I have been a quad fan since getting my first Stretch quad 2 years ago. I read the shapers forum daily and recently found some fascinating info from one of the grandfathers of quad. There are 2 or 3 shapers who are considered the grandfathers of quad by the industry. These guys are often used as the benchmark when another shaper jumps on the quad revival bandwagon. No joke, I know two stories about shapers who tried to copy Stretch boards and improve on them. The following quote from the shapers forum is not from Stretch, but another of the grandfathers sharing his knowledge of quads.
If you have a Thruster with an average sized set of fins... and you add an extra back fin of the same size to make a four fin..where does the sketchiness come from?
It can only come from incorrect fin placement.
Due to many shapers making quads with fin positioning as in 'the good old days' where the rear fins were close to the rail, certain problems arise. To counteract rail-grabbing when using larger fins, they reduce fin size which means the board feels OK when on a rail but when flattened off it has a skatey insecure feel. Toeing the fins in more also helps to counteract the rail grabbing but accentuates the twin-fin 'throw the board from one side to the other' feel as opposed to a smooth rail to rail transition.
Positioning the fins away from the rail at the correct proportion to the tail width at 12" up means that larger rear fins can be used, the board will recenter itself between turns in a more 'Thrusterish' manner but will have all the added benefits of increased drive and holding power. Have the back fins closer also allows for a greater pivotal feel as when the board in forced in a flatter pivotal turn, the back fins help each other to move laterally. Too close together and you lose drive. Too far apart in relation to tail width, too much hold and twin-fin effect. Due to the long standing negativity towards quads of the past, there has been a tendency to try to fit them into the fishy retro market.
Quads are though, logically a better choice for wider shapes, (Sub Vector) the true set-up for full on power surfing. No need to nurse bottom turns and always ready for slamming a full railed gouge.
With correct fin positioning all the same maneuvers you do on your Thruster can be performed on your quad and more.
Unfortunately for the established shaping elite's hierarchal order it's all a bit of a shake up and in some cases with the inquisitional members, a bit of an unwanted embarrassment.
Generally through the 'Thruster' era, having a large quiver of 'Thrusters' was a way to ensure that you had the adequate board for the size of the waves you were riding. To handle the high speeds of big wave riding, a narrower tail is needed to bring the center fin closer to the surface level of the water. You will no doubt remember, thundering through the bowl section light on a rail and feel your tail fin dropping out of the wave, your board drift and roll over and you eat shite.. Maybe it happened in a bottom turn when you didn't weigh the tail down enough to submerge the tail fin. Commonly occurs with standard short-boards of normal width and wider and even with guns. Are you double setting and nursing your bottom turns on large waves or 'two- wooding'?!. Tail fins too close too each other in relation to tail width are no good because they cancel each other out on a heavily back-footed turn and lack drive. Just as is the tail fin of a 'Thruster' no good too far back or forward or of the wrong type, creating a too loose or a too stiff or whatever board.
* Quads are faster due to having all fins driving through turns, or even just running across a wave face. All fins are propelling the board forward much like sails or propeller blades. This is because a board in these situations, is in a constant lateral drift and the water passing under the board, divides between the fins to pass out both sides of the tail while redirecting off the fins, rearward, to form thrust.
* The back fin of a thruster generally sits in the dividing line of this displaced water so provides no added speed but gives direction and control with a dragging effect....
With a Quad you can have looser but more secure guns as you can move the cluster further up the board in relation to the sweet spot. This is not possible with Thrusters as much because the tail fin will move further away from the rail line and will give the insecurity when it leaves the water. Often longer tail fins are used to compensate.
We compensate for the flaws of 'Thrusters' and only after riding a correctly made and correctly finned quad do we realize how limited we were with our three finned equipment. The limits with a quad will be your ability, and leg strength. You will drop into the wave late and with confidence and you will slam the bottom turn, either forward on the board or off the tail with security and added drive.
Like anything made, take your shiny BMW for example, if that wheel alignment is out and the tyres are wrong, it may as well be a shopping trolley. It has taken many years to suss out the nitty gritty, and there are so many variables with quads.