Clide and Don I reported a book data, all in MKS sistem (Meter Kilo Second) Tomorro I will put the picture of the page.
Clide since you managed to measure the various hull, it will be not possible for you to check the paint of your car or the glass mirror in your bath ? That will give a pratical idea about. Chromed spoons and knifes are also interesting… thanks !

As promised, two pages of one book of P. Gutelle french Architect/writer

Having rewieved the previous data offered by Clyde, I wonder if I should read i.e : for a gelcoat : 19 microinch or 190 microinch, the second appear to me more realistic… any scale factor of 10 ?

I found very useful to use this conversion calculator :


Another exemple about the superfinish surface required for this PC cooler :

The roughness bench instrument used to check the superfinished plate surface is revealing some 6.0 µm = to 236.22 microinch

So, where is the sneak ?


Glass 2 microinches. mirror 2 microinches, I understand now that the number we should try for would be very low because of the scaling effect. Much work to be done this winter. Thanks, Clyde

More about…
I knows that this, has nothing to do with models, but it is the only way I have found to shows a surface super finish and the roughness measured value. Just to give an idea ! I’m sorry.
This is the plate under measurement :
- 236.22 microinch = 0.006 millimeter (6 µm) !!! just a car paint !!

So, where is the sneak , when the finish measured on the boat model skins are between a minimum of 12 microinch and a maximum of 65 microinch - average ~35 microinch ?


I think we got a little side-tracked by this discrepancy, whatever it is. The point I think we are interested in is how smooth does it have to be. As I see it so far:

It has to be smoother forward than aft.

The roughness has to be thinner than the flow(laminar or turbulent).

The resistance caused by roughness is only part of the total resistance.

Things I’m not sure of:

Claudio’s page says that the shorter the length the smoother it has to be. I’m having trouble getting my head around that. When the water hits the first foot of the hull does it somehow know the total length of the hull? I can see that the length enters into the total resistance but not that it has to be smoother if it’s shorter.

Again on Claudio’s page it say that the laminar area of the polished 5 m. dinghy is .12 mm thick and we know that the roughness has to less than this but .12 mm is 120 microns, elsewhere they say we need 5 micron or less.

I’m not picking on Claudio’s page it’s just what is in front of me. I’m going back to the library to get the book I was reading to refresh my thoughts.

I’m pretty sure that smoother is better, what I’m not so sure of is how smooth does it have to be to get to the point where you are wasting time trying to make it smoother. Is it worthwhile trying for a 10% improvement in smoothness when smoothness is only 10%(this is a guess) of total resistance.

That’s all my thoughts for now

PS Can anyone point me at a definition of “hydrodynamically smooth”

My balding head ??? :smiley: :stuck_out_tongue:

Can you run over to Clyde’s so he can take a measurement?:stuck_out_tongue:

Well, more figures are in. Clyde does not have many places on his body without hair to mess up the readings. In order to duplicate baldness the reading was taken on the left buttock. Three readings average 107 micro. Readings were taken by wife and tickled. Thanks for the idea guys. The actual measuring device is the same as Claudio’s only older(about 20 years old). Calibrated last week before these tests. Clyde

Just found an article on the Greater Hartford Model Yacht Club website that also may give us some insights to roughness.


That makes it sound worth doing.


I just paste what I wrote at the beginning about “Hydrodynamically smooth”:
The concept of “Hydrodynamically smooth” is produced by the ratio :


L = is body lenght in meters
K = 1.000.000 used for a very smooth surface
K = 1.000 used for a very rough and dirty surface

The K factor is derived by Reynolds numbers, speed, etc.

therefore, while for a superclean 5 meters long boat, the L/K will be :
5.000 / 1.000.000 = 0.005 mm (equivalent to a sprayed painted surface and polished)

similarly for a model of 1 meter, the superclean surface should be represented by :
1.000 / 1.000.000 = 0.001 mm average roughness (Ra)

and consequently using the same simple ratio, for a Fin wide 100mm , the superclean finish wiil be obtained by a 0.0001 mm or 0.1µm average roughness. (Ra)

Personally I do not believe feasable to reach manually such degree of smoothness on our Fin and Rudders, unless …

For what concern the gains or losses I just take this Table extracted from another book …

Unfortunately the comparaison stops at a relative speed of 0.8 were friction is 75% of the total.
Friction resistance is produded by 2 factors : wetted surface and roughness

Actually at lower speed, one would expect higher friction percentage and any gain obtained on surface smoothness should be beneficial at low racing speed, that, I believe, it represents a large amount of time spent on the water.

How much resistance is due to vetted surface rather then to roughness ? I do not know, but as said by Dick, Don and Clyde, smooth will certainly help.

2 microinch for a mirror !!! this is a valid engineering measured value ! How to get there ?


I went reading the Bethwaite book, it is in fact said that someone, for testing purpose, plated the Laser rudder blade with a chromium layer to obtain a mirror surface.

Continuation of finishing efforts. Soling one meter rudder, primed and wet sanded 320 grit paper 19 micro is the best finish sanding and measuring in the direction of water flow. 32-34 micro in vertical direction. First coat paint, wet sanded with 600 grit(used paper) paint must not have any shiny spots as these show up as surface imperfections, 12-14 micro measuring in the direction of water flow. 16-19 micro in vertical direction. Most of the paint color has been removed, next step paint again and wet sand with used 1200 grit. Paint is Wal Mart “Krylon”. Clyde