Re: Rudder Failure - builder error!

Posted by Jeremy on
URL: http://uk-hbbr-forum.61.s1.nabble.com/Rudder-Failure-builder-error-tp3338284p3344979.html

Frogsider wrote

My centreboard is 12mm and seems to be able to take the load.  Sail area is about 3.5sqm.

I'm wondering if I had cut the rudder blade on a diagonal from the ply sheet, instead of with its axis parallel to the edge of the sheet, it might not have snapped so easily.

Now I'm considering using a rudder blade of 8mm aluminium which I hear is easy to cut with normal hand or power tools.  It should be up to the job, and if not it should bend a bit rather than snap right off.

Cutting on the diagonal won't help, as it is only the outer plies on each side that are taking the bending load, the core will be doing very little other than keeping the outer, load bearing, plies apart.

I assume that you cut the rudder with the outer plies running vertically?  if so, then this was about as strong as you could hope to get from that thickness of plywood.

8mm alloy will be heavy and a bugger to cut to shape.  It can be cut with tungsten carbide woodworking tools, like a router, but will be a bit scary, throw sharp swarf everywhere and present a potential hazard from fast flying hot chips of alloy.  It will also be quite expensive.  Even a common, moderate strength, corrosion resistant alloy like 6082-T6 (probably the minimum acceptable in terms of strength and corrosion resistance) will cost a heck of a lot more than plywood.  It's also hard to buy plate offcuts in the sort of small-ish size you want - as standard it's sold in 2.5 x 1.25 metre sheets on the continent (or still in 8ft x 4ft sheets here in the UK).