Building a rudder

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Building a rudder

Postby GreenLake » Fri Feb 11, 2011 2:41 pm

I have the original stock rudder, with its rather interesting idea of foil shape (flat with a ramp at either edge). It's accumulated its share of dings over the decades, but worse, it's developed a crack and is weeping rust. That alone wouldn't have pushed me over the edge, but somebody challenged me to try building a new rudder.

There are other downsides to the stock rudder that make a replacement worthwhile. First, if you look at it carefully, you will see that right where the water hits it below the keel are several square inches of a flat (!) surface acting like a brake (the tapered edge starts a few inches lower). Second, the cheeks between which the rudder blade pivots also extend lower than the keel, and are therefore dragged through the water. Third, it's quite heavy and unnecessary weight off the transom isn't a good thing.

There's an old article floating aroundthat gives dimensions and a design for a rudder using an aluminum rudder head that fixes these issues. From my vantage point, the downside of this design is the use of aluminum. I don't like the looks of it particularly and I don't have tools or experience. Further, it's designed for a fixed tiller (mine tilts up) and for a different placement of gudgeons.

So I decided to adapt it to my purposes and went back to a rudder head more like the one on the stock rudder.

As material I'm using 2 sheets of .5" plywood, glued back to back. For the foil, I used the layers of the plywood (there are about 8 of them in each sheet) as depth gauges. In other words, I marked the expected pattern of dark and light bands that I would get from sanding the plywood into the foil shape.

That part is done, and has worked out reasonably well. A power plane and a good belt sander with 40 grit worked well to get the shape roughed out and keeping the pattern symmetric and straight gave me a good approximation of the final foil shape.

Lots of fine sanding and some applications of epoxy fairing compound later, and I was ready to sheathe the blade in a lightweight glass fabric (3.7 oz.).

That's how far the project has progressed for now. The rudder head is still connected to the the blade. That made working on the latter much easier, since I have an area to clamp on to when working on the blade. It also makes sure that the two are the same width, so after I cut them apart, I can glue on the "cheeks" and the blade will fit between.

Read to the end of this thread for some updates based on multi-season experience with this design.
Last edited by GreenLake on Sat Feb 19, 2011 7:07 pm, edited 3 times in total.
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Building a rudder

Postby UCanoe_2 » Sat Feb 12, 2011 12:39 pm

Pictures, please!
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Postby GreenLake » Sat Feb 12, 2011 7:16 pm

Today, anyone not documenting everything in pictures is almost considered criminally negligent :oops: . Despite that, I didn't take the camera into the workshop, so there are no photos of the early stages. However, I'm happy to share a drawing.

936

The drawing shows a side view and an edge on view (from behind). In the top part, the leading edge is up, and the top of the rudder head is at the left. The drawing is not to scale, in particular some of the horizontal dimensions have been "squished" to allow the drawing to fit.

You can see the rudder blade, the rudder head (both cut from the same piece of laminated plywood) as well as the two "cheek" pieces, which are a sandwich of glass, 1/4" fiberglass and glass. The dimensions of these pieces are shown as a dotted outline. I'm planning to fillet and sand their upper edge so the transition to the rudder head is smooth.

I chose 23" as length for my rudder (as measured from the pivot) and 12" in width. Those dimensions should be class legal. (The tip radius is 6"). The pivot is required to be 1" above the water line, that is achieved by mounting the pintles accordingly (not shown).

On the blade, only the section to the left of the solid line is shaped into a profile. the half circle and the two projections are flat - together with the cheeks they form a hinge. The "spur" like projection above the trailing edge rotates against a stop when the rudder is 90 flipped up (broken arrow). The small rounded corner above the leading edge should help it clear the transom.

The rudder head is designed for a forked tiller that can tilt up. Because of that, it should extend above the pivot hole for the tiller, for better load transfer if the tiller is used in a partially raised position. When I tested this with the existing tiller, I had a bit of a surprise: the plywood laminate is nearly 1/16 " narrower than the old rudder head. The latter is a bit over 1" and the plywood only comes to .95". I will need to build up that area a bit because I don't want to build a new tiller...

Currently, the project has reached the point where I've separated the head from the blade (which was previously shaped and glassed). I've drilled the pivot holes oversize and lined them with nylon bushings. I've cut out two "cheek pieces" and glassed their insides. The next step will be to glue everything together drill the remaining holes and verify that the pivot works, and that the stops are in the right place. Then I'll fair the top edge of the cheek pieces and glass them over with 6oz cloth for additional strength.

This is the one part of the design where I'm not 100% sure whether it will stand up to the expected stresses, so the plan is to bend and twist things a bit and observe whether the rudder is stiff enough. If needed, I'll add another (partial) layer of 1/4" plywood.

Note: this was not done. An additional 1/4" sounds overkill, see also post from April 21, 2017 below.
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Postby GreenLake » Wed Feb 16, 2011 12:55 am

This is the one part of the design where I'm not 100% sure whether it will stand up to the expected stresses, so the plan is to bend and twist things a bit and observe whether the rudder is stiff enough. If needed, I'll add another (partial) layer of 1/4" plywood.


