# Boxy rails = forgiving rails?

I’ve seen references to boxy rails as forgiving rails, and I’d like to flesh out my knowledge on the subject. I’ve noticed that the 7’0" I shaped recently digs in hard when I do a very hard turn on the inside rail. Upon further inspection, the rail profile in the front third of the board is pretty thick, in other words, boxy. What I think is happening is that the rail is adhering to the water more than I’d like. Anyway, how is this “forgiving?” Thanks.

How much cant is on the rail fins? http://www.blakestah.com/fins/

Hey I’d think the definition of “forgiving” needs to be defined. A big wave board with ecsesively (boy, I can’t spell) thick rails cannot be banked over easily, thus is NOT forgiving. A board made for small, slow waves with knife edges can be pushed too hard over on it’s edges, thus digging and catching, making you fail to complete the turn. Not forgiving. And “boxy” is a tough one. How much is too much? I’ve always like thick boxy rails on relatively narrow boards. ???

the ease with which you can “tilt” a board has almost nothing to do with the rail volume, because for the most part we’re not dealing with displacement operations when you’re turning. The only exception is a slight one: on a longboard moving very slowly, but even then the displacement of a thick rail versus a thin one is negligible compared to the mass of a surfer. I suggest the likelihood of a give rail to catch has a lot to do with the amount of edge versus the pivot point of a board. For all boards, the radius line of a turn, that is the line from the center of a turn through the board, has to be through the fin as long as we discount sideslipping. Now think some more…

Hey Hono You are entitled to your opinion, for sure! You might try a Dick Brewer/Tim Bessell surfboard, with it’s thicker rails. Then go try something with very fine rails. Guaranteed you will notice the dif, if both boards are the same width, and used in the same waves. Ever notice there are two camps on rail thickness? Some guys like the thinner, tapered rails with dome decks. Others like the flat deck and hi volume rails. They hate each other’s boards, but both surf well. As for your second paragraph, you have me dithered, and I will just not comment on it.

i think he is simply saying a curvier outline makes a turnier board. that is, if “turnier” is a real word.

LeeD is correct. Two boards, same thickness and width. Same outline. One has blady thin down rails, the other has thick boxy down rails. The one with the blady thin rails is going to be much more sensitive. Obviously. To say that one is more “forgiving” than the other is hard. One will push under easier, one will push back more. One will float less one will foat more… the term “forgiving” dosn’t really apply. A better use of the word “forgiving” would be: “A softer forward edge is more forgiving.” -Carl

still havent solved peters problem… ok these are the possible suspects for front rail grabing… outline to straight in front of front foot… rocker to flat in front of front foot… rails to soft on bottom edge up front… not enough cant on side fins… tail fin to far back or to big for front fins… or your a kook and mistimed it… regards BERT

Hey Bert, i believed that a softer edge rail bottom in the first 2´ in combination with a small amount of convex bottom are good design to deal with this problem…

i used to race power dingies in river races ,some races are 250 kilometers long and take 3 days ,you shooting rapids ,jumping waterfalls ,weaving through rocks and logs ,and carrying your boat around gnarly obstacles …2 guys in a boat and speeds of 65 kph…i dont do it anymore coz its seriously dangerous …we used to build our own boats ,in the end our boats were looking like giant surfboards and no one could touch us for up and planeing early and sheer ability to navigate sharp turns with speed and control ,the boats are only 1 meter wide and 3 meters long one guy on the motor and the decky (that was me) is up front with handles ,basically pulling it from side to side surfing it to steer…it taught me so much about my surfboards it wasnt funny , if our front rail was to soft and round when we pulled hard to turn the whole front would slow down and dig in and the back would spin out and we would lose all our speed ,as well as that the water would shoot right into my face when i put the front rail in , when we did a hard tucked under front edge with a chine coming off at 45 degrees the water had a release point and shot off instead of wraping ,we drove through a turn without bogging or losing any speed ,the same principals work with boards coz its exactly the same angles of attack and circumstances, a soft front rail plus a convex will pull the front of your board down ,hopeless in a longboard for noseriding ,especially in flat soft waves ,and no good in a shortboard as well ,basically when you got the whole rail buried your gettin more release from your tail rail than from your nose rail so the tail wants to overtake the nose ,coz the nose is being slowed down ,plus it pulls water around onto the deck which then creates more viscous drag as well … it may seem logical to soften the front rail ,but it wont give you a high performance board… regards BERT

hmm very interesting… what kind of power did you use on the dinghies? Outboard? How many horsepower? How were they constructed? (did you put fins on them?) Could one be built w/ ocean-going capability? Sounds like they’d flip pretty easy in surf, but maybe not…

well, Bert …i always shaped those bottom rails you describe…, but about a year ago i closely inspected most boards and those sports rounded front bottom rails…, some with a small amount of convex in the first foot and other flat… i thinked that i was designed the wrong way…and change those front boattom tucked edges…

