It’s hard to let an interesting surfboard design question go by without saying something (when one has something to say).
Regarding thrusters or singles in big waves, I am going to pipe up again in favour of the hybrid tunnel fin setup, which could be described as a tri fin or in line twin, depending upon how you look at it.
Obviously both singles and thrusters have been ridden successfully in big waves, and their characteristics are pretty well known. What would be of interest to most surfers would be a setup which combines the good points of both, without any of the drawbacks. The tunnel fin with singlefin setup (hybrid tunnel fin) is the best of both worlds. A tunnel does the same job as the two thruster sidefins, but with even more precision and drive.
The tunnel also avoids one little drawback of the thruster setup. This one little drawback is due to the toe-in of the thruster side fins. On most occasions the toe-in works very well, providing amazing manoeverability, and it is obviously a major factor in the success of the thruster.
It is no secret, however, that the thruster doesn’t like flatter waves, and it has also been reported by some surfers that they sometimes have problems with thrusters in certain kinds of large wave in particular those large waves with a long slope at the base.
These two characteristics of the thruster are due to the effect of side fin toe-in, and in the case of big waves, to the effect of side fin toe-in with outwards cant.
The thruster doesn't really come to life until it is put on the rail, effectively taking one of the side fins partially out of the equation. When ridden flat in the water the side fins tend to fight one another and cause drag. Surfers who ride thrusters well have developed a knack of riding on one rail or the other, and only using all three fins briefly during rail to rail transitions. This knack is more difficult to apply in flatter waves. What happens in bigger waves is another story.
Big waves mean higher speeds. This is no problem for the thruster provided that it is kept on one rail or the other. If the thruster is surfed flat on the bottom at high speed, the toe-in drag and outwards cant will:
1) slow the board making it take longer to find the bottom of the wave;
2) make the board difficult to put on the rail as speed increases.
So there is a typical situation where the surfer is trying to get to the bottom of a very long slope at the base of a big fast moving wave, in order to find a place to bottom turn, and it is taking longer to get there than it
should (problem number one) The surfer then attempts to start drawing a line towards the shoulder of the wave, but is unable to put the board on the rail and redirect the board. This is problem number two.
Problem number two, that is an inability to put the board on the rail at high speed on long flattish slopes, is
due to the fact that outward cant on thruster fins gives a horizontal control surface component to the fin setup. This horizontal control surface component is known as a hydrofoil. On a thruster, the horizontal hydrofoil component is angled downwards due to toe-in (toe in on an outward canted fin is not just toe -in, it is toe-down), and thus
pulls the tail of the board down.
Now it can be seen that we have two fins, both trying to pull the tail of the board down. What happens when the surfer starts to put the board on a rail, and the reason why he sometimes cannot, is as follows:
A hydrofoil setup (which the thruster certainly is) develops power exponentially as speed increases. For example a hydrofoil of 20 square inches in area will be impossible to dominate at normal surfing speeds on say, a six foot wave (The trick with them is to direct them, not overpower them) So with a thruster we have a hydrofoil area which is only a square inch or so and it is possible to overpower this control surface at normal surfing speeds. At high speeds even this tiny area will develop so much power that the weight applied by the surfer becomes insignificant in comparison to the downforce applied by the horizontal area of the two side fins. Thus the board cannot be turned.
There are further complications which the outwards cant of the side fins cause, in particular a resistance to rail to rail roll because of the outward pointing tips which don't want to move laterally through the water.
The mysterious 'suction zone' at Mavericks is, I contend, due to the mysterious properties of thruster side fins at high speed. In other words there is no such zone.
Big wave surfers are obviously remarkably successful at what they do, and in most cases have obviously tuned their boards to avoid these problems, which is perfectly possible by adjusting them to cope with a particular speed range. The potential for big wave problems still exists as an inseperable result of the thruster setup.
The beauty of the tunnel fin setup is that none of these problems occur, ever. The tunnel has no resistance to rail to rail movement at any speed, and surfs at any angle including flat on the bottom without any increase in drag.
The tunnel fin is also able to hang in to incredible late drops, even those which are beyond vertical. They have incredible drive and are at least as fast as the thruster, and often much faster.
Honestly, Roy Stewart