Are Rudders Really Needed?

On a power catamaran with twin props on each hull, where the props can spin forward or backward, is there any need for rudders?

 

Surely it is not essential but if you have run out of motor (one or both engines), you may find it useful to have the rudder.

 

It's kind of hard to maintain headway if your motors are alternately running forward and reverse to maintain your course. You don't have a power cat then, but some sort of ungainly crab scuttling about on the water.

 

"is there any need for rudders?" Yes.

But why do you ask and on how big a cat are we talking?

I imagine you could modify a joystick program, like those on pod drives for docking, to act as a "rudder" program to continually change RPM's to keep/change course, however this is more complex IMHO than rudders. There are also steering systems for jet drive catamarams so they can steer with joysticks and no rudders.

Many monohull powerboats, especially sportfishermen with "high speed rudders" have to use throttles in slow speed operations because the dinky little rudders are just not effective at slow speeds. This becomes "second nature" but is attention intensive and would get old fast without rudders.

Steve

 

Well you can always go with twin Voith Schneider props

 

If we are talking about a cruising situation (not harbour manoeuvering), then the lowest fuel consumption comes with equal load on both engines. If they have to operate under different load for course-keeping reasons, the consumption of the high-load side increases more than is saved by the low-load side.

This can be recalculated to be compared with the rudder drag when rudders are producing a comparable steering side thrust.

 

I'm primarily interested in a 40ft cruising catamaran, although I'm building two smaller ones.

Without rudders and just control of the prop speed/direction, the boat is much simpler, faster to build, and less expensive. No steering wheel, cables, hydraulics, etc (and you have infinite 'helm stations' with a wireless remote control)
The props are electric, they can turn at any speed, fwd or rev, including variable resistance for regeneration.

I'm trying to think how a rudder would be useful here.
Only thing I come up with is being towed if power is lost.

 

As Baeckmo has explained, the minimum power for a given speed is obtained by engines running in the constant-rpm mode. Any change in rpm due to course-corrections imply a loss of energy to the acceleration of moving parts of the propulsion system and to the propeller drag. That energy doesn't come entirely back due to friction, drag and hysteresis of various mechanical parts. That's the first issue.

The other is - if, for any reason, one of the motors dies or one of prop blades gets damaged, you won't be able to steer your boat anymore.

So, perhaps a hybrid solution might be useful in your case. Course-keeping done by a pair of modulating electric motors and two kick-up rudders raised up and ready to get in, in case the motors alone can't cope with the sea or if one of them fails.

Cheers

 

The problem is that, in a boat of medium size, the force that makes the rudder blade is so large that can not be managed without the help of a hydraulic element. So rudders "remove and replace" are not, in general, a solution.

 

of course they are not needed if you do not intend to steer the boat. you can float around all day without a rudder on your party boat, and than drop the outboard and point it back to shore when done for the day. many water craft have no rudders: boats like canoes and kayaks do not need rudders because the paddle can function as one when needed. Floats and rafts do not have them either.

but if you want to maintain control of the direction whenever you are making headway, you will need a way to steer it, and rudders are the most effective way to do that.

 

What about keeping the motors running at optimal speed and having in each hull a rudder fixed at a 15° or so turning angle but fully within in the hull. Steering would rotate one of the rudders out of the bottom of the hull exposing more blade and exerting more or less turning force depending on the how far the rudder is "pushed" down. Simple, but probably only adequate turning at speed.

 

I like your line of thinking, modulating propulsion is much more effective than any kind of rudder, especially when the props are far apart.
But there are two legal issues: one is the law in your country that may require an emergency steering device, the other is Murphy's law (what can go wrong will do so at the worst possible moment).

If you consider electric propulsion with a wireless remote to be simple and inexpensive, constructing independent electrically operated rudders should be a piece of cake.

In case of an electrical failure in one drive, having rudders makes the difference between running around in circles until the batteries die and limping home.

 

I have a toy electric remote controlled RC boat that has two props and no rudders, so i'd say it is 'child's play'.

 

Failure recovery is a major concern for a cruising boat.
One can steer a catamaran without power or rudders in emergency by dragging a drogue by a rope. Unless you have a rudder that is broken which forces the boat to turn, like the Alpha 42 that recently had to be abandoned.
So I'd rather not have rudder(s) that can fail and cause a worse problem.

 

You have simply reversed the order of priorities there. Usually, it is rudders that are used to steer the boat in normal conditions, engine steering being used to assist in tight turns and in case of rudder failure.
Here you assume that rudders are more likely to fail, which is imo an adventurous assumption, and hence want to eliminate them completely. I am not sure it is a wise way to go, but it is your boat (as long as you don't put other boats and persons in danger).