What do "tender" and "stiff" mean?

[river251]

Future sailor here. I keep seeing terms "tender" and "stiff" referring to boats. What do these mean? Which is preferable?

Thanks
Jim

 

[johnalison]

Future sailor here. I keep seeing terms "tender" and "stiff" referring to boats. What do these mean? Which is preferable?

Thanks
Jim

A tender boat is one that heels early to the wind, and a stiff boat one that resists heeling. A stiff boat could be one shaped like a shoe-box and with a heavy keel but it's a bit more complicated. Some cruising boats can be fairly box-like and stiff up to a point but with little ballast will tend to blow over with more wind. My current boat is almost semicircular underwater when seen from ahead, but with 42% of the weight in a lead keel resists heeling after being initially 'tender'. Generally, cruisers don't want a tender boat which has to be reefed in only a moderate wind, but achieving stiffness by building a shoe-box won't make a boat fast. Racing boats achieve stiffness by having very deep keels, or modern tricks like vanes, but in cruising it all comes down to compromise.

 

[Wansworth]

This is the art of the naval architect a balance between tender and stiff apart from other factors.

 

[Buck Turgidson]

And some boats were designed specifically to be initially tender so they heel early and offer a longer water line length which will give better speed. These boats were designed to fit within certain racing design rules of the day but the best architects found ways of "twisting" the rules ;-)

 

[river251]

Thanks much, I think I understand. A wide beamed boat with a full keel like an Island Packet would then be stiff.? Would an older Beneteau First or a Contessa 32 be tender, both used in racing?

 

[RupertW]

Thanks much, I think I understand. A wide beamed boat with a full keel like an Island Packet would then be stiff.? Would an older Beneteau First or a Contessa 32 be tender, both used in racing?

For the First it would depend on how flat the stern section is - very flat means initially stiff.

 

[Quandary]

Thanks much, I think I understand. A wide beamed boat with a full keel like an Island Packet would then be stiff.? Would an older Beneteau First or a Contessa 32 be tender, both used in racing?

Like all things boaty, it is not quite as simple as that!

 

[shaunksb]

A stiff is the name we usually give to a dead sailor.

A tender is a much smaller boat that is usually rowed at an angle of approximately 45 degrees to where your bigger boat is.

______________________________

 

[RJJ]

To avoid confusion, boats are also described as stiff if they are literally stiff i.e. the stiff hull resists the loads of the rigging with minimal distortion. Some boats actually do bend visibly under load, typically if their rigging is wound up very tight for racing and the hull is built down to save weight. In extremis they snap in half, but that's most unlikely/will never happen on any conceivable boat you or I would own. You can view this out of curiosity but there's no need to be scared by it.

So more usually we are indeed talking about heeling. A stiff boat can carry sail better as wind builds; a tender boat will heel more readily. A tender boat will often need reefing earlier. That's about what you need to know as a beginner or for short offshore passages like a Channel crossing.

When it comes to longer or ocean passages, where the risk of unexpected severe conditions is much larger, you get down to how a boat may survive those conditions. And there's more to it; "she is a stiff boat" is just fag-packet slang. You start to reckon with a boat's "ultimate stability" or "angle of vanishing stability" which is entirely different from her readiness to heel in moderate conditions.

 

[Soton sailor]

From a stability perspective, stiff and tender refer to vessels that are more stable and less stable. For a given sail area and wind speed, a tender vessel will heel more than a stiff vessel. It might therefore seem that a stiff vessel is better, but stiff and tender are also linked to roll speed. A stiff vessel will roll upright again very fast when the wind stops, and therefore tends to have quite fast, jerky roll action, albeit through small angles. A tender vessel may heel further, but she will roll in a slower, more comfortable way.

 

[geem]

In my experience its a meaningless expression that tells you nothing. Initially stiff can be down to hull shape but get sailing and the same hull can be putting the deck under when a boat that felt tender at anchor can be stiff under the same conditions. Mast height/sail area, keel depth/weight, hull shape all influence whether a hull is stiff or tender. A sailing boat can rarely be summed up in one word, but I have been on a couple I would describe as crap?

