New Boats 2005

'Laminar sections'

I would be a little reluctant of these.  In order to produce a section that will do that, firstly it would have a fairly short chord (I found my book and at 4 knots in still water the lamina layer will last at most 50cm along a surface, and that is in still water, with a very smoothe surface).  Given that the water, especially near the surface where most of a foil is operating, is never still the chances of having any truely laminar flow is unlikely at best.  The other thing is that in order to maintain the laminar layer the section has to change in thickness very slowly along its chord so as not to get seperation.  The upshot of this is a section that is very thin for a large amount of it's chord (towards trailing edge).  This makes for an inherently less stiff foil (not good for performance in waves) and also more likely for you to put a foot through it when you capsize. 

I would like to be proved wrong about this but from what little I know on the subject my personal view is that they are a bit of a waste of time. 

(This is really not doing my degree much good, but then again these posts are probably the most use I am going to get out of it! )

Lamina Flow...

Lamina flow in the real world is rare, but in principle does occure everywhere that a fluid passes over a surface.  From a dinghy sailing point of view there is little that can be done other than keeping your hull smoothe & fair. 

Lamina flow begins with a uniform flow, that is a stream of fluid flowing with every molecule moving in a straight line (or an object moving through completely still water).  In reality this is impossible if nothing else due to the rotation in water linked with waves.  However the principle is very powerfull when it comes to the mathematics involved in calculating fluid flows. 

As water flows past a surface (eg hull, rudder) friction with that surface slows the flow, to the point where the water directly in contact with the surface does not move in relation to it.  The speed of the water (in relation to the surface) increses as you move away from the surface back up to the speed of the free stream (ie: before contact with the surface).  Because the speed of the water at any given distance from the surface, is different to the speed of the water next to it, both closer to, and further from the surface, there is a shear force produced between these 'layers'.  This shear force disrupts the liniarity of the flow and causes the water near to the surface to become turbulent. 

As the flow contiues over the surface, the fluid flow close to the surface actually smoothes out and forms a very thin layer of laminar flow.  This is where having a smoothe hull is important.  If the imperfections in the hull surface do not stick out above this new layer of flow (called the 'laminar sub-boundry layer'), then the surface can be considered as hydrodynamicly smoothe.  Both mathematicly and in the real world this has great benefits in terms of efficiency through the water. 

The transitions between these types of flow are dictated by a quantity called 'Reynolds' number' (Rn).  It is a function of the density of the fluid, its velocity.  As you move along the hull from bow to stern the value of 'Rn' increases and when a certain value is reached (dependent, in part, on the roughness of the surface) flow changes from being laminar to turbulent. 

Lamina flow is better in terms of drag, but the tranistions between types of flow on the surface create more drag.  So in reality getting the lamina sub-boundry layer established early on is the best option.  At a boat speed of around 5 knots lamina flow will only continue over the first two feet of the hull and that is assuming that the water is completely still.  In the real world the best you will probably get is about six inches of lamina flow on the bow of a hull.  This means having a hull which is smoote enough to be hydrodynamicly smoothe is the important thing to worry about.  If you are on the plane the mechanics change completely and lamina flow does not really enter the 'equation'. 

I think most of this is right, (I'm doing it from memory because a friend is borrowing the best two books I have on the subject).  I will read up and may update this tomorrow, for now I think this covers most things.  If you have any questions do ask, I'll see what I can drag up. 

Enjoy. 

Ok thanks that makes it really clear !

That was a brilliant explanation !