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After looking at the very vertical rudder and keel designs with flat bottoms on the Mark V boats, the idea occurred to me that there is an analogy with the wingtips of airplanes. One source of drag (in either air or water) of this foil shape is vortices that spiral off the tip trailing edges. Most airplane wings designed for efficiency have little winglets at the tip to reduce the vortices and thus the drag, and older planes are being retrofitted with these. And we know that many modern boat designs use wing keels. So why not attach small "winglets" to the bottom of the rudder and perhaps keel on our boats? This probably wouldn't be of too much benefit to the older Mk 1 and 2 designs due to their swept back shape. Small winglets could be cut and folded from sheet stainless steel, and in the case of the rudder, easily bolted to the flat bottom surface. I am picturing something with swept back leading edges and negative dihedral. This would need to be designed to shed weeds since dragging a streamer of weeds would not increase boat performance. But this alteration could potentially increase the lift and control of the rudder and reduce drag. Does anyone out there have the hydrodynamic/aerodynamic education and skills to properly design such an appendage? What about the question for us racers what would happen to our PHRF rating with such a custom modification?
Bob, Heatwave Mk V
Passenger aircraft manufacturers offer winglets for their aircraft, particularly the long-range models (Bombardier's Regional Jet has them too, but as it's reputed to be a thirsty little pig, that may be an act of desperation on the part of the designers). According to Salon.com's Ask The Pilot columnist, however, there is no consensus among major airlines as to whether the fuel savings are worth the installed price – some fit them, some don't. That may change with escalating fuel costs, but the present situation suggests that the benefits, in aircraft anyway, aren't in the same fuel-saving league as, for instance, serving scotch from foul-tasting plastic bottles or making you breathe stale air for 8 hours.
There is a long discussion of foil vortices and end plates in Steve Killing & Doug Hunter's Yacht Design Explained, a book that strikes a nice balance between depth and comprehensibility. I won't attempt to precis any part of long discussion of keel design other than noting that the greatest relative benefit is obtained by a foil with a long chord and low aspect ratio – which is not a Mk V.
During the last America's Cup, there was an extended discussion in Sailing World of the art of placing winglets on keel bulbs that suggested that this is not quite as straightforward as might be imagined – a little out of place, and they're almost pure drag. Moreover, in waves when the boat is pitching, your end plates are going to be operating at small but significant angles of attack; since they're flat plates, they'll stall easily and that will generate masses of nasty turbulence.
Finally, if I were a measurer faced with a wingletted boat, I think I'd "improve" your number by about 10%, just to be on the safe side. That would probably clip most people's wings.
David Weatherston
Towser, Toronto
C&C 27 Mk IV
David Weatherston
Towser, Toronto
C&C 27 Mk IV
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David,
Thanks. I have ordered the Killing and Hunter book. Sounds interesting.
Living in an environment of shallow water - with thankfully soft bottoms - I can assure of one major disadvantage of wing keels. Riding with a friend of mine who grounded his wing keel boat on a mud bottom was a real experience. Normally not a big issue, everybody here learns kedging techniques, etc. The additional suction of the keel in the mud acts like an anchor. Fortunately, it was low tide. After two hours it finally popped out, the whole boat jumped about 6 inches
Warren Smith
Serendipity
Galveston Bay, Texas
I am most of the way through reading the book, "Yacht Design Explained; A Sailor's Guide to the Principles and Practice of Design", by Steve Killing and Douglas Hunter, W.W. Norton, ISBN 0-393-04646-X. This book is very interesting and highly recommended. After reading this, I am not so keen on adding "wings" to my keel or rudder bottom. It sounds like without proper design and tank testing there is no way to be sure they would help. But I have a new idea - small anti-ventillation plates on the rudder a few inches below the waterline. The Mark V has a transom hung rudder and this can lose effectiveness due to air being sucked down the low pressure side at high rudder angles. Does anyone have a comment about this?
David,
Thank you for the reference; I will see if I can get a copy.
Does the book go into details of rudder design?
I am starting to make a new rudder for my Superseal and I am convinced that the balance area should be kept out of the prop wash.
In a serious naval architecture book, (Basic Ship Design) square ends for ships rudders is recommended, but why is not explained.
I have only experienced ventillation on dinghy rudders, particularly on an Enterprise, and only then when bearing away in heavy weather. Getting the "boat under the mast" solves this.
