If you have a ten dollar head wear a ten dollar helmet (Evil Knievel)
Having done a lot of reading, and looking at standards and design of motorbike, bicycle and military helmets (including bomb disposal!) I concluded that any helmet worth wearing on whitewater -- especially if you capsize a lot (hey, don't we all! :)) - should have these characteristics:
My standard:
Kevlar/vinylester or carbon/kevlar composite shell, at least 3mm preferably 5-9 mm thick
15 mm stiff foam liner -- this is what absorbs the impact so must not be too soft -- it you can push your finger through to the shell then it is way too soft. Should be very firm, and EPP (expanded polyprop) or minicel.
Ear coverage -- don't want'em ripped off, after all. I like my ears the way they are.
3 or four way straps
Straps joined to helmet are widely spaced and prevent slipping helmet -- vital to keep your hat on, and not slipped so exposing eg. forehead to rocks.
Bright colour -- natural carbon or black or dark blue or white-helmeted swimmers are nigh on invisible ...
Faceguard -- paddlers actually incur more face injuries than they do head injuries ...
No drain holes -- not ideal as they can weaken the shell a little and are not needed in a properly-fitting helmet.
Liner foam comes right down to edge of helmet as that's where many of the hits are taken.
In more detail:
1. Shell
The terms used by manufacturers are confusing -- 'kevlar / resin / vinylester / carbon / 7 layers / glass / etc, and I suspect they refer to a wide range of composites and hence a wide range of strengths. The same with the plastics like PE, ABS and PC which would be highly variable in their strengths, depending on formulation.
In general, the shell should not flex a great deal if you squeeze it in your hands. This generally means a composite with kevlar in it, at least 3mm preferably 5-9 mm thick
2. Foam Liner
At least 15 mm of stiff foam liner -- this is what absorbs ths impact so must not be too soft -- if you can push your finger into the foam all the way to the shell, then the foam is basically useless. It will absorb a small hit, the most of the energy incurred in a big hit will get transferred straight to your head. Get a helmet that has a STIFF foam lining. You will feel some of the smaller hits more, but will have much more protection for the big hits -- the ones that can render you unconscious and of course it is these that lead to fatalities.
EPP (Expanded polypropylene) is the foam of choice for muli-hit helmets, although EPS (expanded polystyrene) as used in motorcycle and bicycle helmets gives the most impact asorbtion -- it just needs to be replaced after a hard hit.
One thing I have not yet seen is multilayer foam? Surely a three layer makeup with super dense on the outside leading to softer on the inside would cope with a range of hits. You really feel the small impacts with the hard foam helmets, withthe payoff that you won't be knocked unconcious when you hit a big one. But why not have the best of both worlds and have an extra layer of soft foam for the small knocks too?
3. Straps and Fit
Helmets slip all the time. Adjustable 3 or 4 way straps are a requirement. Heads are funny-shpaed things and you must test and add little bits of foam until you get a good fit. If the helmet slips back while you are in a holem exposing your forehead to rocks, then that is bad news!
4. Facemasks
I would argue that a facemask is essential for both playboating and certainly for creeking. Facemasks are unpopular, but anecdotally it seems that the face takes more hits than any other part of the head. People are hit in the face by their own and other people's paddles. They also expose their face to the riverbed when laying on the back desk in a capsize position -- the increasing popular 'back deck roll' which is often very fast, making up for the increased face exposure, but which does not always work first time ...
There is concern amongst boaters that facemasks might snag on branches etc, causing neck injury. The face mask should not have too large a gap in the mesh to avoid this.
I also looked at polycarbonate transparent face masks used in industry, but they seem a bit flimsy. The ones use for bomb disposal might be thick enough. However, there are considerable issues with fogging and water droplets reducing visibility which would be unacceptable.
5. Occipital locks
These are all the rage now: lidds.com, prijon.com and shredyready.com all have a band under the back fo the helmet that can be cranked tight with a dial.
6. Colour
Should be bright. Natural carbon or black or white-helmeted swimmers are nigh on invisible, making rescue much harder. I hear that in Canada pfds (bouyancy aids) have to be a bright colour.
Yet there are many dark coloured helmets on the market right now.
7. EPS as shell would be replaced anyway
Prijon are a manufacturer who use EPS in their kayak helmets. I imagine that they feel the impact absorbtion is excellent, and that a kevlar shell would need to be replaced anyway after a hard hit, so why compromise by using multi-impact foam?
http://www.prijon.com/Katalog_D/DEUTSCH/Zubehor/sicherheit.htm#Wildwasserhelme
8. Full coverage
Helmets with ear coverage make up make sense for both playboaters and creekers. However, there is an enromously strong fashion trend away from full coverage. But as I said, there is a large group of paddlers (playboaters) who paddle most of the time on artificial white water courses, weirs, or selected stretches of river that have few rock obstacles and which they know extremely well, so maybe they can choose for themselves ....
9. Rustproof metals
Surely no-one would make a helmet that would get rusty parts? Worth checking for anyway.
