rudder

Well, I am building a new rudder for my boat. It will be a hung rudder (not supported by a shoe). Previous rudder was supported by a massive 70 kilo shoe bolted to the keel. Rudder was flat plate steel with top plate, wedge in the trailing 15cm. The dimensions, were it rectangular (not quite, a trapazoid), were 55cm chord by 69cm depth.Sorry for the small print in inches - I made this for my machinist but, basically, the new will have less chord length, only 46cm and the shaft will be 7.62 cm 329 stainless. I am trying to clean up waterflow and get rid of the shoe and am planning a foil-shaped rudder. NACA0015 near top (have to fudge a bit the top couple of inches but I don't want to lose the full diameter of the shaft abruptly and up high), and nom.12% near the bottom.

I'm probably going to come in a bit on the lower cord length as I had more than enough rudder before and am worried about the loading on this thing just hung from above. The hull will be the top-plate of the new rudder (There is only a 7.7 degree deadrise here and the rudder will almost touch on the "V"). Any thought on canting the rudder post forward (under the boat) 7.7 degrees to maintain hull contact throughout sweep - worth it? Never seen it done and it seems a no-brainer to me so there must be a reason why not...When you need the top-plate the most (hard over) is when you have the widest gap on a conventional set-up.
I am engineering the rudder to shear off at the bottom of the post rather than damage the shaft or hull upon major impact and suspect that carbon fiber will help in all regards. I can force a break to happen at that point by cutting some of the fibers there in the green stage at layup and not laying too many large fibers across the fault-line and laying an extra ply on the top portion but am intrigued by the brittle/ stiff aspect of carbon.
Ideas on getting the right amount of carbon in the lay-up (don't worry about the carbon taking all of the initial flexural load until failure as the glass layup alone will be massive, nominal 5/8 wall 1708, BTI foil shape with stainless spokes welded to post.
I have not been able to locate rudder side-load numbers but have a pretty good idea it will be strong enough. I don't want to foam or otherwise fill the thing. I want it easily removable.
Also, ideas on not unraveling the top portion of the rudder if the bottom fails and input on any aspect are appreciated. Thanks, Mark

 

I'm sure the back-angled rudder will work fine. Especially if not very extreme. My own 19 ft Alacrity sloop has at least a 10 degree back-angle, though transom-hung.
Have you considered creating a breakaway point halfway up the rudder so that you'd still have a half-rudder in case of a grounding? Losing a whole rudder isn't a good thing especially when you find yourself on a lee shore with more of the same.

 

Thanks but you missed what I said about the rudder post angle. And I have more than considered a break-away "I am engineering the rudder to shear off at the bottom of the post rather than damage the shaft or hull upon major impact"

 

I didn't miss what you said about the rudder post angle. And regarding your having mentioned a breakaway rudder, I noted that too. I only suggested the idea of a rudder plate that breaks further down, leaving a portion of the rudder still viable.

 

Thanks, Alan. I suppose that I am just not that good of a communicator.

"...shear off at the bottom of the post," I don't really know another way to state this...
Perhaps, "shear at sixteen inches from the top of the rudder" or "shear at 14" from the bottom."
Your alacrity rudder cants aft. I said forward.

 

True enough. I see you meant the bottom of the rudder angled forward. And I see what you meant about the shear point being about mid-height of the rudder, which was my suggestion anyway.
Sorry, Mark, I misunderstood.

 

It's late and I'm not communicating well. Thx
Maybe this'll help. My eyes are blurry from staring at the screen an hour doing this with stupid "paint" but maybe... I can visualize the canted post (hypothetical) benefit but can't seem to get it on paper.


Playin' around - forty minutes of that hour were trying to do a prop from tired memory. Now I know why they are almost always "represented."

 

If the rudder was supported with a shoe, the hull is moste likely not engineered for the load you are putting in it. You are engineering a rudder without considering where it is installed.

 

You have a different rudder balance when you tilt the shaft. You have to rework the hull for a tilted shaft, but you probably have to do that for the load anyway, as Gonzo suggests.

 

Thanks for the strengthening reminder. If it wasn't before, it is now!

 

Yes, this boat never having had a hung rudder, I am in the position of having a clean slate. It is a special case in that it was built with the same laminate schedual as the 50' Delta passenger vessel. I am glassing in a heavy wall post tube to hold the Delrin bearings and the upper support is a 3cm FRP plate glassed to all four stringers and the inside of the transom and a plate between the innermost stringers. How do I calculate the side load on this rudder so that I can tell how weak I can make the shear-off point (8 tons, 40', 510 sq.in. 24 knots, 35° rudder angle)? Thanks

 

How do you manufacture the shear point without unduly weakening the rudder, resulting in material fatigue related failure under normal operations?

 

Well, it doesn't have to be weak, per se, just weaker than every other part. My bigger problem is making an accurate jig to be able to duplicate the bolt holes on subsequent rudders if I break the first two. I am starting with two rudders. I am making a male mold the shape of the post/spokes so I could always pop out another one. I don't entend for this to break off every time I hit a four by four or whatever. It is going to be immensly strong and the only scenario I envision breaking it is if making a mistake in rocks and backing over one.

 

Thanks.

 

I guess I didn't answer your question directly, tho. I am going to lay up wet and cut through (with a quilt-cutter - like a pizza wheel) a few plys at the bottom of the shaft while that ply is green. That's why I was interested in carbon - for its brittle aspect. Jimbo got me lined out on a supplier and I think I can make this snap cleanly. If not, well, I guess I just trail a wad of **** hanging where my rudder once was but I fully intend to not let the shaft bend in the boat and bind the post - thats why the 3" 329 stainless shaft. It doesn't taper to 1.5 x 3 until several inches into the rudder which will stiffen and support the shaft. That point where it tapers into the flat (the part inside the rudder) will be the second weakest point. The whole idea is that I get home and don't damage the hull.