Select to expand quoteTardy said..
ha ha ..its good someone is progressing with a shape ,for guys that sail in shallow areas ..well done ..
just a question ...with the sizes ..is there a area that comes with the fins ..as this matters also ..not that i know much about
your spunky fins ..and can you tell us more about the shape ..the change ..is it more for speed and DW speed ,as it seems more
swept back ..which i guess will be easier to push down wind ,and is the hollow to make them lighter or stronger ...sorry for the questions ..just interested ..
Tardy thanks for interest, I will answer these as best I can - probs tomoz. Watching the Olympics and all these fellas that run slower than I can sail
Apologies for the delay in answering Tardy. The short answers to your questions first; longer and in depth boring explanations to follow later on.
- The V4 has a large surface area approx 350cm^2.
- The long curved rake is primarily to control the flow on the leading edge (and to clear weed) not speed.
- The initial prototypes will be solid as they are cast from a jobber pattern. However the weight saving on a fine foil such as this is only minimal (<100grams) and given the increased manufacturing cost and complexity to place a hollow, it may not be worth doing. On the bigger fins the hollow allows for a more uniform material thickness which in turn makes casting cooling issues easier to manage, along with the weight saving. 
No worries in delay ,i drank 2 cartons of beer waiting ..but I'm ok now .
that explains it .
yeah i wondered how you would go with the hollow centre .cost .some surf board fins have honey cone inside to reduces weight and still have strength
maybe that would be more $ effective and easier to build ...dunno ..I'm a buyer not a designer ..but some times I have ideas,
thanks for the come back ..they look fantastic anyways ..November in Shark bay ,these shallow fins are a must ,with the low tides up there ..
i would love to buy one of you bullet proof fins ..after 30 knots all weight disappears..
At the outset, I just want to reiterate that I am no expert authority on hydrodynamics. I have no computational 3D fluid analysis of my own. I am good at reading and researching, but because of a lack of formal training, I am guilty of making extrapolations and assumptions that demonstrate 2+2=5. That said, here are my thoughts on the design process.
In three words: it's Kato's fault. ( as a separate issue, this lengthy ramble is all beer swilling Tardy's fault)
Yes, Kato is freaky good, and yes, LG is freaky good, but to manage 38 knots on the FF28(mod) was still so much faster than I could comprehend was possible. To my mind it was the equivalent of breaking the sound barrier in an A380. So time for a big rethink and revisit my assumptions. A chance comment by Pacey about NACA 16 series foils got me started. The NACA 16 series are not as efficient lift versus drag foils as the ubiquitous Eppler foils, but they have very low drag at low angles attack. They have been used extensively in propellor design and there is a stack of info about pressure distributions etc available.
The leading edge shape: Maritime propellors have very rounded leading rake. This is very effective at countering leading edge cavitation. They are not the most efficient leading edge design for lift versus drag, but they do present the possibility of better handling at high speed at low angles of attack because of their resistance to cavitation bubbles forming on the leading edge.
Angle of attack is an area where my previous assumptions were wrong. I looked at old videos of mine taken from the mast head when on a speed run. I measured angles of 2-3 degrees. An old research paper suggested the same. I checked with Martin Love and he confirmed a much lower AoA was indeed correct. (As point of reference, a high rake delta wing doesn't start forming attached vortex lift until angles approaching 15-20 degrees. At lower angles, it is the foil shape and traditional flows that influence the amount of lift on a delta wing.)
Most traditional triangular 'Delta' fins on the market have low amounts of foil shape. They have relatively large amounts of surface area compared to raked elliptical wing designs. Despite this apparent surface area drag handicap, they have proven to be fast but not particularly forgiving in turbulent flows. The delta shape projects a small frontal area. Given frontal area on a streamlined body increases exponentially as the velocity increases, this adds to the effectiveness of this design.
So, this plus Kato's effort on a 500cm^2 plus fin had me thinking maybe I should consider a slippery foil shape that presents the smallest frontal area and not be so concerned about the surface area. Similarly, perhaps I should not be so dictated by the low pressure lift side of the equation, rather focus on the high pressure displacement lift side of the foil for the lateral resistance I need.
The FF18V4 has its wide point a long way back, so if you angle the fin at 2-3 degrees the low pressure side is almost parallel to the flow for the first half of the foil and the the water only has to recover a relatively small degree of pressure differential to satisfy the Kutta condition at the trailing edge. The down-side is that it produces low levels of lift.
