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Paperback - 192 pages
Frank Mulville

Emergency Rudders

This article is one of two taken from a seminar on Self Steering and Emergency Rudders given by Rob Macfarlane to the Singlehanded Sailing Society in San Francisco. The purpose of the seminar was to inform and prepare entrants for the 2002 TransPac singlehanded race from San Francisco to Hanalei Bay, Kauai, in Hawaii - a distance of 2120 miles across the Pacific.

These articles are reproduced here by kind permission of the author, Rob Macfarlane. For more information, visit the Singlehanded Sailing Society's website.

Emergency Rudder

There are basically two types of emergency rudders:

  • "Get home" rudder that will allow you to control the boat well enough to make port.
  • "Racing" rudder that will allow you to continue sailing the boat hard.

Getting an emergency rudder

The only off-the-shelf emergency rudder I'm aware of is ScanMar's attachment for their Monitor wind vane. I consider this a "get home" rudder. It's small, easy to stow and set up. If you already have a Monitor vane on the back of your boat then you should consider their design a viable system.

The other way to get on is build it yourself (or have someone build it for you). Building it yourself is fairly simple if you have experience in fiberglass composite construction. Hiring it out is the same thing, just more expensive. Arne Jonsson Boat Builders in Alameda have the expertise to build one for you.


Very important - don't purchase or, if you're so inclined, design and fabricate an emergency rudder without then trying it out. Trying it out does NOT mean hanging it on the transom while you're standing on the dock, and it does NOT mean motoring up and down the estuary while steering with it.

At the very least, get out in the bay and try sailing around with it.

A far more accurate test of the system is to sail out to the lightship, drop all sail and install the rudder while bouncing around in the swell and wind, then head out and loop the Farallone islands. By the time you get back to the lightship you'll know a) how to install the thing, b) how to trim the sails to use it, and c) whether nor not it REALLY works.

What you do NOT want to be is Pyewacket - blow off your rudder in the middle of the Atlantic, set up the emergency rudder and then an hour and a half later have the emergency rudder blow off it's bearings and disappear behind the boat. If you're Roy Disney you make for the nearest island and call in your private jet to drop off the delivery crew and a new rudder while you and your mates jet off home. That's a little outside our league and the reality is you're looking at a long, long time to analyze your emergency rudder in excruciating detail as it gets you to your destination.

Nor do you want to do a Robin Davie - get to the middle of the South Atlantic, shed your rudder then discover that you've left some fairly important parts to your Monitor E-mon system at home. While you can then proceed to amaze your friends with how resourceful you are at fabricating parts out of thin air to get it to work, you will also be disgusted with yourself for doing something so silly.

Design Considerations
  1. Blade size (underwater surface area)
  2. Attachment method to the hull - how will you install the rudder out in the ocean?
  3. Stowage - where are you going to keep the blade when you're (hopefully) not using it?
  4. Jettison the primary rudder.
  5. Self steering for the emergency rudder.
Size and rotational axis.

The rudder should pivot on a vertical axis, located at the back of the hull on center line. The underwater surface area in profile should be the same as the existing rudder on your boat, with a minimum surface area of 1/2 existing rudder area. Aspect ration should be roughly 3:1 (height : width). Leading edge is parabolic, trailing edge is thin and flat, not a knife edge.


If you've installed an el toro rudder with the boat in the water you know how difficult it to control the blade while lining up the pintles & gudgeons. Doing the same thing in the open ocean is not just difficult, it is dangerous - so pintles and gudgeons are not a good solution. Simplest installation mechanism is via a cassette box mounted on a vertical hinge axis at the transom. The rudder blade drops down into the cassette box (much like a dinghy centerboard drops down into the centerboard trunk). The tiller is attached to the cassette or the top of the rudder blade, your choice. Hinge can be a length of 1" diameter aluminum dowel.


You have to keep the rudder somewhere on the boat, normally down below stuffed away in the forepeak or the transom. Some people have suspended them up against the overhead in the main cabin (good if you're short). Don't build a 9 foot long blade when you have only a 6 foot space to show it.

Jettison primary rudder.

The primary rudder may not in fact break away completely. The drag and turning force the broken or bent blade causes can significantly hamper the ability of the emergency rudder to steer the boat. Have a way to a) get rid of the primary rudder and b) plug the hole the now gone rudder stock has left in the hull. A sledge hammer and a long wood dowel is all you need to push a rudder blade down out of the boat. A large wooden bung may be sufficient to pound into the hole, or you can carry 18" of wooden dowel the same diameter as rudder stock which you can push back down in place of the stock.

Self Steering.

If the emergency rudder can be hooked up to a self steering mechanism you'll be much happier than if you have to stand in the back of boat and hand steer the entire way to wherever you're going. While not a big deal on a full crew boat, it can be very valuable for a singlehander.

NACA 0015 shape

For a racing blade a good shape is a NACA 0012 or 0015 series. 0015 is thicker than 0012 (good structurally) and more forgiving (more difficult to stall). The shape numbers are provided below. For a get home blade any flat board will do provided it's strong enough. To keep construction simple the design will have no taper - just straight parallel front and trailing edges with a chopped off bottom (exotic elliptical rudders need not apply).

