Steel blade rudder

[Kez]

I prefer a rudder made out of a SS blade and silver brazed to a 3/16 shaft to a wedge type rudder. Most wedge rudder have a very thick trailing edge (up to 1/4"). I have found that a blade rudder with a separate prop blast pick up can add 3 to 4 mph over a thick wedge type rudder. I know many will disagree but that is just my experience

I could not find my old post anymore but I remember someone commented that if the shaft is close to the center of the rudder blade, handling will suffer. I am making a rudder for a new project. Is there a rule of thumb where the location of the pivot point of the rudder needs to be?

Thanks, k

 

[Buckshot]

I have experimented a little with leading edge placement of the rudder blade vs the pivot pin. I like the way a blade works or “feels” with the L.E. Slightly in front of the pivot center, makes it more sensitive. Further behind makes it less sensitive or “numb”.
Just what I’ve found.

 

[John Beardslee]

I found years ago that if you put much lead on the blade you’ll get rudder flutter. It happened at different speeds for different riggers. We went back to zero lead ie: leading edge of rudder center of pivot pin. With strong servos that have incredible speed and programmable radio’s available today I think you can dial in just about any feel or sensitivity you’d want.
Thanks John

 

[Buckshot]

Doesn’t take much.

 

[LohringMiller]

During our SAW testing we found that substituting a hammer knife thin wedge for a standard aluminum wedge was worth 6 mph around 100 mph. The heat treated steel gave the thin blade plenty of strength. We ran a water pickup that was only submerged at low speeds. That was plenty for SAW.

Loheing Miller

 

[Kez]

Thanks everyone for sharing your knowledge and experience.

Buckshot, a rudder blade with a welded shaft will automatically put the leading edge in front of the pivot point. If I silver braze the shaft to the leading edge of the blade, the joint won't be as strong.

John, how far forward can I place the LE of the rudder before I will run into fluttering problem? I would believe it is a percentage of the blade width? The blade is going into a 20 sport hydro so and it will not reach rigger speed.

Lohring, I have found a thin blade with 3/16" of the leading edge sharpened to be faster than a thick wedge rudder. I never reach the kind of speeds you are doing. But in the high 40s, I have found the thin blade will add 3 to 4 mph. I have also found that a prop blast pick up (with the pickup above and to the right of the prop) provides enough cooling.

k

 

[Daniel's Racing]

Reduced wedge might be doable but a thin blade doesn't work so well for most applications.

Never had any luck with the knife blades.
If you remove the wedge there is so much dead band around center the boats won't even track straight.
Without the wedge keeping both sides of the blade working and in full contact with the water the boat has nothing to keep it going straight.

Worthless adventure for nothing. With a flat blade it does nothing until it has several degrees of angle and it starts to work. So you go off in one direction then correct and go off in another direction with no manageable drivability at all.

 

[LohringMiller]

The knife blade we used is a narrow wedge. We ran it on a wide variety of gas boats. A gas cat ran one in our club's heat races last season with a prop blast pickup. Rust is the main down side.

Lohring Miller

 

[dwilfong]

I will pass on some advice I was given.
It is the angle of attack that determines how much drag a rudder has.
It dose not matter how long it is from leading edge to trailing edge it is the angle it shows to the on coming water.
A blade needs to be so wide at the back to have the hole in it. So the longer you make it from front edge to back edge will make the angle shallower.
A strait angle from front to back and sharp edges on the back so the water comes off the blade strait. It should be a wedge with strait sides.
Just some food for thought.

 

[Jim Allen]

A "true wedge" shaped rudder can definitely produce a speed increase. A "true wedge" will also provide a "lifting component" that is directly proportional to the wedge's running area. The wedges thickness & its width, plus its length & speed through the water will all affect how much lift can be generated. Rudder pivot points placed behind the rudders leading edge make the rudder's blade more sensitive & vice versa. I prefer to place the pivot point in line with the leading edge. It is also necessary to have the rudders leading edge perpendicular to the water line at running speed. A toed in rudder blade will pull the tail in when the blade is turned & vice versa.

One distinct advantage of a steel AISI-01, machined rudder blade, is that the complete blade can be hardened & tempered to a full spring temper (blue color). The photos show a typical hardened & tempered blade that has been in use for many years. After tempering, an application of Precision Brand steel black prevents any rusting.

Jim Allen

 

[LohringMiller]

According to Hoerner's Fluid Dynamic Drag, the drag of a wedge in cavitating (or ventilated) flow is proportional to the wedge leading edge angle. The "suction" from the low pressure at the blunt trailing edge keeps flow attached through small angles of attack. We found that the limitations on wedge rudders had more to due to strength than anything. I've bent aluminum blades on big gas hydros. Hammer knives come heat treated and could be cut into suitable shapes.

Lohring Miller