The high pressure side presents a long rounded foil shape that displaces flow forward and laterally, in turn providing lift. Because this is a displacement type lift, the larger the surface area exposed to the flow, the larger the amount of lift force generated. Hence the FF18V4 has a large surface area for a fin of its depth, but projects a small frontal area at low angles of attack, and should be a nice and slippery solution on a downhill run. At higher angles of attack however, it is nowhere near as efficient as the traditional Eppler like solutions.
Thin foils always suffer in unsteady flows and turbulence as the flow readily detaches across the entire low pressure side foil surface. Often this occurs when a leading edge separation is joined by a low pressure separation migrating up the foil surface from downstream. The combination of the fillet, rounded leading edge, rounded rake, and a foil that has reduced low pressure peaks, should combine to keep the flow attached in a broader range of conditions than would be expected from a thin foil delta. I do not expect the fin to be able to provide the same level of tenacity of grip as the standard Fangyfin foil section.
With that in mind, I expect the fin to be found wanting in normal reaching and uphill conditions. I expect it to hang on and not spin out unless adversely loaded, but its pointing ability and speed will be less than a standard Fangyfin. The pay-off is hopefully its performance on the downhill, where it should be fast, stable and capable of dealing with chop at the end of a run with relative assuredness.
Finally, the structural stresses in the fin are borne by the the 'walls'. The loading on the centre core of the fin does very little, hence it can be hollow. The only reasons for making it hollow are to making casting easier and reduce weight. However, in thin foils, a hollow makes the casting process harder and there is a greater failure rate. This in turn pushes up production costs. By virtue of its size, a hollow in a thin foil does not offer much weight reduction. Thus whilst a solid FF28 is unusably heavy versus its hollow counterpart, a solid versus hollow FF18 is 100 grams at worst and probably a lot less.
So that is my ramble through the mess of my brain when it comes to fin designs for Budgewoi. My next step is to test the theory and see what I have got right and wrong this time. I hope to get a couple of castings done and I will send one over to Budgewoi for some crash testing as soon as I can.
Fang that went so far over my head I thought it was a meteorite, I don't have one of your fins but mate I'm more than impressed with what you do, and that's no joke mate 
I was like you Bob ..I had to read it a couple of time to get my head around it ...I fully understand it now ,
looking forward to hearing how the new SB fangman goes .
A picture might be worth a thousand words and make up for my complicated explanations. In the simplified pic of a FF18V4 in cross-section below, assume the fin is operating at close to its ideal angle of attack. The blue lines are the Low (L) pressure side of the fin. The foil shape does not deviate the flow much, and the lowest pressure occurs well back on the fin. Compared to most foil shapes it is not particularly effective at providing lift, but it does create very little drag at low angles of attack.
The red is the High (H) pressure side of the fin. The very long leading edge roof part of the foil provides a large surface area for interaction against the water and acts to push the fin laterally.( in our case, to windward, against your back foot pressure). The lack of disruption of the flow by pressure differentials leads to less chance of turbulence fluctuation across the surface and thus is more stable and less drag at low angles of attack. ( at high angles of attack the opposite is true, and I would expect the fin to be a very draggy and temperamental sod).
This high-pressure side lift dominance is almost the opposite scenario that most fins provide. A 'normal' fin creates the majority of its lift on the Low-pressure side. In this case, the maximum foil thickness is bigger and further forward. This creates a far more effective and efficient low-pressure region at the expense of less area for the high-pressure side to work on. As the angle of attack increases, the advantage of a low-pressure lift style fin becomes greater. The downside is the amount of frontal area exposed to the oncoming flow at low angles of attack is greater, which in turn increases the drag.
The FF18V4 is only expected to be better at low angles of attack where large amounts of lift (and resultant drag) is not wanted, for example on a speed run. On other points of sail, I expect the fin to be sub-par and the board will need to rolled over onto its leeward rail to provide more lateral resistance.
Seeing that foilshape reminds me of a fin YoYo made, unfortunately he couldn't get it to work, hope you have better luck.
Select to expand quotedecrepit said..
Seeing that foilshape reminds me of a fin YoYo made, unfortunately he couldn't get it to work, hope you have better luck.
The optimist in me hopes so, but the realist thinks that if it was any good a far cleverer fella than me would have done it 30 years ago.
Select to expand quotefangman said..decrepit said..
Seeing that foilshape reminds me of a fin YoYo made, unfortunately he couldn't get it to work, hope you have better luck.
The optimist in me hopes so, but the realist thinks that if it was any good a far cleverer fella than me would have done it 30 years ago.
Fangman, keep at it as people like you make the world a better place. I for one am happy to buy/test whatever you put out.
a maybe corny but true, quote from Thomas Edison courtesy google:-
Genius is one percent inspiration and ninety-nine percent perspiration.
