Great discussion. It is fantastic that Rick and Peter are starting to realise the boat's potential and are trying new ideas.
I have had a few off list emails asking why I prefer the beam hung rudders and don't use dagger/lee boards.
Underhung rudders are vulnerable if you hit something, They also increase the draft, and if they are in the ends of the hull are difficult to access. It is harder to see and clear weed and plastic bags from them and they are another couple of sq m that needs to be antifouled and scrubbed.
If the beam hung rudders are in the right place, the aft one (upwind) is locked and acts as a daggerboard. The front one steers. On Rare Bird and Elementarry, the locked aft rudder is aligned fore and aft and there is no noticable leeway. On El, the loads on the aft rudder are low enough that it is used to steer as well. As soon as you are not sailing hard on the wind on either boat, you can lift the front rudder and if you are in a hurry, also lift as much as you can of the aft rudder. This is less wetted surface than two small rudders. Surface piercing foils can easily have end plates fitted if required, but looking at them in operation, the difference in height of the water on each side is minimal )never more than 100mm) and there is no flow from one side to the other, so I have not bothered. If required, it would be more efficient to make them a little longer.
Leeboards/daggerboards:
Ono has them, Arttu likes them. So does Rick. Maybe I should have another look, but when I have tried them in the past, they were always more hassle than they were worth. 3 foils may or may not be more efficient than 2, but they are 50% more things to go wrong and be maintained.
I agree about the flat bottomed hulls and the schooner rig. Both are used on the latest designs. Not sure about the vortex shedding from the Tom Speer foils. Tom reckons they are similar in performance to a NACA section. Hopefully, we will soon see.
Rick, please keep the information and feedback coming. As you pointed out, harryproas are still developing and all information is good.
rob
On Sun, Jan 12, 2014 at 11:56 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Luc
You make it seem more complicated than it needs to be. I doubt a pod would make much sense.The dagger board pivots in the deck as close as we could fit it to the lee hull. It swings up one end and is stowed under the deck clear of the water, hard against the lee hull. THe slot in the deck extends past the top of the board so that it can also fold up the other way if it shears the shear pin upon collision in that direction. Because the board pivots at one end the shear pin end can come above the deck if it collides with something. This photo shows the deck slot without its cover with the top of the board at the right hand end in the photo:The shear pin happens to be an 8mm bolt that was on hand.Two relatively shallow rudders built strong enough to support the weight of the lee hull for beaching mounted outboard of the crash bulkheads. Rudders able to rotate 360 degrees under flat bottom hull. Rudders are a standard symmetrical foil section - NACA0015. It is possible to get by with one rudder but having two means installed redundancy. If you consider the potential hydrodynamic forces on a rudder at 20 knots then that will define the strength rather than the possible loads if beached. The top plate on the current rudders is to avoid loss off efficiency once they open the gap between the top of the blade and the round bottom hull:These top plates are not needed if the hull has a flat bottom.
The drives need to be as far apart laterally as possible to maximise steering moment under power at low speed. We have a simple rope lifting system and restraining system that works well for day sailing. Raising and lowering takes seconds. For cruising the restraining method could be improved and the lifting rope could be replaced with something that is faired or even wire with smaller diameter.Fairing for the brace on the dagger board is being done right now. The brace attaches to the board 150 below the water level so it does not add much drag.The boat has solar panels on the cabin roof for charging drive batteries and this meets the needs for day sailing. Usually the batteries are recharged by the end of the day. No need to refuel in this mode. Longer term there are plans for a DC diesel generator that will plug directly to the drives so electronics and batteries can be bypassed for sustained motoring and back-up power. The components are already on hand for this.I expect schooner rig would be a further improvement but I have not looked at this in detail. Would be a big job on this boat.The 18m proa has a big cabin that presents high windage. Windage is a key consideration. My choice would be for a smaller sleeker cabin but it would have less space.The boat is light for its size with very large deck and cabin. In survival conditions I believe it would be necessary to ballast at least the windward hull so it does not get picked up by the wind. We have given some thought to a ballast tank but nothing beyond that.We have not had the boat out of the bay but already it is apparent that 1.5 to 2m waves will hit the beams. A feature of the big tris is their beam clearance so they can climb over 3 to 4m waves rather than crashing through them. So clearance is a also a consideration. Particularly the height of the beams into the lee hull.None of this is complicated. There are unique subtleties but no more complex than any other sailing boat.RickOn 12/01/2014, at 10:23 PM, <lucjdekeyser@telenet.be> wrote:Thank you, Rick. Your reasonings behind the different components are very instructive. I have the impression that this setup is optimized for cruise and may be less effective when shunting. I suppose that the drag from the dagger brace is relatively negligible.Somehow, I get the impression that I need a "third" hull or a Swiss knife like pod to keep all options open with the in water appendages like rudders, boards, tender, propellers, turbines, anti-capsize wands, ankers, (and the foils probably inevitable in the future) ... with fixed or floating endplates, fairings .., For beaching and surviving big seas, nothing sticking out but the ankers/drogue; for cruising, minimal rudders; upwind, push down a deep board; shunting, lots of rudder; in light wind and docking/mooring, propellers down ...So, what about extending the leeward hull deck towards a pod about a meter or so to windward. This should provide a good platform for installing the different components in their proper longitudinal place and for concentrating all kick up/down, push down/pull up and rotation operations?Luc
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