Building a rudder

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Moderator: GreenLake

Postby GreenLake » Wed Feb 23, 2011 4:06 pm

For reference, here's a picture that shows a schematic view of how to use the alternating plies in the plywood in helping to shape the foil.

When you sand at an angle through plywood, the result is a banding pattern of alternating lighter and darker stripes. By measuring where the plies intersect the foil curve, you can project the location and width of these stripes.

The plywood I used had 7 full plies and one split ply (top and bottom) as shown in the small inset. The rest of the graphic is schematic.


Marking the stripes on the surface is not necessarily ideal, since these markings will be sanded off. But by marking them on some wood on either side, you can use a straight edge for a quick check whether you have sanded enough.

For final shaping I used two different methods, both done with shining a light from one side. For the first, I placed ruler lengthwise, parallel to the stripes, and observed whether there were any low spots (light shining under the straightedge) or corresponding high spots. Sanding with a very long and hard sanding board along the lengthwise direction helped fair the foil.

For the second method, I used a printout of the foil shape 1:1 onto a piece of cereal box. I cut along the curve with an xacto knife and then soaked the edge of the cardboard in epoxy. That allowed me to use sandpaper to get a fair curve on this cardboard template. I then used this template in the direction across the foil, again with use of the light, to find out high or low spots.

The high spots I sanded off as before, the low spots I filled with QuickFair.

After way too many passes of that (for my taste) I decided at one point I was done and proceeded to apply paint.
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Postby GreenLake » Thu Feb 24, 2011 8:54 pm

I've made progress on the new rudder. After a few coats of EasyPoxy (a one-part Polyurethane) I assembled it for the first time to take some "before and after" pictures.


It's kind of shocking to see how gratuitously inefficient the stock rudder is. Forget the fact that the foil shape is rather "random", and let's look at all the other sources of drag that are present.

The two red lines roughly indicate a non-shaped area of the stock rudder that is immersed in the flow. This is a rather large percentage of the foil and can't be but a noticeable addition to the drag.

The red circle indicates where the stock rudder presents a completely flat surface to the water flow. Removing these two gratuitous sources of drag should give some improvement - whether or not I'm able to get the most benefit of the new foil shape (that depends a bit on how good a shape I managed to create).

The right half of the picture shows the replacement rudder from the same perspective. The "cheeks" no longer drag through the flow, and the entire immersed area of the foil is shaped.
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Postby jdoorly » Thu Feb 24, 2011 9:40 pm

That's some ssssslippery looking rudder!

(I guess the website decided to let me post again! I think I overflowed the preview iteration counter.)
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Postby GreenLake » Thu Feb 24, 2011 9:58 pm

Thanks, Jay.

I can't wait to try it out, myself.

Our short winter racing season is over, though, and the weather's taking a cue - these photos were done between snow showers. Looks like I'll have to be patient for a while.
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Postby algonquin » Fri Feb 25, 2011 2:02 am

Nice work. You have inspired me to take another look at my rudder before the season begins.

A dramatic improvement over the stock rudder ! I like the raised shoulder design you included on the top rear of the rudder. It should help reduce turbulence on the upper rear edge.

Your new design appears slightly thicker at the leading edge over the stock piece. Also was wondering if you were going to tackle the centerboard this year ? Brad
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Postby GreenLake » Fri Feb 25, 2011 5:20 am

Thanks, Brad.

The rounder entry is on purpose. The NACA00xx profiles all have a nearly semicircular profile as the leading edge. There's an equation for that somewhere, but what I did was to plot a circle inside the plot for the profile I chose. 1/4" diameter gave a good match, and that's what I sanded the blank down to. I think I made enough allowance for all the coatings. Will see.

In oder to get lift, the point where the streamlines divide is not on the center line. If you had a knife edge entry, some flow would have to navigate backwards over that edge - which is a way to visualize why the entry shouldn't be too fine. (Read Arvel Gentry's papers on for the real scoop, remembering that hydrodynamics and aerodynamics are the same physics).

