Not according to what I read. Ventilation has to do with the same mechanism as cavitation, that is low pressure peaks. I am discontent with my understanding of the subject but it goes apparently so: Only the chordwise flow "counts" in the scheme of creating that low pressure peak. In a swept foil the chordwise flow is slower than if it is unswept. Could be wrong, I need to read more on it anyway. Hence you could be right that only fwd sweep works, or that it is better. Shorter windsurfer foils are often swept back, but this could have to do with stall, don't know. They did lots of experiments to prevent ventilation, but in the end I think they found the best remedy was better and thinner section shape, maybe low area loading and narrow tails (more sinkage/better endplate).
Yes, the rubber needs to be soft enough to bend but not allow fore/aft movement. Of course too much up/down wouldn't be good either. It could work on a small one at least. I'm going to try it on a bigger one unless I can think of someting better. Seems I can scrounge up enough sqm of stuff to build a 9m with the Tornado going into an ama. I am just now leaving town and visiting someone with a wood lathe. Planning on getting made a mandrel for mast foot. Cylindrical at the bearings and conical inbetween. Some extra length for overlap to box section spar. 12m mast, 2 ton meter max RM, is it insane with 60-70 cm Center to Center on the bearings? How about upper bearing innerdiameter 120-160mm (what is enough/better?), lower innerdiameter what, 60? 80? mm. How about bearing height, is 80mm a nice overkill?
All I know is it should be a lighter mast than Sol (250mm upper bearing?) but heavier than el (80-90mm?). Should like to know what you'd guess were good dimensions for that mastfoot.
Regards to all,
Sigurd
From: Rob Denney <harryproa@gmail.com>
To: harryproa@yahoogroups.com.au
Sent: Tuesday, February 26, 2013 11:17 AM
Subject: Re: [harryproa] Proa rudders.
Yes, the rubber needs to be soft enough to bend but not allow fore/aft movement. Of course too much up/down wouldn't be good either. It could work on a small one at least. I'm going to try it on a bigger one unless I can think of someting better. Seems I can scrounge up enough sqm of stuff to build a 9m with the Tornado going into an ama. I am just now leaving town and visiting someone with a wood lathe. Planning on getting made a mandrel for mast foot. Cylindrical at the bearings and conical inbetween. Some extra length for overlap to box section spar. 12m mast, 2 ton meter max RM, is it insane with 60-70 cm Center to Center on the bearings? How about upper bearing innerdiameter 120-160mm (what is enough/better?), lower innerdiameter what, 60? 80? mm. How about bearing height, is 80mm a nice overkill?
All I know is it should be a lighter mast than Sol (250mm upper bearing?) but heavier than el (80-90mm?). Should like to know what you'd guess were good dimensions for that mastfoot.
Regards to all,
Sigurd
From: Rob Denney <harryproa@gmail.com>
To: harryproa@yahoogroups.com.au
Sent: Tuesday, February 26, 2013 11:17 AM
Subject: Re: [harryproa] Proa rudders.
Clever. If the rubber was stiff enough, it would work, I think. If it wasn't, it might be sloppy which is bad news on highly loaded foils. Only drawback is that it does not lift, but I guess a case could be included pretty easily.
Not a biggy, but the sweep required for balance is aft, although not much with deep, narrow foils with the gudgeons close together. Forward sweep is required to delay ventilation.
Let us know if you try it.
rob
rob
On Thu, Feb 21, 2013 at 2:48 PM, nosupersnail <nosupersnail@yahoo.no> wrote:
Refer to folder in files, "proa rudders".
These are cambered symmetrical sections, linked rudders.
They kick back and are balanced.
Limit the sweep (rake) with weak string or releasing cleats.
At the end of the post is a detailed description in case the
drawing is not clear.
Regarding the section, I think that one of Tom Speer's
sections is a good bet. It should probably not have more than
1% camber, otherwise it will be outside its groove at zero or
slightly negative lift.
It could have a good thickness, 12-15%, for stall resistance
and high lift, or it could be thinner, 8-10%, for ventilation
resistance. So, I am guessing P30112 is good.
It needs to have a good size. It should have a high 2D L/D at
low lift, which means it can be made big and burly with little
profile drag.
I think these sections might be more demanding on accuracy and
finish than the blunt NACA 0012 for instance. But on the other
hand they should not ventilate as easily if kept inside the
groove. The sweep that is necessary for the balance, should
also delay ventilation.
Description of the drawings.
The top attachment is like a windsurf mastfoot joint. They are
made of rubber, with a wire or rope through them.
Attached on one side of the wire is a disc with a bolt on it
that screws into a nut embedded in the top hull-mounted brace.
On the other side of the wire is a disc with a bolt that runs
through the rudder, and then has a groove in it. The groove is
for locking, by the blue thing. The blue thing is held by bungy.
And fair everything much better than the drawing!
Questions and comments welcome.
__._,_.___
.