A Semi-Gun is a specialized surfboard designed for larger than average waves…….say in the 6'-10' or double overhead plus range.  Depending on location, bigger waves are usually faster moving, more powerful and more hollow than smaller waves. Bigger waves could accurately be termed waves of consequence since the effects of any mistakes made whilst surfing them are significantly magnified when compared with those occurring in smaller waves. Particularly when reef breaks are involved, failure to successfully negotiate large waves can result in serious injury.

It follows then that surfboards designed for larger surf must successfully address the issues of wave size, power and shape. If this is done correctly, the surfer will be able to confidently deal with whatever comes his/her way.

Wave size.
Large waves have typically traveled from storm centers a long way away and are faster moving as they approach the break than smaller waves.  Therefore a Semi-Gun needs to paddle very efficiently to allow early entry into the wave, and to be of a length proportionate to wave size.  Volume and length are the main variables of interest here.

Wave power.
Large waves naturally have more power than small waves.  Therefore a Semi-Gun ridden in large surf will be propelled much faster than a surfboard being surfed in small waves.  Consequently the board needs to contain design attributes to allow the surfer to maintain control at high speeds.  The primary variable concerned here is rocker.

Wave shape.
For a number of reasons large waves are usually steeper and more hollow than smaller waves.  Again the Semi-Gun needs to be able to accommodate these conditions and ensure that the surfer has control.   Primarily this entails narrower planshape dimensions and adjustments to rocker.

DESIGN BACKGROUND

Planshape/Outline
As you can see in the photo, our Semi-Guns have a continuous free flowing curve through the outline with no straight sections or hips at all.  The effect of this outline is to allow smooth and predictable transitions between turns….for example between coming off the bottom and laying the board over on the rail for the first turn.  The nose area is pulled in, but still retains sufficient area to enable the surfer to move forward on the board if desired to draw out a longer turn….thus having multiple sweet spots.  Boards which are too narrow in the nose can only be surfed off the tail because they cannot support the surfer's weight when forward and hence are restricted in their length of turn.   Also obvious is the rounded pintail whereby the tail area is significantly reduced in comparison with small wave boards like the F-SKATE or V-SKATE.  Reduced tail area is critical in large waves since it allows the tail to be buried during turns at high speed, rather than sitting up on top of the water and spinning out or "chattering"on the wave surface. The narrow tail actually bites into the wave face and helps hold the line through each turn. Tail shapes in my Semi-Guns are invariably rounded pintails……there is no good theoretical reason for using squares or swallows.  Although a rounded squaretail would have a touch more drive……in practice this is inconsequential since there is abundant wave power to propel the board.

Rocker.
Outline and rocker (bottom curve) are the two most important determinants of surfboard performance.
You can see from the picture here that there is significant rocker in the Semi-Gun.  
This is in stark contrast to our small wave boards such as the F-SKATE and Powerglide.   As wave power increases, so should rocker.  Why?  The less rocker a board has the faster it will go and with more drive…..but it will be relatively stiff and hard to turn.  In small waves we use less rocker to compensate for low wave power.  In big waves with abundant power…..a board with low rocker would be really fast but almost impossible to turn and control.  Therefore we use more rocker in Semi-Guns to enable the surfer to turn and position the board.   In essence the heavier use of rocker allows the board to be slowed down in turns from the tremendous speeds which are generated.  Whilst there is gradual rocker throughout the bottom……there is a relatively straight section located somewhat behind the halfway point.   This is the engine room so to speak and can be used to increase speed when necessary by weighting slightly forwards.  Because of the steeper character of most big waves….extra rocker is also required to facilitate late takeoffs.   Additional tail lift enables the tail to fit into the steeper wave curve on takeoff…….and allows the nose to rise simultaneously so as to minimize the chances of nose diving.

Bottom shape.
After many years of surfing and shaping I have developed a bottom shape which produces predictable and confidence building results.  A very slight roll in the nose area ensures that there will be no rail catching up front during full rail turns.   This roll graduates into a flat area and then into a fairly shallow single concave under the chest area.  Concaves promote speed by allowing the formation of an air pocket between the board and the wave and thus reducing friction.  Concaves also have the effect of drawing the board up the wave face.  This ability to be drawn up the face in a controlled manner allows the surfer to ride high in the wave……which is the fastest part of the wave, and to generate tremendous speed runs which have obvious benefits.  The concave gradually flattens out and is replaced with a soft Vee under the backfoot area.   The Vee facilitates rail to rail turns at speed.   So overall we have a very fast bottom shape which still allows for control and direction changes.

Thickness/Length/Rails/Fins and things.
As you can see in the foil picture, the Semi-Gun has the majority of its volume under the chest, and thins out significantly in the nose and tail. The tail in particular needs to be relatively thin to allow it to be buried in turns rather than to ride up on top of the water. Its perhaps obvious but nonetheless very important that you can paddle in early in big waves.  There is nothing worse than not being able to get into the wave in time and going over the falls.  By contrast, early entry allows you to set yourself up for what's ahead and creates tremendous confidence.  The exact thickness of each board depends on many variables which I take into account with each person.   Likewise with length……the board needs to be long enough but not too long, and depends on variables such as where the board will mostly be ridden/whether the surfer has other boards/level of experience and their weight.
Rails are low soft in the nose area blending into a slightly harder rail through the center and then very hard in the last third of the board for drive and bite.  The rails through the center need to be fairly neutral or else catching will occur in choppy conditions.  The rail foil.....the thickness of the rails themselves is also very important.  Too much thickness and the board will feel "boaty" and will lack response/too thin and the board will catch through turns and will feel unstable. Complicated huh?  
Fin positioning is critical.  Too far forward and the board will lack drive/too far back and it will feel too stiff and will be very hard to control at high speed.

It's obvious then that a successful Semi-Gun contains a marriage of many subtle variables. When these are combined appropriately by an experienced shaper the surfer will feel confident in larger waves and will look forward to testing his/her limits.

Happy surfing
Grant Miller.