There are only a few things to consider before you make your rocker table:
i) Do you want to be able to adjust the table for different rocker lines?
ii) Do you want to be able introduce concave into the cross section of the board?
iii) Will you (at some stage) want to use very stiff material in the board such as denser wood like paulonia or cedar?
Adjustability: If you plan on making a few different boards you will very likely want to experiment with the rocker line and so making it adjustable is a good idea.
The right rocker for the type of conditions and riding you want to accommodate involves a lot of proof by 'Trev Reckons' and so we owe a great debt to the big names who churn through tweaked prototypes until the board 'feels' right. We'll explore this more in the design section but for the moment the main decision is how easily adjustable you should make it.
There are a couple of approaches here.
At the more sophisticated end I've seen a metal plate (this the actual 'Mold') with holes drilled around the perimeter each 10cm. At each end there are solid pieces of wood about 10cm high. Holes are drilled through the base and long bolts (15cm) passed through the base up through the holes in the plate. With nuts under and above the plate the level can be adjusted so you get exactly the rocker line you want. This is the ultimate in adjustability.
You could simplify this a bit by not suspending the mold (plate) on a bed of bolts but rather using different height blocks under the plate and using clamps to lock it in place. This example, http://www.youtube.com/watch?v=WoMySXm6heY&feature=related , takes this approach.
I've gone for what seems to be a bit more robust approach and which borrows from the BroKite video http://www.youtube.com/watch?v=wPGhSevDs94&feature=related (brilliant video form the good ol' boys of kiteboard making). They cut out of marine ply 2 sets of both the female of the rocker and the male version of it (the 'Jigs') so that the mold surface can be clamped solidly along the edges between the two matching surfaces. I think that this will have the advantage of being able to use very stiff mold surface (1/4" ply for example) so that stiff core material (denser wood) can be bent over it without distorting the mold.
The downside of this approach is that you need to cut the Jigs each time you want to change the rocker.
Concave does a couple of important things to the board. Firstly, it causes the rails to dig into the water at a sharper angle and so you get better edging and hence upwind ability. Secondly it makes the board stiffer. The way to understand this increased stiffness is to image concave taken to the extreme where the last couple of centimeters of the board's long edges are perpendicular to the deck. These would form an I-beam along the length of the board which would stiffen it considerably. The effect of normal levels concave (5 -1 0 mm) is obviously not that pronounced but the same principle is at work. The FGI document on sandwich material gives a detailed treatment of the increased moment of inertia that results from introducing the concave.
In terms of how to introduce concave into the mold, my approach has been to cut a female version of the boards centre rocker which will be higher than the Jigs that will be clamped along the side of the mold surface. The height difference is the concave height and the curvature of the concave profile will derive from the material properties of whatever you've used as the mold surface.
The 'art' in designing this centre profile comes in when trying to calculate how much higher the centre jig should be. As the mold surface will be wider than the board you will need to estimate how much the mold will slope down between where the board edge will lay and the point where the mold is clamped. If there is 20cm from the board edge to the Jig then you may need to make the height of the center rocker jig relative to the rocker in the clamping parts 2x the required amount of concave. This will need to be tweaked in situ to make sure you have the right amount.
Using stiff core material
This has already been alluded to above. If you try to bend stiff core material over a softer mold surface then the mold will deform and you will get a board that is different from what you designed. As I'm interested in using wood cores at some point, I opted not to use the perspex (Plexiglass) mold material used in the video above.
In the end I opted to use 4mm plywood as the mold material and just coated it in acrylic paint from Supercheap auto so that it sealed the pores in the wood. I'll still be using mold release to avoid the resin sticking to the mold. 4mm will be fine for the PVC foam and probably balsa core. However, I'm not so sure it will stand up to other types of wood cores. Only time will tell.
Building the rocker table
1. 2 x rectangular frames at least 70cm x 160cm - these will be used top and bottom to hold the male / female rocker line Jigs for clamping mold surface. I found that single metal bed bases were an easy solution for this as they had the weight needed to easily bend the mold surface over the center rocker jig. They are convenient because there's no construction needed and they are very stiff.
2. Sheet of 4 mm ply wood ( 2.4m x 1.2 m sheet for $23 from Hardware and General in Brookvale). This is the mold surface. Alternatives to this include melamine coated MDF or perhaps with laminex applied.
3. 1 litre tin of clear Acrylic paint (supercheap auto - $20)
4. 3 pieces of chipboard or MDF 10cm longer than the target board and about 20cm wide. These are for the rockerline jigs. I found a load of cupboard doors on council cleanup day the were perfect. Once I decided on the rocker I cut along the line with a jigsaw and this produced the male and female Jigs.
5. 10 G clamps - These are used to clamp the rocker jigs to the frames in (1) above. You could drill and bolt it together but we had a $2 shop having a closing down sale and so got 10 for $10.
6. A piece of angle iron as long as the frames in (i) This will run down the center of the bottom frame (just off center so that the jig itself is perfectly central) and will be where the centre rockerline jig will be bolted. You can buy strips of this at Bunnings for about $15 but again a piece of metal bed base is perfect for this.
..... apologies for no photos in this post but I'll put some up in the next post to show the actual construction.