rudder discussions

[Tim61N]

Ebb,
From what I have gathered in my reading of rudder hydrodynamics, the reason for the doing away with feathering and keeping a flat blade would be to prevent the blade from stalling when turning. A feathered blade shape when turned at an angle would induce the fluid to create a vortex on the low pressure side of the blade, thus causing the blade to stall and the boat won't turn. If the boat were always moving in a straight line the ideal blade shape would taper off in an infinitely small edge to have the least amount of drag. I'm sure this is a very oversimplified understanding as there are a lot of variables involved, but makes sense to me.
Here is the spec sheet of the Silicon Bronze I got a quote for at Alaskan Metals it is a PDF, hope that comes across. I also included the spec sheet for Aluminum-Nickel Bronze which seems like another possible choice due to its strength and lack of zinc, but I have not seen much discussed here.

[ebb]

Tim,
Take your word on modern rudder design. The flat I'm refering to is the extreme back trailing edge (does not come to a knife edge), which should be flat and both its edges sharp. That is: this flat edge is more or less 90 degrees to the 'flat' of the rudder sides. It is only about 1/4" wide. You could only get this edge using modern composite materials.

The blade itself, the 'flat' part should have a chord to it, exactly the same each side. But what that chord is when upgrading traditionally flat, actually flat sided original factory Pearson rudders is open for discussion. Maybe it's not open to discussion.


Watch it with Alaska.
I personally advise Everdur 655 silicon bronze ONLY.
655 is the number. The correct number says it all.
Silicon bronze is a commonly available REAL bronze. Most available off-the-shelf screws, bolts, rod, bar, allthread, nuts, washers - if they say silicon bronze - are probably made from alloys close to 655. Not to aluminum nickle bronze, mango bronze, or petunia bronze.
There's nothing wrong with Monel if you're rich. Monel has no aluminum in it, last time I looked.

Not recomending anything, just saying what I did and why.
When ordering rod and bar you have to order by number - and sometimes by hardness.

Everdur 655 is the rolls royce of silicon bronzes, it is a specific controlled formula. We're really lucky to have it and regular silicon bronze availabe to us serfs. All the best wood boats of the past are fastened with it. To me it represents aromatic pipe tobacco, cuban cigars, scotch, leather, oakum, served rigging, highend varnished carpentry, and money. So it's prejudice on my part. And I don't smoke. I still think it is the best material, after much discussion here, for the rudder system on the A/C. Each to his own!!!

[mrgnstrn]

ok...a pictoral with commentary:

First, my representation of plywood as seen from the end-grain side.

[mrgnstrn]

next, put the widest set of dado blades into your tablesaw.
the one at the woodshop i go to is just shy of 3/4"
you also need a "carriage" for it. This is like a wide, shallow, open top box that has the correct hardware to slide in the grooves of the tablesaw.

This will allow you to prop up one edge of the plywood. you want to prop up the end that will be the thick part of the taper.
The black line in my drawing is the like the top of the carriage, or alternatively, the top of the tablesaw.
The red line is the cut line.
I should have put an arrow to tell the story better, but the plywood, the block propping up the edge, all move relative to the blade.

Now, this basically puts a groove only as wide as the dado, so shift the plywood ~1/2 inch into or out of the page, and run it though again. Repeat many times. Many many many times. Figure for a 48" long board, 1/2" at a time, with a few thrown in for giggles, ~100 passes. But really, once my jig was set up, ~10 minutes for the whole 24"x48" piece of plywood.

Now make two of these (you need will want to sandwich them together).

[mrgnstrn]

now you have one of these:

[mrgnstrn]

so, take this piece of plywood and cut out the profile of the rudder:

The shaded portion is the part you keep.

[mrgnstrn]

So, now put some bolt holes in....well half of each hole:
Use an appropriate sized cove bit (shown), and put a cove into each of the two halves. how you line up the two cuts is up to you. i made a few jigs to handle it. and lots of marks on each half to line everything up.

then you put the two halves together, and voile, perfectly lined up bolt-holes, down the centerline of the rudder.

[mrgnstrn]

ok, so there you have my method.

a few more points on the table saw method:


propping up one end works well, until you have make a number of cuts, like ~1/3 of the length. you can tell that the right side of my diagram has support before the cut, and the height of the thing you prop it with says the same after you make the cut, because the right side really doesn't get cut.

but the left side of my diagram needs to be propped after it is cut, because it isn't setting on the table saw anymore. So this is easy, just make sure to support the thin part of the board after you have cut it.

If more diagrams are necessary, just let me know.....

[CapnK]

All -

Thanks for this thread, am learning much.

[ebb]

Thanks Capt mrgnstrm,
Tahell with theory - this is the real stuff! Thanks from all of us. Methods and methodology forever!