If I find the right occasion, I might try back-to-back comparisons, perhaps with photos of the wake... no promises. In order to be able to do something like this I didn't reuse the existing pintles. That also allows me to keep the old one as a spare, or for use when launching where ground contact is a real issue.

The centerboard is an interesting issue. The DSA just worked with Cape Cod Shipbuilding to create a mold that allows mass production of CBs that meet class specification while having a nice foil shape.

There's a real temptation here to just order one of those and skip all the hassle...other than that, the same techniques I used would apply to creating a centerboard - except that it needs to be strong enough to hold 1-2 crew members when trying to right the boat.

One of the articles I looked at recently, perhaps the one posted by K.C. from Duckworks, had the suggestion for a groove on each side of the CB at the location of maximum chord to accept some supporting laminate. They used carbon over glass, but to keep with class restrictions, perhaps biaxial would give enough support.

Last summer I did an interim measure, using 3M high strength filler to fair the existing CB and to provide a rounded leading edge and a more triangular trailing edge to my existing CB and to give it other TLC like a nice polish. Can't be sure that it helped, but given how dinged up it had been, I think what I did, didn't hurt at least. :)

That made the rudder the "low hanging fruit".
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Postby Baysailer » Fri Feb 25, 2011 5:47 pm

I looked at your photos and the first thing I think is it doesn't look like mine. My rudder has a very symmetric profile including the lower tip looking at it from the side. It's also stubbier looking than yours. So I looked at the handbook and it shows the same stubby rounded rudder I have. I may be wrong but your rudder may not be class legal if that means anything. I'm sure it's more efficient though and will provide a better control where it counts.

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Postby GreenLake » Fri Feb 25, 2011 7:02 pm

Isn't it funny how photos can mislead?

You've seen the drawing, and if you compare the tip detail you'll see it's a true 6" semicircle (and those dimensions should match the class requirements - I checked the handbook, so unless I made a mistake somewhere...).

The rudder tip is definitely not asymmetrical in the plane, only the profile is (it's thickest at 1/3 of the chord). Now, why would it look that way in the photo? The only explanation I have is that it's a trick of both the lighting and the way the blades thickness is not constant.

Here's the full side view, which gives you a better perspective - although I must warn you that the photos were not made from a tripod nor with controlled settings, so small scale differences are to be expected. Even here, the way light and shadow interact with leading and trailing edge suggests a small asymmetry that just isn't there.

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Postby GreenLake » Fri Feb 25, 2011 7:23 pm

Just found my copy of the fall issue of Day Sailer Quarterly again and re-read Mike Gillum's article on painting the bottom of his boat. Some good instructions and definitely applicable to the task of finishing a rudder as well. However, he had one resource that's difficult to duplicate: a willing and expert brother to lend a hand. :D
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Postby GreenLake » Sun Apr 10, 2011 6:02 pm

We had an unexpectedly nice, one-in-a-million spring day here, recently, with sun and a Goldilocks breeze, so I took the afternoon off to try out the new rudder.

The first thing I noticed was how much lighter it is. I didn't have to wrestle it over the transom. The second thing I noticed was that I had forgotten the small stop that keeps the tiller from sagging. No biggie, a piece of packing foam on the rear deck and we were in business.

The first test usually is the U turn away from the dock (it's a pretty narrow channel and I've had rudder and/or CB stall on me there before - not pretty). It's hard to compare, but I'd say the new one works at least as well as the old one in this situation.

The winds that day were perfect for reaching the length of the lake both ways, so I barely tried to go higher than a close reach. There weren't any strong gusts, so I didn't get a chance to really put a load on the rudder, and I don't think I went above hull speed, but probably close to it several times.

Again, no hard conclusions, but so far, I'd be prepared to claim that the wakes showed a bit less "churn" at the higher speeds compared to what I remember and operating the tiller had just a touch of "smoothness", that I didn't quite remember that way.

