Sailboats, wide sterns and bow-down trim - what is too much

Hi everyone,

I’ve been analyzing modern production hull shapes with wide sterns and high interior volume. One trend I’m struggling with is the longitudinal trim shift as the boat heels.

On the hull I’m currently modeling, I’m seeing a significant bow-down trim as heel increases.
I’ve attached some stability and trim analysis images. I’ve tried balancing the LCB (and hull generally), but I can't seem to flatten the trim without fining out the stern and losing the "balance".

So, my question: Those with design or racing experience, what are the "acceptable" limits for bow-down trim at 15–30 degrees of heel? Is there a point where form stability is outweighed by poor handling, or do we just slap twin rudders on and call it a day?

I’m a bit short on real-world comparison data, so any insights would be appreciated.

 

You are right, with such hull you have more bow down trim with heel than with a classic one. Look at the Dolfi 32S example in the document attached « Gene-Stab Sailboat 3.5_Examples » : page 20, I showed the trim evolution on 0° -180° heel range, looks like yours with similar magnitudes.

 

Thanks.

 

… The reason is, due to the dissymetry fore - aft, the hull no longer heels by pivoting around its central axis but around an oblique one roughly from bow forefoot to leeward rudder root. The davantage is this maintains the bow forefoot close to the water surface. The drawback is a very dissymetric heel waterlines, the hull moves forward in the water like a snowplow, but it seems that the high propulsion power generated thanks to the righting moment so provided can compensate the extra drag.

 

Something like in the attached screenshots in this post?

 

Indeed

it's a very old problem.

The bow dips due to:

(A1) the sailboat's heel, and

(A2) sailing in waves downwind because the center of gravity is forward of the center of Flotation (through which the Pitch axis passes), and furthermore, the center of Flotation moves even further aft (relative to its calculated position with the sailboat stationary) due to heel and/or the waves that wet the entire stern. Moreover, the vertical component of the Froude-Krylov force in a 12-14 m yacht is located aft of the center of Flotation.

And

as the bow dips (A1 + A2) ...

(B) the Munk Moment (the hydroDynamic Yaw Moment of the Hull) increases dramatically, and furthermore, the hydrodynamic center of the keel is forward of the center of gravity and center of buoyancy, through which the Yaw axis passes.



hydroDynamic Yaw Moment

Roll/Heel + Leeway/Yaw

Without (!!!) Pitch

With Pitch (Bow down) = aprox. 2 (!) x

.

 

Yes exactly, again see the Dolfi 32S at heel 20° in this document page 77 .

 

(1a)



(1b)

Outcome

(1a) "Freaking Frustrating" (british gentleman = trad. "Fuxxx Frustrating")

(1b) The very expensive autopilot is unable to control the yacht, condemning the user of the brand-new yacht—recently purchased and awarded "best cruiser" by the specialized press—to 16 hours at the helm in typical summer conditions.

(2) The yacht heel in a very unpleasant way: knock down

(3) The yacht heel and the mast goes into the water

(4) The yacht capsizes.

 

Solution

Good longitudinal balance:

L_CG > L_CB > L_CF

+ and

(A) Create hydroDynamic Lift with

(A1) Force, Speed
(A2) Angle of Attack, and
(A3) a well-constructed wing with Hull flat surfaces and a good wingspan, because Lift force depends on the square of the wingspan



Problem

This solution is very expensive above 1 Ton

2025: 3-3.5 Ton Sport Yacht = 180-240-300 K Euros

---

Square–cube law - Wikipedia https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

The law of the square and the cube is undeniable.

In other words: the idea of making slow yachts in the shape of a high-performance sailboat is, to begin with, ridiculous, and ultimately absurd if the centers are left as they were in 1979.

 

For Slow Yacht
(10 sqm per Ton) ...

Classic Solution is better

 

the Longitudinal Position of The Center of Flotation (L_CF) ...

of this 1-ton Plywood Epoxy windsurfboard is at 58% LWL

(like a 1979 yacht)

But

its Center of Buoyancy (L_CB) is at 60%

and its Center of Gravity (L_CG) is at 62% (!)

Its excellent longitudinal balance makes it almost almost almost a British classic.

Furthermore, the hydroDynamic center of the keel is at 62%: the keel is Neutralized



And the longitudinal metacentric height is about 18 meters = 3 (!) x LWL

And

it's very very Fast because it's small and light

Now, If ...

(1) a yacht is longitudinally unbalanced, and on top of this

(2) has the keel working forward of the Yaw axis, and to top it all off ... is

(3) the typical 5-10 ton snail-wagon carrying its house on its back ...

 

The question is, how to design a 5-10 ton yacht?

In other words: how to design a slow yacht without falling into all the bad habits?:

(1) Royal Navy destroyers from the late 19th and early 20th centuries

(2) Motorboats

(3) Dinghies: a 470 with a Japanese crew hoist 50-80 (!!!) square meters per ton, and the crew can even trim the hull while moving around

(4) MiniTransat: hoist 30-60 (!!!) square meters per ton

(5) Class 40 and IMOCA boats: high-performance racing with astronomical budgets

Today, imitating a high-performance sailing boat is practically impossible above 3 tons and also costs 240-300 thousand euros

In my opinion, there is no solution to the whim of a slow yacht (10 sqm per Ton) that imitates the shape of a high-performance sailboat.

4 M Euro

Finot-Conq 56: A sailing sculpture for four million euros https://www.yacht.de/en/cruising-yachts/finot-conq-56-a-sailing-sculpture-for-four-million-euros/

 

4 K Euros (second hand)

Surfing controlled by a clumsy tiller pilot with little more brain power that a crystal radio

Crystal radio - Wikipedia https://en.wikipedia.org/wiki/Crystal_radio

Top speed Surfing with total Control:

12-14-16-18-20 knots

19.19 LWL

The old man and the sea



Slow Boat
Classic Solution

 

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

(H) Hull hydroDynamic Yaw Moment: Munk Moment (Pitch + Yaw/Leeway + Roll/Heel) = 600 Newtons

(H) Munk Moment (Yaw/Leeway + Roll/Heel) = 300 Newtons



H: Hull Yaw Moment
K: Keel
R: Rudder

And these are figures for a small, lightweight boat.

I mean

+ the bow-down effect is usually devastating for Max Michael Munk Moment

+ you have to take into account the enormous lateral force that a modern Keel can create

+ if we consider the orbital current of the wave (for example, 3-4 knots) on the rudder blade of a slow Yacht traveling at, say, 7 knots ...

= "Freaking Frustrating"

 

Max Michael Munk Moment
(Roll + Yaw)

I average a handful of different estimates, each reasoned along different lines, because I tend to exaggerate and because I'm passionate about the subject.

This is the simplest estimate.

Multiply by 2 to get a rough idea of the effect of the bow dipping.

The simplest solution for a yacht as slow as a snail, carrying its house on its back, is the traditional approach: wide bows and narrow sterns ... or the scow.