Just wondering if anyone out there is experimenting with Centreboard shapes and lengths, particularly the latter. Has anyone used a really short board and if so, what are the pros and cons (apart from not running aground so often!?
High (effective) aspect ratio is pretty critical to lift vs drag ratio. Also low area and short chord foils are prone to stalling, and in practice are hard to get working from a dead boat situation. There is plenty of info out there if you want to read about the subject, would suggest that this has become quite an exact science, and no longer for seat of the pants design.
Andy Hay - Business as Usual
Most of the evolution is currently towards higher aspect ratios, with a crane to extend the pivot point so that the plate pivots on a large circle (giving the required reduction in area as the plate is raised). Gybing boards are possibly back too. Tabs went into vogue a few years back, but seems that that didn't really work.
Short boards? Possibly the Merlin is too slow to make them work, but with the planing upwind mode, this could work in breeze. Not sure that there would be enough "bite" off the line in the high, slow mode off a start line. could swap boards depending upon wind conditions of course.
Good luck & let us know how you get on.
I wouldn't say trim tabs were ever "in vogue" but they were an experiment that promised rather more they delivered. They died a fairly quick death!
As Andy says the current development work is with high aspect boards which are rather longer than the conventional centreboard which had remained more or less unchanged for years. I wouldn't say it's proven that this is the way forward yet, but they aren't slow.
Nobody has properly gone down the short board route yet to my knowledge, at least not recently. I don't consider sawing the end off to get round Tamworth SC doing it properly! Guy Winder did an article in the magazine a while ago about foils and he mentioned he had been using a smaller board, but i'm not sure whether he was referring to length or area.
Peter Mason 847
Well, there is plenty of fluid mechanics info available on how to design a C/B from scratch and, just to make sure you stay close to reality, there is 60-odd years of empirical experience with Merlins. So, I do not think just making a specially short or long one is going to release a burst of performance.
The principal task of the C/B is to control leeway and the drag of C/B itself is not necessarily the key thing. To do this, it needs firstly adequate area, say 0.3 to 0.4 m2. With this sort of area, a high aspect ratio will also tend to reduce leeway for a given side load but it will be more prone to stall at low speeds.
Turning to the drag, a deeper board will rapidly have lower induced drag but, for a given area, the friction and base drag will go up.(Roughly 50% of the frictional drag occurs in the first 15% of the chord.) So somewhere in here is an optimum depth for a particular set of conditions and sailing style.
A rough calc. for my boat shows that at 2.0 m/s, the viscous drag on the C/B is about 3.6 N and the induced drag is about 2.5 N, i.e. they are nearly equal in value with the induced drag the smaller. I would reckon that this is close to the optimum, bearing in mind that controlling leeway is vital. This C/B is quite small at 0.3 m2 and 1.075 m depth.
If you want to experiment, you could try a sharp trailing edge as any thickness has a huge effect on drag, or you could try crescent shaped tips and straight trailing edges, or best of all, you could try and minimise the dreadful necklace and hairpin vortices which occur where the C/B goes into the hull. These account for a lot of drag.