The Offshore Voyaging Reference Site

Twin Rudders Have No Place In The High Latitudes or Maybe Cruising

We just got an interesting question from Françoise and Jean-Michel who are looking at buying a new boat.

Question

Boreal’s daggerboards can be broken as you can see on the n° 42 ep ” Bushpoint ” on You tube. It was last year on a Boreal 55 in the north of Norway. Even if it was not a great issue , it can happen. The owner fix it himself.

My wife and I are looking to build a new boat to change our Ovni 43 and we ask Patrick Lenormand, the yard manager of NYS ( Caen France) to build it .

We have chosen a Cordova 45 ( design of Jean-François André , well known french architect ) . His choice of a twin rudder system concern me .

He said that he put two strong skegs welded on the hull to prevent any damage. My worry is also about the drag of these skegs in front of the rudders.
I know that a boat is always a trade-off but I don’t want to make a mistake.

Answer

You are right to worry about the twin rudders and that’s a great question because it exposes something we are often told: that we can fix a basic design problem by just making it stronger.

I used to believe this, too, until the engineers that comment here helped me understand it better.

Sure the boards on the Boréals can break. Boréal themselves talk about this.

But that’s a a good thing, not a bad thing. It’s a feature, not a bug.

A key thing to understand about boat engineering is that saying we will just make it stronger, is actually bad engineering.

There is no way to build a skeg strong enough to withstand even a six-knot impact from ice, a rock, or a big log (big problem in many places). The key point is that something must flex and bend to absorb force.

So if we make the skeg super strong and it does not flex, then maybe it rips right out of the hull and the boat sinks. The force must go somewhere.

So based on that, here are the key points that I think about:

  • When a Boréal board gets damaged, it breaks and the hull is fine. With skeg-hung twin rudders, a bad collision with a rock or ice can hole the boat and the stronger we make the skeg the worse that problem gets.
  • A Boréal can operate fine with no boards (I have sailed one that way). You could even sail all the way home with no boards. Bend a twin rudder and skeg and the steering is jammed until we can haul the boat and then we need skilled metal workers to fix it.
  • When in tricky waters (rocks or ice) on a Boréal, we can pull up the boards and the rudder is protected by the huge keel box. We can never pull up twin rudders, they are always down there ready to get damaged.

For me, based on some 30 years of cruising, particularly in the north, I would never do so in a twin-rudder boat. Heck, I would not own a twin-rudder boat for cruising, but that’s me.

And by the way, the reason Boréals will never have twin rudders is because the chief designer, Jean-Francois Delvoye, went to Chile in a twin-rudder boat and broke a rudder on a rock.

After that he sold the twin-rudder boat, and designed and built a boat with a single rudder and keel box for himself. That was the first Boréal.

A good example that will help to explain this point about the force being absorbed by bending is the A40 keel.

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