I bottom out in particular parts of my local lake, and am considering making up short rudders, basically by stretching out the trailing edge, and making it out of 20mm ply with 3 coats epoxy then paint over the top. Design is in the attached image with the original planform on the left. This has the same below waterline area as the original. Profile is NACA0009 bottom to NACA0006 at the top. Original is uniformly NACA0008 with a taper top to bottom.



Any comments? Would this profile work?

Yes they would turn the boat.

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Nacra 5.2
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golfdad75

Yes they would turn the boat.

LMAO!

I was once sent rudders along with some castings I was buying, which were my real need. Apparently this guy had a problem hitting his as well. His solution? Just cut 6 inches off the bottom! But only of one! Ha!

I am sure your way is at least more thought out!

Thought about cut-downs but I think it would result in a heavy weather helm, and probably loss in effect.

I've thought it out, but not sure if the thinking is right. Will make them and let you know how it goes.

Damien

Damienb, you have done considerable research on these foil shapes.

Before you start, have you looked at the percentage of total surface area in front of the rudder pintle in each design?
This is where helm is affected by rudder rake, though I am not sure whether it is surface area or volume of the foil we need to compare. Maybe there is a foil theorist in the group.

Is the new design similar to the original?
This is just an informal version of measuring twice before you cut once.
My two cents.

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Sheet In!
Bob
_/)_____/)_/)____/)____/)_____/)/)__________/)__
Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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Too late already started. I'm still using 20mm ply but glassing on top to get it out to 23 mm.

The weather aspect should be the same due to the wider and thicker tip. There's a fair amount of rake available if really needed.

The standard prindle rudder is on the left and my proposed modified version in the middle. I think the standard design is good. Stalls a bit easily on hard turns but otherwise a nice performer.

i have a Iroquois mkII 30' . i sail a lot in the bahamas and cut 6 in. off of the rudder blades. the only adverse effect that i have noticed is that if i put the main up and try to back off a mooring the rudders do not have enough bite to turn the boat and sail off the mooring. under power there is no problem. one note the blades are adjustable fore and aft but i have not noticed any difference in the position since i shortened the blades.