Emergency Rudder

April 16, 2014

The SHTP Race Committee requires an emergency rudder.  This is what I came up with.  I still need to fabricate a tiller head and make a tiller that I can use with my autopilot.

This weighs 30 pounds, but it will not fail, and it's really easy to mount.

This weighs 30 pounds, but it will not fail, and it’s really easy to mount.

A long, long time ago I used to sail dinghies that were kept on moorings.  We’d grab the sails and rudders from the boathouse, row out to the mooring, and rig the boats in the water.  In any wind at all, getting the rudder pintles mated to the transom gudgeons was a challenge.  Same with my Lightning.  I didn’t want to deal with this in a seaway during the SHTP, so I had a support welded to an old outboard mounting bracket.

The rudder is permanently mounted to all of this, so all I have to do is hook the jaws of the outboard mount over the edge of the transom bracket and clamp it down.  That’s a whole lot easier than trying to fight the torque on the rudder.  Also, the transom bracket has about a foot of vertical travel, which means that much less rudder area in the water while I’m securing the whole contraption.

Speaking of torque, anyone with a tiller can feel how much of it there is on the rudder and its fittings when you back a boat up.  There’s a lot.  I got curious, so I tried to work out how much side force would act on a rudder if it were locked down sideways and dragged through the water at six knots.  I’m terrible at math, and it appears there are a few different formulas, but the results I got were shocking.  The lowest number I got was 228 pounds of force.  The highest was nearly 600 pounds!  I went with the higher number, and beefed up my transom and outboard bracket with large, half-inch backing plates inside and out.  The rudder hardware is stout.  That half-inch aluminum plate on the rudder mount spreads the top and bottom rudder fittings pretty far apart, which should reduce torque loads on the fittings.

Through-bolted with 5/16 bolts and 1/2 inch backing plates inside and out.

Through-bolted with 5/16 bolts and 1/2 inch backing plates inside and out.

I’m no engineer, and this whole setup could probably have been made lighter, but I feel good about the design.  It works well at the dock.  I’ll test in on the water in some weather as soon as I get a tiller made.  I use the outboard to get out of my slip, so that will mean getting into the bay, then taking the motor off and stowing it, and then mounting the emergency rudder.  Seems like a PITA, but I need to know how quickly I can do it if the need arises.  For me, it’s one less thing to have in the back of my mind freaking me out.  I’ve done untested, and it has never worked out for me.

Well, that’s my emergency rudder design.  Any input is certainly appreciated.  If anyone knows the real, super-authoritative formula for calculating rudder loads, I’d really be curious to know how my numbers compare.  Let me know.

BackBeat out . . .

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: