[MHml] [Harrys, heavy weather and jordan drogues
cateran1949 at yahoo.co.uk
Thu Nov 30 01:45:02 EST 2006
I thought for a wing mast to work, the boom has to have a flexible coupling to the mast. Therefore some sort of locking arrangement would need to be made .
Also I was considering the cog before the centre of effort (in this case the centre of air resistance). When a gust hits, the mast blows to lw. The inertia of the mast encourages the mast to rotate to bring the front of the mast to ww. The mast has some elasticity and comes back a bit.The centre of effort in this case is the due to the restoration elements in the mast The inertia of the mast now encourages the mast to rotate in the direction dictated by the cog relationship with tthe restoration forces.
If the cog is behind centre of effort when the gust hits, the mast will attempt to rotate to bring the mast tail to wind, and you end up with a very expensive bull roarer.
. Lower down , there is not the same realtionships as the bottom of the mast doesn't fall away to lw. The main thing is to keep the total moments of inertia as low as possible to increase response and avoid overshooting, and to keep the flow over the 'feather' damped so that oscillations don't occur. Robert
Rob Denney <proa at iinet.net.au> wrote:
Good advice, thanks. The boom with the sail furled on it will hopefully do
the job, although it will require a huge lump of ballast to move the cog
forward of the rotation axis. What is likely to happen if the cog is aft of
this? In conditions bad enough to not have any sails up, I will have a
drogue (the only piece of safety equipment which I consider necessary but
the committee doesn't) out, so the boat should be bows into the waves and
----- Original Message -----
From: "Tom Speer"
To: "Informed discussion of multihull issues"
Sent: Tuesday, November 28, 2006 2:39 PM
Subject: Re: [MHml] [Harrys, heavy weather and jordan drogues
> rob dalton wrote:
> > ...Not sure about having the wing masts in a blow ....
> It may be useful to have some sort of tail you can attach to the mast to
> make it weathervane when under bare poles. I realize just going to the
> lee hull and messing with something like that is going to be very
> difficult. A wingmast that flutters in a high wind can produce a huge
> amount of drag and have the windage of a round section much bigger than
> the thickness of the mast - maybe even an effective diameter greater
> than the chord of the wing mast. But if you can keep it feathered, the
> wingmast should have far less windage than a typical mast. However, it
> must respond rapidly to gusts - if it lags, the lift will be
> considerable. Just having the pivot ahead of the aerodynamic center of
> the mast is probably not going to be stable enough or damped enough.
> It's also important that the mast be ballasted so its center of gravity
> is on or ahead of the pivot axis.
> Tom Speer
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