question about rudders
I’ve read several threads about rudders for shunting - vexed topic with no easy solution it seems. My question is slightly different: given Atlantic geometry, what are the pros and cons of putting the rudders on the ama, (at each end), with some sort of lock so the fwd one is inline or allows fine trim tab adjustment?
Someone posted an idea for linking tillers on end rudders using a flexible pole, and then pushing/pulling the center of the pole to steer both rudders at the same time. It might be ideal for your Atlantic ama rudders situation. It might also make an autopilot easy to set up.
thanks, I saw that. But its not the issue I have. The problem with pointing a normally trailing rudder forwards is that it is dynamically unstable, any bit off line will extort major forces and huge drag . Catastrophic failure would leave the bow ‘rudder’ at right angles - bad. Someone suggested mounting the rudder far enough beyond the hull that it could swing 180 and ‘trail’ at the front, nice idea but the necessary gap between rudder and hull might produce a lot of drag. Balancing the rudder like a balanced skeg, almost 50/50, seems interesting.
Rudders located at ends of Ama (Atlantic).
I’ve been kicking this idea around for a few weeks and it seems to make sense. Assuming a fine deep V ama, easy build in ply, say 24’x 2’ beam by 4’ deep, vertical ends. Lots of wetted surface I know, but …
Cut the last foot off each end, and attach back to hull with a vertical ruddershaft, appropriately faired, allowing for 90 to 120 deg of swing. I’ll figure out the construction later - a Wharram cord binding would work, or a more conventional design with solid shaft in rudder and top and bottom ‘socket’ bearings. Or better perhaps, the other way around, with sockets integral to structure of rudder and shaft stubs in hull. ... Add a long well fixed tiller along the top, which is lockable straight ahead when at bow end (and add extended control rods to cockpit).
OK, so I don’t have a deep foil but the whole Deep V ama is a foil. And I have substantially reduced dangers of impact. (Of course, a spare rudder or two will be in the kit).
So my question is hydrodynamic - what are your opinions about the general idea of steering at the ama - on an Atlantic? It seems to me to make good sense - the more heel, the more rudder is in the water. But clearly, the rudders are way out on a radius from the turning axis. Is this a bad thing?
Simon,
Have a look at some of Rob Denney’s proas: http://www.harryproa.com/
Whilst strictly not Atlantic, interesting that he has tried all sorts of systems in all sorts of places, but they are all still on the ama.
If you are thinking of counterbalanced rudders, Dave Gerr in his book: “The Nature of Boats” suggests (through years of trial and error) 17% counterbalance with 20% as the absolute max. With regard to rudder sizing, you can pick and choose between his various options but 0.45 x LWL x D (canoe body) would seem to be the most appropriate one for a proa.
My only other comment is that a using long tillers to leeward in a narrow ama could mean, even with the forward one locked more or less centrally, the aft one could end up dragging in the water if you are doing a big luff up in heavy weather…....I am also not a fan of bow rudders, I remain to be convinced that they give you enough area in rough conditions. And you are absolutely right about malfunctioning front facing ones.
Rob
Hey Rob
thanks for quick reply. I’ll check Denney’s and Gerrs ideas.
If you are thinking of counterbalanced rudders,
My current thought is no. Vertical pivot at ‘leading’ edge (going the right way for a rudder). Nested/faired into hull in an almost semicircular channel.
My only other comment is that a using long tillers to leeward in a narrow ama could mean, even with the forward one locked more or less centrally, the aft one could end up dragging in the water if you are doing a big luff up in heavy weather.
run that past me again? Maybe I should elaborate. Longish tillers (say 4’) linking to a pushrods (1/2” ss) which cross the boat to mid point of a tiller on the main hull, pivoted its end. An old school bell crankish kind of arrangement.
I am also not a fan of bow rudders, I remain to be convinced that they give you enough area in rough conditions. And you are absolutely right about malfunctioning front facing ones.
I’m proposing always locking the bow rudder. But some incremental (i.e. screw) adjustment might be handy for trimming. Shunting means changing to the other tiller.
Simon