What do "tender" and "stiff" mean?

[Buck Turgidson]

Will also depend on which keel and rig configuration the original owner chose. A deep lead keel with a std rig is going to be stiffer than a shoal draft tall rig version of the same hull design.

Yes and it's not the stern section that matters in fact having a wide arse isn't optimal. It causes the centre of buoyancy to move aft as you heel pushing the bow upwind and down into the waves.

Fat all over is the new normal in the Mini 650 class which are becoming more rectangular with each new design.

 

[RupertW]

Yes and it's not the stern section that matters in fact having a wide arse isn't optimal. It causes the centre of buoyancy to move aft as you heel pushing the bow upwind and down into the waves.

Fat all over is the new normal in the Mini 650 class which are becoming more rectangular with each new design.

It was a specific question about Beneteau First 30s - they started with round hulls then went wider and flatter undersides at the stern with an extreme (for the time) First 30E, then ten years later the move to incredibly fat wide sterns started and hasn’t stopped getting more extreme.

 

[William_H]

Will also depend on which keel and rig configuration the original owner chose. A deep lead keel with a std rig is going to be stiffer than a shoal draft tall rig version of the same hull design.

In my explanation I tried to differentiate between stiffness being more related to initial heeling as opposed to self righting moment relating to much greater angles of heel.
I called the latter the pendulum effect for self righting. A shoal keel of same weight as a deep keel will have much the same stiffness as the deep keel. Only as heel increases will the difference be seen. Trigonometry will show actual difference but 45 degrees plus is where you see the difference a lot.
There are of course other disadvantages of shoal draft keel like less efficiency at resisting lee way with more drag in doing so.
Re Zagoto above. Heeling does reduce pressure from the sails that causes the heel but that aspect is good. But not significant (trigonometry again) however it has a detrimental effect in moving the drive from the sails away from the centre line of the boat so causing a turning force which must be counteracted by the rudder causing drag. In addition when the boat heels we find the shape of the hull in the water becomes asymmetrical so causing more turning force to the point where on a reach you can lose control of the direction the boat is going. Called a round up. or broach. So much worse because the rudder also loses sjde area in heel and becomes inefficient
Another detriment to boat performance when heeled is that the keel will tend to present less sideways area to the water so the boat drifts to lee ward. Not good when beating to wind ward. Again this is insignificant at small heel angles but of great concern beyond 45 degrees (trig again)
After all that, heeling has a huge use in that it allows the boat to take sudden gusts and just lean over until the gust passes. So ideally we set sail area for some heel but allow for more heeling in a gust. Unlike a catamaran which in not heeling takes the additional forces of a gust in strain on gear and potential distaster without warning. ie a lead mine tells you when it is in trouble while a cat gives no warning. ol'will

 

[Laminar Flow]

OK it's getting a little muddled now.
So let's introduce Righting moment. This is the normal term for measuring the "stiffness" or lack of.
It is the moment described by the mass of the centre of gravity times the distance between the centre of gravity and a line between the centre of buoyancy and it's metacenter.

Lets ignore the metacenter initially and just say buoyancy = up Gravity = down and when the boat is upright they are one above the other so there is no distance so no RM. Every monohull is like this regardless of hull shape.

However, as soon as a force acts on the yacht to induce heel then the yachts underwater shape changes and the centre of buoyancy moves. It moves until it is displaced by a distance large enough that the force of the yachts centre of gravity x that distance is equal and opposite to the force acting on the yacht. You are back to equilibrium.

For example I step aboard my boat and it heels towards me. Just enough for the righting moment to equal the force i'm exerting (via gravity) onto the boat.

The shape of the underwater profile and the vertical distance between the C of gravity and the C of buoyancy will dictate the lateral displacement between the two and therefore the moment arm.

So a boat with a low centre of gravity and a high centre of buoyancy will create a longer arm therefore heel less than the same hull shape but with a shorter arm.

Now the pedants will all be screaming that the arm is measured to a line between the c 0f buoyancy and the metacenter but for this thread I'm simplifying for the OP.

Lets think about hull shape then.
If we have a perfectly cylindrical hull shape (with internal blast) then there is no change in underwater profile as the hull heels and no lateral movement of the centre of buoyancy. In this case only the height difference to the c of gravity gives the arm required for our righting moment.

If we have a wide flat underwater profile then as we heel the centre of buoyancy moves laterally and this is also away from the c of gravity, happy days, we get a long arm very quickly so we don't need much heel to balance the force or we can make do with a lighter keel and more heel.

Now if the force we want to counteract is the total reaction on the sail caused by wind we get another effect as the boat heels the effective sail area reduces so left to it's own devices in a perfect world the monohull will continue to heel until the righting moment equals the force of the wind, and underwater we have a similar reductive effect as we heel because the effective lateral area of the keel is reducing allowing the boat to slip to leeward which reduces the wind force too.

So if we have a wide hull with therefore high form stability it will not heel as much for a fixed wind force compared to our narrow folkboat. But because it isn't heeling as much it is under higher load. It requires stronger rigging which requires a stronger structure which all adds weight which needs more buoyancy.

My Twister is VERY similar under the waterline to a folkboat, I've CAD modelled both from table of offsets the difference is a little more draft and therefore a little more beam at the waterline, rather like a massively overloaded folkboat. How much? About double the displacement but it's amazing how little is required to achieve that.

I also have about double the keel weight.

So it's not surprising that at 30° heel angle my righting moment is about 18KNm compared to a folkboat which is about 9 KNm.

A first 25,7 has a 30° RM of about 9 KNm with only 620kg of Keel weight. A Pogo 8.50 is @ 14KNm with an 850kg keel. Wide boats, light keels.

So what use is this RM value? Well it dictates how much force is required to make the boat heel (30° in this case) and that dictates how much sail you need to achieve that. Sail area, rig strength, hull strength all dictated by the righting moment. And of course sail area = power= speed.

Very good.
I would just like to add, and in terms of the OP's question, that stiff/tender is in reference to initial stability. It has nothing to do with ultimate stability.
To give the Op some practical reference:

Slocums's Spray: beamy, shallow and initially very stiff. Average angle of heel under sail around 10 - 15 degr. Angle of vanishing stability about 100 degr.

Classic, narrow and deep keel British design, say a Robert Clark design of 1960, initially tender and will sail happily at 50 degr. heel. Angle of vanishing stability: close enough to 180 as to make no difference.

Modern boats are somewhere in between: much stiffer (less likely to heel) than a narrow, deep keel classic, but with an ultimate stability of between 120 and 135 degr..