Rudder theory

shaggybaxter said..
G'day Archenar,
I don't have skills in making rudders sorry, but I would like to have a go at it one day, it's interesting. Musing out loud about some general thoughts..
I note Lydia beat me to it, but I was wondering what was the issue with the original design, was it not enough bite?
If so, I would be leery tapering the rudder tip too much, as when you're heeled you want surface area to still bite on the water, so makiing a nice triangle shape (exaggerated) kinda defeats the purpose, you would lessen the effectiveness from both the heeling and the top of the rudder being further out of the water.
Off cambering the trailing edge to make it asymmetrical definitely helps cut down on cavitation and drag, but this is more a function at higher speeds, I am unsure if the benefit is realised at cruising speeds,
Looking at my rudders, I notice two things, (aside from they are short :) the leading edge has a slight taper from shaft to tip and is quite blunt (the chord is fat too). The trailing edge by comparison is vertical and a lot thinner. The helm is weightless across the wind range, in actual fact the quicker we go the lighter the helm. At high speeds it's a go-kart, twitch the helm and the response is immediate, The stubby draft becomes evident with too much sail up, but that isn't the fault of the rudders per se, that's me being lazy and not downsizing or trimming to suit. No rudder should make up for a lazy or over powered trim in my simple mind.



It was interesting to read up on the loss of Shockwave recently, where a well engineered new keel let go. I know this was a keel and not a rudder, but it has some interesting parallels.
They had gone for a deeper draft, and to offset this they had removed weight from both the keel and the fin. These mods actually lessened the RM (righting moment), so theoretically this should be less load, it all sort of makes sense on paper. Then the keel fell off. No concrete evidence as the boat is still floating around out there, but the preliminary thoughts appeared to believe the larger surface area from the deeper draft negated the lighter weight and still overloaded the keel joint, resulting in the keel departing from the boat. One race a few years back saw three new rudders fail (and not from collisions), so not going too extreme seems a sensible and logical approach.

I was recently getting educated on tubercles, where they serrate and add protrusions to the leading edge of the rudders. This improves the efficiency of the rudder stall angle, allowing grip for longer period. The idea is copied from humpback whales, which are known for their maneuverability (apparently!)
Us mere mortals don't need to worry about tubercles I think, but the science is interesting !



It does sound like fun, could you keep us posted on what creation you come up with?
Cheers,
SB