keel hung rudder

[Tala]

Hello,

I was wondering whether maybe someone has some experience with keel hung rudders? Pretty much all the information I can find is about modern rudders, and I'm not sure how much applies for traditional type rudders. I am currently designing a 25m long keel schooner and am not sure how to go about the rudder. In Skene's elements of yacht design it says the rudder area should be 8-10% of the underwater profile area, but in my case that's 55m² so the rudder would be at least 4.4m². That seems very big to me and would give a very wide, low aspect ratio rudder with huge steering torque. I'm attaching a drawing, would be great to get your opinions.

Also, is it better to go for a NACA 0010 Section or just fair the rudder to the hull?

Thanks

[masalai]

That is one very BIG boat... Go for a walk around some of the yards to get a feel for things and ask some of those with boats of similar style and "could I please have a look inside"? - most non commercial owners of recreational sailing boats would be delighted to show a fellow boatie around...

[Stumble]

Tala,

Is there a reson that you are using a keel hung rudder? I am not a designer, but as I remember it they have really fallen out of favor since they are not nearly as efficient as an independant rudder. I also remember that they really don't provide as much protection as was once thought, though I can't remember why I am thinking that.

[LyndonJ]

You might want to consider a balanced rudder with the bottom bearing on a projecting lower arm. A boat that big can benfit considerably in manouvering with a balanced rudder. Also the torque is reduced for the steering gear, lighter gear and a lighter shaft.

As drawn you will either have twin props or a large drag producing cutout for a single prop.

Hey Stumble
A lower support or a well designed skeg or partial skeg are all much more robust that a free hanging rudder. Modern design are abysmal for rudder failure, it's almost endemic amongst racers and causes a surprisingly high dropout rate, even two recent sinkings of large expensive boats . A lower support can take a lot of force from the rudder eg from a boat washed back onto a shoaling bottom, free hung rudders commonly bend or break.

[LyndonJ]

This sort of thing is better than the older style you drew.

Although the top of the rudder would be better a bit closer to the hull. The foil is just a NACA 0012 if its metal you can use the rudder itself as the main strength member for bending and torsion and then shaft doesn't go right through. note the slots for the plug welds on the two internal frames.

[Perm Stress]

If rudder is attached to aft end of keel, with minimal slot, the only requirement for profile is smooth blending with the keel forward of it.

As to maneuverability, I witnessed a two International Dragons converted to cruisers, berthing in close quarters one after the other. The one with original keel hung rudder turned with smaller radius as one with separate spade rudder. The keel length of Dragon is however, only ~30% LWL, not ~90% as in sketch shown... and bottom of hull itself is fairly flat both forward and aft of the keel...

For the design shown, rudder as in post #5 could be significantly better.
But it all, as always, depend...