Moderator: GreenLake
GreenLake wrote:Nice pictures. They give a good view of your design.
I like the use of the bungee cord to provide a measure of "give" in the system in the event of a grounding. (And it pulls the rudder back into position after it got deflected, e.g. due to ground contact, weeds or whatever).
In theory, one limitation of this kind of design is that the force on the rudder increases with something like the square of the speed as you sail faster (e.g. get on a plane). When you pre-stretch the bungee, you effectively set it at a certain (not very large) force. As you sail faster, the force on the rudder increases rapidly, until it matches the force on the bungee. At that point the rudder starts to deflect. Even if you pull the bungee a bit tighter, you postpone that point only by a little, because the force on the rudder rises so quickly with added speed.
That's where an auto-release cleat has an advantage - it requires much higher forces to release, more like the forces experienced only during grounding. (An auto-release cleat also stays released, so if you hit an oyster bed with your rudder, for example, unlike with a bungee you won't press the rudder down while you drag it across. In fact, for an alternate design, you might use a bungee as an uphaul instead to fully raise the rudder after the cleat releases).
With your motor you can push your boat up to hull speed (and then some) so presumably you checked the downhaul up to those speeds. So this theoretical drawback might only show itself if you got your DS on a plane.
Now one thing I don't like is the location of the forward attachment of your bungee. As it is, the flow over the top third of your rudder will be disturbed by the bungee, which adds drag and cuts down on the already not very great efficiency of the stock rudder design. I also can't see a reason why that location improves the function of the downhaul - so if I had to guess I'd assume this was chosen for symmetry? (For the uphaul, the location does increase the lever arm a bit and, being downstream of the rudder blade, it would appear to have less of an effect on the flow).
BTW, I thought the purpose of a heaving knot was to make the end of a line heavier for throwing it. There's something called a heaving line bend (https://en.wikipedia.org/wiki/Heaving_line_bend) that appears to be designed for joining ropes. That knot, or the double (triple) sheet bend, or the racking bend would seem to be appropriate for trying to connect a line to a bungee. The last of these would seem to be the most secure given the way bungee cord can be tricky to tie (see links in the Wikipedia article).
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