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-   -   Drop rudder design (https://www.sailnet.com/forums/gear-maintenance/70028-drop-rudder-design.html)

basssears 11-19-2010 01:23 PM

Drop rudder design
 
Struggling a bit with rudder planning, curious on everyone's opinion.

I have a Nimble 20, current rudder fits the look of the boat nicely, but the "drop" is just a single sided slab of aluminum, which bangs and looks ugly. Want to keep the current rudder but upgrade the drop.

Here's the current rudder:
http://www.pictureofnectar.com/outside/stock_rudder.jpg

Plan #1:
http://www.pictureofnectar.com/outside/plan1.jpg
...was to be like the Meadowlark rudder, use cheek plates on each side of the main rudder attached to the drop. Problem with this is either they're huge cheek plates with lots of turbulent gap between the cheek plate and the drop rudder itself, or I move the pivot point all the way to the back of the main rudder, and I'm not sure how that would work performance wise?

Plan #2:
http://www.pictureofnectar.com/outside/plan2.jpg
...would be a piece of plywood on each side of the main rudder, but then solid (well, epoxied & screwed layers of) plywood at the bottom end, maybe with some strapping / flat bar aluminum or bronze for strength. The plus here is I can keep the pivot point farther forward so the drop rudder is in line with the rest of the rudder. But Iím not sure if that matters, and I'm not sure if design #2 would be strong enough with the two pieces of plywood on either side of the main rudder?

Most of the other Nimble rudders out there:
http://www.pictureofnectar.com/outsi...ble_rudder.jpg
... have a single piece of metal (prob. aluminum) sandwiched in the middle of the main rudder, but short of building a whole new rudder (which I don't really want to do) I don't see how I could gracefully cut a slit in the main rudder, get it sealed up again and then install a piece of metal in there.

Any / all opinions welcomed, as usual!

Thanks.

-- Bass

Faster 11-19-2010 01:47 PM

How about carving a section out of the existing rudder, adding fixed cheekplates and having a pivoting drop section the same thickness as the rudder, properly aligned with the rudder itself? Rather than 'cut a slit' in the rudder, you just cut out the appropriate shape, reseal the new edges, then add the cheeks. Drill the cheeks for a pivot pin and fashion a rudder extension that fits between the cheeks.... the added thickness would allow you to do some shaping besides.

GaryHLucas 11-20-2010 07:56 PM

A thin section like your current drop is greatly preferred for reasons of drag, your suggested options look to me like a reduction in performance. The problem you have currently looks like a poor pivot pin design. What you want is for the drop blade to be very flat on the side that contacts the rudder, and for the rudder at that same point to be very flat. Then you want a pivot bolt with the least amount of clearance possible.

I'd buy a sheet of Nylatron from McMaster Carr, cheap and self lubricating, and make a large (6" diameter) washer out of that. Put a large diameter stainless fender washer on the outside with a Nylatron washer about 2" dia. under it. This will stop the electrolysis between the washer and the aluminum blade. On the other side of the rudder use a large stainless fender washer, and double nuts to allow you to adjust the clearance and lock it. Adjust it until you feel drag at some point in the blade travel, then back it off about 1/4 turn and lock down the second nut.

You'll have no bang and it'll work smoothly. GO SAILING!

Gary H. Lucas


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