thought

Originally posted by mike ellis isnt it short foils which stall easier barty

To over simplify a *very* complex subject then, for the same area and section, the longer and thinner the centre/daggerboard then the lower the drag will be, the lower the range of angles (ie leeway) it will operate efficently at, and the more easily it will stall. This is complicated by the section being dictated as much by making the board thick enough to survive capsize recovery and a billion other factors.

One of the big limits on the foil properties is that it needs to work effectively coming out of tacks with tons of leeway and very little speed. Very low drag foils are rarely good at this and need more skilled handling, and you can readily reach the point where you lose more tacking than you gain in between tacks with reduced drag. For example learners boats need thick and relatively low aspect ratio foils because they're easy to sail with, don't break if the crew jump on the end, and the drag is less of an issue.

Everything else being equal mike, a high aspect (long skinny foil) will stall more readily than a low aspect (short stubby) foil

Aye, all the losses are drag.  Long foil stall easier so its a compromise between performance (lift v darg) and ease of use

All lifting surfaces; i.e. wings, keels, c'boards, rudders etc all suffer from losses of lift.  These include tip losses where the low pressure flow jumps over the end to join the high pressure flow (this is why commercial aircraft have little winglets on the end of each wing), venting when you have a rudder or c'board/keel operating close to the surface of the water and of course turbulent flow due to stall or surface finish. 

Now if you consider the aspect ratio of a keel or centreboard, that is the span (depth in our case) compared with its breath then a keel with a small length but a long root chord (the length against the hull) will suffer these losses against a greater proportion of its span and therefore will be inefficient.  A keel/c'board with a long span and short root chord with suffer these losses against a small proportion of its span hence being efficient.

Consider a jet plane verses a glinder.  A jet plane has dirty great big engines which can provide the speed to use smaller (i.e lower aspect ratio) less efficent wings.  A glider has no engine so it has long, high aspect ratio wings to be as efficeint as possible.

So to answer your question a c'board with a short span but long root chord would be easy to steer in a straight line, be hard to tack and would slip sideways!!!

Long spans, short chords tach easily, don't slip to leeward as much but stall easily so its a compromise between reliability and performance.

Hope this helps

Ok maybe not closer to the wind, but surely it would affect upwind performance as there'd be more to stop you going sideways? And I didn't mean just a long centreboard, i meant one the same length but running the length of the boat... sorry that wasn't clear.


Edited by HannahJ