Subject: [harryproa] Re: Aerodynamics and sailing performance
From: "Robert" <>
Date: 2/26/2007, 7:27 PM

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 take
the point loading of the gaff,
--- In, "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 comfortable,
> 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 we
> 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-around
> sleeve luff and a short gaff. Usually loose-footed due to the huge
> 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; plus
> 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-cars.
> Aerodynamics - this has been an interest of mine since boyhood, and
> 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 uploaded
> the photos folder 'Aerodynamics and Sails' .....
> This illustration is meant to focus on the importance of very small
> things in the overall drag picture - a sailing boat, for example.
> Depending on the Reynolds Number....
> (
> (
> ....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 the
> worst case. It can, too. Some shapes are worse than a round wire.
> 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 stiff.
> 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 pole
> increases rapidly with diameter. Flexibility to shed wind load in a
> 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 Rob,
> a pestilentially picky perfectionist when it comes to sail shape.
> Wrinkles may be common - but I prefer them on other people's sails,
> 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:-
> 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.

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