Good morning. Let’s see…to Blakstah; the cant is 4º, mid-range for what Speedfins suggests. The toe-in is probably more than necessary, about 3/16". To Bert Burger. I think your power boat analogy gets to the root of the problem. With more speed, you would have more pronounced effects. You said: “if our front rail was to soft and round when we pulled hard to turn the whole front would slow down and dig in and the back would spin out and we would lose all our speed ,as well as that the water would shoot right into my face when i put the front rail in ,” This is what I’m seeing. I figured that the rail was too smooth and round and therefore making it easy for the laminar flow to continue around to the deck. No release for the water, so something has to give. In this case, the board kind of submarines. As far as being a kook, well, you’d have to judge for yourself after seeing me surf. Thanks for the input.

sounds to me as if the “digging in” may also be related to the rocker in the front of your board, as in, not enough of it. Thick rails in the front third could be related, i.e. thinning rails can provide more rocker at the rail than at the stringer… by the way I’ve always thought of “boxy” rails as describing their shape, not their thickness - they are more squared off at the deck & bottom. Boxy rail shape can be found on a thick rail (more typical) but also on a thin one…

What Bert wrote is exactly right in my experience. A tucked edge in the front 1/3rd of the board creates lift keeping the nose and edges free. A softer edge will allow the front edge to dig in (catch). I know it seems that an edge will catch, but in practice the opposite is true. Anytime you increase lift in the front, you reduce catching. Anytime you reduce lift in the front you increase front edge hang up. Theory of Balance.

Also a full rail will resist the wave face reducing the catching aspect. A thin rail resists less, creating the opposite effect. Again this has much to do with the balances between bow and stern lift and resistance.

KeithM Yeah, I think what I ended up with is really round rails up front, not boxy rails, now that I think about it. It seems as though what one would want to avoid is allowing water to wrap up around the rails to the deck. That would essentially mean that the water was releasing over the deck, which would submerge the board or cause it to spin out. Okay, so you need something to apply pressure to, and that’s where the relatively flat bottom comes in? Otherwise, there’s no way to lean into a turn without directing ALL of the water flow up past the rail–like trying to surf a beach ball. As far as rocker goes, there’s plenty up front…which leads me to get to Honolulu’s point about the pivot point. On a typical roundhouse cutback, there’s lots of rail in the water, so the pivot point isn’t really the fins, but maybe the midpoint between your feet? So you’re depending on the rail to hold you in at least to a small degree…not just the fins. More complex than I realized. Thanks for all of the input everyone. Peter

Greg An edge creates lift, and therefore doesn’t catch. A full rail reduces catching, but a thin rail sinks and therefore catches more? As a rail gets thinner and thinner, doesn’t it approximate an “edge” no matter the shape of the rail? I’m trying hard to relate aerodynamic lift with a surfboard rail. As air moves over an airplane wing, the air moves faster over the top, reducing pressure and providing lift. As water moves over a rail, it seems to me that it would slow over the deck, even though the deck has a longer route for the water to traverse. So…if the water is moving faster over the bottom, that would provide lift vectoring down, right? In other words, faster water flow on the bottom creates lower pressure and “suction” towards the ocean surface? So zero water speed along the bottom and high speed along the rails would tend to move the board towards the rail… Interesting. I’m looking forward to your manifesto whenever you get it into book form.

Moving water creates lift. There is lift on the bottom of the board (we do ride planing hulls) and there is lift along the rail of the board, most notable during turns which is known as resistance (because the rail resists the wave face). Then there is the balance of lift and resistance between the nose and tail of the board. In the case of the bottom lift this balance would be known as pitch. As in all water craft the more lift in the stern the lower the bow (or nose) rides. A low riding nose will of course catch easier than one that rides high. The one that rides high won’t plane as easy though. Obviously wave shape, fin set up and surfing style have to do with the performance of boards with differing pitch angles. Pitch is the primary aspect studied in planing hulls in yacht design. The fact that we plane on a curved surface (and that those curves can vary greatly) makes our design aspects much more complicated. In the case of resistance there is also a balance. Too much resistance in the tail in comparison to the nose will cause the board to track, spin out and catch in the front. Too little and the board is not responsive out of turns (lacks acceleration). Again wave shape, rider style and fin set up have much to do with this. There is also balance of weight which can make a board very responsive (light nose) or increase drive (heavier front). These three aspects are the basis of the Theory of Balance which I first wrote about 25 years ago. Adding edge, in the nose or tail adds lift. It also adds to resistance. This is why edges in the nose, catch less.

I had an HIC Eric Arakawa designed board, it was 6’4" x 18" x 2.25" with low boxy rails, soft all the way to the fins. The interesting thing about it was I could do these big slashes on the face and be totally extended and the thing would hold and my momentum would bring me back over the board on the way down. I can’t do those turns on my current boards - I think the long straight soft rails were what made it so good for that move - becuase it was a bit long I never had the nose burying problem. The boxy rail also gave the board a bit of volume. The drawbacks of the design were it didn’t seem to drive off the bottom well, so I gave it away when it started getting a bit thrashed. However I’m thinking now, the lack of drive was due to the fcs fins being a bit too flexy and that I should have held on to it and tried it with carbon fins.