On Thursday, November 12, 2020 at 8:32:33 PM UTC, Walter Martindale wrote:
On Monday, 2 November 2020 at 05:37:52 UTC-7, carl wrote:
I take your point on weeds, but don't understand why fins should be getting deeper - which is what James says is happening.
A more sweptback shape, without the sharpening (it's pointless - it doesn't reduce drag but does impair performance) of the leading edge, would be better at shedding weeds, not to mention the polythene bags & other detritus
I recall one day on the Avon in Christchurch, NZ, when one 8+, rowed by the OURC men, shredded three coxswains' hands right to the tendons when they reached under the boat to clear weeds off the sharpened fin. Not my program but I went over with a file
and took the edge off the fin. Oddly nobody else cut their hands. The crew still won their race if I remember correctly (it was pre-2000). One of the stupidest things I've ever seen is people sharpening a fin on a boat.
Sharpening the leading edge of a fin is hydrodynamically daft. If you were going to do anything, a gentle, curved radius would be best, but unnecessary as it's pretty close to that anyway. There is some small (imeasurebly, pathetically small) benefit to
sharpening the trailing edge of underwater appendages but not worth even getting your file out when they are only the size of the fin on a rowing shell.
No serious water vehicle should ever have a flat plate fin, and definitely not a flat plate rudder. Though again, when a rudder /fin is as small as they are on rowing shells, the drag improvements in going to a slightly smaller symmetric foil fin are
very small. If you replace a flat plate with a proper foil, it will produce more lift (in the case of a fin lateral resistance) for its surface area so can be made a bit smaller, so a bit less drag in a straight line. When steering though the foil is
vastly superior to a flat plate- it can maintain laminar flow across its surface at much higher angles, this means a win/ win of both better steering and less drag!
Note that foils for rudders and foils for fins should have a different profile. Foils for fins are (or should be) thicker (than, for example in the case of a racing dinghy the foil for the centreboard). That's because a thicker foil may have a tiny bit
more drag in a straight line than a thinner foil of the same depth and chord, but a thicker foil keeps attatchment and laminar flow across it at a higher angle of attack (so will stall less readily)- so is much better as a rudder. It's leading edge would
not be remotely sharp, for most of the same reasons the leading edge of the tail of a Boeing 747 is not sharpened by guys with files on stepladders after every flight!
When a rudder is placed just behind a fin, as Carl says, the two are to an extent fighting each other- and because they have (or should have) different profiles to do different jobs, working out what is going on gets very messy, although computational
fluid dymamics would probably be able to do it for you. However, why bother? No racing yacht has had a skeg hung rudder in about the last half century for this reason. Surely the solution is a balanced spade rudder of the correct size that gives both the
lateral resistance you require in a straight line and the steering power you need when altering course? It has the profile it needs to work as a rudder, and when you turn it you turn the whole thing. Your rudder in a straight ahead position is your fin.
I like dipping rudders. One profiled and angled for turns to port, the other starboard, You want to make a little turn to port, you dip a little bit of port rudder tip, big turn to stbd, you immerse your whole stbd rudder etc. (They both slip out of the
water totally when your steering is straight ahead- so zero drag). Make them bigger than you need, so if you ever need to do a BIG sudden turn- for tactical or safety reasons its taken care of, but you're not paying any drag penalty for that superpower
when you're not using it. Easy to set up with rudder lines working against bungees, so for the cox it's the same, pull one line to go one way, the other to go the other! But I digress. I expect FISA have them well and truly banned. Or probably would if
someone turned up with one. I always wonder about them when I watch the Boat Race or Henley though, as not sure how closely these events' elderly stewards run them to FISA's dreaded 'Definitions of rowing'.
Back to your NZ crew. I bet they also sanded the hull down with 1000 grade sandpaper, or didn't sand it down, or sanded the aft half of it, or polished it up to a mirror finish, or coated it with some hydrophillic, or hydrophobic etc. etc. snake oil
product displaying a distinct lack of knowledge re: boundary layer theory.
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