(For C&C 27 owners info, the Superseal 26 is a Ron Holland design and is rather similar to a MKV, except that 90% of production had lifting daggerboards and internal ballast)
Frank Marsden
Frank:
I think this book will answer many of your questions on foil design. I cannot recall anything on ship rudders, but the recommendation in "Basic Ship Design" may be a function of practicality more than anything else (though I seem to recall that US Navy cruisers of the 40's had a rudder with a low-aspect-ratio ellipse).
CS Yachts in Canada used to build a 27 drawn by an English designer, Raymond Wall. The CS27 had an outboard rudder that would blow out when running dead downhill at hull speed. It was the weirdest thing - I've had this happen on boats before when heeled hard over while beating or under chute, when the rudder ventilates wildly and the rudder simply becomes useless, but this was with the boat upright. We'd be sailing fast and suddenly the rudder would be gone. We thought it was the owner, but we put a really experienced helm on the tiller and it was the same. He could feel the change before it took full effect, and would push the tiller back and forth with his fingertips while cackling wildly. Then we'd wipe out.
The last time this happened, the owner accused me of causing the broach. He said I'd set the spinnaker inside out. There's no dealing with a remark that foolish (and he an Air Canada senior captain - seriously!), so I apologized and none of us never sailed with him again.
David Weatherston
Towser, Toronto
C&C 27 Mk IV
David Weatherston
Towser, Toronto
C&C 27 Mk IV
Offline
The Steve Killing book has a whole chapter (about 18 pages) on rudder design. This is generally interesting but may not be specific enough to help you actually design a rudder. Overall, the book is well worth having. Amazon.com has it.
David,
This is where I am at right now. The existing rudder is seriously cracked, and as it is GRP (chopped strand mat only) and with a heavy filler, I have decided it is scrap. It weighs 92lbs.
I have bought some vertical grain Douglas Fir, now in strips 2 5/8" x 2", four off.
These will be profiled to the centre part of the section NACA 0012, epoxy bonded and sheathed epoxy/fine glass cloth.
I then intend to bond on leading and trailing edges to a design similar to the Hustler 25.5, which has impeccable handling.
Reason for doing it this way is so that I can easily alter the rudder shape if it does not feel right.
The issue I need to resolve is the amount and position of the balance area.
Frank Marsden
Mano Amica, Superseal 26
Weymouth, Dorset, England
.
Frank:
The fellow you need to talk to is Ralph Ainslie from Dartmouth (NS, not UK), who contributed information on the rebuild project he did after Hyperion lost her rudder in a hurricane. I think you and Ralph actually corresponded at one point on converting a Mark I to a Mark III-style rudder. Ralph has a friend who is a real marine engineer, as opposed to an opinionated yacht-club-bar knowitall like the rest of us. This fellow specifically helped Ralph on the balance issue. Hyperion is listed (with Ralph's e-mail) in The Fleet.
David Weatherston
Towser, Toronto
C&C 27 Mk IV
David Weatherston
Towser, Toronto
C&C 27 Mk IV
Offline
David,
Thank you, action taken.
I have said that the Superseal is like a MKV. I just noticed a significant difference, Superseal has fractional rig (7/8th)
Frank
There is a short but fascinating article in the August 2004 issue of Scientific American magazine (page 18-20), about a study done of humpback whale fins. These fins have a series of bumps along the leading edge. Studies of model fins in a wind tunnel, comparing otherwise identical foils except that one had a smooth leading edge and the other had a model of the humpback fin's bumps, show a decrease in drag of 32%, an increase in lift of 8% and a resistance to stalling to a 40% larger angle of attack. The improvement in performance is attributed to "pairs of counterrotating swirls", which serve to keep the flow attached to the upper surface of the fins. The author speculates that "the next America's Cup victor might tack more sharply using a bumpy rudder." Well worth reading!
This has never happened to us. We have hull #25 bought new in 1975. We have raced Annapolis to Bermuda 6 times and done the Newport to Bermuda Single and double handed race back. We did add the modification supplied by CS to add several inches to the leading edge of the rudder. We did a heavy air run up the Chesapeake Bay and covered 40 NM in 4 hours. I guess that it was a knot of current with us in this spinnaker sleigh ride. Had to replace the guy during this run.
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