  • Chord length is length of section from front to back.
  • X is a percentage of chord length from the front.
  • Y is a percentage of chord length, measured perpendicular from chord.
X 0 1.25 2.5 5.0 7.5 10 15 20 25
Y 0 2.367 3.268 4.443 5.25 5.853 6.682 7.172 7.427
X 30 40 50 60 70 80 90 95 100
Y 7.502 7.254 6.617 5.704 4.58 3.279 1.81 1.008 .158

The loads generated by a rudder are very high, and typically the first failure of any emergency rudder is the attachment point to the hull when the lower gudgeon or pintle breaks off. The blade itself rarely fails. Refer to Paul Kamen's article to calculate the blade loadings for your boat. Note that loadings are dependent on the speed of the boat squared, so the design speed (a number you think appropriate for your boat) is critical. I think Paul's numbers are too low - I hit 8 knots with my emergency rudder in my 33' boat, and am using 16 knots as design speed for the 45 footer, not Paul's suggested 10. Beetle hits 10 knots too easily. (NB - the above refers to another document, not published here)

The other contentious number for shaped blades is the Coefficient of Lift. A blade on the verge of stalling (such as when you're surfing down a wave and about to broach) will generate more load than the same blade simply dragged sideways through the water down that same wave. The increase in side load is accounted for by the Coefficient of Lift. NACA shapes will typically generate 1.8 times the load, so using 2 in your calcs creates a small safety factor. Using 3 creates a nice large safety factor. Use 3.


The construction approach outlined here is for a do-it-yourself composite blade of fiberglass over foam. Metal structures are usually custom built, and should be stainless steel or 6061-T6 aluminum. The cassette box is solid fiberglass layup. Aluminum is lighter and easier to shape with typical wood working tools than stainless steel. If you haven't worked with foam/glass construction before, make a test piece first (e.g., shape a bit of foam, laminate a full layup schedule onto it) to see how the materials work.

Select the foam

After designing the blade, purchase foam thick enough such that two layers of foam will be slightly thicker than the complete rudder blade (e.g., for a 3.75" thick rudder, get 2" thick foam). The purpose of having two and only two sections is a) you can access the center of the blade for insertion of material, and b) there will only be one seam line in the foam, and that seam line will be on centerline. If you laminate a blade from more than two layers of foam you will have a seam line at you will have to sand; you will discover that hardened spooge is much harder than foam and therefore it will not fair evenly during the sanding process (the hard spots will be proud, the softer foam will be cut away too much), resulting in extra unnecessary fairing at the end.

Insert support sticks in the foam

Lay the two layers of foam on your table and draw the rudder profile on the top piece of foam. Select two hardwood sticks which are longer than the entire blade by 2 feet. Separate the foam halves and cut a groove to accept the stick in one piece of foam using a router or table saw. The sticks should be as widely separated in the foam as possible, with the caveat that you must not cut all the way through the foam when making the groove. The sticks provide a means of hanging the blade when you apply the fiberglass laminate (the sticks do nothing structural, they are just a convenience.)

Laminate the foam together

Laminate the two layers of foam together using a resin/microballoon mash (aka spooge). You want the foam to be flat, so the table should be flat. Place a polyethylene tarp over the table and place the bottom foam blank on the tarp. Paint the grooves with spooge and lay the sticks in. Spooge should flow out of the grooves, leaving no air pockets. Use the foam roller to apply a coat of spooge to each foam blank. Lay the blanks on top of each other. Place weights or use clamps to press the foam together while the spooge is drying. Spooge should squish out of the seams, indicating there are no air bubbles in the laminate. Wait 12 hours before removing the weights.

Remove the upper box section

Saw off the foam above the portion of the blade that will be shaped, and save this piece - it will become the top of your rudder. Mark the end you cut, and the orientation towards the nose of the rudder so you can later determine how it fits back on.

Create the section templates

Create two section templates, one for the top and one for the bottom. As this rudder is the same section throughout, the templates will be identical. Take the NACA foil numbers and plot onto a piece of paper at full scale. Use spray glue to attach the paper to a flat board of masonite or plywood and cut out the shape with a bandsaw or saber saw. Fair the profile by sanding. Mark the templates at equal chord points (e.g., 10%, 20%, 30%...)

Attach the templates to the ends of the foam blank with screws or glue. The templates should be exactly parallel to each other and perpendicular to the foam blank.

Shape the blade

Create a long board sander with a length of straight hardwood (3/4" x 2" works well) that is two feet longer than blade. Wrap the narrow edge of the board with 60 grit sand paper held in place with staples. Lay the longboard across your foam blank so the longboard lines up at the same percentage mark on the top and bottom templates, such as from the 20% chord point at the top template and the same 20% chord point at the bottom. Sand evenly in the vertical direction of the blade (top to bottom) to cut into the foam until the long board just touches the templates at each end of the board. This will cut a grove through the foam that has exactly the correct depth for the section at the particular chord point your are working at. Continue to do all points on both sides of the blank. Be careful, foam cuts very quickly. When you are done you should have grooves cut in the board, with raised areas in between. Use a surform tool to remove some of the extra material, then go back around the templates with the long board, working at areas in between the chord points you did the first time.

