Fear of tipping over is inherently primal.
The boat terminology you're talking about is actually called initial and secondary stability, this is translated into what is called a "capsize ratio,

until people get used to the effects of a boat tipping (heeling) that primal fear of falling over, losing footing, balance, is pretty overwhelming, those people try to rationalize the fear, uncertainty, or impending doom feeling by using numbers and words to justify the way they're feeling because, their subconscious hasn't aligned itself with the realistic action of a boat under sail.

As people start trying to self educate, & start shopping boats and they encounter some of these numbers and stable or tender discussions even though if they don't quite understand the numbers are just more about design & theoretical characteristics but! they will try to fit their fears in with the numbers

So we will see opening statements like; is such and such boat tender?
Is it blue water capable?
Is such and such boat stable?


Of course all this is dancing around the even stronger primal fear of CAPSIZE! fllipping over and being tossed out of the boat!

Coastal boats are generally lighter and more agile (tender) then heavily ballasted
full keeled ocean going craft.

This these discussions will often lead into statements like "oh so and so sailed his little Catalina 27 down to Bermuda!"

But but generally speaking if a boat is used within its designed parameters it usually is pretty safe.

Hope this helps or maybe get you even more confused!

It is said my hunter 34 is a tender boat but the same people that say that do not realize the boat has a huge Mainsail and know very little about how to balance a rig or reduce sail size.

 

A tender boat will heel more easily. Which means from a sailing POV, you need to think about reefing sooner. This could be because the rig is a bit larger and/or the ballast is a bit lighter

A stiff boat could be the opposite. heaver ballast or a smaller rig. And less likely to heel

Froma hull POV, a wider hull will be a bit stiffer but a skinny hull would be more tender. Think John boat vs canoe John boat would have more hull stability and be stiffer, a canoe would be more tender as it has lesser hull stability...

Shoal drafts could be more tender since they have less righting moment than a boat with a deeper keel all other dynamics being equal

 

The terms "tender" and "stiff", when used correctly refer to the form stability of the hull. It doesn't really have anything to do with sail area or keel.

In a nutshell a tender boat tends to heel more easily due to it's hull shape. A typical heavy cruiser is likely to more tender than a race boat. The race boat is able to carry much more sail area because it has more form stability, ergo it is "stiffer".

Look at the shape of a modern high performance boat with it's very wide, flat hull and transom. That shape is going to resist heeling much more than a narrow, rounded hull with a narrow stern.

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Schock. Thanks for the correction. Always though the combination of everything helped determine if a boat was tender or not.

 

Some older boats are "tender" by design, as they have a longer waterline when heeled. Length at the water line is a factor in a sailboat's theoretical maximum speed. There was an outdated racing design scam that had a short waterline, when standing upright at the dock, but a longer one when heeled over underway. Therefore, they could sail faster than the book said they could and these boats had handicaps they could outperform. More modern hull designs have plumb bows, wedge shaped hulls and hard chines at the transom, making maximum use of interior space for length overall. They don't need to heal as much to attain that waterline.

Have you ever heard the joke that goes something like the reason young children can walk across fence posts and hang from trees, is because no one has taught them about gravity yet? I don't think a new sailor could easily tell a tender from stiff boat, if they had little to nothing to compare it too. Very few keel boats are so tender as to be a problem. As Denise said above, one needs to learn the boat is not going to capsize, even if it is tender, and how to manage sail area. That goes for all sailboats. Shopping on the stability numbers is not a great idea. I don't think there is even consensus they are correct.

 

In short, a "stiff" boat tends to heel less and sails best up to about 15 degrees of heel. A "tender" boat sails best up to about 30 degrees of heel.
Both are perfectly safe for coastal sailing. If you are new to sailing, walking around at 30 degrees might be disconcerting.

That said, it is only an rough approximation and lots of exceptions abound.

 

A tender boat will heel due to external forces or list due to internal forces (like sitting on the rail) more readily than a stiff boat. Stiff boats have relatively high initial stability and if heeled, will return to a state of equilibrium rapidly.

Factors that influence a boats relative tenderness include the height of the centre of gravity above the keel as well as form stability or the shape of the boat.

A boat that is too stiff or too tender is not desirable. Stiff boats tend to give jerky uncomfortable rides and things can break. A boat that is too tender may heel excessively.

Factors affecting the relative height of the centre of gravity can include weight distribution including depth and weight of the keel, weight of the rig and placement of machinery and tanks. A boat that may be relatively tender with the rig up and slack tanks may become a stiff boat with the rig removed and tanks pressed up.

Form stability. Relatively wider boats tend to be stiffer than relatively narrow boats. Catamarans for example could be considered stiff, but some catamarans will be stiffer than others. Relatively beamier monohulls can be stiffer than relatively narrower monohulls.

The relative stiffness or tenderness of a boat is independent of it's ultimate stability or tendency to recover from a capsize. A catamaran for example may be relatively stiff, but incapable of self righting after a capsize. A narrow monohull with deep heavy keel may feel initially tender but recover rapidly from a capsize.

