FF18 Budgewoi Special V4. (...and an exclusive ' Limited Edition Low Hyperbole' SB post from me)
The latest design to hit Fred's foundry soon is the result of feedback from the dedicated crash test dummies on our eastern shores. The fin is a complete re-design of the existing FF18V2. Time will tell whether I have got it wrong and it is an anchor, or if it lives up to the design aims; to produce the lowest drag concomitant with benign handling. There is enough displacement ( high pressure side) lift to allow dogging up to the top mark and thereafter it is a slippery AF shape that can handle less than ideal conditions. Accordingly, the maximum performance efficiency occurs at very low angles of attack.
The fin has a number of anti cavitation strategies borrowed from maritime propellor research and design. The design intent is that the large radius and elliptical planform leading edge will delay leading edge cavitation, while a very long roof area and a maximum low pressure point well behind the mid chord line will further delay sheet cavitation. The foil profile design is less susceptible to surface area roughness effects than the previous FangyFins. This is to give greater flexibility of finish to those who wish to do the finishing themselves and provide a fin that only needs to be treated with a modicum of seasonal care and maintenance whilst still maintaining performance.
The location of the maximum thickness aft, decreases the velocity gradient (and hence the pressure gradient) in the mid-chord region. The resultant favourable pressure gradient in the mid-chord region promotes boundary-layer stability and increases the possibility that the boundary layer remains laminar. Laminar boundary layers produce less skin-friction drag than turbulent boundary layers but are also more likely to separate under the influence of an adverse pressure gradient. However, it is accepted that with Reynolds numbers to the order of 10^6 at the intended operating speeds, the flow is likely to transition from laminar flow irrespective of the design features.
Despite maximum chord thickness located well aft, viscosity effects may trigger cavitation bubbles in the flow separation zone behind the maximum chord thickness. The progression of these bubbles toward the leading edge and resultant sheet cavitation is hopefully retarded by the above strategies.
There is a small section of tip roughness to interfere with tip rollup and vortex formation. (Not shown on drawings) This further reduces the efficiency of the fin, but is an effective measure in countering the downstream tip vortex bubble migrating on to the foil and initiating sheet cavitation.
On a stream-lined body, with increasing velocity, pressure (form) drag increases with the square of the chord thickness, whereas friction drag (surface area) decreases as a proportion of total drag. Accordingly, the chord thickness is reduced to minimise form drag, but the surface area has been increased to provide adequate lift at low speeds.
The fillet has a varying radius that has been largely copied from America's Cup strut/foil designs and has proven effective in previous FF prototypes.
The base section is minimised as per the FF18V2 to take advantage of the fillet loading and drill guides for bolt placement incorporated in the casting. The base has been moved rearward a little more to offset the aft movement of the peak low pressure point with the foil change. The model number is now imprinted to make identification easier.
Well done all you dedicated scrollers! You got to the bottom of the page and discovered there really was a paucity of adjectives to be found. In any case, once I have a 3DPrint and casting it will be hitting the water. Stay tuned for whether I have created a lemon or something a little more tasty.
I think those mods might have been effective.
. I suspect that someone, probably in the eighties, has done this sort thing before and I just wasn't paying enough attention back then...