And with plywood one would not use a beltsander to shape a completed plywood rudder (as I wrote earlier ) because you want to keep the veneers intact. With mrngster's method the carved sides are the ones that are glued together face to face keeping the full veneers on the outside, making for a stronger unit.

Now I would like to see recorded here for prosterity a series of photos showing this alternative rudder being assembled. A series like this does not exist anywhere on the web or in a book that I know about.

And as folks who have not done it befor will say, there is nothing obvious about the steps or the process.

The tapering of the panels can also be done simularly with a router (from the top) - messier than the table saw method.

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The conundrum:

I believe a blade built in modern times should have some foil shape, more wing shape, some live curve to the surfaces, any opinions? Any naval architects in the crowd? Tried to explain earlier: when you start at the shaft fatter than the shaft with the wood - and bring that same dimension you started with further aft and then gently curve the sides to the tip of the blade - you'ld have a stronger rudder with longer bolts if desired. More wood.

And if curved correctly the rudder would then be able to generate lift. Rather than providing resistance when changing course and kind of slewing the boat around. IE, the curved blade uses both sides continuously to do its work - and the slab sided blade gets the boat to change course by sticking the wood out to halt that side , while the free side gets to continue on round.

Putting it bluntly:
Really wondering if adding a recognized modern chord shape to our rudder surface would get the A/Cs to sail any better, faster, or more responsive. Or should the blade be left dead slab flat as it came from Pearson? Or should it be the mrgnster taper???

[Tim61N]

kp,
Great technique! Far less dust that using a belt sander and very accurate.

but the left side of my diagram needs to be propped after it is cut, because it isn't setting on the table saw anymore. So this is easy, just make sure to support the thin part of the board after you have cut it.

I suppose some long strips as thick as the height of the dado blade double stickied to the table would be a good prop.
What will you use to glue to two sides together, epoxy or something like resorcinol?
Tim

[willie]

Just thought i'd get in here~~
Several subjects to cover, first off i gotta brag about going sailing this weekend!! Starting the season early, as it's looking like there won't be enuf water this summer to make spit, let alone float my boat. Pretty dry winter here in Oregon. So had a chance to go, weather was nice, had a bit of wind, so off we went. Awesome!

So on this rudder shaping/chord/lift making idea...which i think we covered somewhere before we had more experts come aboard....

I've heard it said the keel generates lift, by it's shape (chord?) like an airplane wing, or a sail. And possibly the rudder can be made to be a continuation of that shape, but i don't see how. My keel goes pretty straight back there. Of course it's been rebuilt, but i just continued the lines on back. Sure it tapers down at the rear, but not much. I don't see how making it with a chord will improve anything. A clean bottom, flush fittings, and no outboard hanging in the water to create drag and added weight in the stern will do so much more to improve performance than a wing shaped rudder. I can maintain steerage with my boat down to a snail crawl, and she coasts along forever. Anyway, just doing a little rambling here on what i think is more important. And i'm still struggling with the keel shape generating lift. I don't see how, with both sides being shaped the same!! On a wing or sail, you have the chord side, and the other flatter side, creating difference in pressure/air speed? I just can't see that on our keel. Yeah, it's nicely shaped to slice through the water, but lift? Come on someone, please explain!

On carving a new rudder, which i'll be doing when my rubber coverd mahogany finally gives out--or maybe the shaft will first--think i'll just use my stihl chainsaw. lol
Actually, here's a little work i did for a bit of nautical theme in the kitchen over the winter. Well, i did cheat and use a makita grinder w/sanding disc for the finish shape. Was going for an Airel when we started, ended up with something like a ketch!

Ya'll keep rebuilding, i'm going sailing! Thar is light at the end....

[mrgnstrn]

Tim:
Yes, thin strips stacked up as high as the dado blade is set would support the cut part of the plywood.
I found that 1/8" thick hardboard work wonderfully. Hard board is the same stuff as brown pegboard, but without the pegholes. And 1/8" is pretty good for most adjustments. after that 1/16" thick aluminum flat bar works well for any finer adjustment than that.

I used West epoxy to glue the two faces that I had cut together (cut face to cut face) so as to leave the unblemished faces outward. (of course, I went ahead and blemished the faces by putting 8 bronze screws on one of the faces to pull the two faces together enough for me to tighten down the machine screws/bolts and nut. oh well....)

