Because there are two rudders, and because they are angled out, to be maximally effective when the boat is heeled over, the steering linkage is rather intricate. Not counting the rudders and the wheel, there are 5 moving parts. The tie-rod and the aft whipstalk will be fabricated using 1-inch aluminum square stock, 1x¼" stainless steel strips and #10 stainless steel mechanical screws. The tillers will be made of 1.5" aluminum rod with flats milled into the ends at the right angles for attaching the stainless steel strips. The connections will be made using ¼" stainless steel bolts and Acetal washers. The entire assembly is intended to last for the life of the boat without any maintenance.
One interesting feature of this linkage is that it makes it possible to switch between wheel steering and tillerpilot steering at the pedestal. The wheel can be disconnected from the steering linkage simply by pulling back on it, causing the pinion gear splined to its shaft to disconnect from the rack which it drives. This is useful, because it lessens the inertial load on the tillerpilot from the angular momentum of the wheel. My preferred tillerpilot (Simrad TP32) can generate up to 100 lbs of force, but with considerable power drain and wear. With the wheel disconnected, the force required from it will be low, because the linkage is of lightweight aluminum and the rudders are balanced.
Another interesting feature this linkage makes possible is the ability to slow down the boat by toeing in the two rudders. That's right, unlike just about every other boat, QUIDNON will have a brake lever! The lever at the steering pedestal will drive a cable leading aft, which will act on the steering linkage to toe in the rudders. This feature will be useful when running downwind in overly boisterous conditions, because it will slow down the boat while pulling down the transom, lessening the likelihood of broaching or pitch-poling, and making it unnecessary (in most conditions) to use any of the usual techniques for slowing the boat down, such as trailing warps or deploying a drogue or a sea anchor.
Referring to the schematic diagram below, the green vertical plane on the left is the transom. The two rudder shafts are mounted to the outside of it, using brackets. Each rudder shaft is joined to a tiller through a horizontal pivot (axes of the pivot points are shown in blue) and each tiller is joined to one end of a tie-rod using vertical pivots. At its center the tie-rod is joined to the aft whip-stalk using a two-axis pivot (left/right and fore/aft). Steering action is indicated using red and green arrows (red for starboard, green for port).
The aft whip-stalk is connected to the steering shaft through a fore-and-aft pivot. The steering shaft is mounted to the deck by brackets and rocks left/right along the centerline of the boat. It penetrates the aft wall of the pilot house and runs forward to the steering pedestal.
The forward whipstalk is connected directly to the steering shaft inside the pedestal. At the top of the forward whipstalk is a rack, which is moved left/right by a pinion gear splined to the wheel shaft. The forward whip-stalk also holds a pin (not shown) for mounting a tillerpilot. Before engaging the tillerpilot, the tip of its telescoping arm is snapped onto the pin, and the wheel is pulled back on its shaft, disengaging the pinion from the rack in order to reduce the load on the tillerpilot.
Magenta arrows indicate the action of the brake lever. When actuated, it introduces a bend into the aft whipstalk, making it shorter. In turn, this causes the two tillers to pivot down. Because of the angle of the rudder posts, this has the effect of towing in the rudder blades by a few degrees.
Referring to the diagram above, the axes of the tillers incorporate a 10ยบ twist, each in the opposite direction. Because of this deviation from horizontal, when the tie-rod (shown here in cyan) is pulled down, the rudder posts (yellow) rotate inward. When the rudder blades are towed in, they pull down the transom and induce drag. Note that it will still be possible to steer the boat even with the rudders toed in.
Does it have to be aluminum? Why not galvinized steel or stainless? Or are you assuming that the loads won't fatigue it?
ReplyDeleteIs the aft whipstalk being *bent* or pivoted? If it was pivoted (with the right
mechanical linkage), you could get the same effect, without the bending.
I chose aluminum because it has good stiffness for the weight (which is important to keep down, to reduce inertial loads) and because it's easy to work.
ReplyDeleteNothing is bent.
This one gives me a "hmmm" instead of an "eep". Is there a cotter pin or some similar "mechanical fuse" at the connection to the rudder post? I'm having trouble seeing what happens when there's a sideways shock to a rudder.
ReplyDeleteThat's a really good point!
DeleteLast time my rudder got a sudden sideways jolt it snapped my tillerpilot in half. I'd rather that didn't happen, as I lost a $600 piece of electromechanical equipment (which insurance paid for, but whatever).
The simplest approach I can think of is two tensioned springs that hold each tiller straight unless there is an excessive load, and then gives way temporarily, snapping the tiller back into place as soon the excessive load is removed.
I'll draw something up.
No complaints about the rest of the linkage?
When I think about this problem, my subconscious keeps insisting on hydraulics. It's not telling me why, just yet. If it tells me, I'll tell you.
DeleteWell, if there is a solution using hydraulics that is cheap, lasts the life of the boat without any maintenance, undergoes graceful degradation and can be repaired anywhere in the world using bits carved out of driftwood and lashings, then it would work. If one hydraulic leak anywhere renders the boat unsteerable until it is rescued, towed into a marina, and consultants are flown in, then your subconscious is in need of Freudian analysis.
DeleteIt says, being a subconscious it prefers Jungian therapy. And by hydraulics it means shock absorbers paired with your tension springs.
DeleteShock absorbers... But wouldn't that just delay the release response in case of overload? And isn't water itself a good enough shock absorber?
DeleteSo, the aft whipstalk isn't bent, just moved a certain way to toe in the rudders. Great. Some of the serious bicycle folks I know (and one of the best TIG welders out there) jokingly refer to aluminum as the 'false metal', because of fatigue issues in bicycles.
ReplyDeleteAny turnbuckle-like stuff in there, to tune the linkages?
I won't use aluminum where forces are concentrated. I'll use stainless there. With bicycles, there is the desire to make things extra-light. It's less of an issue here, so I'll use rod instead of tubing. There should be plenty of strength to spare.
DeleteThe need to do "back-end alignment" is a good point. I'll discuss it in tomorrow's post.