The blades will not hang all the way down, but rest against a stop at a 30º angle from vertical. If they are allowed to hang all the way down, then the lead slug weighing them down becomes less effective, and they are bound to become deflected by the stream of water rushing past, resulting in erratic steering. On the other hand, if they doesn't hang down far enough, then the angular momentum they would build up from the stern of the boat moving up and down will be enough to cause them to bounce up, again resulting in erratic steering. The angle of 30º is a good compromise between these two extremes, and will be further tuned to produce neutral steering, so that the rudders trail in the water, and produce a small amount of feedback to the helm, but so that moving them takes minimal effort.
Neutral steering is produced by placing the horizontal pivot point of the rudder blade forward of the rudder's vertical axis. This will place enough of the entire rudder's surface area ahead of the vertical axis to balance the forces.
Each rudder blade will be bolted to the rudder body using a ½-inch bronze bolt with large fender washers, and two disks made of polyoxymethylene (Delrin) sandwiched between the rudder body and the blade to provide friction-free action. The rudder body will be constructed of solid fiberglass (roving and mat covered by cloth) while the blade will be made of two pieces of ¾-inch plywood screwed and epoxied together and covered in 3 cloths of fiberglass. A puck of lead will be embedded in its lower trailing edge.
When running downwind in heavy weather, with the centerboards pulled up, it will be possible to toe in the two rudders to slow the boat down. Not only will they add drag, but, because the blades are cambered out, toeing them in will force the stern down, making pitchpoling or broaching less likely. A typical unpleasant situation involves a storm that makes heaving to difficult, with ample sea room, so that running downwind under bare poles is a good choice, but the boat is too fast even under bare poles. The usual procedure is to deploy a sea anchor or to trail warps off the stern. QUIDNON will offer another choice: apply the brakes by toeing in the rudders. Of course, this requires the rudders to be built very heavily.
When motoring, both of the rudders will be well out of the way of the stream of water from the prop. This, based on my experience, adds about a knot of speed given the same throttle setting when motoring at slightly below hull speed. I have never understood why designers place the prop directly in front of the rudder and incur a speed penalty when it is so easy to offset it slightly. Love of symmetry?
Wouldn't common copper plumbing pipe be a cheaper solution for a sacrificial tie rod, over bronze? Or would the copper be too soft for the purpose? Maybe upsize the copper pipe for necessary strength?
ReplyDeleteWe'll see how the money equation works out. Copper pipe is certainly cheaper, but will it be strong enough? The diameter is limited to about 2.5 inches before it starts getting so fat that it interferes with the flow.
DeletePerhaps the reason the prop is put directly in front of the rudder is that if you start the prop at a full stop, the water jet of the propwash is directly put on the rudder, and if the rudder is turned, the force is greater. The rudder and prop are on the centerline for symmetry in control; a prop or rudder offset might have different responses.
ReplyDeleteIf the rudder is not in the propwash, that could reduce maneuverability. I am very much liking the dual rudder setup for reliability. Dmitry's design takes design cues from the A-10 Warthog, lots of simple lines, redundancy in all the important parts, and pretty much bulletproof. Ironically, the A-10 *does* have some asymmetric features; in the front landing gear, which is offset(!), so left and right turning radii are different.
You could, in theory, add a pipe/mount in the *middle*, just aft of the prop, so if you *wanted* maneuverability, you could move one of the rudders to be in the prop wash. Once you are out of a region where you needed it, you move the rudder back to the outboard side. This might be more hassle that it is worth. I doubt you are putting maneuvering thrusters in the bow!
The other advantage of using the outboard, is that in theory (did I miss this?), the outboard could be lifted up (if on a rail setup), so the prop is out of the water, and you reduce drag while out to sea, but I don't know if that is worth it. Those rails would have to be able to handle seawater.
I'd say that the reduction of drag isn't the primary reason to be able to raise the outboard while underway, longevity (& resale value) because the outboard on a coppered boat becomes the sacrificial anode by default. So anyone who lives on Quidnon; docked, moored or during a lengthy crossing; is going to want to pull the outboard above the waterline when it's not going to be needed for several days. The alternative is to mount the outboard using and insulating mounting system, because I'd doubt that wood (particularly while wet) is a good enough insulator to prevent signs of early corrosion below the waterline. It might, but if not, the built-in anode that comes with outboards is not going to be sufficient in saltwater.
DeleteHowever, that reminds me of another thought. The outboard engine well, while not being used otherwise, would be a good place to keep a sailboat drag-driven generator; for powering a DC mini-fridge, lights, electronics, etc. Probably not worth it though, unless a mini-fridge is what you want, since solar panels can do the rest just fine. If a drag/prop driven refrigeration compressor exists on the market, it might be worthwhile just for refrigeration for long crossings; but if that is something that would have to be built from parts, then it's probably not going to fit into the 'overall economic efficiency' paradigm Dmitri has already set.
I saw a design that included a weighted trap door on the outboard well, once you lifted the OB, you would drop the door, smoothing out water flow across the bottom. To lift. it had a line through a pad eye so you could pull it open and secure it.
ReplyDeleteSort of like landing gear doors, or like the automatic covers on high end convertibles. How big does the port in the bottom have to be; just enough to clear the prop? Or larger? From the original picture, it seemed like a square well.
ReplyDeleteDmitry commented on the engine page that the rudder placement didn't make much of a difference.
As the rudders are widely spaced, there's a considerable differential in turning radius between them, potentially leading to drag while turning and poor tacking.
ReplyDeleteCatamarans and other wide hull, double rudder installations have solved this problem by using various approaches to Ackermann geometry (see also Ackermann angles). Simple solutions include coupled tillers, curved to embody corrective geometry.
Lots of good, on-line info on the subject.
Love the toe-in idea! Is that yours? If so, I'll credit you when I pass it on!
Dave Z
PS... Prop wash bounced off the rudder IS often used for steering, to the extent that displacement power boats around where I live have ridiculously small rudders. Their tight-quarter turning is always punctuated by large surges of power and a rolling boil at the stern.
So, you've got Ackermann geometry in regular sailing mode, and then the toe-in option. Some sneaky and neat mechanism should do this (seems like you would shorten the tie rod somehow). This gets you toe-in drag, but still allows them to act as rudders.
ReplyDelete