Hi every body.
my last puzzling questions :
How much is rough the surface of my Hull ?
How to measure it %
What could be the best achievable smoothness in order to reduce friction drag ?

Hi Claudio
I just happen to have a copy of Lars Larson and Rolf Eliassons" Principles of Yacht Design"(It was actually in the car waiting to go back to the library).There is a section on roughness and a graph that charts speed against roughness. At 1 m/s the permissible roughness is 100 microns, at 2 m/s 45 microns at 3 m/s 27 microns and 4 m/s 22 microns. They also say that the surfaces should be smoother towards the bow and can get rougher towards the stern.They say that 400 grit sandpaper has a grain size of 25 microns. This doesn’t mean that you should sand with 400 grit(although for us that would work). That is just an indicator of what 25 microns feels like. We’re in the 1-3(I wish)m/s range so it looks like we could coat our boats with 150 grit sandpaper and be fine. They don’t mention anything about a hull being too smooth except for the front of the fin and rudder where (I get the impression) that it can’t be too smooth. Any imperfection here can mess up the laminar flow. My take on all this is that the boat should be smooth and the front of the rudder and fin should be polished. It is a very interesting book, you should try to get a copy.

Hi Don,

So, to avoid misunderstandig , 100 microns equal to 0.1mm (1 micron = 1/1.000.000 of the meter)

One of my books, discussing about Reynolds numbers and frictions, suggest that a body of 1 metre long should have a roughness of about 1 micron. and possibly less for fin and rudders.

The book suggest one exsample : the car body paint have a roughness in the order of 5 microns, a boat of 5 mt long, if spray painted as the car body, should be OK , this surface is called “hydrodynamically smooth”.

According to the book, the tollerable roughness is function of the lenght, not only the speed.

The concept of “Hydrodynamically smooth” is produced by the ratio : L/K

L = lenght in meters
K = 1.000.000 very smooth
K = 1.000 very dirty surface

therefore, while for a boat of 5 meters : 5.000 / 1.000.000 = 0.005 mm of average roughness is very good,

instead, for a model of 1 meter : 1.000 / 1.000.000 = 0.001 mm of average roughness will be also very good but difficult to obtain.

For a normal speed of 3 m/s and for 1 meter boat, according to your book , 27 microns are OK, while according to my book, 1 micron, is necessary to consider the surface Hydro…smooth.
The difficulty increase for fins and rudders that have shorter chords as mentioned above.

Thanks anyhow Don, once again books are not telling the same things and the puzzle remains.
Hoping to ears others opinions.

Did it give a Reynolds number for this?

Edit: My book is for full-sized boats so that may have a large bearing. What we need is two identical boats,one as smooth as a babies butt and one coated in sand.

Don ,
always according to one of my books, the Reynolds number can be calculated by the simple formula :

Rn = V x L/ v

V = speed in m/s
L = lenght of the body
v = water viscosity at 15°C of 1.19 x 10-6

Therefore for a model of 1 meter long sailing at 3m/s, the Rn number will be :

Rn = 3m/s x 1meter / 1.19x10‾6 = 3 x 1.000.000 /1.19 = 2.520.100

If I’m wrong let me knows…


Lot’s of work being done on this … swimmers are using textured suits for instance … smoothest is not always best (a shark’s skin surface is extremely efficient) … work has been done as well by various sailing syndicates … nothing has caught yet however, and given the computing resources required to figure it out: it’s still a rich man’s game … it does challenge orthodoxy however

HI Tmark,
dolphins ejects substance from the skin similar to polymers and compensate the pressure variations with their muscles, the power developped by a dolphin is 2 HP and can go as fast as 20 knots a reaching up to 40 Kn, Our engines of the same power are good to go fishing !!
Shark are also using muscles even the skin is some what rough plenty of small needles and used as a sort of “turbolators” to maintains the flux as turbulent as possible, therefore retarding the separation.

3M developped for US rowing boats a special film composed of triangular longitudinal grooves of 0.8 of millimeter.
They won the silver medal at olympic games.
Somebody else tried to make semicircular grooves but did not worked.
US and Holland developped a system ejecting polymers from micro holes in the bow, was forbidden by the Rules and it is not clear to my knoledge that worked well.

So, several research are taking place expecially for military purpose (submarine, torpedos, etc), but higher the speed smoother the surface shall be to avoid heat up.


What I was trying to say was when they were determining that a 1m. shape should have a 1 micron finish what speed were they testing at? Was it was a foil for a catamaran or for something going 1 m/s? I can see a high speed shape needing that kind of finish.

actually the models used in test towing tanks do measure only the full drag including wave and friction. They cannot split the two drags. Speeds are variables.

