Rudder failure

[VicS]

I have been asked as Technical Advisor to the Owners Association to comment on the rudder failure in these pictures

http://i148.photobucket.com/albums/s20/bandit1312/IMAG0594.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0593.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0592.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0591.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0590.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0589.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0588.jpg
http://i148.photobucket.com/albums/s20/bandit1312/IMAG0587.jpg

and to suggest whether it can be repaired or if it means a new one.

It appears that this is a poor attempt to make lifting rudder which has failed because the ply side cheeks were not strong enough. The original rudders were made of solid mahogany and were not lifting ones.

While it might be possible to cobble something together to keep the boat sailing I have suggested that a new rudder is called for. I have suggested that if ply is again used in the construction of a lifting rudder it should not only be heavier but that it should be cut with the grain diagonal, at least for the cheeks, for greater strength.

I have also queried the balance area and suggested that it should be between 10 and 15% and that based on my own experience should not infact be more than 12.5 % (The original rudders were not semibalanced but many have been modified.)

I would appreciate any comments especially on my suggestions about cutting the ply with the grain diagonal and on the balance area.

 

[William_H]

I presume this is quite a small dinghy as the design looks quite inadequate even for a small dinghy.
My 21 fter has a swing rudder that is about 1.5 metres long. The structure of the cheeks is welded aluminium although later models of my boat use welded stainless steel tubing.
Anyway I suspect that the cheeks are too small in the vertical dimension. In so much that if the blade has cheeks on each side from the tiller to the water line then there is much more sideways support. Likewise the cheeks should extend to the aft edege of the blade. Which should be full thickness where it sits in the cheeks.

The rudder blade takes a large sideways bending action so the cheeks need to carry the load both to the pintles (especially the bottom pintle) and to the tiller. Thickness of cheeks is the only way to give stiffness in holding the cheeks tight to the blade.
(My boat lives on a mooring so I actually put a bolt through the cheeks and blade to clamp the whole together to reduce play) (obviously not practical in a dinghy)

I was involved with a fleet of 10ft training dinghies. I made a few rudders. The cheek arrangements were a piece of wood the thickness of the blade with 3/8 inch plywood glued and screwed right through one side to the other which was from the tiller to the water line. There was another piece between the blades at the top with the tiller attached to the top. So the blades were kept together at the front against the hull and it the top.

We had a few blades broken but not the cheeks. Of course if the boat is sailed with the rudder not fully down (which seemed more often than not) then the loads on the cheeks blade and tiller are a lot higher.
Looking a the first picture I can't see where the pintles are attached. But clearly the cheeks should be more a box (sqare) shape extending further back to support the blade and held one cheek to the other at the top. Then the cheeks should be thicker.

The question of ply against the grain I don't know about but my gut feeling is that it will help but not by much as to make it worthwhile.

Yes rudder balance is very desirable. Your 15% sounds good as mine is although I havn't measured it. My tiller loads are very mild for a boat on its ears with spin up in a strong wind so I always advocate rudder balance. I hope I will be doijng just that this afternoon, the last of our summer races.

I can't help feeling I have missed something from the design in looking at the photos. But yes the rudder is repairable by redesign of the cheeks and tiller attachment. olewill
Another last look at photos.
Ah I see now the metal ring is the lower pintle and the rudder blade is pivotting at the bottom of the cheeks and tiller is removed.
Yes you want a longer blade which can pivot from up high near the tiller with cheek support right down to the water line.

 

[CliveG]

Just to give another possible solution.

This is the rudder set up on our Evolution 25.
it is a vertical lift rather than a pivot.
For trailing we lift the blade out completely.
The rudder blade is hollow aluminium but could be ply.
It would be difficult to get any sort of balance on this sort.
We don't have any issue with heavy weather or lee helm.

 

[colvic987]

looking at your pics, and the failure of the ply, you could still use the stock of the rudder.

A) Cut off the other cheek and reseal the ends and epoxy up and repaint
B) manufacture two pieces of stainless to cover each side of the stock, and cheeks, do not skimp on this with a small piece of stainless at the bottom, as it will fail again.
C) Drill the holes and epoxy up then redrill, at the four corners of the the stock and hole in cheeks, bolt it up using washers and spacers (if needed) at the cheeks.
D) machine out excess if able to, to shed weight. Or use aluminium..