The friction in the pivot was easily enough to hold the rudder down, and because I don't need to be able to raise it, I can afford to keep that bolt rather tight.

If I did it again, I'd make the rudder head narrower, by trimming some of the aft portion. As it is, the tiller doesn't swing as far around as before, making for a slightly less compact configuration for storage.

I'll need to go out again, with crew, in some stronger gusts to see what happens when the DS develops more noticeable weather helm. But not today - gale warning. 8)
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Postby GreenLake » Thu Aug 18, 2011 3:43 pm

In the meantime, I've sailed in various conditions and waters of different degrees of hardness :) Here are some things I found out.

If I had planned beforehand on the number of times I went fishing for rocks, I would have changed the design up-front to include stronger glass reinforcement for the leading edge. That would have limited the damage.

After filling the dents, I retrofitted a bit of glass tape over the leading edge. If I had incorporated this from the start, I would have sanded down the leading edge a bit further and perhaps put yet another thin strip of glass there.

(That said, I'm really happy I bothered with replicating the kick-up design, as I don't sail under class rules, there had been the temptation to go with a simple board...)

The one part paint I used is proving too soft. It's easy to apply, but it is a bit sensitive and I've had to touch it up.

The bolt needs to be tightened well, or the rudder will come up in higher boat speeds. Novice help will not readily appreciate how tight...

Finally, I realized I made the neck of the upper part a touch too wide - the tiller doesn't fold back as far as before. If I were to redo this, I would shave about 1" from the back.

Apart from these issues, I'm really happy how well this turned out for a "first attempt".
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Re: Building a rudder

Postby GreenLake » Tue Aug 26, 2014 4:35 pm

This is an update that I should have posted earlier closer to when it happened.

After about a year or two of using the rudder, I noticed that some of the joints had split. This affected both the main joint between the two sheets of plywood that make the rudder blade. Looks like that joint had gotten starved and allowed water to get in. I dried out the rudder, opened the split, re-applied epoxy (SystemThree GelMagic from the cartridge, if memory serves). Similar separation affected one of the cheeks of the rudder head. After re-glutting and re-sealing all exposed wood with epoxy I covered the repair with white spray paint (since I had no more EasyPoxy left). That has worked surprising well.

The separation of the cheek (or cheeks, can't remember whether it affected both) may have started when I hit a rock, or the concrete ramp at our dock, hard enough to bend the gudgeons. It also appears that I may have sanded through the epoxy sealing the inside of the rudder head when fitting the blade without noticing. Some of the cracking was clearly due to swelling. As the boat is stored on the trailer, the rudder always dries out and none of the wood held enough moisture to be soggy. After these repairs, the rudder has held up fine.
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Re: Building a rudder

Postby GreenLake » Fri Apr 21, 2017 2:34 am

This is an update after 6 years of sailing with the "new" rudder.

After I fixed the issues mentioned in the previous update, no serious damage of any kind. I have sailed the boat about 100 times for a total of 2-300 hours in the water. Conditions were mostly light to moderate, but have sailed on occasion in winds up to 25kts.

Inspection shows that the paint (one part Petit EasyPoxy - no epoxy in it despite the name) has worn of on some spots on the leading edge, exposing the glass sheath.

The "cheeks" of the rudder head show faint stress cracks in the paint, extending aft from the bracket for the lower pintle. This is a challenging spot, because the bracket needed to be "sunk" into a groove as it was not wide enough to fully span the width of the rudder head with the cheeks in place. (The paint in that area of the head looks different from the paint I have on the rudder blade itself; if so, it would most likely be SystemThree WR-LPU, which I have seen as rather unforgiving in that respect; it simply does not stretch).

To make the rudder head stronger would involve adding an additional layer of glass cloth (or simply heavier grade cloth) and finding a way to taper down the cheeks so that the cloth layer can continue under the bracket. (The outermost layer handles the bulk of the loads, the plywood just acts like "core".

