Rudder: lower bush.

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Sealong

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10m tiller-steering sailing boat.
I have taken out the rudder to inspect the top bearing and lower bush and extracted the lower bush (Delrin, 86 mm OD, 68mm ID) without damaging it. There are salt-like deposits on the rudder stock which I will clean up with fine emery cloth before reassembling the unit.
I would like some advice on what sealant or adhesive to use on the outer diameter of the lower bush which is a loose fit in an aluminium tube that is bonded onto the inside of the hull.

My initial thoughts are to use some sort of Sikaflex, but I am not sure if this will have adequate shear strength.

Any recommendations will be much appreciated.
Chris
 
Hi Roger (howzit!) and thanks for your reply.
Unfortunately I am not getting a particularly warm feeling from the conflation of "sometimes" and "propshaft".
I am rather hesitant to use epoxy since I doubt it will bond successfully to the derin.

Best wishes
Chris
 
Sealong said:
Hi Roger (howzit!) and thanks for your reply.
Unfortunately I am not getting a particularly warm feeling from the conflation of "sometimes" and "propshaft".
I am rather hesitant to use epoxy since I doubt it will bond successfully to the derin.

Best wishes
Chris
Click to expand...

The other times bushes are a press fit or held in with grub screws.

Mine are vesconite and are a light press fit but have a flange to stop then pushing out

A epoxy glue does allow less accurate machining (boring) as the bush can be lined up on the shaft nd the Epoxy take up any inaccuracy one set
 
The answer is recomeneded by the adehesive manufacturer ADHESIVES for DELRIN and ACETAL.

How do you bond DELRIN with an adhesive to dissimilar materials?
One of the properties of POM that makes it difficult to bond to is its low surface energy. Acetal can be bonded to itself as well as dissimilar materials using BONDiTTM brand adhesives manufactured by RELTEK LLC. BONDiTTM products are among the few available that can be used to bond acetal to itself, and to other substrates for harsh environments and long term deployment without debonding.
Industry producers of acetal generally report it cannot be bonded without special surface conditioning, due to the high degree of polymer chain alignment and hydrogen pairing on the surface. However, DELRIN CAN be bonded to itself and to other substrates without significant preparations (light abrasion is usually recommended) using BONDiTTM brand adhesives manufactured by RELTEK. With light abrasion some of the paired hydrogen can be broken up to create tertiary hydrogen (connected to a tertiary carbon and not sterically hindered or paired with another adjacent hydrogen) which are chemically active and increase the surface energy. BONDiT B-4x series epoxy products are designed to take advantage of the tertiary hydrogen to form good bonds, producing with a acetal/steel lap joint a typical lapshear strength of 350 PSI to 450 PSI (pounds-per-square-inch) depending on curing method.
Click to expand...
 
Chris , sorry I don’t have a solution for you , is it worth asking the same question on the naiad owners forum
chris B
 
I am just about to replace our lower bush.
It is made of delrin as was fitted and used by Hanse with Sika. I am using the screwfix equivalent. In my case I am having to fit a new longer bottom bush as the original bush is unworn after 20 years. However the aluminium shaft has worn to give excessive play.
I have deliberately commissioned a new bush to be made an interference fit with the rudder tube over the original bush length. The extended part will have a reduced diameter to allow a decent thickness of CT1.
Some people also have the bottom flange drilled and countersunk to allow screws into the fiberglass hull to guarantee anti rotation whilst the Sika sets hard.
I see it as being penny wise to replace the bush if it has shrunk so much it no longer is an interference fit! A new bush can easily be machined up by any good local machine shop.
Steve.
 
Thanks for the very useful replies. To provide some further clarity, the clearance / interference between the outer-diameter of the bush and the inner-diameter of the aluminium tube may be adequate, at present I do not know. The reason the bush came out with the rudder stock was due to the interference of the inner-diameter of the bush on the rudder stock, which had a build up of salt-like deposits. I will clean these off enough to remove the bush from the stock, measure up the respective diameters and that will allow me to estimate the clearance / interference on the outer-diameter of the shaft. As TSB240 says, it is probably worth having a new one machined up if it is a loose fit.
 
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