Effect of Friction Lines on the Resistance of a Friction Plane

REPLACES EARLIER POST

Does that imply "viscous pressure resistance" is the integral of the pressure over the body surface in viscous flow with a rigid surface rather than a free surface?
- or -
Is it the difference between the drag of the body in viscous flow with a rigid surface minus the skin friction drag estimate?

They are not the same.

 

Yes, the (double) body is assumed to be deeply submerged and making no waves, i.e. essentially zero Froude number.

 

Leo, you responded too quickly. I changed the question.

 

I have to get sleep, so my apologies for resorting to wiki diagrams.

The components of the total resistance are shown in this page.

http://en.wikipedia.org/wiki/Ship_resistance_and_propulsion#Components_of_resistance

Note, however, that this division into components is not universally accepted.
Many people forget that it is Froude's HYPOTHESIS, not Froude's Law by which we justify separating total drag into wave resistance + skin-friction.

 

You mean you sleep??

It is taken from this:



From Larsson & Baba, "Ship Resistance and Flow Computations"- Advances in Marine Hydrodynamics, 1996.

And if one wishes to be even more pedantic, need to account for air resistance too, unless the boat has no freeboard

 

According to:

I should have used the term "viscous resistance" instead of "frictional drag".

I have replaced the double-body with a body of revolution of identical cross-sectional area.
The background of the method is described in my paper "Design Process Automation for Sailing Yachts" in the proceedings of the 2nd HPYD conference, Auckland, 2006
Meanwhile I have improved the method and use Patel's approach for the boundary layer calculation.
Uli

 

There is an interesting proposal by Min & Kang in their paper "Study on the form factor and full-scale ship resistance prediction method", JMST 2010.

They suggest another way of establishing the 'form factor', but a bit long winded!

 

Remmlinger

Interesting paper you wrote in J.Small Craft Tech. on design loads for keels.

 

Thank you for your kind comment. It's another topic and was discussed in a thread that I opened up in the sailboat forum.
Uli

 

Seems everybody is asleep. While the math gurus are sleeping, I am struggling with formula based Wetted Surface Area calculations (the S or Wsa), the necessary element in skin friction calculation. I am using at least 6 different variations which are;

1. Taylor’s formula revised: WSA = C/5.916*(L*Vol)^(0.5)
2. Baier-Bragg formula: WSA = C*L^2/10
3. Froude formula: WSA = Vol^(2/3)*(3.4 + L/(2*Vol^(1/3)))
4. Haslar formula: WSA = Vol^(2/3)*(3.3 + L/(2.09*Vol^(1/3)))
5. Denny-Mumford: WSA = L*(1.7*T + B*Cb)
6. Holtrop-Mennen formula : WSA=L*(B + 2*T)*Cm^(0.5)*(0.453 + 0.4425*Cb - 0.2862*Cm + 0.003467*B/T + 0.3696*Cw )+ 19.65*ABT/Cb

I find that each one gives different results. So what are you guys using? Or more specifically, which one do I use for a particular hull form?

 

Why not use the wetted surface area calculated from the hull shape? It should be simple to determine if you have the shape in "CAD". If it's a set of lines on paper then measure the wetted girth at each station, and use the appropriate version of Simpson's Rule or other quadrature formula to get the wetted surface area.

If you don't have the hull shape then one of the formulas listed could be used to generate an estimate. Probably best to determine what types of hull shape each was derived from and choose one which best fits the hull shape you're interested in. What is C in the first two formulas, a coefficient selected on hull shape?

 

I agree that is a good way for many hulls, but it doesn't always work well with spheres, or some ellipsoidal-like bodies.
It's often better to sum a large number of small "elemental" areas.

 

Quite agree ACAD is more accurate. Works well for volume too. Beats the Simpson's rule. That's way down the line.

I do most of my preliminary design in Excel. Specifying first the ratios and coefficients, then carrying out dimensional analysis to arrive at a parent hull from which I will work out the displacement, wavemaking, skin friction, air drag, powering, ect. That is when I use software suitable for the study.

The problem is, I can be working on a very small boat, say a kayak then sometimes a full blown ship like an 80 footer.

For example, in a kayak size, the WSA can vary by as much as 15% between the formulas. So I am wondering if all my calculations are off?

 

Are you responding to using the actual wetted area, or to my suggestion to use girths and Simpson's rule? Based on yur comment "It's often better to sum a large number of small "elemental" areas" I assume the latter. In that case I would agree.

 

Sorry for another one of my 4am posts.

Just to clarify: using girths and Simpson's rule is not a good method for calculating WSA. It is better to use elemental areas.