# Sailing yacht - Estimating ballast bulb added resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by Claudio Valerio Parboni, Apr 28, 2021.

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1. Joined: Jul 2018
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### Claudio Valerio ParboniJunior Member

Dear all,

I am trying to estimate the resistance added from a bulb on a sailing yacht in upright resistance.
• I have bare hull resistance calculated from maxsurf resistance
• I have calculated appendage added resistance with the methods described in the book "Aero-Hydrodynamics and the performance of sailing yachts" by Fabio Fossati
• I am now attempting to estimate the added resistance from the bulb
• Fossati's book says that there are some sistematic series in developement (book is from 2009 - I wonder if they are public now...) and that currently there are no immediate or mathematical methods of estimation without recurring to CFD or Tow-tank testing.
• Having calculated only rudder and the keel's fin resistance, I imagine that tip vortex and boundary effect on the bulb changes drastically, which puts another unknown in my research.
• I have looked left and right, but I couldn't find anything, I am just wondering if I am missing something stupidly obvious or if it actually is a complex issue.
• If you are wondering, I am assuming the bulb is a NACA section in profile, with round section shape (to simplify things, I know bulbs are shaped slightly different).
Thank you in advance,

Claudio Parboni

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### AlikSenior Member

Take any book on hydrodynamics or aerodynamics, there are plenty of formulas for stealmiled bodies, such as bulb.
When I developed my VPP 25+ years ago, I just programmed one of these formulas... from Russian reference book.

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### DCockeySenior Member

A common but bad assumption is to try to estimate the resistance of a body of revolution using the resistance of a 2D section with the same cross section. The velocity and pressure distribution around the body of revolution and the 2D section will be fundamentally different with lower peak velocity and minimum pressure around the body of revolution. If the bulb was not attached to the keel and has a long, tapered after-body then the drag coefficient based on frontal area due to the pressure variation around the body will probably be less than 0.010 as long as the leeway angle is not too large. The skin friction resistance due to the shear stress at the surface needs to be added to the pressure resistance.

But a keel bulb is not isolated; it is attached to the keel. This means the flow will be altered by the presence of the keel which will influence separation and possible vortices along the junction. The shape of the junction between the keel and bulb is critical, probably more critical than the shape of the bulb itself.

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### DolfimanSenior Member

In my home-made VPP (SA-VPP), and with reference to Hoerner Fluid Dynamic Drag, I use the formula (28) in 6-17 (Drag of streamlined bodies) , giving the equivalent friction coefficient for the total drag, based on wetted area :
Cdwet/Cf = 1 + 1,5 (d/l)^1,5 + 7 (d/l)^3
About the bulb/wing interaction, the issue is discussed in 7-7 (Wing-tip tanks) : on the one hand, you have an increase of the aspect ratio of the keel wing, on the other hand you can have parasitic drag due to interference between bulb and wing tip. So, in absence of either a sophisticated design of the bulb/wing interface and/or a dedicated CFD computation, one can wisely consider that pro and con effects just compensate.
About the bulb shape with rounded section, the basic approach is to use the Naca 4 digits formulation but with x^1,5

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