Determining the center of lift for a deep vee hull?

Now the numbers finally fit

Mast is around 35% LWL

Hull draft (h) i don't know; but it must be between 30 and 40 centimeters and in any case there are two hulls.

The Angle of Attack (AoA) maybe is about 15 degrees = 10 degrees Leeway + 5 degrees Bow or something like that but you get the idea.

 

Well yes, the Tiki 21 Hull produces a lot of Lift

and my educated guess is that the Hull Center of Lift is ... perhaps maybe ... more towards 25% (?) LWL.

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It is unstable downwind and needs a 'drag device' to ride the storm with big waves.

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In case you want to sail downwind with big waves ...

... it would be much better to use a Hull that produces at bow ... zero Lateral Lift ... and plenty of Vertical Lift

 

Partly depends on the shape of the leading edge:

if the bow is vertical (or becomes vertical when pitching the sailboat) the Center of Lift is far forward (moreover: many formulas are presented so that the result is a Vector right at the bow).

If the shape is of the "Delta Wing" type the Center of Lift is further aft and in some cases much further aft.

 

The two extremes

 

References

Nomoto K, Tatano H, 1979, "Balance of Helm Sailing Yachts, a Ship Hydrodynamics Approach on the Problem". 6th Symposium on Development of Interest to Yacht Architecture, Amsterdam, pp. 64-89

Keuning JA, Vermeulen KJ, "The Yaw Balance of Sailing Yachts Upright and Heeled", SNAME, 16th CSYS, Annapolis, 2003

Hörner, Fluid Dynamic Drag
Hörner, Fluid Dynamic Lift

Slooff, JW, Aero- Hydromechanics of Keel Yacht, 2015

 

"the [HydroDynamic] Yaw Moment is still being under predicted, so CLR [prediction/estimate] is generally too far aft"

(JA Keuning, KJ Vermeulen, HISWA, 2002)

 

Wow

HISWA, 1979 (!)

I don't know where I have misplaced the contribution of Professor Kensaku Nomoto from Osaka.

His sailboat was called "spring wind", in an accident Professor Nomoto drowned, rest in peace.

 

TU DELFT

Balance of helm of sailing yachts - A ship hydrodynamics approach on the problem | TU Delft Repositories https://repository.tudelft.nl/islandora/object/uuid%3A665e754f-128b-4fc3-ac9d-0bc33d9879f3

Osaka University
Department of Naval Architecture, Japan

Presented at: Proceedings of the 6th Symposium on developments of interest to Yacht Architecture, HISWA 1979

 

Hull CLR

If the maximum draft is just aft and it has a true Delta Wing profile, then the lateral pressure is moved well aft (50% LWL) and the sailboat is centered and balanced. And the best part: as the yaw increases the further aft CLR moves because of the Vortex that is created. This is the big difference between a true "classic" and the "modern" (1970-): the "classic" tends to calm down and find the balance, but in the "modern" the more yaw the more it wants to yaw.

And the devastating effect of Pitch on some sailboats: the lateral hydrodynamic force increases, it can increase from quite a lot to a lot depending on how sharp the bow is and, on top of that, the force moves more towards the bow having a greater destabilizing lever arm.

 

I should have made four drawings instead of three, as the drawing above 40% LWL does not fit with the drawings below as it obviously lacks more depth at the bow: more fore-foot.



(Kensaku Nomoto)

 

Balance of a modern sailboat (1979-)

The hydrodynamic Center of the daggerboard, its center of lif (K) in order not to be destabilizing, must be somewhere between the longitudinal position of the center of Buoyancy (LCB) and the longitudinal position of the center of gravity (LCG) of the sailboat.

The effective Aspect Ratio (ARe) of the centerboard is twice its Geometric Aspect Ratio. And the Area of the Daggerboard we extend it to the Waterline, which is the old lazy man's method.

For the Hull Yaw Moment we make an arithmetic mean of three estimates

For the Rudder Force we calculate by the lazy method a Rudder Area x 0.40, and if we want more entertainment then instead of this lazy method we calculate the Down-wash of the daggerboard over the rudder, i.e. how much the water flow diverts it. If the sailboat has two rudders then Rudder = 1 x Rudder

 

Look

It should be noted that the hull of a modern sailboat creates little, very little or no (y = 0, in an ideal fluid) Lift ... but it is capable of creating quite a large turning moment, Yaw Moment ... because of its large lever arm

 

The contribution of the hull of a modern sailing yacht to the lateral force is very little, usually no more than 10% of the total and more like 5%, a figure similar to ... Leeway (radians) x number "pi" x h^2 x dynamic pressure (q = 0.5 x 1025 kg x V^2)

But if the hull is very sharp (s) and vertical walled (v) it can produce quite a lot of Lift. LA: Lateral Area up to say for example 0.60 LWL or up to where the maximum draft is.

 

And the sails of the sailboat

Well, good question

as T. Harrison Butler said ... If the sailboat is hydrodynamically well balanced ... then the sails can go pretty much wherever you like it so to speak. The obsession with fitting the Sails to the CLR is a side branch of a poorly solved problem: hydrodynamic balance: CLR aft of LCG without moving the Rudder, without touching it, without looking at it.

 

The striking thing about these matters is that in the end they depend on things that seem like simple details.

"Wanderer II" at first glance looks like absolute perfection (at least to those of us who admire the classic solution) But the bow is too sharp compared to Butler's, and that explains the difference in behavior. And the same happens with sailboats from the 70's of the last century that at first glance look the same.