Beneteau First 30 - Keel position (Lead)

Hi together .. yes, this is again about Lead and the balance of sailing boats.

Looking at the new First 30 and First 36 from Beneteau it’s noticeable that the keels on these designs are positioned quite far forward, often even in front of the mast or at least aligned with the mast foot.

When applying the extended keel method from Nomoto, as outlined in Principles of Yacht Design, to estimate the Center of Lateral Resistance (CLR) and the main and headsail triangles for the Center of Effort (CE) of the sails, it appears that the lead is actually negative, meaning the CE is located behind the CLR.

Could someone explain the reasoning behind this design choice?

Thank you!

 

Maybe there is no reasoning, other than the experience of the designer demonstrating that the approach works.The problem with theoretical approaches is that they may be derived from data sets that don't extend too widely and aren't universally applicable.Have you found a report of a boat test that describes handling vices?

 

CM: Yaw Moment Coefficient
lambda: Yaw/Leeway

1: Yacht (1970-)
2: Aircraft (1914-)

 

"Could someone explain ..."

---

ape does what ape sees

Planet of the Apes (novel) - Wikipedia https://en.m.wikipedia.org/wiki/Planet_of_the_Apes_(novel)

 

The obsession with "lead" tends to obscure the fundamental problem.

A yacht must be hydrodynamically stable along the yaw axis

That is: the rudder only moves to balance the imbalance caused by the sails.

 

HydroDynamic Balance of the Yacht

We draw the yacht without sails, seen from above, with 6 degrees of Yaw/Leeway and one or two or three speeds

hydroDynamic_CLR =

+ H: Hull Yaw Moment
+ K: Keel Lateral Force
+ R: Rudder Lateral Force without moving the rudder, without touching the rudder, without looking at it

The yacht has Stable Balance if ...

If the hydroDynamic CLR is located Aft (!) of the yacht's Center of Gravity.

 

Once we have the yacht balanced from a hydrodynamic point of view ...

we play moving the rudder...

seeing how much it has to be moved so that the force of the rudder (RF) multiplied by its lever arm balances the force of the sails (SF) multiplied by its lever arm = height (h) between the lateral force of the water and the force of the wind on the sails multiplied by the sine of the angle of the sailboat's heel

 

CLR is not the only centre that shifts as a boat heels, but so does the longitudinal centre of floatation, which by itself can cause a change in the angle of incidence of the keel. Beamy boats in particular, are more likely affected by this.
Additionally, heeling moves the CE forward as well, with taller rigs experiencing a more notable shift.

Whether the rudder contributes to lateral plane and as such should enter into calculations regarding CLR seems to be a matter of debate among naval architects. In my experience, it would appear to and I am not certain that ignoring it's influence is the right approach.

I am furthermore not certain that Nomoto's approach works as well on boats with a deep(er) forefoot and where the canoe body itself contributes to lift in a significant manner.

I have taken note that a number of designs by Britton Chance seem to have their keels unusually far forward, yet they are reputed to be well-balanced performers. Equally, it may be rather doubtful whether in these instances the centre of gravity will be aft of the CLR, dynamic or otherwise.

If we accept that the geometric centres of the hull and it's appendages do not reflect true CLR, the same holds true for the sails. For determining CE, Marchaj offers a more up to date method in "Sail Performance" that goes some way to account for the actual centres of lift on the sails, rather than guesstimating sail balance via some "black magic" fudge factor called lead.

 

We can make the whole thing more entertaining on a peaceful, rainy Sunday afternoon.

But the real issue is simple: it's very difficult to balance a yacht if the keel's hydrodynamic center is forward of the yacht's center of gravity.

 

In the drawing above, "M" (Munk Moment 1 + 2 + 3) is "H"



Munk_1 due to yaw/leeway

Munk_2 due to roll/heel

Munk_3 due to pitch

Max Munk - Wikipedia https://en.m.wikipedia.org/wiki/Max_Munk

H = (Munk_1 + Munk_2 + Munk_3)

 

If a yacht is well balanced from a hydrodynamic point of view... then you could almost say that it doesn't matter where the sails are.

Thomas Harrison Butler in Memoriam

Thomas Harrison Butler - Wikipedia https://en.m.wikipedia.org/wiki/Thomas_Harrison_Butler

 

Everyone against the Helmsman

The yacht heels. When heeling, the hull's symmetry is broken, and the bow angle becomes the angle of attack (AoA) of the leeward wave. "Leeward wave" is an expression used by enthusiasts to refer to the Munk Moment

Furthermore, the keel (K) produces an enormous amount of lateral force forward of the yaw axis.

And on top of that, the force of the sails (FS) joins the party.

Everyone against the poor helmsman

 

Modern (1970-) Yacht without Sails and Big Ocean Waves

 

It would be good if we forgot about "lead" and focused on the essentials:

good hydrodynamic balance

The yacht, while trying to surf, trips over its hull and trips over its large keel.

And tries to capsize.

The frightened crew asks to be rescued.

And the yacht turns (see the end of the video) and crossed alone the Atlantic Ocean.

That helicopter should have been saving lives elsewhere in this tragedy, Azores 2015

 

Is it the keel that is quite forward , or the mast that is quite aft ???