Sunday, October 24, 2010

simple harmonic motion

so no results from the foil testing as yet because i cracked the bottom rudder gudgeon before the first race. sometimes when you say 'that probably wont be strong enough' its a bad idea to convince yourself that it will be. anyway its glued back together and tomorrow it will be beefed up and i should make it on the water later in the week.

The reason I loaded up the rudder and snapped the gudgeon was because of the annoying downwind motion the boat exhibits through our short chop. I drew a diagram to help myself understand what was going on- my theory is that the wave motion is wildly changing the angle of attack on the foil as it goes through the water. This can be up to around + - 5 degrees or more, depending on the boat speed and ride height. The boat tries to follow the wave- up the back and down the face - but there is a bit of lag because of Newton. The flap also complicates things a bit because it is reacting to the lag and not the wave.

The calcs suggest the best thing to do is go faster. The more waves per minute you go through, the less time F=ma has to crash the boat. But the faster you go, the greater the lift from that extra AOA and F becomes a bit more significant. also, there will be a sort of no go zone, where dead square is ok, beam reach is ok, and somewhere in between the effective wavelength gets in a problematic range and causes this kind of crap behavior. Probably people who can sail will be saying 'duh' but this has been hurting me for ages!

I still don't get why 2 boats with very similar setups will respond differently to this. Ncik said its all in the shock chord...

PDF here


n4rkla said...

It's not ALL in the shock cord.

nick flutter said...


Karl said...

Not enough pitch input!

How about a long bowsprit...what would be the effect of planting the mast on the bow and moving the daggerboard forward the same amount or a bit more, while putting the wand out on the bowsprit, with the forestay? Should be more stable in pitch. I think I want one =:-)

Karl said...

Or maybe a tiny damper. Didn't Elton write a song about that? My current one is not so tiny, but no matter - it works well.

nick flutter said...

i think i might have a go at a bowsprit. kind of hard to mount..
also im learning to sail the boat better. maybe its a technique thing. with the tiny rudder i can use bodyweight to get some positive input

nick flutter said...

as a way of rounding off this post - i think increasing the gearing (less wand movement for more flap movement) has gone a lot of the way toward fixing this problem.
thanks Karl.