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Thicker rails are going to be more forgiving than thinner rails . For the water to push down on the deck and destabilize you , first it has to get onto the deck . Any SUP paddler that comes from kayaking will tell you that a round creek boat shaped kayak is more forgiving than a flattish hard railed playboat style kayak . The same physics apply to SUP . An eight-inch thick rail requires the water to travel that much higher to get onto the deck than a fiveinch thick rail . And a rounded deck is going to force the water to apply the pressure gradually , rather than the switch like on / off of a flat deck .
Simply put , a narrower ( within reason ) thick-railed board with a rounded deck rail is going to be more forgiving than a wider , thinner , flat deck rail .
That said , stance height above the water is a major contributor to stability . For every 1cm lower you stand , its about the equivalent ‘ flat water stability increase ’ of a 2cm wider board , all other design elements being equal . This is not an absolute , but it ’ s a nice little rule of thumb . Therefore standing on a six-inch think board that ’ s 34 inches wide has the same ‘ flat water ’ stability as standing on a five-inch thick board that ’ s 32 inches wide .
Clearly , a low standing area conflicts with a high sidewall rail if the board is essentially planar in its design ( parallel deck and hull ), which all inflatable boards are . Composite and plastic boards allow you to recess the standing area as the design is not dictated by the construction method , so that you ’ re at waterline ( or below ) and have a thick rail – letting you eat your cake and have it .
If you ’ re on an inflatable , then you have to make decisions between the extra volume and forgiveness of six-inch versus the lower standing height of a five-inch . There is no correct answer : this is simply information for you to use .
Construction method is a contributor as well . The most stable boards are composite as they are the stiffest . They have no flex at all , either directly under foot as both plastic and inflatable boards have , nor longitudinal and torsional flex like inflatable boards have . Flex in a board amounts to something much like trying to do yoga while standing on a waterbed : you might develop the skills to do it , but it ’ s a lot harder than standing on a solid floor .
Centre of gravity The most stable position on the board is when your centre of gravity is lined up with the centreline of the board . Your centre of gravity is close to an imaginary vertical pole running down through the center of the top of your head , down through your chest following a straight back , out the centre of your crotch and down to the board , landing directly on the centreline of the board ( stringer ).
No matter what your knees , ankles and legs are doing , the goal is for this imaginary pole to always remain vertical ( even if the board is not horizontal ) and pointing at the centreline of the board .
MAIN PIC : THE MOMENT BEFORE THE FALL- BENDING AT THE WAIST THROWS YOUR CENTRE OF GRAVITY OUT FROM BOTH THE CENTRELINE OF THE BOARD , AND OUT FROM AN IMAGINARY LINE GOING THROUGH YOUR ENTIRE BODY THAT ’ S CENTRED BETWEEN YOUR FEET