The reason I refered you back to the other thread is that in that thread I explained that I believe fillets add drag. The theory has to do with the end plate affect and keeping as much of the flow vectors flowing parallel. As you add fillets you increase the area where vectors are compressing and causing more turbulence. If you look at the history of marine architecture you’ll note that prior to the early 80’s keel hull joints were always designed with large radii fillets. As more and more flow analysis was done for the America’s Cup and other high budget development programs, it became apparent that clean 90 degree keel hull joints were the way to go. I ran a boatyard in Newport Beach from 1990-97 and had the opportunity to work with some renouned marine architecs Bill Lee, Allen Andrews, Bruce Nelson, Dave Pedrick and Jim Pughe to name a few. When ever we discussed finishing the keel hull joint the answer always came back no fillet!
Thanks tom - sorry for not re-reading before I spoke up… You know times the thread goes on, and on, and on, and… One, such as myself, may not have read the whole thing, or it may have come back with a new life in a different direction. I will take your knowledge and experience as good advice. As, my own empirical experiece mostly suggests some things, such as “fillets,” may not even be noticeable, where as other things, such as moving a fin up 1/4" can hav profound results. Thanks for all the good sharing. Taylor
Earlier in this thread we described how at speed the area up to 1" deep below the board is a turbulent area. There are alot of different things going on there. And at only 4"+ long the root cord length is minimal compared to the 3’ to 5’ plus keel cord root cord lengths I’m comparing. But, if they don’t help on bigger foils they probably don’t work on small ones either. And yes, moving your fins has a profund affect on how your board rides. But, that doesn’t seem to click with the general market.
rovings… from all high performance shortboards here on the coast the preference is to glass ons… the rovings providing the dampener to the turbulance travelling along the bottom of the board. test and see bert, get some silicone windscreen sealant and wipe it down the sides of your box fin with your finger mimiking rovings…
Gotta disagree with you there Dave. IMHO The rush to glass ons is do to how lightly boards are being glassed today. And people’s lack of understanding of how much moving and changing fins expands the range of your boards. The monocoque construction of glass ons allows everything to flex together. But, to support a foil that is not bonded into the shell of the board vis a vis a box, it has to have fillets for structural support. As a matter of fact, the glass on bonzer side runners are routed into the shell of the board to do away with the fillets.
sorry tom im not rushing with the tide on glassons, please go for a surf today and try the silicone rovings! then tell us what you think.
I thought I was onto something with the bulb keel in pics below.Guess I’ll have to finish it
and try it out,if we ever get surf here.Pretty cool that Downing did it 2 years before I was
born.
Just go try it is not a strong theoretical argument on why it would work. As I said before since the fillets are in a turbulent zone it’s going to be hard to percieve any notable difference on a 4"+ root cord length 1/4" radius fillet. I named names of the degreed naval architects that influenced my opinion. Give me something more than just try you’ll like it.
After 30 years of surfing glass ons, seven years of developing fin systems and thirty years of racing sailboats and studying hydrodynamics, I’ve gotta tell you I believe any benifit felt was imagined.
flex…ithink flex is slow it lets the power off…I work with grenough and i have always had stiff fins…now grenough uses stiff fins…he is working with the twist and no sideway flex and no twist with side way flex
cheyne!!in full agreement there…
tom /dave!!!
tom i reckon dave is feeling something…i have a theory…
i had a board a few years back ,i put some ply in the board around the fin area ,routed slots and set my fins in ,i wanted to set it up with no rovings…mimicking box’s (there werent any at the time)
board went fine ,but after about 5 or 6 surfs ,cracks developed at the base so i decided to finish the fins off with rovings and reinforce them…
next time i rode it ,it seemed to have more hold ? but was a little slower as well ???
my conclusion was .the rovings created more drag …
but also gave a little more hold because the mound around each fin was causing a region of low pressure drag ,thus helping it to suck down to the water a little…
regards
BERT
Drag is going to pull the tail down due to it’s rocker. But, I think you cant get more positive hold without purposely inducing excess drag. It just comes off as a negative to me. Usually, when you are attempting to create lift there is a parasitic drag component that is impossible to get around. These fillets are creating drag without any positive lift component.
