Semi spade rudder construction details

[ldigas]

Pardon my curiosity with this simple questions, but these are things one "knows" until he eventually has to draw them.


What keeps the semi spade rudder from "pressing on" the "rudder leg" (skeg, horn)? I have a semi spade rudder, with two pintles. Both pintles are "towards down" with retaining nuts. But if pintles are "free to move" axially (vertically) what keeps the rudder from "sitting down on the leg" with its weight? Bearings take only the radial (horizontal) force, do they not?
If anyone has an example construction drawing of such a rudder, or some similar which can illustrate this, it would be equally appreciated.

Also, an additional point of interest. In G.Lloyd the rules for Yachts and boats up to 24m (Under special craft) deal only with semi spade rudders that are on one stock (the part with bending moments). Is my understanding correct that for semi-spade rudders with two pintles have a different bending moment distribution, and that a different equaation should be used. I've been unable to find such case in GL rules for yachts under 24m.

[michael pierzga]

The bottom bearing is designed to take "thrust" .

The bottom of the rudder stock is machined "round". Its Domed. Not a small radius, a large radius dome

Inside the lower bearing is a separate piece of stainless steel , an insert, the same diameter as the rudder stock, and with the same " round" "Dome " machined onto the face that will contact the bottom of the rudder stock.

I recently serviced a 100 footers rudder bearing. Ill look thru my pictures to see if I have any of this detailing. Unfortunatly I have no way to make a simple drawing and post
"

[ldigas]

Still having a bit of trouble seeing it.

You mean that inside the bottom bearing (a cylinder) is a small insert, "domed" to a large radius. And the rudder stock is made similar at its bottom end.
So that way the two spheres touch (theoretically, of course) only at one point.

Is my visualization of it even close?

[michael pierzga]

Sorry. I had second thoughts about posting detailed engineering drawings and have taken them down.

Best luck with your project.