Ebb:
Well, funny story about pictures of the process....there aren't any.
it was last saturday when my wife took my 1-year old son to a friend's house for a few hours (read: no parental responsibilities for a while, uniterupted). so there I am, outside on the back porch, preparing to mix epoxy, getting the vaccum bag ready, etc. and I thought to my self, "I should really take pictures". Then I saw the storm clouds moving. Well, I made a cursory round through the house looking for the camera, to no avail. But, it was now or never, so mixing epoxy and spreading and glueing took place before pictures. Luckily I got the epoxy down, the faces together, shaft on and tightened up, the vacuum bag over the whole thing and vacuum pump on before it started sprinkling.

by the way, vacuum bagging is hard. especially in the (relative) cold, raining, and really your first time doing it, and on a large piece. But, eventually i got everything sealed, but it took some liberal application of silicone caulking around the edge of the bag. in the rain.

lastly, on the profile of the rudder, if you look at the rudder as a flap on the end of a section, really it is mostly just tapered, and mostly straight sides. Like look at a NACA section, and imagine the front of the keel is the front of the section, the really extreme aft end has an almost inperceptible curvature to it.
If the rudder wasn't hung on the end of the keel, like for instance, a spade rudder, or a shorter immovable portion, like a skeg hung rudder, there would be a more perceptible curvature.

I browsed through Skene's Elements of Yacht design, and their example boat "Pipe Dream" is a cutaway forefoot keeled boat. The lines drawing in the book shows a taper, and again the inperceptible carvature of a faired in line.

as far as the extreme edge treament and eddies: I think that the overall aspect ratio of the rudder has much more to do with that.
I think that wheather the trailing edge comes to a fine edge or is squared off and 1/4" think has such an inperceptible difference that I don't think one would ever notice the difference. I plan on just using my orbital sander to round it over enough to allow for fiberglass cloth to lay on it and leave it at that.

[CapnK]

I'm tossing in my chips that tapering the rudder won't make *that* much of a difference. Here's why:

A NACA foil of the proper shape for our boats will have it's maximum thickness at a point about 12% aft of the leading edge. The percentage is based on the chord length of the foil. NACA/NASA came up with a number of different shapes for different foils which serve different purposes. The actual overall shape of the foil is based on the speed at which the foil moves through liquid medium - air for a plane, water for boats. So we would want the NACA foil which was designed for 6 mph, more or less (I forget which specific foil # NACA gave this one).

If that is the shape of our keels foil overall, then the shape of the trailing edge - be it flat or tapered - will have little to do with how well the foil works. However, a tapered shape *is* more hydrodynamically slippery, and would work to take away drag caused by an over-wide trailing edge (and its resultant vortices).

Blah blah blah , at any rate, I think we are probably talking in hundredths of a knot/mph difference in the speed gain. I'd say at *absolute most*, a sweet taper would give a 1/10th of a knot, but I seriously doubt it would be that much. I'd bet that a properly steered boat with no taper would outrun a boat not steered very well which had the perfectest of tapers.

Our rudders don't need a foil shape, either, because they sit right behind the deadwood of the keel, and their rounded bar front edge probably works as well as any other shape to help bring water off of the keel and over the rudders surface. Our rudders are like flaps on an airplanes main wing trailing edge, where a rudder hung by itself (yuk) would be more like the airplanes tail surfaces, and thus needs its own foil.

Last thing - a symmetric keel/foil develops lift when the Angle of Attack makes the water striking it hit more on one side than the other. Angle of Attack is the difference between where the chord of the keel (straight line front to back, through the middle) is pointing, and the direction the keel is moving through the water. If your angle of attack becomes too large for your foil, instead of producing lift, it begins to stall and lose power (with resultant leeway), just like when you oversheet your sails.

PS - I don't mean to come off as a know-it-all. Because I *don't* know it all. Here's how I learned some of this: Smaller Com-Pacs (23' and down) come with a transom-hung rudder made from a flat sheet of aluminum - no taper, just some very slightly rounded corners, enough to keep the metal from cutting you. Some individuals had made foiled replacement rudders, and claimed they improved windward performance. Well, with a CP, anything you can do to improve windward performance is a Good Thing, so I set down and larned myself all about foils and all that, and started producing a design which CP owners could retrofit to their flat rudders. Luckily, just as I was finishing up the design process, Joel at IdaSailor stepped in and volunteered to help. Now CP sailors can buy a proper rudder from Joel. Some of my knowledge of aerodynamics also comes from being an aviation buff.

[mrgnstrn]

yeah, Epiphany's right......there are more-better things to do to improve hydrodynamics.....like keeping a clean bottom, or going with baltoplate or something, taking the outboard out of the water when sailing, buying a feathering prop, etc etc etc.

heck, about the only thing I do for hydrodynamics sake is untie my docklines when I want to go out, so as not to drag the dock in the water behind me.

well....that is WHEN I do go out....or IF i do go out (ok, so i am a little jealous of Willie.)