I notice some race boards have the concave that goes all the way to the tail of the board and some it stops around the fin and goes to a V at the tail. Can anyone explain the pros cons of each. I presume the V tail is better for surfing and turning. Also some have a straight concave down the centre and some it follows the outline of the board. Why is this. Nearly all race boards have some sort of concave now.

Concaves are inherently slower in displacement speeds (non-planing), as they increase the wetted surface for the same volume.

But, they also can make the board stabler for the same width.

So board designers found ways to use concaves to reduce enough the board width, while keeping the stability, so that it offsets the added wetted surface, with a gain of speed on the whole.

Agree that it doesn't look right.
Check out starboard raceboard sup 2020 video

I understand the general idea of the concave but would like to know the difference between a concave that goes all the way out the tail. Like the Sidewinder II and a concave that finishes and the tail has a V , like the 2019 Blackfish.
Yes the new allstar is different. Marketing to sell the new model or does it actually work. You could put Boothy on a barn door and he would still do well I think.

The 2019 Blackfish looks awesome board. They called it an inverted v with an edge not concave. Have a good look around the fin box area, shaping lights show the edges. Kai Lenny is making look extremely fast.

Yes SB video is great marketing but a 1m straight edge ruler is great tool to see used on a board.

Those manufacturers would be building a lot of customs with subtle differences and test them extensively before putting them into production.

Guessing you looking at a board build

Yes getting close to finishing the design. It's going to have a concave,but full length or like the Blackfish? I have watched so many vids and reviews. Which one is better for what. I want a good all round board. Quite often choppy around where I paddle most of the time. It's going to be 14x24-5. Hollow wood.

I have the Sidewinder and I don't know about speed increases or differences with one or the other but as Colas said it helps with stability. The secondary stability and ability to bring it back from a tilt is far superior to a flat bottomed board as was my last 14fter.
I did take a pic while it was in flat water from under the water line to see what was happening and the air was trapped under the board in the channel and would that not create less drag as the total surface area on the water is less, thinking Catamaran here so stay with me if possible????

The big concave in the bottom of boards is to create as you say a 'catamaran' effect. Water is under increased pressure at the rails and less in the centre, helping improve stability for a slight drag penalty created by the hard edges of the concave. Hard edges are best for stability but poorest for drag. Rounded edges/surfaces are best for drag reduction, but not stability.

There are a wide range of designs but it comes down to a stable fast design in a wide range of conditions.
Not an easy choice