Extra fins on rudder blade...??

Hi all,
I recently noticed this rudder with these extra two horizontal fins attached to the vertical blade. I'm wondering what these might be for, anyone ?



Thanks,
V.

 

It's an anti-ventilation plate. It appears that particular rudder is near the surface and at the aft end of the LWL on a double ended boat. This means underway, it will likely pull down air from above, so the plate is added to prevent the rudder from losing effective area from this ventilation situation.

 

Depends where the waterline is I suppose, but could it be an attempt to dampen pitching ?

 

I estimate the top of the rudder blade is a half foot below the waterline, more (1') when the boat is moving forward.

Hullspeed of this 21' displacement mobo is around 6 knots:
is any -serious- ventilation likely at such low velocity ??

 

You wouldn't think so.

 

It could just be a bad idea

 

Water aint allways flat and the velocity at the rudder (just behind the prop) can be double the hull speed, especially with rough or fouled bottom..

 

The photo doesn't show enough of the hull, but it can be assemd there's a fair bit of volume forward of what's visible and a significant hole being dragged behind the steep run and deadrise that is visible, which will cause some ventilation. I agree in that it appears a bit dubious at best on this hull form.

 

That puzzles me, a rough or fouled bottom would be more inclined to slow the speed of the flow over the rudder, would it not ? ( neglecting the prop wash, which obviously increases the speed of the flow).

 

Slows the speed of the boat, right. The prop wash is the main contributor on the flow on the rudder so cannot be neglected.

 

Of course the propwash increases the flow speed, but I'm not seeing how a rough bottom does.

 

If you look a a motor boats stern under way you often see disturbed water and wasted energy from the prop wash. I believe that, correctly or incorrectly, those wings are an attempt to direct the wash deeper into the water and give therefore more stearing thrust.

 

Trim tabs?

 

I've put 'fences' on many rudders-- especially flat (plate type) rudders. They double as stiffening, fitted at top and bottom of blade, both sides, and project about 10% of the blade chord. If you want some real 'lift', add a fairing over the stock and a fishtail at the rudder trailing edge. Tried this on a 85' MY and the owner said he seldom had to use his stern thruster.

 

Too wide to be "fences" imo.