Emergency Rudders

[brianb]

Hi,

What are we all using for emergency rudders ? Here is one approach.

Starting construction on a new ER. Past projects I shaped the foam using templates. This time around I am using inexpensive foam wing sections from model suppliers.

Here are shots of joining of two section to create a 6' blade. NACA 12 shape. Next step vacuum bagging the carbon uni directional finish.
www.brianboschma.com/sailing2007/rudder01.jpg rudder02.jpg rudder03.jpg.

With these pre formed sections this goes very quickly.

Brian

 

[tiger beetle]

Brian - that's a great idea. What kind of foam is it, what are the expected loads, and care to share the source you used for the supply? Shaping the foam is one of the most difficult parts of the project, and sourcing foam that is pre-shaped is very cool.

- rob/beetle

 

[AlanH]

I've gone super-low-tech. The rudder is a 2 x10, doug fir, five feet long, crudely shaped with circular saw and surform plane to something vaguely approximating NACA 0045a.56 sub pagaraph 6b. In other words, thick at the front and thin at the back. I bought shaefer J-24 size hardware and bolted it on.
Yeah, it weighs 7-8-9 pounds but it cost next to nothing, and took me two nights in the garage with no-tech materials. To lose 5 pounds and make a really light and hot rudder would cost a lot more and take a lot of time. The theory is that I'll never use it, right? This is a get-the-boat-home rudder.

 

[brianb]

source of NACA foils in foam

Brian - that's a great idea. What kind of foam is it, what are the expected loads, and care to share the source you used for the supply? Shaping the foam is one of the most difficult parts of the project, and sourcing foam that is pre-shaped is very cool.

- rob/beetle

Hi Rob, source = http://www.dbalsa.com/foam_wings/foamwingmain.htm

There are other sources that charge a lot more as they use CNC milling. Some of the others offer a variety structural foams.

These guys use a hot wire cutter and are the lowest cost I can find. I believe he will do custom cuts in a variety of foam types. The foam I used is low density. The loads on this rudder I have not calculated, however my older rudder, a Newland based design using his structural models, was calculated and this new blade is only 1/2 the chord using the same laminate schedule. Put another way, this blade is overbuilt for the pintal loads and foam strength.

 

[Critter]

After making a mess trying to build a foam and glass blade, I ended up with a 2x8 board. I added formed aluminum sheet to the front and rear to give me the leading and trailing edges (and increase the chord to 14.5"), and thicker aluminum sheet epoxied and screwed to the sides to increase the bending strength. The upper part is built out to a rectangular section, and I made a cassette out of plywood, 2x2s and 2x3s.

Since my transom seems kind of thin, I built another wooden framework to carry the gudgeons. It spans the height of the transom, tying in at the top and bottom. Only instead of gudgeons I'm using some anchor chain stoppers that I bought on Ebay. They mate with thick delrin ... gudgeons? brackets? ... that wrap around the sides of the cassette.

Complicated, low tech, on the heavy side, but then my boat isn't an ultralight anyway. Maybe someday I'll take another shot at building a foam blade to get the weight down.

Alan, I can probably bring this monstrosity to your emergency steering seminar if you want. Right now it's behind about 10 feet of stuff in my garage.

Max

 

[AlanH]

Hi Rob, source = http://www.dbalsa.com/foam_wings/foamwingmain.htm

There are other sources that charge a lot more as they use CNC milling. Some of the others offer a variety structural foams.

These guys use a hot wire cutter and are the lowest cost I can find. I believe he will do custom cuts in a variety of foam types. The foam I used is low density. The loads on this rudder I have not calculated, however my older rudder, a Newland based design using his structural models, was calculated and this new blade is only 1/2 the chord using the same laminate schedule. Put another way, this blade is overbuilt for the pintal loads and foam strength.

After seeing all the nifty foam and carbon rudders at the seminar I decided to move up in the world and try making one myself.

I've just bought Dynamic Balsa's symmetrical WY-5 section, $30 plus $13 shipping
11-7/8" chord
66" long
2-3/8" thick

for $40 I get a real foil shape instead of something I hacked together out of a doug fir board. Not that the doug fir board wouldn't probably work, but now I can be sexy and fiberglassed like Brian, Rob and Greg.

I told the owners about the SSS website and this forum and hopefully they'll post here. The foam is compatible with epoxy. The sections come in two pieces...they are intended for model airplane wings, after all. In fact you can also buy half-lengths, in this case, 33 inches. You could then mate that with a 2.5 inch thick piece of rectangular foam if you wanted your emergency rudder to be "box shaped" at the top and foil shaped at the bottom.

 

[solosailor]

What kind of foam is it, what are the expected loads, and care to share the source you used for the supply?

Well their site says it's 1 pound per cubic foot which sounds quite a bit weaker than the higher density foam usually used in composite emergency rudders.

 

[AlanH]

Well their site says it's 1 pound per cubic foot which sounds quite a bit weaker than the higher density foam usually used in composite emergency rudders.

He offers one or two other kinds of foam, as well....or so he said on the telephone. I don't remember what they were, though.

I was under the impression that the strength of the structure depended on the laminate skin, not the core?

 

[solosailor]

I was under the impression that the strength of the structure depended on the laminate skin, not the core?

The strength depends on the core and the skin otherwise we would always use the lightest core. You would have to increase the laminate schedule a lot to make up for the difference in using a weaker foam core. Remember that lastafoam or divinycell comes in weights like 4lbs to 10+lbs.

I would be very concerned about using that foam that is designed for small RC plane wings in an emergency rudder.

If anyone wants to provide me with a piece of this foam I would happy to do a test vs. a scrap piece of lastafoam/divinycell and carbon I have laying around. I would layup a few layers of carbon on each side and test the panels. I think it would be worthwhile before investing in a good amount of money on carbon, epoxy and time.

 

[AlanH]

The strength depends on the core and the skin otherwise we would always use the lightest core. You would have to increase the laminate schedule a lot to make up for the difference in using a weaker foam core. Remember that lastafoam or divinycell comes in weights like 4lbs to 10+lbs.

I would be very concerned about using that foam that is designed for small RC plane wings in an emergency rudder.

If anyone wants to provide me with a piece of this foam I would happy to do a test vs. a scrap piece of lastafoam/divinycell and carbon I have laying around. I would layup a few layers of carbon on each side and test the panels. I think it would be worthwhile before investing in a good amount of money on carbon, epoxy and time.

When it comes in I'll cut off a 6 inch piece (it's longer than I need. anyway) and get you a slice. We'll chalk it up to research!

 

[brianb]

Foam Cores- weak....

When it comes in I'll cut off a 6 inch piece (it's longer than I need. anyway) and get you a slice. We'll chalk it up to research!

Hi all,

Of course if you use a foam core that doesnt have good shear/compression characteristics you need to make up for it. To say "you shouldn't use a RC plane foam core" is simply not reasonable. One simply has to engineer the composite structure properly. Here are techniques used: beef up the laminate structure, add stringers the length of the structure, use a composite outer core - example a combination of corex with an inner and outer laminate applied, and there are many other examples that can be properly engineered.

Having made other rudders the longest time spent on the project was fairing the foam to shape. With these foam blanks pre cut one can really accelerate the project.

Having said all that, two core suppliers supply cores in a variety of low density and moderate density cores. The later being typical of the foam used in rudder projects.

Thanks,

Brian