frictional resistance

[pavel915]

finding the resistance seems o tough to me;
i want to know how can i evaluate the frictional resistance of my ship with best process , when i have done the lines plan.
please help me by telling how can i get frictional resistance in the best way !
thanks.

[RANCHI OTTO]

RF = CF x 0.5 x rho x S x V^2

CF = 0.075 / (LOG10Rn -2)^2 according to ITTC Line 1957

where :

RF = Frictional resistance (pds)
Rho = 1.9905 (mass density of water)
S = wetted surface (square feet)
Rn = Reynold's number = V x L / 1.2817 x 10E-5 (water viscosity)
L = waterline length (feet)
V = Speed (knots)

[pavel915]

thank you very much.
is it applicable for all kinds of ships?
i want to find the resistance of a inland passanger vessel.

[RANCHI OTTO]

This is ok for displacement ships.

For your design there is no problem.

[alpamis34]

Ranchi is right mate,
If it s a planing hull,after a certain froude number it will switch in to planing regime and change its underwater shape ,thus the wetted surface area which will yield new skin friction.

[RANCHI OTTO]

It depends on speed, anyway check the resistance with this diagram for preplaning range (btw. displacement and fast displacement ships)

[erik818]

Ranchi,
I'm new to boat building/designing (and to this forum as well) and I'm not familiar with the abbreviations commonly used. I´m trying to understand the graph you posted on this thread. The 11 lines in the graph, are they representing the length/width for the planing surface (going from 4.0 to 10.0) or something else?

Erik

[RANCHI OTTO]

Lp = projected length of chine (ft)
V = volume (ft^3)
Fnv (Froude number volume) = v(ft/sec) / (g x Volume^1/3)^0.5
R = bare hull resistance (lbs)
W = Displacement (lbs)
R/W = specific resistance


This graph is to be used for fast displacement boats having a displacement near the 100'000 lbs for estimating rushly the bare hull resistance.

If there are some doubts, don't esitate to ask me...I will try to explain better.

[RANCHI OTTO]

Pavel915, already finish with your estimations?

Give me the following input and ,if you are interested, I will estimate the resistance (barehull and total) according to my model basin data base results and various methods:

Type of hull (round/chine/flat aft...)
LWL (length waterline)
BWL (breadth at waterline)
D (displacement)
LCG (longitudinal center of gravity)
NPR (number of shafts)
AW (frontal surface from waterline to roof of superstructure)
Vk (design speed)

[pavel915]

thanks ranchi otto for your care and help.
Now i have understood the frictional resistance formula (ITTC rule) and I have tried michlet, and finding that michlet is a nice program for finding frictional and wave making resitance resistance,
I have read some books,,,, and resistance and powering is now pretty clear to me.
thanks again.

[RANCHI OTTO]

Remember only that hydrodynamic is not an exact science....!

[MauroB]

Look in the book "Principles of naval architecture vol II" (SNAME).

[Leo Lazauskas]

thanks ranchi otto for your care and help.
Now i have understood the frictional resistance formula (ITTC rule) and I have tried michlet, and finding that michlet is a nice program for finding frictional and wave making resitance resistance,
I have read some books,,,, and resistance and powering is now pretty clear to me.
thanks again.

I wish I understood it and I wrote the program :eek:

Google around for the Proceedings of the 24th ITTC and in particular the Final Report and Recommendations of The Specialist Committee on Powering Performance Prediction. The ITTC compared three friction lines: ITTC 1957, Grigson's line and Kensei's line. The amusing part for me is that I can find recent boundary layer experiments results that validate (to about 2%) each of those lines. Which friction line you believe in depends on which experiments you have most faith in.

I think that I now understand flat-plate skin-friction +/- 7% :(

[RANCHI OTTO]

I use standard I.T.T.C. 57 and a CA coeff. of +0.0004 for large hull (over 30m length).
CA is a factor to be added to CF for the roughness of the immersed hull.

[Leo Lazauskas]

I use standard I.T.T.C. 57 and a CA coeff. of +0.0004 for large hull (over 30m length).
CA is a factor to be added to CF for the roughness of the immersed hull.

That's pretty standard. I have also seen estimates that add 0.1% to the skin friction coefficient for every day that the hull is in the water to account for additional roughness due to barnacles and other growths. Over a year that's a lot of additional drag!

Regards,
Leo.

[RANCHI OTTO]

In the contract speed specification I put in...
"The hull has to be in water not more than 3 weeks and has to be clean as the propellers.
The wind Beaufort up to 2...."

Calculating the propeller pitch I put a marge of 2% in the revolutions, so doing in service and with a dirty hull the engines develop the full power...

[Leo Lazauskas]

In the contract speed specification I put in...
"The hull has to be in water not more than 3 weeks and has to be clean as the propellers.
The wind Beaufort up to 2...."

Calculating the propeller pitch I put a marge of 2% in the revolutions, so doing in service and with a dirty hull the engines develop the full power...

Have you come across any rules-of-thumb like the 0.1% for every day in the water idea before? I know that it's extremely rough but I'd love to know if anyone else uses something similar. Is there any standard allowance that you know of to compensate for marine growth on steel, aluminium or other surface materials?

Regards,
Leo.

[RANCHI OTTO]

For me what is important is to reach the contract speed and for this reason what happens after...

The fouling on the hull depends on salinity of water, temperature,etc.

A ship in Iceland has much less fouling than a ship in the Maldive Islands.

Very difficult to have a rule...