Curt,

Having equal areas is just the first step. The difference in shape could still sabotage you. Have you worked out where the centers of the old and new rudders are? The relationship between the rudder centers (center of pressure, center of lift, etc.) and the boat's other centers is what determines weather or lee helm.

The other thing about the difference in shape is that your new rudder has a much higher aspect ratio than the old one. That means (as a rule of thumb) that it is much better suited for higher speeds than low. Or, to put it another way, at low speeds the new one may stall out and you'll be stuck in irons.

Then there's the whole question of which airfoil shape to use on the new rudder. Have you worked that out yet? You could maybe compensate for the low speed stalls with the right shape, but then there's the question of how it works at higher speeds, the total drag, etc.

When I converted the standard rudder on my brand-X skiff to a kick-up, I left the blade untouched in position and shape. All I did was cut it loose and added cheeks to hold it in place and an axle for it to pivot on. Didn't have to recalculate anything.

Good luck,

Laszlo

   Laszlo,

Center of pressure:
Agreed, it moved down and forward, but not a lot more than where the COP is on the oprional NE Dory kick up rudder.   I asked John that question and his thought was moving the COP several inches would not impact performance on a boat as narrow as the NE Dory, but do you think I should angle the rudder back more to move the COP back?

higher aspect ratio:
Nope, this I didnt factor in at all, maybe I should?  I was mentally (again) going off the optional NE Dory kick up rudder that is less of a barn door and more of a fin.  Do you think I should make the new rudder shorter and wider?  That would be easy in my design.

airfoil shape
I had planned on about a 3/4" radious rounded leading edge with a trailing edge tapered over a couple inches...your thoghts?

Thanks for your comments!

Curt

Rudder shape (planform) won't effect weather/lee helm- that's overwhelmingly driven be foil location (centerboard and sail). Having the rudder's CE dramatically behind the pintle axis does increase helm load- a different issue than weather helm. The deeper higher aspect rudder will always outperform the shallow draft 'barn door', even into the low speeds. Think sailplanes, or that solar plane currently (pun!) circling the globe.

Deeper draft does impose some increased bending loads, especially around the area of the lower gudgeon- my eyeball says your lower gudgeon will see around 2x the load it does now. If this boat has a record of rudder hardware failure at all, consider stacking another gudgeon below the first and make sure your lower pintle engages both.

Then there's the question of engineering the plates and pivot. First, the plates need to be stronger than a half-thickness of your rudder. I think you can devise a simple experiment with found lumber, glued into a sort of tuning fork shape to mimic a cross section of the rudder and plates and stock, and as this bent against the stock you'll see the plates need to be a lot closer to the full thickness of the rudder to prevent them snapping.

Or, use a stiffer material for the plates, like the bronze plate of your example pic or by very heavily reinforcing the single layer of ply you propose with many layers of glass. You'll then want to prevent these stout plates from crushing the rudder core: drill fill drill for the bolts, and add extra glass layers to the surface at the head of the rudder, or taper the bottom of those plates like an external scarf and glue them to the rudder blade.

Foil wise, this is still a flat plate concept- round the leading edge in a sorta parabolic fashion, and taper the last few inches. There's no use pursuing some true foil sectional shape with a blade this thin (unless you regularly exceed maybe 15 knots...)

 Dear mystery builder:(I think it timed out and lost your name)

lower gudgeon will see around 2x the load:
No, it's never had a problem, but I could widen the top of the blade of the rudder if that would reduce load?

engineering the plates and pivot
I have plenty of 3/8" Okume with me, so I could easily double up the plates and put glass between the layers of each cheek, sound good?

Thanks! 

Curt

Gudgeon load: I'm worried about the part on the boat, as well as the rudder itself near the lower pintle. If this rudder was engineered (it obviously wasn't, but if) for the loads the old shape could generate, then as you change the shape to make bigger loads it's obviously smart not to drastically reduce safety margins.

Plates: no, glass between a wood sandwich is unhelpful. I'd put maybe a dozen layers of glass (about 1/8" thick) on the inside (against the rudder) face, and at least two layers outside (if you want the plates clear/varnished, otherwise equal each side). I'd add about a half dozen layers to the top of the rudder blade, staggering their length an inch/layer.

Can you tell I distrust plywood's strength when used as a foil?

Lower aspect ratios typically have better stall progression characteristics, so their performance at low speeds and high angles of attack (such as slow tacking) is typically better.

Nemochad,

How many foils have you had break to make you so mistrustful? I'm right there with you on the futility of putting a tensile layer in the neutral axis of the core, but that glassing schedule seems more appropriate for a battleship than a NE Dory :-)

Laszlo

   Funny, I developed a laminate design spreadsheet for a class of home-built boats, and got to see the results of eyeball efforts versus those that fully considered the pressures a rudder can create.

There's a reason clc specs pretty conservative and beefy rudders!

 Update...comments welcomed!

I've made a fully functioning prototype out of 3/4" CDX roofing plywood (I will actually try sailing my dory using this prototype). Here's a VIDEO showing the action of the uphaul line and downhaul bungee.

From above comments:
--I will add a second lower gudgeon to the boat transom
--I beefed up the cheek plates, used 3/4" ply and made them bigger.

The square hole is for a 2x2 tiller that will be pined with a removable dowel.

If "sea trials" using this prototype work I will make one out of Okume wrapped in glass, and better hardware.

https://picasaweb.google.com/1147177type.)87929554738224/BuildingWoodenBoatCLCNortheasterDoryRudder?authkey=Gv1sRgCOH5w_Pfs7D5zgE#6298867682568845330

Suggestions/comments?

Thanks!

Curt

   Curt,  I'd like to do the same/similar modification on my rudder. Do you have any comments on how yours is working; assuming you made the change.  Also, anything you would change?

Thanks, Barry