Rudder Angle

[C08]

I am thinking of fitting a bigger engine (outboard) to my catamaran. The new engine is a little wider than the existing engine and this may limit the maximum angle of the rudders. This may not be a problem as the rudders do +/- about 50 degrees so I may have a bit to play with. What is the generally accepted max rudder angle, if there is one?

 

[knuterikt]

On my keelboat the angel is 36 degrees each side, that limit is imposed by autopilot drive unit.

 

[NormanS]

It's usually accepted that 35° is the maximum efficient angle.

 

[Yorkshire Exile]

On my keelboat the angel is 36 degrees each side, that limit is imposed by autopilot drive unit.

Yes, but how many angels can dance on a pin head?

 

[Boathook]

I am thinking of fitting a bigger engine (outboard) to my catamaran. The new engine is a little wider than the existing engine and this may limit the maximum angle of the rudders. This may not be a problem as the rudders do +/- about 50 degrees so I may have a bit to play with. What is the generally accepted max rudder angle, if there is one?

Which outboard and new or second hand? I'm interested as I may be renewing the outboard on my Cat.

 

[TSB240]

It's usually accepted that 35° is the maximum efficient angle.

You may be right about efficiency. However our rudder will in fact turn through 180 degrees and can be completely reversed if required. My party piece is to actually turn our boat in its own length through 360 degrees at slow speed. It takes a lot more than 35 degrees of rudder to achieve this!

 

[Neeves]

I can turn our cat in, roughly, its own length - without using the rudder, but then we have 2 engines. It not a party piece, it normal.

Jonathan

 

[rogerthebodger]

It's usually accepted that 35° is the maximum efficient angle.

Much above 35° the rudder will stall and as such have little effect/"lift" in the same way when the angle of attack of an airplane with gets too high the wing will stall and give no lift.

 

[wombat88]

Just a thought...when altering rudders do not change the angle of the leading edge. Only really possible on boats with lifting rudders.

If you rake back the rudder blade (for example to avoid a prop) you run the risk of changing the boat's balance when under sail.

The simplest example would be on a sailing dinghy where raking the blade back induces more weather helm and bring it forward reduces it.

 

[anoccasionalyachtsman]

Much above 35° the rudder will stall and as such have little effect/"lift" in the same way when the angle of attack of an airplane with gets too high the wing will stall and give no lift.

That's initially true, but as the boat starts to turn the turn radius can be reduced by increasing the rudder angle again, and repeat. This is how tight turns work and theoretically, if you could keep the momentum long enough it should be possible to get the boat travelling backwards and still turning.

 

[anoccasionalyachtsman]

Having watched the engineless 5.5 Metres mooring in Cowes recently I really really want a 90 degree turning rudder. It stops them dead just when you thought you were going to see several £100ks of utter beauty have its front fall off.

 

[thinwater]

That's initially true, but as the boat starts to turn the turn radius can be reduced by increasing the rudder angle again, and repeat. This is how tight turns work and theoretically, if you could keep the momentum long enough it should be possible to get the boat travelling backwards and still turning.

Exactly. Rudders stall at 17-21 degrees, depending on smoothness and design, but once the boat is turning sharply, the angle can increase. I'm quite sure I can beatifically use more than 35 degrees at times. Fin keel and dagger board boats can really spin.

 

[rogerthebodger]

I can spin mine without using engine or rudder. I just use the bow thruster.

 

[thinwater]

So this has become a "mine's bigger" thread?

CO8: Is this one engine or two?

 

[C08]

I have one 20 hp engine at present on a 8M Catalac but would like to replace it with a 30hp as 20hp struggles against strong tides and a headwind. I have a wheelhouse with a lot of windage!

 

[Boathook]

I have one 20 hp engine at present on a 8M Catalac but would like to replace it with a 30hp as 20hp struggles against strong tides and a headwind. I have a wheelhouse with a lot of windage!

Look at the Yamaha 25 high thrust. It's gearbox drives a much larger diameter prop for displacement boats. There seems to be very few engines at 30 Hp with high thrust props. Quite a few local 8m's seem to do very well with high thrust 9.9hp.

 

[knuterikt]

Yes, but how many angels can dance on a pin head?

Sorry, English is not my native language, good for you that you write flawless English....