Ideally a boat would be neither excessively stiff or tender.

 

The benefit of secondary stability people don't understand, the more the boat tips the more it resists tipping unlike a boat with a typical flat bottom that will feel very stable until it goes "over" all at once.
This may or may not be true of larger boats but on smaller craft it's very true, I've always loved the "tippy" feeling of a canoe of which just strikes mortal fear in people.

Picture it! Somebody holding on for dear life they're so afraid they're going to roll over and die In 12"
of water!

 

As has noted, the term ' tender' as properly used in naval architecture strictly applies to the stability of the boat. But colloquially, people will also apply the terms stiff or tender to how easily a boat heels.

In yacht design, stiffness generally is applied to form stability. Form stability refers to the stability that comes from the movement of the center of buoyancy due to heeling. You can visualize form stability by thinking of a sealed tube with the same weight and length as a wide plank. If you rotate the tube on its long axis, the center of buoyancy does not movement. But if you rotate the plank the same angle one edge of the plank is pushed deeper in the water and the other edge lifts out of the water. As that happens, the center of buoyancy moves towards the deeper side, and since the center of gravity does not move the lever arm between the center of buoyancy and gravity increases and develops more stability on theplank, while the lever arm does not change on the cylinder.

Pretty much all boats fall in a zone between the plank and the cylinder. Boats that are closer to the cylinder are tender, while boats that are closer to the plank are stiffer.

Whether a boat is stiff or tender impacts a lot about the behavior of the boat. As broad generalities:
A stiffer boat can use its stability to carry more sail area relative to drag and displacement to improve light air performance.
A stiffer boat can carry more sail area in a building breeze and so don't need to be reefed as soon.
The generality easier driven hull form of a stiffer hull form helps them in heavy conditions.
The stiffer boat will tend to heel in phase with wave face. In a short chop that means that it rolls faster but through a smaller angle. A more tender boat tends to be out of phase with the wave train. That means it rolls more slowly through a wider angle, and with more abrupt stops at the ends of the rolls.
The reality is that a tender boat tends to be sailed at steeper heel angles which is harder on a crew, but that is a little offset by the slower motion. Tender boats tend to be less forgiving in changeable conditions, and tend to develop more weather helm. But the extent of whether that is true is influenced heavily by the rest of design of the rig, hull shape, keel, rudder, weight and buoyancy distribution, and so on.

Jeff

 

You can't have stability without gravity. Gravity and the vessels centre if gravity has to play a role. You can put the beamiest boat in the world in the vaccum of space and spin it about it's axis with 2 fingers.

For a vessel to be stable on the water the centre of gravity must be below the metacentric height. A neutral metacentric height results in a vessel with no stability. A negative metacentric height results in a boat that capsizes or rests at an angle of loll.

The role if a ballast keel on a sailboat, aside from providing lateral resistance and providing a foil shape is to lower the centre of gravity and thus increase the metacentric height (GM) resulting in a more stable- stiffer- boat.

Form stability- beam- influences the length of the lever that is applied to the weight or centre of gravity acting in concert with the force if buoyancy.

When a sailboat loses it's keel at sea, the centre if gravity goes up, the metacentric height decreases, the boat becomes excessively tender and capsizes.

The keel and weights aboard a vessel as well as form stability influence how stiff or tender a boat is.

To make a boat stiffer, increase beam or lower the centre if gravity. To make it more tender, decrease beam or raise centre if gravity.

Check out sailboats that have lost their keels, you will see lots of images of upside down boats, both with and without good form stability

 

Guess we'll just throw lay person terms out the window
IE weebils wobble but they don't call down!

 

Well okay then... I've just gotten a thorough education on a topic that I hear thrown around a lot without any real context. Thank you.
I'm going to have to read through these answers again to be sure I got a firm grasp but at least now when I hear the reference, I'll know better.


But do they have good form stability?

 

I think JeffH summed it up well with his cylinder vs plank analogy.

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Plank and cylinder are a great way to describe form stability. If you want a stable cylinder, put some fixed ballast inside of it. If you want a stiff cylinder, put a deep heavy keel on it

Consider a bulk carrier. When empty they roll, big tender rolls. If going out in any kind of weather a prudent skipper will take on sea water ballast to stiffen her up. More weight, more wetted surface, so a loss in efficiency is the penalty paid for a stiff boat.

Take on a load of wheat, high volume low density cargo will sink the ship bodily, but doesn't do much to stiffen her up.

Unload the wheat and take on a low volume high density cargo like taconite, now you have a really stiff ship.

4 different load conditions, going from very tender- light ship, to a very stiff ship, hull form remains constant, what changes is the height of the centre of gravity.

Sailboat stability is no different from ship stability except that sailboats have external ballast to stiffen them up (keels), and recreational sailors mostly don't have automated ballast control systems to stiffen up their boats if the soup is spilling