Vacanti keel question - nose diving

I'm reading Vacanti, Principles of Yacht Design, Marchaj etc in preparation for keel fairing on my Express 37. I'll probably use the 63A012 section or something very similar. I'm still tracking down the section.

Vacanti says on p. 96,

A well known yacht had a tendency to nose dive while running before the wind ... The problem was not poor buoyancy in the bow but very poor foil selection on the keel.​

While I understand his point I can't follow his logic. I understand that upwind the lift/drag of one keel section would make it preferable to another. I understand that implementation and maintenance issues might make a 0012 section a rational choice. Downwind you want least drag.

But how would one section make the bow more susceptible to digging in and submarining?

Would there be any similar effect from the keel for digging the bow in upwind?

 

That had to be a REALLY poor section...
I so want hear how Vacanti found out that..

 

Very, very interesting.

I'm not familar with "Vacanti". Can you provide a title or other information about it?

 

Keel and Rudder Design article in Pro Boat Builder. You can find the PDF here:

http://www.sponbergyachtdesign.com/Articles.htm

 

Sounds like it's producing lift aft of the center of motion, causing the stern to rise and the bow to dig in.

 

With any keel boat would possess bow diving tendency just only becasue CE of drag of keel position is well below WL. If drag of keel is high, and keel has high span then more bow diving moment will occur.

 

Particularly with a long strut and a poorly designed bulb.

 

The reference I find on Goggle
Lister

 

Olsonist

Choice of foil section depends on the planform, surface roughness (of the foil) and likely sea surface conditions.
Most theory of aerfoil sections goes out the window in a seawy. The advantages of a 5 digit NACA foil over a 4, quickly evaporate and nice lift drag polars have very little real application. Aerfoils are designed to delay the separation of laminar flow but it's seldom that you'll get much laminar flow across the keel in that sort of boat. Consequently changes to the planform are usually far more effective than changes to the section over a simpler 4 digit NACA.
The grim reality of boats not hard stored and polished between races is that the laminar flow will be lucky to more than 300mm. As soon as you encouner a short chop the resulting boat movement trips the flow anyway.

All the theory ( Vacanti for example) is based in ideal foils in an ideal flow with an ideal surface. Even expensive tank validation of CFD prediction is based on a smooth water application.

 

Mike, to add more - selection of keel section and planform should be done only using VPP analysis; there is not much sence to play with section itself. Of course, VPPs are far from perfect, but at least they allow to evaluate efficiency of keel in range of heel angles, speeds and yaw angles as cummulative on boat's overall performance. I.e. keel designed for best lift or minimum drag is not always the best keel.

 

VPP analysis is anexcellent tool for making tradeoffs among keels with different characteristics. Its use should avoid the mistake of making decisions based on non-dimensional characteristics instead of dimensional quantatites.

The equations of those performance characteristics and their linkage to geometry have to come from somewhere. They can be based on interpolation to systematic test data (such as Delft series), math theory of various types, experimental sectional data, ad-hoc assumptions, analyst's intuition, or ???

 

VPP's are based on mathematical model of quasi stationary sailing condition, i.e. balance of non-inertial forces and moments. These models are based on physics and supposed to be correct; of course they need validation from experimental data.

I have developed several versions of VPP myself; one can get the hydro/aero model and algorithm work but often results from different VPP's are not compatible.

 

What I tried to explain in my note above was that VPP's need equations for the forces and moments of the sails, hull and keel, rudder, etc as functions of velocity through the water, wind velocity, heel angle, leeway angle, sail trim, etc, and it's those equations which have to come from somewhere. For a VPP to be useful as a design tool those equations have to be related to physical characteristics and that's where the interpolation to systematic test data (such as Delft series), math theory of various types, experimental sectional data, ad-hoc assumptions, analyst's intuition, etc comes into play.

 

VPP actually consists of 3 parts:
- equations of forces and moments, usually balance of X (longitudinal forces), Y (side forces) and MX (heeling moments).
- mathematical models of components, used in those equations; say resistance of hull as function of speed, heel and yaw; same for appendages; same for Y component; same for individual components of aerodynamic forces, etc. These components can be defined in different ways; most of commercial VPP's have options to select them.
- solving algorithm for equations; this is purely numerical method for system of equations.

So I don't know where is 'intuition' here

 

The "intuition" can be in the mathematical models of components IF the equations are set or adjusted based on what the person setting them feels is correct. A user might decide to increase or decrease some parameters in those models based on "intuition". Or the person who wrote the code may add some adjustments to the equations based on "intuition".

The important point is the math models of the components have to come from somewhere.