 

[William_H]

Stiffness to my mind is the resistance to heeling. Not so much the same as self righting ability. A keel as a pendulum only has real effect from about 45 degrees on. You can use trig tables to see the effect as heel angle increases. Obviously at lay down 90 degrees the keel has the most power to right the boat.
No stiffness ie initial heeling is more related to hull shape in cross section. 2 extreme cases explain. A large log of wood circular has no tendency to stay up one way. It just rolls. A sheet off plywood floating on the other hand stays flat and is very resistant to tipping over.
Now in boats a catamaran is in effect a big flat bottom in terms of stiffness. To heel the boat one must in effect raise a hull. The total weight of the boat is what resists this heeling. A boat with a long deep keel and slab sides ie vee shaped cross section (no chines) will be very tender leaning initially quite easily. With further heeling the pendulum effect of the keel keeps it from going over.
So looking at your average yacht the bottom is kind of semicircular but very much spread at the turn of the bilge (chine) towards a flat bottomed shape.
The reason is that a very flat bottom presents a large surface area which gives drag at low speeds. The semi circular shape has minimum surface area but no stiffness so a compromise is reached to stretch the curve of the chines to achieve stiffness. Chine is the term for the turn from bottom to sides of a boat. Can be very pronounced as in a mobo or very rounded in some sail boats.
So imagine that the boat heels so that body of the boat at the fat sides goes under water providing buoyancy and so tending to lift the side. This lift produces stiffness and provides resistance to heel from just the first few degrees of heel. The total weight of the boat including ballast aids this stiffness. This ballast if on the tip of the keel aids in self righting. But many boats have ballast under the floor and it still aids stiffness but less effective at self righting.
Regarding the sinking of the One Australia above no not one of our finest hours. I believe the boat had 12 tonnes of lead in a bullet on the bottom of a very deep fin keel. The rest of the boat weighed 3 tonnes in structural carbon fibre. One theory is that the hull was weakened when towed out in a steep choppy wave. The design of the hull was to resist the sideways force of the keel no one thought of pitching causing problems. The second contributory problem was that one of the jib sheet winches had failed and they carried the jib sheet aft to a winch designed for spinnaker or running back stays. This huge load from near the stern caused the banana to bend.
However even our sedate GRP boats can bend under extreme fore stay /back stay load. As many owners report the toilet door jambs when they load up the rigging. ol'will

 

[johnalison]

It is easy enough to imagine resistance to heeling as a static problem, but a sailing boat is not only moving but subject to variation in wnd pressure and wave action. When the need for speed is also taken into account, this means that the simple model is incomplete and may be meaningless. However, I think that the terms tender and stiff can still be useful if used in a general way and not taken too literally.

 

[Sharky34]

There's me thinking ybw had wandered into massage parlour land, from the OP title.

 

[William_H]

A couple more comments on stiffness versus tender.
In a sea way with large waves coming from abeam the boat will roll and heel when floating ona slopy patch of water simply because the water supporting the boat is sloping. However the pendulum effect of a deep heavy keel may minimise this effect.
Now it might seem to OP that stiff is good and tender is bad. Certainly temder is an undesirable attribute. But everything in yacht design therefore in your choice of yachts is a compromise. My own little boat at 21ft is very tender by most standards. This because it is a trailer sailer so weight is kept to a minimum it being about 950kg and also from the hull shape. (known as form factor). When I climb over the side form the dinghy it heels alarmingly. Yet the up side is that it sails very well and with crew weight on the gunwhale and right sized sails will handle strong winds well. It also performs very well in light winds. I love it despite being tender and have raced it for 40 years. It does however, with light weight bounce around a lot in rough water. Not so good. But is no problem on trailer behind the car.
At one stage I wanted to race in night races calling for cat 5 safety standard set by Australian Sailing. This mandated a pull down test to test for self righting. Despite only 100kg in the drop keel and most ballast under the floor it did meet the requirments. (requiring about 30kg down pull at the hounds to hold it down at 90 degrees) However interstingly the test is specified with no crew on board. Now if I sail with 4 people on board the total weight approaches 1/3 of total weight. If as has happened many times it is laid over (usually on a reach with spinnaker out of control) the crew end up standing on the wrong side of the boat and it does not pop up so quickly. Only when crew realise the problem and climb up onto the gunwhale does it come up. All very alarming with water pouring into the cokpit but not really a safety concern. good luck with your search for a boat ol'will

