Sharper Rails

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Sharper Rails; sharper rails
Topic Started: Apr 17 2008, 10:08 PM (291 Views)
stevyd	Apr 17 2008, 10:08 PM Post #1
Member Posts: 15 Group: Squad Member #995 Joined: March 21, 2008	Mark and Harry were talking about rails on ski design at weekend but I thought afterwards it might be adapted for HP boats. I f both rails were built out approx. 10mm the full length of boat at allow them to be sharper could this allow better carving and holding on steeper, wallier waves and possibly more speed. Its all new to me but if it might work Id try modifying my boat, if it doesnt work can always sand them down again. any thoughts welcome



chris-uk	Apr 18 2008, 01:41 PM Post #2
Advanced Member Posts: 33 Group: Members Member #44 Joined: July 6, 2005	Take a look at the DB Fusion, Nathan tried this concept on that boat but his new boat has dumbed these rails down. I'd have a chat to someone who has one of these boats. Chris



AndyMcC	Apr 20 2008, 01:55 PM Post #3
Advanced Member Posts: 175 Group: committee Member #799 Joined: April 24, 2007	Is this the style that you went for on the boat you made Harry? Andy



Harry	Apr 21 2008, 06:32 PM Post #4
Advanced Member Posts: 1,288 Group: Admin Member #1 Joined: May 9, 2004	Hey Stevie, Don't get confused between rail sharpness and rail thickness. A sharp rail has an abrupt change in shape and a distinct edge, or corner, between hull and deck (2). A soft rail blends the change between hull and deck with a smooth curve (3). Rail thickness is the depth of the rail in the vertical plane, or the distance from deck to hull measured straight down, especially where the rail can bite into the wave (1). So a thick rail has more volume and a thin rail has more "bite". Older WW boats, for example, have soft, thick rails. In fact they don't really have rails at all. Consider the shape of a Pyranha Rotobat; cylindrical in cross-section with a gradual curve from hull to deck with no distinct divide between hull and deck (4). The whole, rounded side wall of the kayak could be considered to be a big fat soft rail. This type of boat doesn’t surf very well . A progression from this would be the Dagger RPM. Still very rounded but the ends have been thinned out to give a sidewall shape that will actually bite into the wave face. Although still "thick" compared with a true surfcraft rail this type of boat will surf much better than a rotobat (5). Next step would be the first flat bottomed playboats. These still have a thick rail. In fact most have a fairly high, vertical sidewall but they also have a distinct edge between hull and deck, although it would be hard to say they have a sharp rail when compared to a surfboard or waveski. This edge will give some grip on the wave face and when combined with even more thinned out ends giving more "rail" these boats can surf quite well. I'd say that the Wavesport Forepaly and XXX are amongst the best non-surf specific boats for surfing (6). Now on to modern surfkayaks. To my mind there are two distinct rail types here. One with a rail shaped into the deck of the boat that comes in from the side some distance before meeting the more vertical side wall of the cockpit area (10). The other type has no distinct, protruding rail but tries to keep the thickness of the whole kayak down to keep the rails thin (11). My boat and Nathan's Fusion are good examples of the first type, many IC boats are the latter and most Mega HP designs fall somewhere in between with the side wall sloping down to a thinned out rail area. It's important to realise that where the more defined edge of a playboat gives more grip than a rounded hull kayak (7&8) this does not carry over to surfkayaks, waveskis or surfboards. In fact it's almost the opposite that's true. As these craft grip the wave by the rail biting in (disregarding fins) a more rounded rail will actually suck water round it dragging the rail deeper into the wave and providing more grip (12). A sharp rail will allow water to release from this sharp edge without being dragged round a curve. This means the rail is not sucked into the wave so much, reducing grip but also drag allowing the craft to plane faster (13). So you could easily give your Neutron sharper rails all along its length by pouring on some resin or filler along the edge of the hull and, after it’s set, sanding it to a sharp edge at the rail and blending it into the hull towards the centre of the boat. No need to add extra width in order to achieve this. However, conventional, proven surfcraft design suggests that the rails should be sharp from somewhere beside the fins or hips to the back of the boat allowing efficient water release and therefore speed. The rails should gradually become softer from the hips to the front making the boat (or board) more forgiving to surf. A sharp rail all the way to the front isn’t really desirable. Rail thickness and the variation of this thickness from the font of the boat to the back (foil) is also important to think about. On a surf board, thicker rails, with more volume, don’t bite into the wave as much, they cause more drag so they are not as high performance but are much more forgiving. Thin rails cause much less drag and allow much more of the rail to be used (the board can be railed right over and even be buried into the wave up to the board’s centre line or further) but they are much less forgiving, demanding a higher skill level from the surfer and better surf conditions. The big problem with a surfkayak when compared with a board or ski is that at some point the deck gets in the way. As the rail sinks into the wave it either gets thicker very quickly or, at some point, meets a more vertical side wall – depending on the design of the kayak. This inevitably causes more drag (a lot more) than on either a waveski or surfboard (14). This is one reason why a kayak can never reach the performance of a ski. It’s crucial that the rail thickness and foil is right on a surfkayak for your skill level and for the type of waves you surf. If the rails are too thin, they will only work on a fast, wally wave and only then in the power pocket and with the exact right edge applied. A soon as the kayak slows down the thin rails immediately sink into the wave up to a point where they meet a much thicker part of the kayak, this quickly increases the drag by a great deal really slowing the boat right down. A thicker rail keeps this from happening when the kayak slows down a little. The extra volume doesn’t let the rail sink in so deep keeping the thickest part of the kayak out of the wave. This allows the kayak to be surfed in a greater range of conditions by a wider range of paddler ability with little loss of top speed on the steep waves. As to modifying a new Neutron, or any boat for that matter, I think it’s really hard to do well, blending in the changes to the existing shape, so not worth the effort. Sorry, but you did ask ;-) Attached to this post: diagram.jpg (33.91 KB)



stevyd	Apr 21 2008, 08:26 PM Post #5
Member Posts: 15 Group: Squad Member #995 Joined: March 21, 2008	Youve spent some time with that reply. Will have to read it a few times to get my head round it. Thanks very much for the detail, might be best to look at it again next time i see you before i make any changes. But may be best to leave alone as you say as could limit the use to very specific conditions. I have put in a 2'' foam seat in which seams great so far, cant wait to see what difference it makes on bigger waves. Anyway, thanks very much again, speak soon


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