10. Inflation
I would love to see inflatable outfitting, as per some of the football helmets. Fit is so vital.
11. Drain holes
I don't see why a properly close fitting helmet would need drain holes. And they must weaken the shell. They also let heat out which can be an important safety problem when you consider hypothermia is a common risk for paddlers in cold weather.
12. Head Injury Statistics
There have been relatively few deaths attributed to head injury (about 5% of all whitewater deaths according to my analysis of AWA records 1975 - 2000). However, all kayakers take head knocks, and I wonder how much a slight knock that does not leave a noticeable mark on either helmet or user has contributed to cases of bad judgement or a lack of alertness which in turn led to an accident that was classed as something other than a head injury. Perhaps the term 'Head impact-attributable accidents' would be better.
13. Example helmets
Finally, here are what I think are the best helmets on the market right now. However, this opinion is based purely on specs as these helmets are not all found in the UK:
EPP Foam Liner:
Lidds: Nato http://www.lidds.com/nato.html
Shred Ready: Full Mental Jacket http://www.shredready.com/fullmentaljacket.asp
Sweet: Rooster http://www.sweet.no
Minicel Foam Liner:
Grateful Heads: Edge/Hard Hit/Drop Zone http://www.gratefulheads.com/hlmt-features.htm
PS Composites: Alpaca or Monitor http://www.customkayak.com/html/accessories.html
EPS Liner:
Prijon: Protector would also qualify if user paints it a bright colour. Note that this helmet has an EPS (polystyrene) liner, which means it will have the best impact absorbtion of all, but will need to be replaced after a hard hit. http://www.prijon.com/Katalog_D/DEUTSCH/Zubehor/sicherheit.htm#Wildwasserhelme
Plastic helmets with EPP:
Sport Helmets: Cascade Full Cut http://www.sporthelmets.com/sporthelmets/WITS.htm
Prijon: Surf 2002 http://www.prijon.com/Katalog_D/DEUTSCH/Zubehor/sicherheit.htm#Wildwasserhelme
14. Further reading
This article is a decent introduction:
http://www.paddlermagazine.com/issues/2001_4/article_123.shtml
And David Weber has done an excellent review of Lidds' helmets which also covers some good basics:
http://boatertalk.com/article.php?id=76
And this is also a nice page: http://www.middletonsboats.com/clothing_safety/headgear.html
15. Conclusion
After all this though, what do I wear now? I have stopped wearing my Protec Classic, which has very soft foam and a very soft and flexible plastic shell. Instead I have the Prijon Surf 2002 (£35) which is a stiffer plastic and which has the same liner system as the top of the line Protector (£95), and I have added a black Palm polo facemask (£15) which I modified to use three attachement points as recommended by Wilko below. However, as I stated above, the important thing it to find what is right for you. I would be looking at the Sweet Rooster and the Protector and the Full Mental Jacket if my head was smaller and a different shape. Figure it for yourself and make sure you have a good helmet!
Because you've only got one brain ...
What I learned about adding a faceguard is that it's important to stay away from the edge of the helmet when you make any holes in it. I prefer melting for plastic helmets, as that lessens the chance of the edge of the holes starting a crack. I have also drilled holes in a couple of Wildwater helmets and then put a small soldering iron through each of the holes shortly to make the edges smoother.
When you drill holes through a composite helmets, I recommend using the pre-drilled holes for the helmet straps when possible, adding a little resin to the edges of the drilled holes to prevent the fibres around the hole to soak up water. Go for the stainless steel rings, bolts and nuts, and don't forget to file off the sharp edges of extending thread and nuts. It's rather easy to lose some skin off your fingers when you brush past the sharp extended edge of a steel bolt. I always use steel rings (are they called "washers" in English?) to spread the force of the pull on a bolt over a larger surface area of the helmet.
I dislike the faceguards with big openings (like one rail down over the nose and one from ear to ear), because they still leave a lot of the face open to injury. Also, the risk of snagging becomes somewhat bigger than with the smaller size hole wire frame guards. I prefer the darker faceguards (black plastic covering) over the lighter ones (chrome, stainless steel or white plastic covering), as they reflect the sun so much less, thereby being almost unnoticable when wearing them.
I always had issues with the older models of faceguard available requiring the holes to be drilled rather close to one another. Recently I bought a new model which has only three holes, one in the middle and one on each side of the faceguard. It looks just as sturdy, but is easier to attach to a helmet with a small visor. I attached one of those to my girlfriend's Wildwater helmet.
I've added "canoe polo" style wire frame faceguards to a number of helmets (Prijon Korsica, Romer ?, Wildwater, Prijon (older model), Grateful Heads Dropzone), and failed to do so with some other helmets. I look for helmets without drainage holes, but besides the strength issues which Edwin already addressed, I also think that because of the holes, less inside area is covered with foam, so that more of the inpact force is transferred to a smaller area of foam on the head. More importantly, I dislike a cold head. The holes not only let the warmth of my head escape more easily, it also lets in too much cold water.
I've probably forgotten a lot of things I should mention with regard to faceguards, but it's getting late here...
Wilko