Continue this until the entire surface of the blade conforms to the template section. Hand sand the board at the front and trailing edges using 100 grit sandpaper. The front is more important - you want a parabolic nose. If it looks about right, then it probably is.

Reattach the top section

Round the edges of the rectangular chunk of foam you saved from the top of your blank. Glue this piece back onto your now-shaped foam rudder with spooge. Apply fairing at the joint to smooth the transition from the shaped blade to the squared upper box.

Suspend the foam blade over a work table that you can walk completely around (this is where the sticks inserted into the blade come in handy - hang the blade from these). The nose should be up, the trailing edge hanging down.

Drape fiberglass over the foam, aligning the fiber orientation to be 45 degrees off axis (see drawing). Cut the glass to fit, leaving at least two inches of fabric hanging below the trailing edge of the rudder. Continue until you have reached the glass thickness you calculated. Number the fiberglass sheets and mark centerline of each sheet along the centerline of the nose.


Do the entire lamination in one process from start to finish (allow at least two hours, so start early in the evening). Mix a pot of spooge and roll it onto the foam blade. Mix a pot of resin and lay out the DBM on the table. Wet out the DBM on both sides and drape it over the blank.

HINT: wet out one side of the DBM with a foam roller, turn the cloth over and wet out the other side, then roll the cloth up. Lay the rolled up cloth on top of the blade with the marked centerline on the center of the nose, then unroll the cloth along the top of the blade. This keeps cloth distortion to a minimum.

Continue until all the cloth is applied. Work quickly - the resin will typically begin to set in 30 minutes or so, depending on how much you catalyzed it. It is a good idea to mix small pots of resin as you move through the job rather than one large pot for the whole layup.

Use the hard roller to press/roll the cloth down so it evenly contacts the foam blank. The roller squeezes out air bubbles, excess resin, and makes the cloth lie flat and even. Allow the resin to dry overnight (24 hours is better).

Final shaping. Sand off any rough bumps, drips, etc. from the laminate. Shape the trailing edge with a sander. Fair in shallow spots with low-density spooge. Paint the blade if you want.

Construct the Cassette, pivot point, tiller stock.

When the blade is complete, you need to create the cassette, which incorporates the forward bearing.

Wrap the box-shaped top of the rudder in two layers of wax paper - this is the mold release and leaves a thin gap which allows the blade to slide into the cassette. Using the same procedures as for the blade, cut and mark the cloth to wrap around the box, then laminate the cloth around the box.

For the hinge point, add a piece of hardwood to the leading edge of the box so the bearing point will be 1-2 inches forward of the rudder. Take the metal bearing/support, wrap it in two layers of wax paper, and laminate it to the front of the cassette.

Wait a day, then remove the rudder and hinge from cassette.


You may need to wrap a piece of cloth around the trailing edge of the rudder if the rudder skins did not get laminated together.

The blade will float, so you'll need to incorporate a pin running horizontally through the cassette (and blade) to keep the blade from rising when immersed in water. Harken FastPins work well.

  • foam Divinycel is best (closed cell). Size the thickness of the blade such that two layers of divinycel are slightly thicker than the finished blade.
  • resin Modified Vinyl Ester (MVE) works very well. Stronger than polyester resins, much cheaper than epoxy.
  • fiberglass Knytex 1708 DBM (double/biaxial with matt) is very good. The cloth is two layers of glass strands oriented at 90 degrees to each other, with a fine layer of matt sewn on one side. There is no weave to the cloth, so no wasted crimp space. Cuts nicely and doesn't fall apart. Four layers of 1708 will yield 3/16" - 1/4" of final layup thickness.
  • hardwood 3/4" x 3/4" oak make good support sticks. 3/4" x 2" makes a good sander
  • template masonite or 1/4" plywood
  • respirator get a good one, fiberglass dust is nasty stuff.
  • hard roller 3" wide is fine
  • foam roller disposable 3" rollers of fine foam
  • roller tray use a tray with disposable plastic inserts
  • gloves buy a boxful of surgical gloves
  • mixing pots plastic or paper pots are ok, but NO waxy pots
  • acetone get a gallon. good for cleaning tools, not good for cleaning hands.
  • brushes cheap 1" bristle brushes, you go through them quickly
  • fairing West System, get one large container of medium density
  • resin MVE resin, get two or three gallons with plenty of MEKP catalyst
  • wax paper get some from the grocery store or the kitchen
  • tape 2" wide blue masking tape
  • plastic tarps polyethylene tarps keep the garage floor clean
  • marking pens Sharpie Fine Point, black
  • scissors inexpensive kitchen scissors are ok, the fiberglass will ruin them
  • sandpaper 60 grit sheet paper for the long board, 100 grit for hand sanding, 180 for final sanding