If the rudder looks ok and is not starting to rot then resheave in epoxy after it has been totally dried..

It should last you another decade..

 

[fluffc]

Having a 'balancing' element to a rudder will increase the loading drastically in relation to what the helm feels.

So, if this rudder was heavily balanced, a helmsman could pull it apart; not noticing that he is overloading the rudder because it is 'balanced'.

IMHO if a heavily balanced rudder is 'needed', then there is something wrong with the rig setup of the boat.

 

[srp]

I'd make a new one. The other (unbroken) rudder cheek probably has the grain running in the same direction as the broken one, so that will be the next to break if you keep it. As you say, the grain sould be angled, not running vertical/horizontal. This applies to the cheeks, stock and blade in my view. The mild steel fixings obviously have to go, and I would bond the whole lot together with epoxy. If rudder failures on this type of boat are common because they are underspecified (ie ply too thin) then it would increase the strength considerably if it was sheathed in woven glass cloth and epoxy for little extra cost. I'd be tempted to sheath it anyway.

 

[Bilgediver]

I think Vic that the failure was helped by the fact that the swivel pin has such small shoulders and so when side loading was experienced by the side that failed the pin pulled out through the other side rather than get it to share the load.

I would suggest larger shoulders(washers) at the ends of the swivel pin if you go this route so that side loadings are fully shared on both sides of the stock.


John

 

[Strathglass]

As you say, the cheeks are not strong enough.

I used to build boats and tried all sorts of things to reduce rudder weight. -with varying sucess.

The main strain on the rudder stock such as the ones in your photos will be when the boat is being sailed with the blade trailing before it is down in it's normal position.
This would open up the jaws of the stock and initially weaken it.

The best stock would be one fabricated from alloy or stainless.

If this cannot be done and you have to use ply then use it at the diagnol with the outer grain of the ply running from the bottom of the transom upwards and aft at 45 degrees.

Use as thick ply as you can to give rigidty preferably all mahogany or similar strong ply. Any 'marine ply' from a builders yard will probably have voids.

I would have thought that in the long term the rudder blade it's self should be lengthened above the pivot bolt hole as this would add considerble strength.

I don't think that changing the ballance area would have any effect on the strength of the assembly other than removing any feel from the helming and beause of this hiding the huge forces being imposed on the rudder/stock assembly when the craft is being badly sailed.

Iain

 

[VicS]

Thanks for all the comments. The rudder is from a 19ft bilge keeler. I don't know why the lifting rudder was made
I think the owner is intending to make a new fixed rudder and I will advise him to largely copy the design of the originals but incorporate a small balance area as that has been found to be beneficial. If he uses ply I will advise hime to use good qulity marine ply that is thicker than the mahogany used for the originals and to consider cutting the ply "on the diagonal"

I think elegant solutions with stainless steel frames for a vertical lifting blade will be ruled out on cost. It will cost nearly as much as the boat is worth I suspect

 

[aitchw]

Looking at image 590 in particular I would say that the failure was the result in no small part to lack of maintenance and care during it's service. The surfaces of both the top of rudder and inside the cheeks appear to have virtually no protective treatment left and I suspect the failure of the cheeks was the end result of the ply taking up water. I agree that the overall design lacks strength but it is not much different from many even quite large dinghy setups. Better pretreatment of the ply and occasional drying and retreatment would have at least delayed and probably avoided this failure.

I agree that cutting the cheeks with the grains at 45 deg would also help but the fundamental problem is lack of protection from water ingress.

 

[VicS]

[ QUOTE ]
result of the ply taking up water

[/ QUOTE ] Very probably. I do not know anything about the history of this boat but I am fairly certain it is new to the current owners. It may even be their first season with their first boat so a broken rudder out in the Solent is not a very good start!

The trouble is that some of these boats have remained in beautifully maintained condition some even still with the people who built them 30 years or more ago while others have changed hands many times and suffered from lack of decent maintenance or from badly designed modifications like this one.

I will make the point about protecting ply underwater if it is used for the replacement.

 

[aitchw]

[ QUOTE ]
It may even be their first season with their first boat so a broken rudder out in the Solent is not a very good start!

[/ QUOTE ]

That's very sad. I hope it does not turn out to be one of those disillusioning events that lead to abandoning sailing as a pastime.