Depending on how it is mounted, fillets MIGHT stiffen the fin up noticeably. This would be quite easy for the surfer to perceive.
I have significant doubts there is a positive impact independently of this. My recent experiments with flexier fin materials are really clear. Foil stiffness matters a lot.
I’m trying to think of ways to apply this to my finbox - to keep the rotation highly controlled, and the cant as stiff as possible.
Blakestah,
Fillets definitely stiffen fins in glass on applications. That is their structural function. Silicone fillets are going to do close to nothing for the stiffness of a box mounted fin. The primary focus of Red X was to get the most efficient method for transfering torque from the tip of your fins through the tail of your board. No other system fits as securely. So, optimizing stiffness is a high priority. Adding drag and not gaining appreciable stiffness is just steps backward.
agree fully tom…speed is everything ,plus i dont run what youd consider tail rocker,so the drag was rovings and nothing else,and while i used the term hold ,i could also use the term hold back,
so yea ,now i would say just plain old more drag,
im gonna try another photo ,
its one of my team guys hes off to the world longboard titles in france in a few weeks …i hate deadlines …
hope he smokes em tho…
regards
BERT
I want to apologize to everyone and especially to Dave about the negativty that I’ve brought to this topic. I try to keep an open mind to what everyone has to say, usually. In designing plastic injected parts, I work with fillets everyday to resolve structural dilemmas. But, I’ve spent many hours working under boats and at drafting tables making structures without fillets inorder to minimize parasitic drag. I guess after spending so much time and effort working fillets out of hydrodynamic structures, to be told here’s a quick and easy way to make your board ride better by adding fillets. I would hate to think that I wasted all that effort. It’s a small factor on your surfboards. But, it’s a significant factor on 70’ racing sailboats.
Rick Vogle, who used to live on Kauai was involved in swordfish fins for years. I don’t think he is working on them now, but Kokua posted one picture of them in the archives. Does anyone else have his fins?
Cheyne - Thanks for your reply. I have been under the impression, from Dale S., that George G. worked with flex quite a bit, and with the Paddle fin, twist. I understand what you’re saying about flex letting power off, in my experience - say at the apex of a true “bottom turn” - but what is your take on the feeling of the “push back/power return” as you let up out of the turn? My understanding is, that (“push back/power return”) is what Greenough (via. Dale S.) was working towards - thus the twist in the “paddle fin” - also, it has the nice short root cord length. I do know part of his thing was over all flex - in the spoons - a notion I am still wrestling with, with my 3 piece-flex tail (heavy sigh) some times I wanna just get a mat off Dale and give up the rest, but I got a vague theory on evolution, standing up-right, balance, etc… Gotta say, I’ve also appreciated the “star” fin, although I haven’t had the opportunity to ride one - did see a guy ripping Upper Trestles years ago, with a nice smooth style, and when he came in he had the star fin. Ok enough of me.
Peace, joy, love, etc. Taylor E. Olson
pic attached shows my crude FCS cutways…the center fin was also cut out. The base length of the G5s are now 3.5 inches. Base foiling is still pending.
Ocean here is freakin flat…need a trip to Central Am soon…anywhere except CR is fine with me. I could always try a boat tow just for fun…
Gents, this is one of the most interesting threads Ive come across in a while…thanks for the info.
meecrafty,
I'm not sure from this picture how you refoiled the trailing edge. But, if you start to get a hum at speed make sure you taper the trailing edge such that it is no thicker than .060".
meecrafty,
I'm not sure from this picture how you refoiled the trailing edge. But, if you start to get a hum at speed make sure you taper the trailing edge such that it is no thicker than .060".