Aluminium rudder stock - bearings choice

I guess the simple solution would be to figure out how much your deflection is over the length of your bushing, and put that much clearance between the diameters.
Never understood why cutless bearings made any sense as rudder bearings. Probably because they're easy to find and cheap.

 

Curious
What does the 6061 grade do underwater? Seems to be used quite a bit, lift strakes and such on aluminum fishing boats
Tom

 

Aluminum works fine in fresh water, we have a 40ft wylie uldb in the area thats been using an aluminum shaft for the last 30 yrs, also have an x boats one tonner that came from salt water with one of the nicest rudders in town, the whole thing is aluminum. When i worked on Kialoa, the composite one in 1980, it had an alluminum rudder, im pretty sure the shaft was also. A friend built the last 3 Santa Cruz 27s, so,about the same size boat as the OPs and it used solid Extren for the shafts which is a pultruded fiberglass rod.

Steve.

 

Aluminum works fine in fresh water, we have a 40ft wylie uldb in the area thats been using an aluminum shaft for the last 30 yrs, also have an x boats one tonner that came from salt water with one of the nicest rudders in town, the whole thing is aluminum. When i worked on Kialoa, the composite one in 1980, it had an alluminum rudder, im pretty sure the shaft was also. A friend built the last 3 Santa Cruz 27s, so,about the same size boat as the OPs and it used solid Extren for the shafts which is a pultruded fiberglass rod.

Steve.

 

Dear Steve,
did your friend use self-aligning bearings for fiberglass shaft supporting?

rgds,
Ugo

 

Yes you need a self aligning bearing with a inbult shaft seal at the bottom and a simular self aligning bearing at the top as well the quadrant will clamp and key to the shaft and hold the rudder from falling out of the boat . we used bearings that were made and sent from france in the 8 Korean match racing yachts we made . i dont have a manufacures name or any photos . the outer case of the lower bearing had a lower flange and a tappered housing and a seal housing all as one . the rollers inside were a very hard plastic

 

Ugo, im pretty sure he would have just used something like a uhmwpe sleeve bearing top and bottom, i will give him a call today and find out for sure, on a boat like yours i expect that you are tiller steered and have a fiberglass tube from hull to cockpit sole with a bearing top and bottom bonded in. Whenever i do this i have the bearings machined and have a couple of extras made while they are set up.

 

A self aligning bearing is to allow the shaft to bend in use and not stick.


This is for racing boats with light weight tiller style rudder shafts.

 

6061 is common in the US ostensibly owing to the aero industry.

Until like likes of Austal et al, starting using 6082 on large volume and a regular basis in the US..it was pretty hard to find any mill supplying 6082 as stock items, only 6061. We had a licence agreement with several US yards in the 90s, was next to impossible to find 6082...so we ended up exporting to the US from our own yard.

6061 is high in copper. Copper and aluminium are not the best bed fellows!

Most Class societies no longer accept 6061 under the waterline, unless there is a proven track record of the arrangement and well documented.

I have not used it since the early 90s and even though it is still widely used (generally in the US) I do not recommend it. 6082 is a superior alloy and was tailored made for the marine industry.

 

U

thanks a lot.
Just had a look at SC27 website, and got some interesting info.
From the drawings I figure out that:

• Rudder area is close to 0.38m2.
• Bending arm is close to 0.65m.

very close to my design

two different rudder configurations have been built:

1. SS pipe SCH80, 1'1/2 (metric: 48mm Outer diam, 5mm wall thickness)
2. Solid Composite (pull-truded fiberglass Extren) bar, 2'' (metric: 50mm Outer diam., E range = 30-50kN/mm2)

both have plain delrin bushings.

Since SC27 composite shaft deflection is likely to be similar to my predicted deflection, This shoud mean that plain bushing should be Ok for my application as well.

that's why I selected 6082 T6 alloy for my design.
Thanks

Ugo

 

Thanks
Interesting I checked online from local suppliers and no listing for the 6082 grade, I would think if it is a better grade and approved for underwater it would be available.

 

Nice assumption, Taz, but since you guys can't even manage to convert to the metric system like the rest of the planet has, I doubt that the reason nobody carries 6082 has anything to do with 6061 being better. Far more like NIH combined with lack of interest in supplying a (perceived??) non-existent demand.

PDW

 

Everyone in the marime industry here refers to 6061-T6 as aircraft alloy,its a marketing thing, my understanding is that it extrudes well so pretty much all readily available, off the shelf extrusions such as angle, channel, rod, tube etc as well as most, if not all mast extrusions. Any other alloys you have to order in but they are not hard to get.

 

It is not just about having a marine grade ally.

1) 6000 series is next to impossible to remove all porosity. Thus any welded joint which requires to be 100% waterproof shall suffer. Whilst the strength calc's can prove that 6082 is fine, after welding (since it losses 50% of it strength when welded), the amount of porosity often makes the joint impractical. If you are doing Class approved welding. If you're doing back yard welding, this may not matter as you wont be testing it....but it will haunt in you later.

2) 6000 series has lower fatigue life than 5000 series. When you weld 6000 series it gets worse. And when you expose the welded joint to sea water spray or otherwise, the "allowable" design stress is roughly 10% of the proof/yield stress. It is very poor in fatigue underwater.

Thus, you sure you still want to use 6082?...I wouldn't!

 

Not sure what the metric system has to do with it

Might be there are better options than 6061 or 6082
5086 and 5456 ? both available in round bar