Displacement tailed planing hulls 101: Tunnel or single fin?

Displacement pintails when correctly designed allow the tail of the board to sink during turns and stalling moves, greatly increasing the versatility, control and balance of longboards.  When force from the rider is released the tail lifts again automatically due to a combination of planing and buoyant lift. 

It is vital for such tails to have sufficient thickness to allow both a low drag shape ( with soft catch free rails ) when submerged at higher angles of attack and to provide sufficient volume for the required buoyant lift.  A narrow low area tail is also needed both for clean underwater running and in order to reduce rail to rail resistance from  athwartships planing force leverage. 

We have used this principle with great success both on single finned longboards and those also equipped with tunnel fins. The single finned boards use hull based lift only, with a higher proportion of buoyant lift over planing lift. The tails on these singles are easier to sink and have a corresponding softer spring back as the tail lifts, all else being equal. Because of this the ratio between the force required to roll the board from rail to rail  and the force required for fore and aft trim is altered in comparison with the tunnel finned tail.

Here's a single finned displacement tailed board, the Ghost 12, sinking the tail during a bottom turn, the aft 6 feet of board is beneath the wave surface, this is achieved with moderate pressure from the rear foot and most importantly from a central riding position which allows excellent balance and gives the tail more leverage during spring back: 

     

A reduction in weight on the rear foot will bring the board back into down the line trim in a 'self tending' fashion, here's the Ghost 12 again:   

Dragon 13-9 with twice the weight and nearly two feet more in length utilising the displacement tail. Greater weight increases the force required to lower the tail since the longer heavier nose has a stronger counterbalancing effect. The longer tail also has more leverage, which reduces the depth to which the tail submerges.This is appropriate for the Dragon since it draws longer lines.

In contrast here's the Makaha 12-9 which has a four inch diameter tunnel fin. The planing lift from the tail is greatly increased, the effect is somewhat like stronger or tighter rear suspension on a vehicle:

Both options work admirably and the advantage which the system has over standard longboards can't be over stated.