I have not failed. I've just found 10,000 ways that won't work.
Many of life's failures are people who did not realize how close they were to success when they gave up.
Select to expand quoteFlex2 said..
Many of life's failures are people who did not realize how close they were to success when they gave up.
I can relate to that!
Cheers Flex. Have no fear, I enjoy the process too much. There will be plenty more bits of ally hitting the recycling bin before I am done. Now, what we really need is a weedy season to play in!
Thirty hours of 3D printing at $10 per hour gets you two halves ready to trim back and glue together. The print material is PETG, which is the printable version of the stuff used to make plastic bottles. The base flanges have been added to each section to make the pieces stable whilst printing. Fingers crossed I don't make a mess of it now.
Fangman, Any reason you are using PETG over the cheaper PLA? PLA would probably give a cleaner finish. PETG is stronger but I don't think you need strength. In my understanding you use these to make a sand mould. At that price though, two or three fins and you'd have your own printer. If it helps I'm happy to donate my printer to the cause (print volume 250mm x 210mm x 210mm)...good quality PETG delivered in 2 days (Perth supplier) costs $34/kg. You can get 30% cheaper if you wait. PLA is around $28/kg delivered quickly.
Select to expand quoteFlex2 said..
Fangman, Any reason you are using PETG over the cheaper PLA? PLA would probably give a cleaner finish. PETG is stronger but I don't think you need strength. In my understanding you use these to make a sand mould. At that price though, two or three fins and you'd have your own printer. If it helps I'm happy to donate my printer to the cause (print volume 250mm x 210mm x 210mm)...good quality PETG delivered in 2 days (Perth supplier) costs $34/kg. You can get 30% cheaper if you wait. PLA is around $28/kg delivered quickly.
That is very generous Flex - I will defo take you up on that! I can trade you bits of plastic for chunks of ally.
(The current fin is under 200 grams so the saving to me is enormous!)
The reason I used PETG is pretty much that is what the printer suggested. I had told him it needed to be tough enough to put up with the heat and hard life in the foundry, and be stable/accurate when printed with no sagging of the foil or lifting at the ends. Although initially, it is just used to make a sand mould, if the design is successful and there is demand, a proper casting pattern is made from the print too. For that reason I like the print to be tickety-boo just in case it is needed for the real deal.
In the meantime, if I make a mess of this print, I will be in touch really soon.
there is only 15degC melting point temp difference between PETG (230degC) and PLA (215). If you want temp then better to go ASA (265) or Polycarb (275) which are pretty much the limit of hobby printers. PETG is fully recyclable but PLA is corn based and supposedly fully biodegradable (but only at temps above 65degC). PETG is a lot more 'stringy' but strong and flexible but PLA is much more brittle but super easy. Either way, each fin print should be around $6-8 of plastic which is cheap enough to do piles of them before you commit to Al. Anyway, just sing out if you need print time. You have my number and I'm down your way lots. (still think you should get a printer for xmas/birthday/whatever)
Looks like quite a good quality print, well done to the guy tuning the machine. PETG vs PLA -- I think whichever the machine is tuned for, is the one to use. These patterns get sanded and covered with bog anyway, so I don't think it really matters which you choose.
However it's a travesty to print it in white. Clearly the guy needs to watch more Lego movies.
Looking forward to seeing the results Fangy
Select to expand quotenebbian said..
However it's a travesty to print it in white. Clearly the guy needs to watch more Lego movies.
...and if it can't be black, it should be very, very, very, dark grey.
The travesty continues; I didn't have any spray bog in very, very, very dark grey... The finished print is now off to the foundry next week for casting.
Waricle bravely put his hand up for some crash testing of the FF18V4 and having lived to tell the tale I thought I had better get some posts and pics sorted. First up some pics of the V2 and V4 together. The added surface area of the V4 is shown.
I am concerned about the sharpness of the leading edge on the V4. To my eye they look too sharp, but the NACA 16 series, on which it is based, does have a fine entry in comparison to other FF shapes. In theory all the other incorporated design anti-cavitation measures should allow for a finer edge, but as usual, time on water will be the true test.
Anyhoo, this one is on its way east for some comparative testing between V2 -V3 - V4 to see what conclusions can be drawn about the different designs.
Fangy, thanks for sending these test fins over to the east coast for testing at Budgie. I have attempted to follow your description of the design process. But I have to admit that some of it I don't quite follow. Not to worry I will do my bit and will try and best describe the performance of each one.
The state government keeps changing their mind on when we will be able to get to Budgie. The latest prediction is 1-Nov as Sue says.
Until then I look forward to lining them up on my desk and admiring them as works of art
