Sigurd,
In terms of ventilation, I believe one difference between the
current variations of harryproa foils and a windsurfer foil is that
the sailboard's foil is not fully exposed to air. Instead, it has a
large endplate on the top in the form of the board itself.
Granted, when the board is planing, there will be some air up
there, but not the same way as with the proa foils.
Swept forward, the foil pushes some water up the front edges.
It's not immune to sucking in air, but it's also not encouraging the
air. Swept back, the water flow isn't helping to reduce
ventilation.
Still, I'd rather have a kick-up foil that's slightly larger, in
order to make up for ventilation, than one that goes through the
hull, which could either sink the boat or render it unsteerable when
grounding or running into a submerged log.
- Mike
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.
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.