Claudio and Don, i have been reading this thread with great interest. My other career was in quality control and one of the variables we had to quantify was surface finish. I can borrow a portable roughness tester from work and take some readings, will post the readings after the local regatta this weekend. Will use a skid type of profilometer. Clyde

Fantastic Clyde,
we have a point in common, because I was me to a Product Assurance Manager within the European Space Agency for 25 years. Very often travelling to US for professional bussiness.
I have seen many times the Rugosimeters, during various inspection on mechanical finish, unfortunately I was non involded in navimodelism at that time.
Today, since retired, I’m far away from that world.

Will very nice to have some data, including a piece of glass, or a chromed knife or spoon, or the car paint, what ever you may judge for comparaison.
My secret idea is to try a galvanoplastic surface treatment of my carbon:epoxy hull, based on copper and chrome, followed by a super polish and finish as to have 0.2/ 0.5 micron average rugosity.
Now is not anymore a secret !!!
Very, very good


Aother element of smoothness is the waviness factor. A surface, may or may not have perfect compliance with whatever curve or flat is under study. (Most likely not) Another question, that is relevant, is how much undulation is permissable. Undulations or waves will have two dimensions: amplitude and frequency. When those dimensions are very small they become synonomous with roughness. Waves with small amplitude and long frequency is the other end of the scale.

Surely the presence of waves will have an influence on boundry layer behavior. That brings us to the dolfin and shark thing, where their skin is presumably manipulated to reduce or increase drag as appropriate to the situation at hand. The mysteries deepen:confused:

Well, the data is in and it may cause more questions than it solves. The variation in finish can be extreme(all measurements in microinches at .030 cutoff and.500 inch stroke) . First example, Tom W. ODOM: gelcoat hull 19, fin 32, rudder 24. Tom L. ODOM: gelcoat hull 23, fin 29, rudder 19. Ted L. Soling One Meter: hull 19, fin 22, rudder 44. These boats actively raced, did not check for variation as it was cold and raining yesterday (ice still on the deck at Cherry Lane). At home measurements: Nightmare Trimiran: main hull(sanded epoxy) 14, 12, and 16, floats(painted) 21, 29, 24, 31, 21, and 22, fin(sanded helicopter blade) 22, 20, 24 (found one place on the fin that measured 54 to 60 was a bubble in the surface) rudder(sanded helicopter blade) 17, 15, 18. Logic 50-800: hull 31,29, 30 fin 22,25,24 rudder 36, 42, 40. Fairwind unfinished: (hull and fin wet sanded with 240 grit paper) hull 62, 64, 61 fin 65, 61, 63, rudder(unfinished out of the box) 14, 14, 13 Soling One Meter: (hull, fin and rudder wet sanded with 500 grit paper) hull 32, 31,32 fin 34, 34, 33 rudder 33, 33, 34. These finishes are much higher than I would have guessed and remindes me of what the salesman told me when training on the profilometer, “do not trust your eyes or your fingernail when measuring surface finish”. Maybe with more data we can find a better correlation between surface finish and speed or lift. I know that I am going to spend some time polishing and buffing this winter. Clyde

This discussion brings to mind the old “dimples on a golfball” issue, where the surface roughness of the dimples causes the boundary layer to be thicker, allowing better attached laminar flow & delayed turbulent separation of the airstream. This allows a much straighter flight path than a perfectly smooth ball. How much of this translates into hull drag in water is discussed in the following article written by Christopher H. VanEpps, a sailor who also happens to be an aeronautical engineer at Lockheed Martin (thanks to Dick Lemke for bringing this to my attention in this thread: ).

1 microinch= .025 microns
30 microinches=.75 microns acording to this site

Claudios car finish was 5 microns which is 196 microinches.
Clydes measurements were in the 30 microinches range.
Something doesn’t add up. Either that or Claudios car example is finished with 40 grit paper. What is wrong with my reasoning?


I simply reported what was written in a book, I never got the opportunity to measure it.
It will be nice if Clyde could and try to measure also the car paint, if possible.

1999 Plymouth Voyager 11-12 micro (white paint no clearcoat). 1999Mercury Sable 5-6 (baked clearcoat). My wife recommended a universal item that we all can relate to- porcelain toilet tank top 8-9. Clyde

Claudios car is 5 microns and Clydes car is 5 microinches and yet my conversion site says---- boy I’m confused. Claudio is there any chance that your book is meaning in microinches, not microns. I can’t think of anything else. I looked at two or three sites and they all say that 1 microinch is .025 microns. I’ll do some more digging

The reason I used a fingernail as a negative example for measurement was that veteran machinist’s use their nail for a gage. Not a good idea, below about 40 micro this is very subjective. Unused 100 grit sandpaper 970 microinches, 150 grit sandpaper 640 microinches, 240 grit sandpaper 400 microinches, 320 grit sandpaper 330 microinches, 400 grit sandpaper 260 microinches, 600 grit sandpaper 200 microinches. Perhaps Claudio’s measurement is in metric? Most paint would be below 20 not the same as 600 grit sandpaper (200). Clyde

Interesting reading