What is the Best Rudder Configuration – Spade, or Skeg Mounted?

Iron LAdy Reef Meeting 100 31

Years ago the debate about the most effective rudder for steering was settled, and the cantilevered spade configuration was the winner. But what about hitting debris, running aground, and catching nets you might be thinking.

First: structure. We can tell you from our own experience, and the feedback from owners who have tested our theory, that a properly engineered spade rudder is stronger and more damage tolerant than skeg or keel hung fins.

There are three keys to this strength. The first is a substantial strong rudder shaft. Our norm is to use twice ABS and/or Lloyds Special Service rule as the baseline stiffness specification. Next comes sufficient hull strength and gusseting of rudder bearing carrier(s) where they connect to the hull for long life and abuse. The last item is a frangible rudder tip, so that it goes away before the rudder shaft itself is damaged, leaving the upper two thirds of the blade with which to continue on.

The photo above is the rudder on the FPB 64 Iron Lady, which spent several hours pounding on a reef. It will give you a feel for how tough these blades are.

Next comes catching things on the rudder. Here we have a swept leading edge to shed, and almost total protection from the full depth prop skeg.

The alternative is a skeg hung rudder, or a spade with a beam from the trailing edge of the keel to the bottom of the rudder. The problem with both of these approaches is damage tolerance. If either is bent from pounding, then the rudder is jammed and you have no steering.

We’ve cruised well over 250,000 miles now with spade rudders, and done our share of running aground, and we are happy to report we’ve yet to have a problem.

Posted by Steve Dashew  (March 30, 2012)

3 Responses to “What is the Best Rudder Configuration – Spade, or Skeg Mounted?”

  1. Steve Says:

    First, thanks for the remarkable effort you pug into these posts. It provides a lot of food for thought.

    I know you are pretty far down the road on design on the Wicked but I was wondering if you had ever considered a hybrid drive system with diesel genets and electric final drive? It might be interesting to look at if you ever have a free moment. I once had the opportunity to sit in on a design review for a medium size ice breaker and the we had settled on such a system. It creates some interesting options as you can concentrate the mass of the generators wherever you want – if you want to get really radical you can mount the electrics drive in azipods (probably too complicated for your purposes)

    It is an interesting thought exercise though since it allows one to challenge one of the main design features of a vessel and move the mechanical spaces wherever you want them – within limits.

  2. Steve Says:

    I should also not that we saw considerable gains in efficiency (predicted) due to the tractor configuration of the prop and the increased thermal efficiency of the gensets designed for constant speed operations.

  3. Steve Dashew Says:

    Diesel electric are not yet reliable enough in this power range for our type of service.