I've now glued the cheek pieces onto the head and added one layer of mat and one layer of 6oz cloth. (Actually, it could even be a tad heavier, I no longer have the package, anyway, not the light weight stuff but something considerably beefier).

For the pivot, I'm using a 3/8" carriage bolt. I cut a square into one of the cheeks for the head, not adding any reinforcement. On the opposing side, I glassed in another nylon washer, just because.

Then I put the two pieces together, and found that I didn't leave enough of a gap between head and blade, so out came the belt sander to remove sticking points.

I then suspended head and blade at their ends and leaned heavily on the middle section. That gave a load of 150lbs. If my thinking is correct, that would correspond to 300lbs of lift on the rudder blade (since the lift acts over the entire length, not the end point). With the tiller giving a 10:1 lever, 300lbs of lift would require 30lbs of helm. The rudder holds the 150lbs fine, bending a bit, but I don't think it would hold substantially larger forces.

So, is the test good enough, or do I need to beef things up a bit?
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Postby GreenLake » Thu Feb 17, 2011 1:07 am

I now believe that I tested this too soon, so the epoxy hadn't cured far enough. It takes 7 days to reach full strength, and I tested after approximately 18-24h. I just tried it again 24h later, and now I can see less bending even though I applied closer to 200 lbs this time.

It still has a slight bit more give than the stock rudder, but the difference seems less. (There's a slight bending at high loads, but no wiggle in the joint).

I found a discussion of maximal rudder loads, and if I plug in 8kts and a rudder area of 2sqft, I get a value between 300 and 400lbs.

That should match a tip-only load of 200 lbs, so, if I believe all these formulas and calculations, the new rudder should be strong enough.
Last edited by GreenLake on Thu Feb 17, 2011 4:20 pm, edited 1 time in total.
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Postby ctenidae » Thu Feb 17, 2011 10:12 am

GreenLake wrote:That should match a tip-only load of 200 lbs, so, if I believe all these formulas and calculations, the new rudder should be strong enough.


Only one way to find out for sure though, right?

I'm reminded of James MAy on Top Gear:
He asked, "How long with the tires on a Bugatti Veyron last at 253 mph?"
The engineers said, "Well, we know how long they last at 240 mph."
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Postby Baysailer » Thu Feb 17, 2011 11:53 am

Greenlakes,

I've been following this a bit and have some input. I've made 3 centerboards for my buccaneer over the years the first one used plywood like you're describing. The only fiberflass used was tape over the edges to keep delamination down. I used 3/4" and it held up quite well, especially considering the little time and money I put into making it. Second one I put time into using cedar and covered all of it in glass. It warped pretty bad since I used 1"x12" cedar planks. The third one I used white oak 1"x1" and epoxied them together. Formed it but never covered it and it's still working well.

Second I think you're way overestimating the forces on a rudder. The same buccaneer has the original thin mahogany rudder and head assembly and it's never shown any issues relating to stress and I'm sure it would snap under 200-300# loads.

Good luck and let us know how it works out.

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Postby GreenLake » Thu Feb 17, 2011 4:46 pm

Fred,

thanks for your "real world" input on rudder loads. I'm beginning to agree. The maximum load (other than ground contact) is when you go forward at top speed and suddenly put the tiller down 90°. At that point you have the drag force of a flat plate. That's what the estimate is about that I linked from my previous post. Using maximal values for speed and a value for the rudder area that's rounded up slightly, the numbers would approach 300lbs.

But it seems rather unlikely that I will ever experience that scenario and the forces drop with the square of the velocity, as well as with tiller angle.

Having said that, the old rudder has its lower pintle bracket bent out of shape (sideways). That's something that could only have happened in the water, and it's a reminder that it's possible to strain the rudder rather severely. I have no idea how and when this happened, but I assume it was due to some cause like ground contact rather than sailing.

The stock rudder held without any indication other than the bent bracket. I'm pretty confident the new one will as well, even if its maximum strength should happen to be a tad lower...

Status: I've faired over the head, so the thickness grows more smoothly between the top of the head and the cheeks. Once it's painted, it will look much like the stock rudder head, and only someone who knows what to look for will spot the difference.

I've printed the foil section onto a piece I cut from a cereal box and cut it out to use as a template to check the foil shape on the rudder blade. Looks like I got very close with my method of using the plywood plies as depth gauge.

Cutting the foil with an exacto knife along a printed line isn't smooth enough. My plan: soak the edge of the cardboard in a bit of epoxy, let it harden and sand it smooth to get a strong and accurate template.

Depending on the results, one more pass of fairing on the blade.
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Postby jdoorly » Thu Feb 17, 2011 5:32 pm

Hi Greenlake, I'm following your rudder build closely and expect to do the same next winter (still too much to do this winter). Is the foil a standard NACA or can you describe the chord, etc.