For an "expedition"-style rudder head I might change the design to use not only an additional outer layer of glass for the "cheeks" but also to not end them in a straight line across the rudder head; instead the idea would be to taper them into a teardrop shape with the "tail" to run up the middle of neck of the rudder head (which could then be cut a tad narrower so the tiller can fold over more completely for storage). The idea would be to give a more gradual transition for the loads. Possibly go up a mm or so on the strength of the plywood for the cheeks, if a source could be found.

I see no indications of stress on the rudder blade itself.
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Re: Building a rudder

Postby GreenLake » Sat May 19, 2018 11:50 pm

I've had something bump into my boat, hitting the rudder against the hull when fully deflected. Most likely at the dock. Details as to when and how are a bit murky, but it could have been the "swim step" on the transom of some motorboat; when I got back from parking the trailer, they were tied down very closely stern to stern at the dock and my newbie crew was fending them off. When coming out the the water one set of pintles had lost its screws on one side, and later inspection found the part bent. Either happened then, or unnoticed during the last sail in the previous season.

Either way, the bracket for the pintles, which should be a nice U shape, had a bit of an "S" in one of the flanges. That definitely looks like shock load, not stress from sailing with it.

In the area between the lower pintle and the bottom of the rudder head there's only about 1" wide glue joint between core plywood and what I call the "cheeks", the two plates between which the rudder head pivots. That joint had cracked with the blow. I decided that the current design had held up well enough to sailing loads, so I just dried everything, re-glued the crack and then changed the 3/4" wood screws that held the pintle to #10 machine screws (through bolted).

My thoughts on how I would improve the design if I rebuilt it from scratch would be to lower the rudder head a fraction of an inch where it forms the stop for forward end of the rudder blade (and to remove an equivalent amount on the blade end). That would increase the area along which head and "cheeks" are glued. Further, I would redo the grooves (channels) that I had to cut for the brackets that hold the pintles. These don't span more than 1", hence the need to have a narrow spot there. Instead of just taking material out of the cheeks, I would make a deeper / wider trench and line it with fiberglass, extended smoothly all the way around to the top of the "cheeks". That way, there would be a continuous load-bearing skin, even if depressed.

I'm not a mechanical engineer, but that approach promises to be ultimately stronger to me.

rudderhead-fix.JPG (20.89 KiB) Viewed 8662 times

The sketch is very schematic, and I might ultimately make the inner part of the new glass (blue) a bit thicker than shown.

For now, based on sailing experience so far, it's not needed for the conditions I typically sail in.
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Re: Building a rudder

Postby tc53 » Mon Feb 04, 2019 6:19 pm

After recovering from shoulder surgery, I am ready to begin building a new rudder head over the next couple of months. I am happy with my current blade, which was not damaged when the head broke last summer.

As near as I can tell, the old head was made with 1/2" ply (presumably marine grade) cheeks on either side of what appears to be a piece of 1" thick solid wood (mahogany?), with no evidence of fiberglass anywhere. After considering building the new one out of aluminum, I've decided, essentially, to replicate the original, though with improvements.

Greenlake, it sounds like you used two sheets of 1/2" ply for the head portion. What are the advantages of ply over solid wood here? And am I reading correctly that you used ¼” ply for the cheek portions? Seeing that your project was completed nearly 8 years ago, have these cheeks stood up well?

It sounds like you glassed the inside faces of the cheeks before gluing to the head portion. Had you put any glass on the head at that point? After gluing the cheeks to the head, did you cover the entire assembly with that 6 oz. glass?

Other questions:

What type of glue did you use to bond the cheeks to the head?
How did you attach the pintles to the new head? (on my old one the plate or strap portions of the pintles went into slots between the cheeks and head, with the bolts going through cheeks, head and pintles).
When installing your nylon bushings, did you glue these in?

I’m sure I will have more questions as the project proceeds, but this is more than enough to get me started.

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