Rudder bush

[TWINSCREW]

Greetings, I have to remake the top rudder bush on our 33' power boat, I will be turning it from Acetal and the rudder shaft is 1.75'' diameter, can anyone advise the clearance required between the shaft and bush?
Thanks.

 

[Dipper]

Slight deviation and not answering your question but have you considered UHMWPE instead of acetal? Advantages include high abrasion resistance, low water absorption and low coefficient of friction. I made a lower bush for my sailing boat rudder from it this year. It's the same material they use for human joints. See here: http://www.directplasticsonline.co.uk/UHMWPE_Rod.html

 

[srp]

Difficult to answer without more information. Will it be lubricated in any way? Is the bush a press fit in it's housing? If so you'll have to make an allowance for a slight deformation when it's pressed into place. I would suggest you look at the Vesconite on-line calculator (www.vesconite.com) to give you the dimensions - although you're using acetal not Vesconite I think it would probably be accurate enough for a rudder bush.

Edit link to metric rudder bush design page http://www.vesconite.com/design/bush/marine/DAB_rudder_metric.htm

 

[DinghyMan]

Non-technical answer: next to bugger all

Technical answer: approx 0.2 to 0.4% of diameter, so 0.125 to 0.25mm, so effectively just a nice fit

I would turn to size for a top bearing as any swelling due to moisture would be minimal and I've made a few acetal/delrin bearings reamed to size with no clearance and not had any issues but as mentioned above these were not pressed in bearings and for those you need to consider how the bearing will be fitted and allow for them deforming. Also I would clean and polish up the shaft carefully as well and lubricate initially.

 

[TWINSCREW]

TWINSCREW Says thank you for your quick replies, much appreciated.