fin you need a reynolds number to be less than 5×10^5
This number is a ratio of the inertial forces to viscous forces and is used in everything in fluid dynamics.
For a fin of length 10cm, travelling at 15 knots, your reynolds number is going to be 767000 which means that you will have turbulent flow. In fact, you will have turbulent flow for the last 3.5cm of your fin.
20knots, more than half your fin will experience turbulence.
30 knots over 70% of your fin will be in turbulence.
The faster you go, the more turbulent the flow will be.
That may suggest at first that our fin should be long but narrow ? Is it right? In order to avoid turbulence.
Lets perform mental experiment and take that sharpest katana blade. What will be resistance if you run it trough the water at 40-50 knots
a) perpendicular ( vertical)
b) parallel to our direction ( horizontally)
Obviously total surface area remains the same.
What will be difference in lateral resistance of vertical and horizontal blade?
and another consideration regarding surface directly
(((((Without going into details I could speculate that regardless how smooth your surface becomes , eventually the gross of the interaction between boat/ fin surface and water depend on quantum effects known as Kashmir effect. The mentioned distance about 0.1mm may further firm the suggestion. Our goal could be then eliminating this Kashmir effect if possible. In order to do so I imagine that we need to separate this two surfaces by the distance that quantum effect becomes neglected. I imagine that we should be able to create precise grooving along the flow. This way water may have direct quantum contact only with this apexes of the grooving, space between filled with gas/air trapped between that doesn’t provoke Kashmir effect. Ideal material allow then create ultra sharp apex , then groove/ valley between. I am not sure if continues groove is needed or just hill , sharp needles like.
Translating this to our simplified technology I imagine that surface of our fin / or bottom of the boat should be ideally coated with hard material grooved precisely along. Instead of mechanically grooving ideal could be nonomaterial of specified surface structure already.
Practical simplified method could be hard coating our fin then sanding along the flow.
Waring is required when playing with Kashmir effect. Wrong size of structure may cause opposite effect magnify the force to the point that surfaces becomes almost "glued" together ! look gecko climbing your ceiling )))))
Nope DB, only 85! Gee, I'm only up to 6 Alfas - Sud, Alfetta 2.0, guila sprint GT, Autodelta Alfetta Gtam, 2 x Suds - yep not much in common...
so, I'm going to leave the finish matt for awhile with the intention of another coat of 2 pack urethane on the bottom, but sprayed this time and then finish to a gloss - for ease of maintenance and then work out a refinish of the top. I'd like to keep the graphics, so a mat clear with some more anti slip.
