|Subject: [harryproa] Re: Aerodynamics and sailing performance|
|From: "brag_rotor" <firstname.lastname@example.org>|
|Date: 3/10/2007, 6:15 PM|
Excellent points, Mike.
I'm another technophile - sailing a gaff-rig ! <grin>
Think that the unstayed carbo mast can take a gaff comfortably if it
is designed for the purpose. Serious taper is probably not that
necessary if a flexing gaff can be used to dump gust loads and the
mast diameter can likely be increased without significant drag
penalty since the mast is inside the wingsail.
Might consider a wall thickness increase, but from the website pics I
-think- that the EasyRig mast looks to be a lot tougher than our
2.5mm wall thickness aluminium tube that is doubled-up at cap and
foot, but not where the gaff bears.
Even allowing for scaling of forces with size the EasyRig looks well
capable, with those substantial carbon 'planks' and skins in and out,
but I have not done any calculations since I lack data. Hard to
estimate the mast loads anyway - all the sources I have seem to
conflict with one another. <sigh> Aircraft spars are easier....
You are dead right about the cylindrical nose section of a Wharram
wingsail not being the most efficient, a foil-section wingmast
should indeed be better, but as you point out, much harder to align
The little analysis we have done with tuft testing shows that the
leeward leading edge of the mast/sail seems to be free of the
separation that one would expect on a single foil of that shape -
unless we roll up the genoa.
My hypothesis is that the convergent flow to leeward - in the slot
between main and genoa - is somehow responsible for maintaining
attached flow here. A pressure gradient issue, maybe.
Looking at aircraft wings with slotted leading edges I see similarly
bluff noses where their main wings start, so there is some precedent.
I'm still working my way through the archive using dialup for now,
apologies if I cover old ground.
All the best, Ben
--- In harryproa@yahoogrou
> Rig efficiency:
> Due partly to Robert's comments posted in this group's archives, I've
> spent a lot of time looking into Wharram rigs, and as a result have
> become a big fan. This wasn't easy for me, either, because I tend to be
> a technology nut. There are times I love to pull strings and watch the
> rotating mast line up with the fully-battened square-top mylar sails,
> all telltales behaving, and feel a light multihull surge ahead with that
> little bit of extra drive. The performance is only half the fun -- the
> other half is in the gadgets and high-tech materials.
> Thinking about going to a gaff rig, on a round mast, with soft dacron
> sailcloth just seems wrong in some ways. Nonetheless, there's a lot to
> be said about it.
> Your comments about frontal area and cord width are certainly valid.
> It's possible to come up with a more efficient foil shape than that
> presented by a round mast section. There are countless debates about
> this rig on the net, the more detailed of which get into aerodynamics
> that are over my head. But the conclusion is often the same -- a good
> rotating wing mast, rotated properly, will be more efficient than the
> Wharram rig.
> But there are two downsides to that argument.
> One is that it takes a lot of work to rotate the mast properly, and
> this has to be adjusted every time the apparent wind angle changes.
> People with rotating masts rarely have them at the right angle for
> optimum efficiency, and therefore often lose the first foot or two of
> their mainsail due to mast turbulence. This means that most of the
> time, in real conditions, the Wharram rig is more efficient than the
> rotating wing.
> Good racers, with adequate crew, will of course not worry about this.
> They'll have the knowledge and extra hands to keep everything adjusted
> throughout a course. But for everyone else, that soft wing sail has
> real performance benefits. I suspect that the sail area high up,
> enabled by the gaff, also adds to the speed of the rig.
> The second downside is cost and complexity. Sail track, cars,
> hinged/rotating batten pockets, mast rotators, and a rotating mast
> bearing all add a significant amount of cost amount of money, and all
> add additional failure points. The Wharram rig might not be as simple
> as Rob's elementarry rig, or perhaps a junk rig, but it's magnitudes
> simpler than a bermuda rig.
> Wharrams aren't perfect, but they offer a unique ratio of performance
> to both cost and ease of use. The fact that this is done with soft
> sailcloth makes them even more impressive.
> Unfortunately, I have worries about how to implement that gaff on an
> unstayed carbon mast. There would be additional point loading high up,
> an additional halyard, and perhaps an issue with chafe. The mast might
> have to taper less than it does on Rare Bird.
> Resale price:
> I imagine someone else might answer this, but I'll reply just the
> same. The prices on used harryproas are not rock-bottom because they
> are all quite new, and all quite handmade. This is very different from
> the market for ten-year-old fiberglass boats made on production lines.
> The big question is why handmade boats of this size and speed cost so
> Take a look at what it costs to build a 12m catamaran or trimaran, and
> then compare that to Blind Date, and there's a significant difference.
> There's just no production line, yet, for Rob's designs.
> - Mike