Spade Rudder Strange Behaviour

Modern spade with a foil that has a slight hollow aft section, well built out of a mould.

The symptom is one that I have become aware of, not sure if it’s always been there or not. When the boat is stationary if you move the tiller from side to side it’s almost as if the rudder blade is loose on the shaft. I am not discounting the possibility, but I find it highly unlikely since I built the rudder and I am aware of its construction which is fairly substantial.

There is a definite slack/bite feeling as the tiller is moved but when I tie it off to one side then get under the boat and try and wiggle the blade its totally solid????

The only plausible explanation is that the water is attaching and releasing to the rudder, Am I way of here or is there something else I should be considering? It really is very strange.

 

The tiller or the rudder moves laterally in relation to axis of the pivot point? Or is it the resistance or force necessary to turn the tiller arm/rudder changes as you sweep it?

If you are tying it off and then checking the rudder for play, you may be binding up whatever is loose. I would bet on it being a rudder shaft bearing. They are the usual culprits.

 

It comes from lateral movement, i.e. perpendicular to the shaft axis. Compounding the confusion is that when travelling at 7kts under motor the effect disappears. Logically is would appear that the water flow at that speed would hold the blade amidships and the tiller could still move or be sloppy but instead the opposite occurs.

That why my theory is that its something to do with the water attachment and the relationship with the balance point.

I will have to haul it and have a good look.

 

Check the lower bearing and shaft for wear. Spade rudders are notoriously hard on lower bearings and, depending on shaft material selection, susceptible to fretting wear.

Edit : I see James said it first.

 

Reporting back the bad news is that I am now making a new spade rudder!

The previous rudder had a hollow shaft with a reinforcing insert, outer tube about 2.5" X 1/4" and the inner sleeve also about 1/4". The stock failed with the outer tube fracturing 360 degrees, given that it was 30 plus years old and raced pretty hard it not a bad life but still I am disappointed as I just went through a rebuild using the old steelwork. Only luck stopped it from falling off. I must take all responsibility as I should have "done it properly" Looking really close it seems that the inner had a few thou of clearance and the engineers opinion was work hardening along with a stress riser created by a pin weld between inner and outer shafts led to the failure. So a warning to those with hollow shafts, those old boys tried every trick to keep them light and sometimes the trial and error paid off but I consider myself lucky it failed in sheltered water.

Here is my question, I will be replacing the stock with 2.5" solid stainless but I am not sure which way to go, a respected NA has advised using 55mm or just over 2.25" in 2205 but since my tube and bearings are for 2.5 I would need to go up a size. I have the option of getting some CG 316 for a few hundred versus $800 for the 2205. Any input on whether 2205 is necessary, I know its much stronger but do you need a Bentley to go the shops when a Toyota will do? The other hollow shaft was 316 and failed primarily due to poor construction so my thinking is leaning towards the more economical option. What are your thoughts?

 

You don't really gain any (well, ok much) strength by using solid stock vs tube. Stress concentrates on the surface, most of the interior material is just along for the ride. With good solid fabrication, the 316 tube is more than good enough and will save both weight and cost.

 

Yes you are right but practical experience in spade rudders over a long period of time here in NZ has shown that hollow tubes are prone to failure, I dont understand the science but I do know that weight for weight hollow tubes are stronger than solid but perhaps there is deformation to account for? Also all the stress is concentrated within a few inches at the bottom bearing. I would imagine that if I could get thick wall tube of at least 1/2 it would be an advantage weightwise but alas I cannot.

 

What type of failure was it?
You can tell by looking at.
Where it started, how it propagated.
I suspect your anecdotal solid-shaft data is more based on load-cycling mitigation.

 

Long term corrosion due to the environment and neglected maintenance mostly. Either galling around a bearing or a weld that entrapped an impurity that sets off chemical erosion that creates a stress riser in the surface.

 

Do not use a stainless alloy, all of which have crevice corrosion/stagnant water issues. Use K Monel (Monel 500, preferred) or Monel (Monel 400), this is what the material was designed for. High yield, high ductility, excellent workability, excellent weldability, no corrosion, moderate cost. Only Inconel 718 is better at a much higher cost. A monel shaft with a bronze or HDPE bushing is effectively forever. If the difference in shaft price is less than a haul out you are waaaaaay ahead.