What is purpose of fullers on blades?
 

Probably a very simple answer to this question, but can anyone tell me why some sword/knife blades have fullers and some do not.
Also what is the purpose of this? Various sources call these a "blood grove" but I have my doubts that this is the reason.
Stu

Lightening/strengthening the blade, and/or ornament.

Ornament is obvious. Sometimes fullers are cut into blade to remove flaws.

Lightening and strengthening deserve a few more words. A blade is usually strong enough in the front-to-back direction, but side-to-side stiffness depends on tradeoff between weight and strength. Stiffness depends on the cube of the thickness, so a little bit more thickness can have a large effect on the stiffness. But thickness adds weight. So, enter fullers - a blade of the same thickness with fullers has a smaller cross-sectional area. There is less steel, so there is less weight. If the thickness of the blade is the same, the fullered blade is almost as stiff as the unfullered.

How to make a blade lighter with a fuller: take a blade, and cut a fuller into it. You remove steel, and the blade is lighter. You only reduce the stiffness by a small amount.

How to make a blade stiffer (i.e., stronger): when forging the blade, forge the fuller. You have the same amount of steel, and if you make the blade the same width, it will be thicker. The weight is the same, and the stiffness is greater.

The usual analogy is the I-beam - not as stiff as a solid beam, but a much higher stiffness-to-weight ratio.

Some people will claim that cutting a fuller into a blade makes it stronger. No, it makes it weaker. Forging a fuller can make a blade stronger.

The same kind of weight/stiffness benefits can be obtained from hollow-grinds, T-spines, raised ridges, Z-section blades, etc.

Some martial arts people like fullers because a fullered blade makes more noise when they swing. The noise is sensitive to blade alignment and speed, so it's useful audio feedback.

AFAIK, just to decrease the weight without compromising stiffness.

I have heard opinions that they actually increased blade stiffness by approximating profile of the blade to the I-type configuration, but several engineering souls said that it was nonsense, and that stiffness actually decreases but not by much.

Beyond that I am totally ignorant: math was never my strong suit:-)

Ooops....
You were too fast , or I was too slow:-)

But I am glad I was able to transmit the wisdom of the professionals without major errors:-)

Strictly speaking, a fuller is a depression which is forged into the blade with a fullering tool. Anything cut or ground into the blade following rough forging by mechanical means is called a groove.

That's new to me!
So fullers are in fact better than grooves?
Thanks Oliver!

Thanks for info so far, but I am still not sure WHY they are there. Most European (military) swords do not have them and do not seem to suffer from either weakness of heaviness. So the question really remains as to what purpose they serve.
Stu

I think Timo answered the question quite concisely above.

Thanks Oliver. Yes I see and understand the reasoning outlined by Timo.

Not sure what time period you are referring to but most of my European military swords have fullers. All of my 18th and 19th century cavalry swords do as do most of the infantry ones. There are a few exceptions; British 1833 General Officers sword, Drummers swords, some hangers and cutlasses. Pipe backed blades were a slightly different attempt to add stiffness without compromising weight too much but were not used for long.

The groove makes the sword lighter (faster and better to handle) and also allows control about the mass distribution, used to adjust the inertia and gravity (point of balance) of the sword.

Why fullers?
 

This is not a very educated answer, just logical guesses.
As some of you mentioned before, weight distribution and especially reducing weight while keeping stiffness and size intact make lots of sense. Steel beams and construction elements in modern use are never a solid bar, but have I, H or hollow pipe shape for the same reasons.
The second possible reason is related to the "blood groove". A straight stab may make a dagger or sword hard to pull out because of the suction forces. Fullers could make a weapon easier to extract. In a way, like some modern chef knives have depressions on the blade to prevent vegetables from sticking to the blade by suction.
But like many other features of weapons, function has become fashion and individual cases would be hard to explain because the functional aspect may no longer be relevant. For example, the central ridge construction has been carried over from bronze to steel weapons, where, at least in some cases, are not necessary anymore.

Sorry to come up with a recurrent source:

https://en.wikipedia.org/wiki/Fuller_(weapon)

This because in my home language we mainly (only) use the term 'goteira' (from the Latin gutta =drop ) as for blood dripping, although we (some) are aware this is a fantasy.
It would however be interesting to know how such connotation was born ... semantic wise.

Hi Fernando,

it's the same in Germany. We call it blood groove (Blut-Rinne). I think this term was invented by civilians or maybe authors of adventure books about war. My own father is one of the people which really believe, that the groove was made to led the blood away from body with more efficiency or similar stupid stuff.


Roland

Roland, this is so common among so many peoples around the world since the early 20th century, after the need for bladed weaponry was past.

But Fernando your word goteira and the English word gutter have the same Latin root ie 'gutta' ... and of course a fuller has the same appearance as a gutter but not necessarily the same function

This is a commonly held belief especially amongst those who have been in the armed forces. It is usually a fantasy promulgated by macho training sergeants to young recruits to scare or impress them ... even I was told this in the 1970s by several NCOs when I was in the Joint Services .