Thanks.
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Postby GreenLake » Thu Feb 17, 2011 6:55 pm

The foil shape should be a standard NACA. From a source I have now forgotten I have a formula for the NACA00 series that I put in a spreadsheet and used the graph function of the spreadsheet to lay out a curve.

I made one graph with exaggerated elevation and horizontal lines corresponding to the location of the plies in the plywood. That allowed me to read off the horizontal axis the edges of the stripes I would see when sanding.

This method worked exceedingly well, except that the tail is a tad too flat, because it's difficult to preserve a nearly flat curve.

At some point, it became clear that the alternating plies were being sanded at different rates, so I stopped and applied fairing compound (SystemThree's QuickFair in my case) to get the sanding to be more uniform.

I ended up using an 8.25% foil, because I wanted a 12" wide rudder made from 1" stock. The closest official section would be the NACA0009. It would have been easy to laminate a bit of veneer to the middle portion of each face to be able to go to a 9% foil so I could have used published tables, but I didn't feel like going to that level of trouble.

I also admit I "eyeballed" the curves for the tip - something that's easier with the "stripes" from the plywood. You can see how these stripes curve and even if I missed the ideal curve, I could ensure that the two sides remained symmetrical.
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Postby jdoorly » Fri Feb 18, 2011 4:17 pm

I made a rudder for one of the kayaks I built a few years ago. I glued 2 pieces of aspen (from HD) together, for width and to gain a lamination joint for anti-warp, and then cut off, by eye, all the parts that didn't look like rudder. I did cover it in 4oz "S" cloth and pox, and purposely bought warped pieces and glued the concaves together. After 7 years it's still perfect, but I believe laminating many strips on centerline and flipping the grain on every other is best.

I tried to plug your rudder specs into the "keel and rudder wizard' in my CAD program but it only goes down to 0010 for the 00 series. The program is a free version of a very capable 'ship' CAD but does any size boat and has some cool automatic features. If your interested you can check it out at

http://www.delftship.net

you'll need to login to access downloads.
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Postby GreenLake » Fri Feb 18, 2011 4:26 pm

Jay,

your post is empty!
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Postby GreenLake » Fri Feb 18, 2011 4:27 pm

I've posted a picture of the plot I used to shape the foil.
937
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Postby K.C. Walker » Sat Feb 19, 2011 1:35 pm

Well, I'm a little late to this party. I made a much longer rudder blade of 1 inch thick mahogany for my old boat, which was 19 feet. The original one was also mahogany and snapped off after about 20 years, right at the rudder head. The original rudder had no fiberglass and I think it weakened from compression on the grain from the side load. The aluminum rudder head on that setup was a little bit sharp on the edge and the original rudder didn't fit as tightly as it should have. I sailed the 2nd one pretty hard for about 5 more years and saw no problems, but I did put a fiberglass tape right at the exit point of the rudder head. I think you're going to be fine for load.

I've been thinking about making a rudder, also. After having faired the bottom and centerboard, it's what I have left on my list of things to do regarding under the water. Of course, now that I've talked to Mike Gillum, and he wrote that article in The Quarterly, I might have to polish the bottom of my boat, as I only took it to 600 grit!

I suppose you've seen the articles in Duckworks?

http://www.duckworksmagazine.com/09/how ... /index.htm

http://www.duckworksmagazine.com/07/how ... /index.htm

http://www.duckworksmagazine.com/06/how ... rboard.pdf

http://www.duckworksmagazine.com/06/how ... /index.htm
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Postby GreenLake » Sat Feb 19, 2011 7:21 pm

K.C.

I wrote a long reply but the editor ate it before I could post it. :( Anyway, thanks for the links. I saw some of them, but after I had started my project.

I went back to my first post here, and added a link to the original Day Sailer Quarterly article that got me started to think about this many years ago. In the meantime I collected a number of references, but in some cases I no longer know where I got them from.

If you are using plywood, especially the kind with many, very thin plies, then I would definitely recommend my method of using the plies as contour depth lines. Whether as the main method or whether limited to checking the result as you get closer. The rough sanding was perhaps 20mins, while the fine sanding and fairing isn't finished and has taken several hours.

If I had to do it over again I would have switched sooner to a longer, stiffer sanding board - that would have avoided some problems with hills and valleys. By sanding only in the lengthwise direction, it's easier to not "flatten" the tail end of the profile - with an orbital sander its really easy to get that part to come out "hollow" or flat.

Whatever the method, plywood sands unevenly - the plies seem to have different degrees of hardness. When I discovered that this lead to a subtle "wave" across the surface I gave up trying to achieve a "natural" wooden look and started using an epoxy-based fairing compound.

Two areas where I relied entirely on the contour lines are the tip - I didn't have a good understanding of its theoretical best form, so I just interpolated by eye, making the lines come in as a curve. The other is where the blade flattens into the half disk at the top. Again, the contour lines allowed me to eyeball this and keep the results symmetric.
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