Subject: Re: [harryproa] Aerodynamics and sailing performance
From: Doug Haines
Date: 2/24/2007, 7:03 PM


How does the sail fold up at the sleeve when you reef or take the sail down?


brag_rotor <> wrote:


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 this
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 will
reef going downwind (a major safety factor) because of the loose
sleeve luff and the weight of the gaff bringing the top down; plus it
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 shrinks
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 telling
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 to
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 sails,
but it is still a disaster in terms of aircraft quality fluid dynamics.

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 imperfections
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 importance
when the sail is hoisted, so it can be properly plump and stiff. Not
too plump, since we do not want excessive drag when reefed or in high
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 batten.

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, I'm
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

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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