Thank you for your explanation. Maybe this is a kind of training sergeant running gag and they always have tears in their eyes from laughing, if the recruits believe that.

My father told this to me when I was six or so and I couldnt believe that. I have been thinking about this for a long time as a child, to find out whats wrong with this theory.

Yes I think it is a common theme with training NCOs especially with young newbies

I've heard it posited that a well shaped fuller or groove can increase shock absorption of a blade. On the surface of it, mechanically this does make sense. A semi circular dish is basically a vault. And vaults are good at redistributing pressure through deflection. The idea of this would be that when you hit something, that you are less likely to feel the shock of an impact through the handle as severely. However I've never seen any experiments that test this. And so I must treat this as speculation.

Blood letting and air pocketing have the same issue to my mind. They are simply theory craft that have not been tested to satisfaction that I have seen. Anything that can be submitted without evidence can be dismissed without evidence. In other words if you don't see proof you are seeing an opinion. That is not to mitigate the value of the informed or trusted opinion. As those can lead to tangential learning.

Why fullers?
 

Just few additional remarks:
I, and others, have mentioned strength/weight ratio as a factor. But this apparently very logical argument has a big hole in it. The main strength parameter to increase (per weight unit) is the lateral stiffness and this is NOT a good idea because it reduces lateral flexibility and increase the chances of bending and breaking. A lot has been done in sword forging to achieve the exact opposite-more flexibility, like in katanas (very functional swords indeed).
Air-pocketing and blood grooves do not make sense either if we consider physics and physiology. Suction (underpressure) does, in theory, because in battle situaton, a sword would have to be inserted and pulled out very quickly (much too fast for bleeding to develop). I beleive in empirical solutions, so this will remain a feeble theory untill experiments are conducted.
Why fullers then?

Forgive me Motan, but it does seem to me that this question has indeed been adequately answered. The blood letting "answer" has been known to be a myth for many years know so i don't think any kind of experiments are necessary to prove this point.
:shrug:

It's very easy to achieve more flexibility: just make the blade thinner. Thin enough, and the blade is very flexible and very difficult to break:
https://www.youtube.com/watch?v=eMAsCuDFSUI
(as long as the steel is resistant to metal fatigue). But too much flexibility reduces cutting and thrusting ability, since the blade will buckle instead of cutting/penetrating the target.

Stiffness is proportional to the thickness cubed, so the deflection of the blade by a given force is A/d^3, where d is the thickness, and A is some constant of proportionality. The strain (i.e., the proportional compression or elongation of the steel) is proportional to the deflection and the thickness, so deflection*B*d, where B is a constant of proportionality. The strain due to a given force is then A*B/d^2. If the strain exceeds the elastic limit, the blade will either break (at least crack, even if not completely break into two pieces) or take a set. Both things we wish to avoid. Note that a stiffer (i.e., thicker) blade reduces the strain - it's stronger, and less likely to break. It does not increase the chances of bending and breaking, but reduces them.

At this point, one should ask why, if this is the case, the thin very flexible blade doesn't break. The answer is that it's very difficult to apply a given force if the target gives with the force. Push on somebody hard - you can exert a lot of force. If they move away, you can't push that hard any more (without moving towards them). That's why the urumi survives. But if the blade wasn't able to get out of the way, you'd be able to press a small section of it into a tight kink without needing much force, and it would break or take a set. Since you probably don't have a urumi at hand, let alone one you're willing to destroy, you can try this with wire: compare the force you need to permanently bend thick wire vs thin wire (i.e., make the "blade" take a set). The thick wire needs more force; the thin wire needs to be bent further, but you can bend it further with less force.

The importance of stiffness for (a) supporting the blade against buckling while cutting, and (b) for strength explains why it common to see cutting-oriented swords with thin tips and thick bases. The last few inches near the tip can be about 1mm thick, while the base of the blade can be 10mm thick. A razor blade on a stiff 2.5 foot stick, rather than a 3 foot razor blade.

the 'flexibility' and cutting ability of japanese blades is also a myth. if you are doing cutting with a katana, especially on stuff like bamboo, if you make a bad cut, you can bend the blade. even a master can have a bad day. with a soft spine and a hard edge, it is less likely to snap, which can get you killed in battle, a quick bend the other way can restore a bent blade, at least well enough to preserve your chances of living long enough to replace it.

japanese used layered and laminated steel to make the best of the best of the steels available to them, producing art accidentally in the process. modern mono-steels produce better swords for cutting competitions, less likely to bend or break and if made correctly, sharper - the mono steel allows a wider blade and thus a more acute edge angle.

they generally do not have bohi (fullers) to reduce cutting friction and move more weight balance out to the tip to improve the inertial forces. quick recovery to defend or re-attack is not a major requirement when cutting bamboo or pool noodles, or tatami mats.

fullers improve the strength to weight ratio, and are used for the same reason we use I-beams. a box beam (hollow square x-section) maximizes the lateral and vertical stiffness and strength to weight ratio, but it's be rather difficult to make a hollow square sword - 90 degree edges do not cut well (tho they may work for a stiletto or a pure thrusting blade/ small sword or cane sword).

fullers are used in sabres and swords because they work if done correctly, improving the balance and handling. bohi on samurai katana are also used because of the neat noise they add if you cut correctly (and don't bend your blade).

