|Subject: Re: [harryproa] Re: Aerodynamics and sailing performance|
|From: "Rudolf vd Brug" <email@example.com>|
|Date: 3/2/2007, 3:35 PM|
----- Original Message -----From: Mike CrawfordSent: Friday, March 02, 2007 5:22 PMSubject: [harryproa] Re: Aerodynamics and sailing performance
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.
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 little.
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.
I like the idea of an unstayed mast especially on a boat that
doesn't need to have gobs of sail area to attain good overall speed.
Don't really agree with you on the wire vs foil thingy, may be just
my misunderstanding. I don't have a formal education in
aerodynamics. But doesn't frontal area and cord width play a part in
your apple vs an orange theory ;) I wonder what the out come would
be with the same size sail for given length masts attach to each
If the whole principal behind your designs are minimal material and
hardware then why such the high price tag $$$$$ on used boats ?
--- In harryproa@yahoogrou
ps.com.au, "Robert" <cateran1949@ ...>
> The Wharram rig makes sense to me, possibly with a curved gaff to
> sooth the exit. Not sure of how to set up the unstayed mast to
> the point loading of the gaff,
> --- In harryproa@yahoogrou
ps.com.au, "brag_rotor" <brag_rotor@ >
> > Greetings,
> > This is a hopefully useful contribution to the debate on
> > sail formats for the HarryProa.
> > We have sailed a Wharram Tiki 30 since the '90s, and 'Pilgrim'
> > has provided us with a source of much glee in nailing much
> > larger and supposedly faster vessels on most points of sail.
> > The numbers we can demonstrably repeat (especially since our
> > recent coat of bottom paint) caused some surprise when I posted
> > them last time - apologies for my lack of tact. All boats are
> > an improvement over no boat, and it is not my place to denigrate
> > anybody's design, multihull or monohull.
> > Pilgrim is a small catamaran, and cannot comfortably sustain high
> > speed in rough water - ask my wife! Well, Olly isn't
> > anyway. We will need something more comfy and spacious for our
> > declining years, but I am most reluctant to give up on the
> > giant-killing fun we enjoy. Eaten any Oysters lately? #;^p
> > That's why we're here.
> > We did check our numbers and have also discovered (see PS) that
> > can point and foot well under main alone. So my focus is now on
> > type of mainsail, and its possible application to an EasyRig.
> > The Wharram Tiki Wingsail is a cunning combination of a wrap-
> > sleeve luff and a short gaff. Usually loose-footed due to the
> > sheeting angle available on a cat, the rig is simple to use. It
> > reef going downwind (a major safety factor) because of the loose
> > sleeve luff and the weight of the gaff bringing the top down;
> > is safe to gybe all-standing due to the lack of a boom.
> > The gaff keeps the sail area useful to the top of the rig, since
> > bermudan triangles lose performance rapidly as the sail chord
> > with respect to the mast. A square-top variant is easy to make;
> > the gaff then becomes (in effect) a batten, but cheaper than most
> > modern battens-with-
> > Aerodynamics - this has been an interest of mine since boyhood,
> > as a student I recall our aerodynamics instructor at the CAAE
> > us that even a small wrinkle or rivet can perturb the flow.
> > His favourite example was a wire and a foil, which I have
> > the photos folder 'Aerodynamics and Sails' .....
> > http://au.ph.
groups.yahoo. com/group/ harryproa/ photos/browse/ 1641
> > This illustration is meant to focus on the importance of very
> > things in the overall drag picture - a sailing boat, for
> > Depending on the Reynolds Number....
> > (http://en.wikipedia
.org/wiki/) Reynolds_ Number
n.edu/~asmits/) Bicycle_web/ blunt.html
> > ....the foil could be of the order of 10x the thickness of the
> > wire for the same drag. This can double at very low Reynolds Nos
> > (Re) to around 20x - and sails run at low Re.
> > Which suggests that a 5% imperfection might _double_ the drag in
> > worst case. It can, too. Some shapes are worse than a round
> > This puts exposed masts in a poor light unless they rotate
> > very precisely. Shrouds don't look good, either. They are in
> > the low Re zone for sure.
> > Sailmakers do have techniques for picking up some cleaner flow to
> > leeward of masts by putting a baggy step in the leading edge of
> > but it is still a disaster in terms of aircraft quality fluid
> > There is a _lot_ of improvement to be had over a bermudan rig.
> > The Wharram approach uses a deep sleeve luff, so that the mast
> > 'disappears' aerodynamically inside the sail, like the spar on an
> > aircraft wing. The cut is all-important, since smaller
> > start to matter more once the main sources of drag are addressed.
> > Chris Jeckells made my sails, bless him.
> > The thickness of the mast ceases to be of great aerodynamic
> > when the sail is hoisted, so it can be properly plump and
> > too plump, since we do not want excessive drag when reefed or in
> > wind with the rig down. Fortunately the stiffness of a beam or
> > increases rapidly with diameter. Flexibility to shed wind load
> > gust could be added in the gaff - like a windsurfer's flexing
> > Two Tiki Wingsails are shown in the photo folder, one seen from
> > another boat, and one shot from on board. These are not Pilgrim,
> > and the sails do not seem to be setting as well as ours. A lot
> > of people are relaxed about sails, and as I have admitted to
> > a pestilentially picky perfectionist when it comes to sail shape.
> > Wrinkles may be common - but I prefer them on other people's
> > not mine! Remember the wire and the foil. A 5% wrinkle
> > might double the drag - so how about lots of small wrinkles?
> > The two photos are there to show the Wharram wingsail - more at:-
> > http://www.wharram.
eu/photos/ index.cgi? mode=album& album=Tiki-
> > So what do you folks think, is there a case for using a Wharram
> > sleeve luff/gaff combination on a HarryProa EasyRig?
> > All the best, Ben
> > PS
> > About that run up the coast of Lanzarote in January under main
> > alone - lazy skipper, should have reefed both main and genoa.
> > But we learned something interesting.
> > The tack was 88 degrees on the GPS (and compass - near as
> > one can tell on a compass) and we started footing upwind at
> > nearly 9 GPS knots. Tricky to get a main flying spot on without
> > any genoa telltales, but eventually we had 9 knots -roughly-
> > showing on the GPS. Pretty good, I thought vaguely. It was
> > a pleasant surprise to point so well under main alone, which
> > is not usually our custom.
> > Sea was bumpy, hence the reduced rig.
> > Afterwards I had a look at the GPS points I'd put in and
> > from the times did a speed over ground by hand, which was
> > 8.8 knots, and 8.8/16 is 55% of max wind speed measured
> > 1/3 the way up the mast. 8.8/14 is 63%, so we were going
> > pretty well even without a genoa.