Rudder Ventilation Fences

[raw]

Hi All,

I'd like to drawn on some of the experience that visits this forum.

I am deeply involved in the design and production of semi custom powerboats in the 50-65 foot range.

One of these models, a 60 foot, 35 knot twin screw vessel with prop tunnels, suffers from some undesirable turning characteristics due to rudder ventilation. We have managed to confirm and eliminate this problem using bolt on ventilation plates made of stainless steel angle. The rudders are positioned under the hull.

Unfortnately, this solution is not very elegant, so we designed a new rudder that attempted to move the blade area away from the hull a little further. The new blade also featured a foil section with a nicely faired in stock. (Don't suggest wedge rudders, we have classification concerns to worryu about) For all intensive purposes, the new rudder looks the part, and is a well thought out bit of gear.

Unfortunately, it has not solved the ventilation issue completely, so we are back to ventilation plates. This time they will be weld on, rather than bolted.

My question is this, for those that have been in the situation before, is there any real benefit to running the fence around the leading edge? I have some information from yacht stuff showing this and also a ducane sketch for a hydrofoil one showing the same, but also came across photos showing plates that are merely on the sides.

Thanks in advance for your replies.

B

[Gilbert]

I think you will enjoy visiting the metalmarinepilot.com website.
There is a lot of information there about rudder design. They used to work up specifications for rudders for free if you provide them with information about your vessel; I believe they still do.
Just a comment: I believe the foil section rudder was a step in the wrong direction for your boat.

[raw]

Thanks, for the link, I will look it over. Job is done now anyhow.

Why do you think a foil section is going the wrong direction? Sure a wedge might* be better in the top end speed, but I can't design a good wedge rudder in accordance with the required rules here since the stock strength must extend well into the blade.

Besides a foil section should give better results at slower troll and cruise speeds. Top end may not be high enough to warrant it anyway.

[Gilbert]

Powerboat propellors shove water past the rudder considerablly faster than the boat is traveling through the water and create turbulence along the rudder surface. The rudder is then not operating in 'solid' water and usually the rudder can be turned some to the right or left and have no effect on the direction of the vessel. For good steering, especially with an autopilot, it usually is helpful to modify the rudder to make any movement of the rudder actually cause the boat to turn. I would be very surprised if a foil shaped rudder would produce that result. And I don't think slow speeds will be much different. So that was the reason for my comment.

[lazeyjack]

g,day mate
how close are the rudder tops top the hull?tunnel?

[tom kane]

Boats with props in tunnels only like to go straight ahead,and when the hull is turned by any device the sides or edges of the tunnel (as the hull leans either way) interfers with the water flow to the prop.Similar to having a keel or other projection breaking the water flow.