Release the Twins - Part II

So now for the “Why” twins work so well in SUP Racing compared to traditional single fin set up’s…

Single fins Vs. Twin fins in SUP Race boards when trying to go straight and fast:

1.      Projection

a.      Because Single Fins are in a center fixed position in the tail; fin outline shape, flex, cord length and depth can all dramatically effect projection – projection effects speed in both flat water and catching bumps down wind

b.      Twin Fins are able to “push off” from two separate parallel planes, allowing for one fin to compensate for lateral forces (side chop, turbulence from other paddlers, etc.) and provide more consistent projection (with far less drag due to reduced depth)
 

Larry Allison Elliptical Twin Fin examples - color coded fin box inserts adjust the Fin Cant by varying degrees from center (Blue Insert is Zero Degree for rear flat bottom Race Sups)

2.      Drag

a.      Single fins tend to be longer (or deeper) because with only a single plane to work off of; single fins need to do the multiple tasks of projection, yaw (tracking) and lateral stability – this does not allow for the fins to go much shallower than 6-7” (and be affective)

b.      A pair of Twin Fins can accommodate the same surface area as a larger single fin with much less depth; as little as 4” – my current favorite for most race conditions are 4” deep with a 10” base in an “Elliptical” shape – Depth equals drag, so if you can run shallower fins, you have less drag.



Retro Fitted Twin Fin set up on SUP Race Board with Pro Box Fin Systems and Blue Zero Degree Cant Inserts



3.      Stability

a.      Single Fins again have a single plane to counteract lateral forces like side chop and sub-surface turbulence (from boats, and other paddlers), rebound from shore lines, jettys, docks etc. The plane also being at the tail, makes it even harder for this single point to stabilize efficiently (similar concept as to what is more stable in rough water; a dual hull/keel vessel or a single hull/keel vessel)

b.      Twin Fins again have the advantage of dual planes; this allows for greater lateral resistance (as water pushes and pulls from both sides, allowing one fin to compensate for the other and visa versa) – increased lateral resistance equals greater stability – in order for a Single Fin to increase lateral resistance, it has to be made longer – BUT if you make it longer (deeper) what happens? – Increased Depth equals Increased Drag

4.      Tracking (Yaw)

a.      Single Fin SUP race boards suffer from excessive Yaw – early Single Fin SUP surf boards had tremendous Yaw for example; this is one of the reason’s displacement shapes for race Sup’s came into use; to compensate for the Yaw created by paddling a single blade paddle on one side and then the other – if you create displacement in the nose, the hull aids in compensating for the Yaw and allows the board to go straighter (more strokes per side) and helps the boards “track” and “point” in side chop and windy conditions

b.      Twin Fin race boards do not need as much nose displacement – the two planes of the Twin Fins work off of each other, creating better tracking and pointing. When added to a Ventral fin (a single fin located 20” forward of center balance point) – the three fins work in concert, allowing the paddler to point a board (displacement hull or not) wherever they want to go (regardless of side wind and surface chop). How to use this efficiently requires an understanding of the various environmental forces that a SUP race board can encounter in what I like to call “Mixed Media Races” – race courses that traverse both open ocean, intra-coastal waterways, tidal currents, cross/head/tail winds, etc. ALL in a single race. Twin Fins together with a Ventral fin, exchange “jobs” when set up properly throughout a single race; producing more efficient hydro dynamics for the paddler.
 

(Blue/Green Board to far right) - Twin Fin Standing Start example - within first few yards the board is at top speed (notice the small "jet" effect off tail - that's the Venturi Effect)

5.      Venturi Effect (added bonus)

a.      If you view a Single Fin underwater when paddling a Race Sup board, you will see turbulence (as bubbles and wash) coming off the fin as it alternates pressures being placed on it from side to side with each stroke of the paddle blade.

b.      If you view a Twin Fin set up underwater when paddling, you will see zero turbulence. The water is releasing cleanly and with greater efficiency than the Single Fin. An added bonus I discovered with the Twin Fin set up is a “Venturi Effect” that is generated by the parallel fins. The Twin fins from Larry Allison combined with Pro Box Fin Systems are placed in reverse to how they are positioned on a Twin Fin Surf Board – the “Flat” side of the fin is on the outside of the fin towards each rail – the convex sides are on the inside facing each other – this channels the water between the fins and creates a “Venturi Jet Effect” off the tail.

In summary, Twin Fins in Race SUP boards:

·        Reduce drag (shallower Fins)

·        Increase stability (increased lateral resistance while maintaining reduced drag)

·        Increased projection

·        Enhanced tracking (without increased hull displacement required, allowing for more “planning shapes” in Race SUP boards)

·        Venturi Effect (channeling water, increasing speed – especially time from standing start to top speed is reduced)

If in Florida or Southeast – contact George at www.surfguyssurf.com to get your Sup Race board retrofitted to Twin Fins and feel the difference!

**NOTE: Not all Sup Race Boards have flat bottoms – Twin Fins need to be set at Zero Degrees (meaning vertical to the bottom) – Pro Box Fins Systems by Larry Allison are the only retro-fit fin boxes that allow for different “Cant” inserts – this allows for Zero Degree alignment regardless of the contours of your Race Sup bottom – we tested various Cants with Twin Fins for this application and Zero Degree positioning of the Fins is CRUCIAL. – I have a Morelli and Melvin 14’ Race Sup being retrofitted now and will post more detail on “Cant”; using this board shape to illustrate.  

So let’s see those Twins and see YOU in the water – Aloha!