Creep is not an issue with continuous rotation. The only requirement is knowing the position of the rudder and that is done with a visual indicator on the deck above the rudder stock. The indicator is not easy to see at night because the rudders are a long way from the steering position.
The proa originally had 900 deep rudders on quite flimsy shafts. They got bent before the boat sailed and were repaired. The second time they got badly bent and were replaced with 600 deep rudders with stocks strong enough for the boat to sit on hard ground. Top plates were also fitted to them to prevent vortex shedding over the top of the blade when the gap between the narrow round bottom and the rudder blade opened once the blade came off centre. You could feel the original rudder lose grip once the gap opened up. They also flexed enough to jolt the boat if steering from the front one and the load on the blade shifted from one side to the other. If you were not hanging on during the jolt it would take your feet from under you.
The original rudders gave angle between upwind tacks at best VMG of 110 degrees. The smaller rudders gave best VMG at 120 degrees between tacks but reaching speed was faster. The swinging dagger board was added to improve windward ability. It is designed to swing up if grounded because it is 2m deep with 480 chord - very efficient. With board down, best VMG has angle between tacks of 100 degrees. Operating the board is easy because it has lifting and lowering lines in the middle of the boat.
Managing forces on large rudders is challenging. The linked photo shows what Blind Date ended up using after various failures:
You can see that the engineering involved is not trivial. The rudders have a latch that releases if grounded.
With the dagger board I designed a faired tubular brace that connects to the board at the waterline and is braced back to the longitudinal beam under the deck. The compressive load on that brace at 15kts is 5 tonne.
A HP has natural windward helm. In a two rudder system it makes sense to use the trailing rudder to balance the boat because it almost offsets leeway. If the leading rudder is used balance the boat then it is adding to leeway. With two big rudders it is always faster to lift the leading rudder and use the trailing rudder to balance and steer.
On a fast boat like the 18m HP the rudders need to be designed for hard over at 20 knots with a safety factor of about 3. If they can handle that then it is quite likely they can handle being on the hard although I would not want to slam into something solid when sailing at any speed above drifting.
Rick
On 22 Mar 2019, at 12:57 am, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:Rick:Hydraulics is a big part of what I do for a living, so I'm naturally inclined that way. The beauty of hydraulics is that one has so many options, but the problem is creep, and keeping things in synch when they are not physically connected, which is essentially impossible with an orbital system, though possible with a cylinder based system using specially built cylinders that feed each other.... the volume on the ram end of one being equal to the volume of the opposite end of the other, and flow through valves in the pistons so that when fully extended they can equalize. For practical purposes in most cases a mechanical connection of some sort is needed to keep things synchronized.I have to ask if an unsteered board is needed at all on a dual rudder setup? The two rudders working together should be able to tackle leeway more efficiently than a board..... the real function of a board is to have an axis to pivot about.... You use the rudder to change the AOA of the board, pivoting the entire boat to fight leeway.... On some boats the hull has a significant amount of lift to contribute, on others less.... With a shallow draft round or flat bottom, it seems to me that the value of altering the longitudinal axis of the boat to windward of the direction of travel might have minimal value.I've built a number of rear steering machines over the years........ the shorter the wheel base, the more radical they are ..... more difficult to control, the longer, the easier they are, and I've designed a number of features to dampen them and make them easy to drive at speed. That experience suggests that using the front mounted rudder as your leeway device would give you the option of rear or front only steering for great directional stability...not twitchy, both for extreme maneuvers, and the ability to "tune" one rudder's AOA for leeway. The mechanical aspect is the huge challenge with anything that versatile.H.W.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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