To Timo Nieminen and Kronkew. I also anderstand the basic physics. After all, this is no rocket science. But this does not explain everything. A cutting blade should have a balance between strength, flexibility and cutting power. The question is whether specific blades fit their specific purpose. Beams are made to absorb lateral forces, while cutting tools are generally made to absorb forces parallel to the sides of the blade (although bad hit can exert lateral force too). A blade that has thick and stiff spine will have less cutting power and less flexibility, while a thin blade will cut better, but might brake more easily. I personally think that ethnographic weapons take their shape mostly from tradition and esthetics and less so from function. Can you explain the fantastic shapes of oceanic clubs by their function? Can you explaing the totally different geometries of Ottoman kilij and Persian shamshir by different in their use? I can't.
Just as anecdote: when I was young, I worked in a banana plantation for sevral years. Instead of machete, we used Victorinox butcher knives of the largest size available. Why? Because their cutting power is far superior and you could do much more work in a day. However, they did brake occationally and were expensive to replace.

To David
 

Sorry David but you misunderstood me. Naturally, there is no reason to test the "blood grooves" theory in practice because it just doesn't make sense in the first place. I was talking about something else. I presonally don't think fullers reduce the resistance to stabbing. However, they can, at least in theory, reduce the resistance to pulling out the weapon AFTER a stab if it needs to be withdrawn quickly. Outer pressure should push the flesh onto the blade and the vacuum created by pulling may resist it. Fullers can relieve this vacuum by letting air and fluid IN. This is however all theory. I know these forces exist but I have no idea if they play a role in practice.

the ultimate 'blood grooved' 'shiv' knife: take a piece of steel tubing of appropriate length, bed or chair leg, about a foot should do. cut the tube at an acute angle from one end but not too acute such that you have a sturdy tip & leave al least 5 inches for the grip. sharpen. wrap grip in friction tape. push forcibly into a target containing fluid. possibly an unwilling 'volunteer'. see what happens.

i seem to recall peasants in 'seven samurai' making spears from bamboo like that. ruined the bandits day.

p.s. - to summerize my thoughts, metal was expensive, smithing techniques learned on bronze and copper weapons that maintained stiffness while reducing the required amt. of material for a specific length weapon by adding grooves and fullers would as a side effect produce a artistic appearance that might be carried on even into stronger materials where they technically might not be needed, tho if you are making a few hundred thousand sabres for queen and country, the material savings in steel, weight carried by the trooper and horse, extra calories to carry heavy swords, and transport of spares, etc. could be worth a few shekels.

I've been following this thread with interest, but until now I have refrained from contributing any comments, simply because other people have already said what I could have said. However, Kronckew's latest post is too good for me resist adding a tail to.

When I was a kid --- say between 16 and around 30 --- I regularly hunted wild pigs in the lignum scrub around Moree in North West NSW, Australia. I hunted with a group, usually no more than 6 of us, 6 or 8 pig dogs --- vicious cross breeds that would rip up the owner as soon as rip up a pig. We didn't hunt with rifles, although one of our group usually carried one just in case the pig got a bit a difficult.

Our weapons of choice were Lee Enfield bayonets, yep, the ones with the debated blood groove, and lengths of water pipe or electrical conduit that had a point ground on one end.

The method of hunt was to drive along a track in a ute (utility truck=pickup truck) until we saw a pig or pigs, turn the dogs loose and run through the scrub after them. The pig would usually bail up within a couple of hundred yards, then we would stick it with our bayonets and lengths of water pipe. When it had weakened a bit somebody would jump in and try to cut its throat. It was a lot of fun.

It was not at all difficult to remove the water pipe, that came out pretty much as easy as it went in, except when it got bent by a bone, but the bayonets needed to be twisted to one side before you could kick the carcass of the pig off it.

On a totally different track:- we tend to think of Indonesian spears and other stabbing weapons as being mostly made of iron. Well, the ones that have survived mostly were made of metal. But when a lord called up his populace to bear arms against an enemy, many of these peasant farmers did not have metal weapons, sometimes, not even metal farming tools, so guess what? They used lengths of sharpened bamboo. The picture here is of the pointy end of one of these bamboo spears, together with a metal pointed spear in cover, exhibited in the Den Pasar, Bali, museum.

Bare skin is pretty soft.

Outstanding riposte Motan!!!!
And very well explained......very much enjoying the discussion and elaboration on dynamics and physics here on this topic.

Of course you are right , and the pipe'shiv' and the peasants bamboo spear did not come into being because they retracted easily , but simply because the raw material ( bamboo & piping ) were available in the locality. They just happened to be that shape.