There are a BUNCH of different ways to design and mount rudders, most of which can be adapted to barge/scows. Here, I'll talk about three basic approaches.
The basic problem is that barges tend to be very shoal of draft.
A high aspect ratio rudder (taller than wide... I'm speaking in profile view, here) has to extend below the hull, where it is exposed and vulnerable. Low aspect ratio rudders (wider than tall) need to gain area by spreading very wide, indeed, which increases leverage burden (heavy helm).
But there are many approaches with easy, DIY, and workable solutions. The following are three, core approaches, from which many variants are possible.
|Barn Door Rudder|
A similar system is used by traditional cat-boats, but using a wheel controlled tiller. Should the wheel system fail, they can be extended for full tiller steering. They look pretty nice, too.
Note that, in plywood, box barges (such as TriloBoats), the skeg's shape is identical to the plywood off-cuts from the sides. They can be brought inboard and beefed up with timber before bolting to the hull. A variant would be to extend it a bit with a rudder post mounted vertically along the full length of the transom and skeg.
When taking the ground, with this type, be sure that the rudder isn't going to hang up on even a slight hummock or small rock... that will generate a LOT of stress as the boat settles.
|Post Rudder with Bottom Plate|
This one has its post set at right angles to the plane of the bottom... as it turns, it's edges remain snug to the hull without widening gaps. Particularly at the leading edge, this helps keep weed and such from jamming.
It is balanced, meaning it has some area forward of the post (line of rotation). This greatly reduces strain on the helm, and allows much more area than would otherwise be comfortable. Typically, a tiller would be fitted.
When underway, the bottom plate keeps water from spilling off the lower edge, increasing the rudder's efficiency. I've heard a rule-of-thumb that the added efficiency is the equivalent of adding one side of the plate vertically to the bottom of the rudder. Bottom plates would surely help with barn door types, too.
Advantages are that they can be lifted completely clear of the water, when not in use. The relatively fragile blade is dismountable, for easy maintenance. They can be high aspect, extending below the hull, but will kick back and up if run aground. In heavy, floating weed, we lift it clear and propel/steer with the scull.
This one is balanced, allowing the rudder to be oversized (nice positive effect and adds to lateral plane). The forward tip of the blade, when the rudder is kicked back near horizontal, is just a bit less than the draft, allowing steerage in any immersed position. The aft-set lower tip, and continuous curve (no flat section at the bottom) will roll the boat forward if we've forgotten to pull the rudder, and we settle down with the tide (purely theoretical, of course!).
The lower tip of the rudder needs to be weighted. Pouring lead or zinc works ('spent' zincs are easy pickin's around many grids or yards). We found some commercial, paired zincs that we cross-bolted in place... they're not as streamlined as inset metal, but much easier to work with. If you use zinc, consider epoxy-coating to keep it inactive.
If we're really booking, off the wind, the rudder may kick back some from water pressure, despite the weights. We have a little hook on the rudder post, low and forward, to catch the retrieval line and lock the blade down. We try to remember to undo it before approaching shoal water.
One thing that's worked well is cutting a circle of close-cell foam, and using it to pad the bearing plate. This keeps the whole thing perfectly quiet (can get clunky, without, in slop and bobble).
Planning out the rudder, it helps to include the mounting arrangements. Winging it, later isn't at all easy, as close fits work best. Consider walking around a boat yard or marina (the funkier the better), trolling for ideas. Lots of good stuff on the net.
I see a lot of plans (on the net, especially) that look great on paper, but seem pretty non-KISS. Even something as simple as housing cheeks on a kick-up rudder are harder and more expensive to build, and often swell or warp, creating problems. Long and ingenious holes bored through the body to lead control lines are hard to waterproof and inspect. Be ware!
One mounting system we really like was first seen on James Wharram catamarans.
Matching, thwartship holes are drilled and faired in the rudder-post and leading edge of the rudder, both of which have been faired to half-rounds.
Light line is fixed low (stopper knot, among other methods) laced from the bottom up, tensioning as you go, in figure 8s (e.g., through post to port, lead aft and between post and rudder to rudder starboard side, through that hole... repeat, alternating sides). Fix high (cleat or somesuch).
The 8s need to be kept from slipping, to keep rudder and post in-line. Wharram fills the holes with epoxy, and drills out if replacing. We prefer shouldered screws through the line, just in case, so we can do and redo.
This system is absolutely quiet (the line cushions all moving parts), with no metal to fatigue. Since it's rolling, as the rudder works, rather than sliding, there's almost no friction. LUNA's line was looking great after 12 years! If there ever is a problem, it costs a few bucks for more line.
One variant we saw was alternating hinges of heavy webbing, crossing from one side, between rudder and post, to the other. All the advantages, with possibly easier attachements and less initial work (no holes to drill and fair).
When laying rudders out, I usually just eyeball it. There are several theories as to how much to count toward a boat's balance, how big they should be, how deep, what shape. But looking around traditional designs, the variation is huge, and all seem to steer the boat.
If anything, I would err on the large size. You can always cut it down. You can't have too much steerage, and the downside consequences of too large a rudder - even among optimal theorists - aren't drastic. You can, on the other hand, have too little, and that's a problem!
One consideration is the angle of the rudder post. If raked aft (traditional, and our preference), the rudder will want to swing to center. This means, all things being equal, the boat will tend to go in a straight line when you let go of the tiller. If vertical, it's neutral - less help, though when making way it will tend to center. If raked forward (reverse transom), it wants to flop outboard - a true pain in the neck.
So pool your options, build your rudder and set your course!