Bent Rudder Stock

[kungfoo]

Originally Posted by donradcliffe

The engineer that bent the rudder shaft back straight is long dead. It was from a Cal 28 with a 2 inch shaft, and bent during a hard race back in the late 60's. We got the rudder out in the water, and took it over to a boatyard to fix. No one was around at the yard on Sunday, so we scrounged a 10 foot piece of pipe that would go over the shaft, and slid the shaft under the keel of a big boat in the yard and lifted up on the pipe till the shaft was straight. It turned out to be pretty easy, and he had the experience to decide where to put the blocking. However, the shaft was undersized for the boat and after that we never yanked on the tiller that hard.

Thinking more about your boat, your shaft is 150% of the diameter of the Cal, and would take 5 times the load to straighten. You could probably use a hydraulic jack and a 3 in ID pipe, but you need strong supports to push against, like a large truck.

My other data points are my Bene First 456, which has a 6 foot by 2.5 ft unsupported rudder and a 4" diameter shaft, theoretically 4 times as strong as yours. It went around the world without bending through some pretty big storms, but I never dragged the rudder sideways with a big spinnaker up. I also bent the original SS rudder shaft on my Santa Cruz 27, and Bill Lee gave me a new rudder with a fiberglass shaft.
I also sailed across the South Pacific in company with a homebuilt steel boat, who had a skeg hung rudder with a pipe flange coupling on the shaft above the skeg. The shaft failed going to Easter Island, and he had it rewelded there. It failed again going to Pitcairn, but he couldn't get it fixed until he hit French Polynesia. Their problem was that the shaft couldn't handle the torque, and the welds kept braking. They were double handing, and they had a hole drilled in the rudder with ropes coming up to winches to steer.

Thinking more about your boat, the torque capacity is not the problem, its the bending moment. Putting a weld into the shaft would create a discontinuity, and increasing the outside diameter of the welded shaft would mean new bearings. I think if I wanted to go extended cruising on you boat, I would split the rudder housing, cut the shaft off, and replace it with the same diameter shaft with a thicker wall if I could get it.

Sorry to hear about your friend, he sounds pretty cool. I'm glad you got to have some fun times together.

In our case, bending is simply not an option if for no other reason than that the people with the equipment will not do that.

Inserting a new solid shaft I *think* would actually be *stronger* than it was before. If we go for a 25um interference fit, then I *think* that it would have the bending strength of the original PLUS the solid inner core. I'll be checking that failure mode with a buddy of mine who is a mechanical engineer/sailor.

I'm angling for this fix because we don't have to do any complicated glass/foam working.

[markxengineerin]

Disclaimer, didn't read every post, but my first thought is to sleeve it using an adhesive (G flex epoxy? Loctite?) rather than welds, interference fits, and shear pins. Welds risk overheating the glued joint you already have and will be a weak spot, interference fit is finicky to get right (ever get something stuck halfway installed?), and will put stresses into the glued joint you already have, and pins are extra spots for water to get in over time. Some measurements and calculations could estimate what torsional loads the rudder is capable of withstanding in its original design, and as long as a new design approaches the same numbers and isn't more prone to "age effects", I think you'll be fine. Many will say "just replace it" or "do it right, start from scratch" etc, but a repair is certainly possible with enough care, research, planning.

[kungfoo]

Originally Posted by markxengineerin

Disclaimer, didn't read every post, but my first thought is to sleeve it using an adhesive (G flex epoxy? Loctite?) rather than welds, interference fits, and shear pins. Welds risk overheating the glued joint you already have and will be a weak spot, interference fit is finicky to get right (ever get something stuck halfway installed?), and will put stresses into the glued joint you already have, and pins are extra spots for water to get in over time. Some measurements and calculations could estimate what torsional loads the rudder is capable of withstanding in its original design, and as long as a new design approaches the same numbers and isn't more prone to "age effects", I think you'll be fine. Many will say "just replace it" or "do it right, start from scratch" etc, but a repair is certainly possible with enough care, research, planning.

As you say, interference fits can be finicky which is why I reached out to a professional today about hiring them for that critical step. Expectations are that I'd have about 100um of space I can create by a combination of a heat-gun and a bath of acetone/dry-ice but 100um (4 thou) is not that much and I REALLY don't want to screw up that step getting it stuck halfway through.

I'm looking into alternatives (including an epoxy bond). I'll be checking into the shear strength of epoxies on a recently-roughened 316SS.

Our necessary working-load shear-strength is pretty easy to calculate by asking how many Newtons of force I can apply to the end of my 2-foot emergency tiller. A 100um interference fit is quite sufficient to max that out with the welds and pins being for funsies.

It's also worth noting that any welds might represent a potential corrosion point, but the current system is already welded -- they weld the stock to the tabs inside the rudder so it isn't really a significant change in that sense. All of the welds would likely be located inside of the rudder. We'd probably grind back the rudder a bit right around where the stock enters to do the welds around, and the pins would be welded inside the rudder glass/foam. There's also talk of doing a plug weld instead of a pin that way you don't disturb the inner inegrity of the post and it can fill with water without exposing any of the welds.

I'm with you that a repair is not unreasonable if you do your homework, at least part of which involves posting here and letting everyone tell me how wrong I am :-)

[markxengineerin]

how many Newtons of force I can apply to the end of my 2-foot emergency tiller

I'd use the real tiller for that calculation then multiply it by 2 or 3. Emergency tillers are known to be useless without additional mechanical advantage like roping them to a winch, and you can get a pretty good impulse going with your body weight. I think this number would give you a better idea of what's needed to be "safe".

I don't know of a boring/honing machine that would conveniently fit this job, especially not something able to bore as deep as you might be thinking, so I would be trying to avoid machining the ID entirely. Adhesive first and only look elsewhere if you need to, which I'm guessing you won't.

0.004" of interference on a 2.X" diameter is a lot, not unheard of, but on the high end. Definite risk of something going wrong during installation and leaving you with an expensive mess.

All of the welds would likely be located inside of the rudder. We'd probably grind back the rudder a bit right around where the stock enters to do the welds around, and the pins would be welded inside the rudder glass/foam.

This sounds unrealistic, the types of welding likely to be used here are going to create a lot of heat, which presents risk to resin.

In conclusion, before overthinking, I think you should see if a glue joint will work. Still have to be careful, for example, giving air a place to exit so you don't create any compression during installation or cause the glue to blow out.

Edit: and make sure the ID you're working with is round enough, which to me means the glue can fill the non-roundness and still have adequate strength to accomplish the task.

[rognvald]

". I think if I wanted to go extended cruising on you boat, I would split the rudder housing, cut the shaft off, and replace it with the same diameter shaft with a thicker wall if I could get it." donradcliff

Yes, yes, yes.
Rognvald