Wassa - ava gander [Archive] - The WoodenBoat Forum

Wild Wassa

03-27-2008, 03:59 PM

Stazza, this boat is totally unexpected. I had no idea that the design existed. The Flying 11 bears no resemblance to the Flying 15 so one wouldn't expect a Flying 10 to either. A pleasant surprise. I can see that the boat has the same flat bottom shaped keel of the Classic Fifteen and you need to look twice to see that it isn't.

The author's comment about the cost for a Flying 10 being prohibitive certainly holds true. A new FF is about $36,000AU - $42,000, depending on the type of materials used, so I can imagine that the little boat would be in excess of 20 thou to purchase new.

We are making over the F15 'Rag Dolly' for the Nationals and World Titles. The boat is getting the full treatment. Rag Dolly raced in the 1991 World Titles and was Ian Anderson's boat that he built for the Titles but he didn't get to race her for some unknown reason.

My Skipper Leigh Nielsen has had an assessment of Rag Dolly done by the hydrodynamic engineer John Kennedy.

Here are JK's much editored comments about the mould, foils and characteristics of the boat in responce to some of the questions Leigh asked about RD. You might find parts of JK's reply interesting as many questions about rudders have appeared on the Forum of late.

The mould came from Roy Windybank (in the UK) as a mould IV to Anderson in Western Australia and was in production by Anderson for a time – Rag Dolly was either the last or last but one out of the mould before Bill Shand acquired it. Rag Dolly was built by the WA builder as his own boat for a Championship, World I think, to be held in Brisbane but in fact he did not sail it, it was sailed by a Queenslander. Rag Dolly is a divinycell cored boat.

The mould once acquired by Bill Shand was re-faired by him and became the Shandy Bank which he used for a number of years before constructing a new mould to the latest rules.

One thing to note is that at that time Australia was allocated a series of numbers and the numbers at that time, consequently, were out of date sequence with those used in the UK, which can also help to confuse the mould issue.

Rag Dolly is a Windybank mould IV boat. The only subsequent changes to Rag Dolly were a new keel and new rudder ... I was not over impressed by the rudder as I think it followed high performance boat design principles and is most effective at higher speeds than an ff can normally attain. Mind you I think they missed an opportunity with the keel by doing the same. Max thickness is too far aft 40% approx rather than 30% and is too thin, reduces two things lift and drag, while reducing drag is good, reducing lift particularly on a rudder is not, unless you are much more expert than most of us are. As with all things in hydrodynamics there is a constant trade off between lift and drag. The other and a major problem with keels and rudders is that the tolerances to get a good hydrodynamic surface are far to tight to be able to be reproduced without a high tolerance CAD/CAM pattern milling machine to produce a blank for the mould and at the same time having good knowledge about the shrinkage properties of the moulded product. The most critical element is the leading edge and then back to maximum thickness.

From all I have read and researched the best compromise profile for most yachts I think is the NACA 0012 profile and certainly no less than the NACA 008 profile. My own view is somewhere between 0010 and 0012 with tiller steering depending on your expertise, coming down to 008 makes steering much more critical and I think would be too thin for most of us to be able to effectively get sufficient additional benefit from, wheel steering might make it more feasible, any less than 008 and the lift is too limited and the rudder stalls out too early due to the boundary layer prematurely breaking away from the suction surface as the rudder is moved to turn the boat. These profiles were developed in about 1938. The NACA 00 profiles have a maximum thickness at 30% back from the leading edge a maximum thickness in percentage terms of the chord of what the number is and a parabolic leading edge tapering off to a point at 100%, in fact most are cut back to a square trailing edge of about 2 mm thickness which for a yacht does in fact help lift when turning by preventing the flow from reversing from the pressure side to the suction side.

There was a very good design produced for 5o5’s in the 70’s/80’s and I produced computer drawings of the NACA and 5o5 profiles and overlaid them, the changes were quite subtle and a subsequent article I was reading suggested it was a no go due to the very critical nature of the 5o5 profile, even a slight departure was enough to make it perform worse, departures of no more than a couple of thousands of an inch in thickness were critical. [My early engineering work was in hydrodynamic research]. John Kennedy.

Rag Dolly doesn't get the same degree of lift as the newer Flying 15s, what JK wrote certainly holds true. When we race her, footing, slightly off the wind, because we are a slightly heavy crew and always footing in light wind, playing max boat speed rather than trying to point her too close, she is an excellent old boat. Rag Dolly now has a new rudder over the rudder John Kennedy assessed. Each week she is improving. In the last month we have lifted her game by several hundred metres in Club racing. Older boats can get upto 13+ knots free running where as new boats (the latest Modern Class FFs) to 14.9 knots. The last couple of races we have been hanging in there with the newer boats.

Warren.