Rocker'n roll

Hi Blogosphere it's been a while since I've posted. Sorry about that, we all got knocked down with the flu and Real Life caught up. While laid up at home I had a good opportunity to peruse the Net about sailboard construction. It has certainly been an interesting exercise. Lot of negativity (or maybe practicality!) out there. However, what it did bring home was a couple of points.

1/ For a one-off amateur board builder, there is not much room for trial and error if you want to end up sailing something which is not a total dog. So it really is important to come up to speed with the state-of-the-art. Move away from this at your own peril.

2/ A lot of amateur-built boards turn out too heavy. My interpretation of this issue is that during the production board design process, a lot attention is paid to weight minimization through providing the strength only where it is needed. Another possibility is that production board manufacturers use advanced materials and building techniques that are not available to the amateur builder.

With regard to 1/ there seems to be enough information available on the web to reasonably tightly constrain the shape of a modern competitive longboard. There are tabulated intercomparisons of boards; nice plan views of the various manufacturers boards (i.e here, here, here, here and here); a more limited number of side on views (i.e. or here) for comparing rocker; and many interesting forum discussions on the merits of the alternative board designs. With respect to 2/ I think it should be possible to use a fairly simple sandwich technique to provide a stiff board. High density polystyrene foam- computer shaped in 2-d for the core and an s/e glass epoxy layup. Follow this up with a second thin foam layer on the board top (possibly using vacuum bagging to get a good even bond). Use a second layer of s/e glass on the board top to complete the sandwich and then add a layer of carbon fibre to the working areas of the board. Piece-of-cake ;-)

Now on to the consideration of rocker- which was the primary intent of this posting. I like the definition that John Carper provides in "Shaping 101". This is illustrated in the following figure which demonstrates that the rocker consists of three components. 1- the entry; 2- the centre or planing section; 3- the exit. Getting these three components of rocker working correctly together is critical for the design of any type of planing board, be it a surfboard, the slalom sailboard or a racing longboard. In the case of the latter, an additional complication is that the rocker has to work across all of the board configurations- upwind sailing on the rail with the centreboard fully extended and the mast track forward and the sailor positioned in the upwind straps; downwind planing with the centreboard retracted, the mast track fully back and the sailor positioned in the tail straps; non-planning conditions, sailor's weight over the centreboard.

There is an interesting discussion on the UK sailboarding website Boards regarding the rocker of the Equipe-III from "GraemeF" who seems to have some experience with this issue. To quote

".... It'll probably have a Euro rocker, they always do, it certainly wont have the benefit of the old Mistral rocker profile. Complicated things Long board rockers, it's a three stage rocker.

Half the reason for the long track is using it to transition rocker stages, if they have mad(e) it with the track way back it's because they're using a flat straight rocker, good up wind, O.K. on Euro lakes, but totally useless on the sea.

The reason the current Equipe or Pan Am is still so successful these years later is because of the rocker. "

So the Euro rocker is designed for flat lakes and the Mistral rocker is designed for choppy/open sea conditions. Given the comments of GraemeF, it's informative to consider a plan view of the Starboard Phantom 380, the Equipe-III and the Exocet Warp-X (see the figure). When comparing the Phantom 380 and Equipe-III you can see that there is quite a marked difference between the locations of board components. In particular the mast track front end terminates 21 cm further back on the Equipe, the centreboard is located 11 cm further back and the reaching straps are positioned 12 cm further back.

The differences are even more marked in the case of the Warp-X which has all components located closer to the bow, even when compared to the Phantom. I'd hazard a guess that the rocker of the Phantom and Warp-X may not be significantly different and that the placement of the components on the latter are also influenced by the removable tail fixture.

The comparisons shown in the figure seem entirely consistent with GraemeF's posting and if nothing else, demonstrates the importance of matching component location with rocker characteristics.

The majority of my sailboarding will be taking place on Port Phillip Bay at Elwood sailing club. For any sort of onshore wind conditions (which includes bay- and sea breezes), the area builds up a steep chop. Given this, my board will be constructed with a three part rocker which is consistent with these conditions. The following plot shows the normalised rocker (normalised by board length) which I am intending to use. I've derived this by looking at images of boards on the web and by measuring up a couple of older boards which I have access to. Also shown, for comparison is the normalised rocker of an older and slightly shorter board which was developed for flatter conditions.



You can see by comparing the two profiles that there is a significant amount of additional rocker required for ocean-going conditions.

All the best for now.