An answer to some questions regarding rail shape and bottom curves

There's a lot of confusing misinformation out there in the surfing world, it's really a lot simpler than people make out.For example the downrail/uprail business really doesn't mean a lot. What is most important is the radius of the bottom section of the rail. Attached is a diagram which shows how misleading the uprail/downrail dichotomy can be. Because a parallel profile board distributes volume evenly such a board is always thinner in the mid section of the board, thus a 50/50 rail on a parallel profile board does not necessarily imply a larger diameter to the bottom section of the rail as is usually supposed.The owner of a board with a rail section resembling the one on the right will typically be of the opinion that they have a 'racier' rail then the supposedly 'fat' 50/50 rail on the left, because theirs is classified as a downrail. . . . but nothing could be further from the truth, the thinner board with the 50/50 rail has in this case a lower volume rail, and is a better board in all respects, assuming that there is sufficient buoyancy.There's a lot more to it than this, so I'll try to give short answers !Regarding the horizontal hold and release question, it's never an issue, the majority of the on the face tracking or hold work is done by the fin(s) and the rail is used to hold and release in order to adjust the angle of the fall line and the angle of presentation of the fins as much as anything else. Boards with no fins which rely entirely on the rail for on the face hold can be built but they are problematic, it is better to use fins to do miost of the work in my opinion. The short answer is that we use constant rail sections, between a 50/50 and a 30/70 downrail depending upon the thickness and planshape of the board , and they work perfectly, with excellent hold. A constant rail section is fast, forgiving, and gives good control during turns.Regarding bottom contours, surfboards are planing craft, and thus they do well with flat bottoms or slight concaves. V sections can be used but they reduce planing lift and efficiency with very little gain to offset the loss. Planing craft prefer constant bottom sections, it's better not to have complex bottom curves which change from single to double concaves or V sections, as these create conflicting water flows which cause drag. High speed planing craft always have constant hull sections in the run aft. This means that simple flat bottoms or single concaves work best.Regarding decks I always design flat decks, these allow the board to be thinner in the riding area for a given volume which improves control by lowering the centre of gravity ( this is also one of the main reasons for using a paralle fore and aft profile ) With the parallel profile the decks are concaved or spoooned out fore and aft, but flat across the board. The deck is in fact a perfect replica of the bottom and is always parallel to the bottom, this give the rider exceptional control and security.I hope that helps somewhat, the ins and outs of 'shape ' have in my opinion been woven into an uneccesarily complex set of myths, designed more to confuse the board buyer and blind him with meaningless shaping 'science ' than to make sense of board design. . . . such complication often distract people from more important and basic design parameters.