Extra fins on rudder blade...??

[Munter]

The misalignment between the hull and the plate looks like a source of drag to me. I think the plate is probably a misguided attempt to increase the effectiveness of the rudder.

 

[JSL]

Yes, they are probably less..... I should have checked an actual drawing

 

[Squidly-Diddly]

It could just be a bad idea

How can a combo:

anti-vent,

anti-roostertail,

stiffener,

and last but not least Swim-Step....

be a "bad idea"?
:idea:

 

[Mr Efficiency]

I'd be pretty confident it wasn't a professionally fitted device.

 

[TeddyDiver]

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

Becouse people use the cruising speed they are used to, turn the throttle until the log says 7kn without thinking more. Becouse of the fouled bottom they have more thrust..

BR Teddy

 

[DMacPherson]

Is there any chance it is nothing more than a standing platform for some sort of work that needs to be done over the stern?

 

[JamesG123]

Is there any chance it is nothing more than a standing platform for some sort of work that needs to be done over the stern?

Probably not since the rudder is largely overhung by the stern. It would be quite awkward to stand on that.

I'll go with a ventilation shield/duct.

 

[mydauphin]

May be it is a place to stand on when cleaning the transom.

 

[FAST FRED]

"Hullspeed of this 21' displacement mobo is around 6 knots:"

Doubtful, double enders sink their stern far more than conventional sterns , so hull speed is always lower.

My guess would be surface area to help prevent the stern from sinking under power.