 

[Zagato]

I am just learning about this also but what it leads me to ask is what degree of lean is more efficient for speed. I presume the more upright the sails are the more wind they pick up and the pull on the sails is horizontal but if the rig is leaning over there is less wind and the sails are pulling more into the water. I have 58% ballast ratio on my IF Boat which allowed for a taller mast for light airs. She is tender initially then completely stiffens up, some have never reefed apparently. All well and good but doesn't the terrific energy used to lean the boat over also take power away from forward motion? I would have thought that you would want a trimaran with foils to prevent heal and put up a sail that has a much lareger area at the top?!

 

[RJJ]

I am just learning about this also but what it leads me to ask is what degree of lean is more efficient for speed. I presume the more upright the sails are the more wind they pick up and the pull on the sails is horizontal but if the rig is leaning over there is less wind and the sails are pulling more into the water. I have 58% ballast ratio on my IF Boat which allowed for a taller mast for light airs. She is tender initially then completely stiffens up, some have never reefed apparently. All well and good but doesn't the terrific energy used to lean the boat over also take power away from forward motion? I would have thought that you would want a trimaran with foils to prevent heal and put up a sail that has a much lareger area at the top?!

There's a reason fast dinghies sail flat and at much greater speed (relative to length)

Yes, sails are more efficient upright. But something has to counteract the heeling moment. Any form of movable ballast (water, humans, canting keel) takes skill, complexity and expense to operate, while also posing risks if caught on the wrong side. There's also a reason fast dinghies are often seen upside-down.

If you have static ballast (a fixed keel) then you can't move unless you allow the boat to heel, except deep downwind.

What angle best? On a dinghy, dead flat. On a lead-mine...it depends. There will be some heel angle that balances the rig, the hull and the rudder, it's probably between 25 and 40 degrees. There are other tradeoffs for example the hull overhangs - even if you could sail your folkboat upright you would forgo waterline length. The extra few feet of waterline length when heeled is a deliberate design feature.

 

[Laser310]

Stiffness is a measure of the ability of a boat to resist heeling forces

A tender boat has less ability to resist heeling forces and thus heels more than a stiff boat - other things equal

Stiffness is dependent mostly on the keel and ballast.., as well as the hull shape. Deep keels with big bulbs confer stiffness.., as does a wider hull - especially one that carries the width to the stern.

Stiffness is also a measure of a boat's sail carrying power: heeling reduces the force of the wind on the sails, and consequently limits the ability of the sails to use the wind to produce motion.

Therefore, stiffness is a big factor in the loads experienced by the rigging and hull. Imagine a model yacht on your desk: If you push with your finger on the top of the mast, the force on the tip of your fingerwill increase as you push, but that increase will be limited by the model starting to tip over as you cotinue to push. Now imagine the same experiment, but with the boat glued to your desk. The force on your finger tip will increase as you push, and the more you push, the more the force on your finger tip will increase. This force is transmitted through the whole boat, but especially the standing rigging, the running rigging, the sails, the hardware...

Thus - other things equal - a stiffer boat needs stronger sails, lines, hardware..,

Modern race boats tend to be quite stiff.., and that's why you see them all with carbon sails - the loads on their sails are much greater than the loads on the sails on a cruising boat.

Multihulls take this to another level - racing multihulls are extremely stiff: I've gone ~30kts on a large ocean racing mnltihull. we were heeling less than 10deg. The loads are beyond anything on any other yacht. The loaded sheets are like iron bars. Extreme care is needed dealing with these lines: they will amputate any limb that gets in the wrong spot

 

[Buck Turgidson]

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.

 

[E39mad]

For the First it would depend on how flat the stern section is - very flat means initially stiff.

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.