Double Concave

[Rand]

Hi all,

What is the downside to double concave in the tail?  That is to say, if every design feature has a benefit and a negative quality, what is the negative to a double?

Also, if I understand correctly, a double can increase speed, but when?  Is this an initial acceleration thing that then maxes out, a consistant, always faster feature, or a top end hauling arse kind of speed. 

Lay it on me.

[Dooner]

Like every individual facet of board design, there's no one answer. It all depends on the designers intent, and how it is combined with rocker & outline.

That unfortunately means that it's impossible to say that a double concave board does this or does that. It's a lot more complicated.. But in general, most shapers utilizing concaves (single or double) are doing two things: flattening the rocker through the concave area while maintaining rocker on the rails (& middle in the case of a double) and giving the rail a little more bite by putting a little downward angle at the rail.

[Rand]

Thanks for the reponse Dooner,

Ok, but I think it could be said that most riders who are asking for double in the tail are looking for more speed.  Maybe not.  My understanding is that tail concaves acclerate the water flowing under the board and therefore can increase speed.  Is there another effect?  Also, if speed is the upside of accelerated water flow, what is the downside?

[Dwight (DW)]

Hi all,

What is the downside to double concave in the tail?  That is to say, if every design feature has a benefit and a negative quality, what is the negative to a double?

Also, if I understand correctly, a double can increase speed, but when?  Is this an initial acceleration thing that then maxes out, a consistant, always faster feature, or a top end hauling arse kind of speed. 

Lay it on me.

I'll go to board sports outside surfing for my contribution.

Concaves promote earier planing. It can make some boards faster, but flat is usually fastest, all things being equal. Concaves come and go like fads. They can fix a slow board, or make a fast board slightly slower. Confused now?

Take a slow to plane windsurfer, add concave and it becomes faster. Take a flat fast planing racing board, add concave and it can be slightly slower.

In kitesurfing concaves add comfort and control. But flat is fastest.

[Dooner]

Thanks for the reponse Dooner,

Ok, but I think it could be said that most riders who are asking for double in the tail are looking for more speed.  Maybe not.  My understanding is that tail concaves acclerate the water flowing under the board and therefore can increase speed.  Is there another effect?  Also, if speed is the upside of accelerated water flow, what is the downside?

What part of the tail are you talking about? Are the concaves starting in front of the fins and running full depth right off the tail? Or fading in and out, flat in front to flat or vee in the very tail? These are two different things, and different intents. The rocker in the back half of the board plays a huge part as well.

If the concaves fade in/fade out, it is flattening the rocker in that zone, usually around the sweet spot at or in front of the fins. This will give more lift and less drag compared to a flat bottom (given the same rocker on centerline/rails). This should make the board faster all around, but it will not be as loose, and will not "sit down" in the pocket as well as a flat/vee board WITH THE SAME ROCKER.

[Rand]

What part of the tail are you talking about? Are the concaves starting in front of the fins and running full depth right off the tail? Or fading in and out, flat in front to flat or vee in the very tail? These are two different things, and different intents. The rocker in the back half of the board plays a huge part as well.

If the concaves fade in/fade out, it is flattening the rocker in that zone, usually around the sweet spot at or in front of the fins. This will give more lift and less drag compared to a flat bottom (given the same rocker on centerline/rails). This should make the board faster all around, but it will not be as loose, and will not "sit down" in the pocket as well as a flat/vee board WITH THE SAME ROCKER.

Terrific answer.  That helped me a lot.  You gave a likely performance for the in/out concave, but what of the "concaves starting in front of the fins and running full depth right off the tail".  What would be the difference in intent there? 

Again, thanks!