Aluminum fabrication (rudder).

[firstascent2002]

One of the first jobs that I want to do when my seal 22 gets home is remove the old steel rudder blade and have an aluminum one fabricated..

can anyone help with
1) where I can get this done...its not like walking into B&Q. Need a campany how fabricates in aluminum. Can the aluminum be bought in a sheet and cut at home?

2) The balde swings around a single bolt. Will the aluminum need bracing at this
point...and if so how can it be done?

3)what is the relative strength of aluminum vs steel? My boat carries a fair bit of wheather helm and the thought of being out in the Bristol channel in a blow minus the rudder leaves me feling cold!!

Cheers in Advance

Jamie

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[extravert]

Your profile does not say where you are, but if you are in the wet part of the country, this lot can do good quality marine aluminum work...

Trinity Marine Ltd, Newry Beach, Holyhead Gwynedd, LL65 1YA
Tel: 01407 763855

They rebuilt my rudder for me when I had an all-aluminum Extravert.

If you want a foil shape fabricating out of aluminum, it will be expensive. It is probably cheaper and just as good to use a different material.

<hr width=100% size=1>One day, I hope to own a real boat.

 

[richardandtracy]

Right. Pretty meaty one this.

1) Yes, you can buy sheets for yourself & cut it at home. Unfortunately Al is one of the most sensitive of materials to size of order. Buy less than 100kg, and you'll pay 2 to 3 times the price per kg of 500kg plus. You can cut it with a jigsaw, but don't use an angle grinder as the swarf can clog the disc & make it explode. Home welding with MIG is supposed to be OK (if somewhat tricky as molten Al is very runny), but don't use arc unless you're an expert. I have never done Al welding myself, just steel.

2) No idea without seeing the design. Al alloys are 1/3 the stiffness of steel, so may need bracing even if stong enough to reduce deflections and fatigue.

3) The strength of the Al is very sensitive to alloy and condition. The better Al alloys for sea use are 5083 and 5086 (best). These come in a bewildering range of hardened conditions. F means 'As fabricated' (ie 'No Idea, could be strong or weak'), O is annealed, then H2, H4, H6 and H8 for increasing hardness and strength. If you get a 6082 alloy, then it'll be M (As rolled or extruded, ie 'No Idea'), O, T3, T4 or T6 for increasing strength. Then extra tempers like T6511 indicate strain hardening & stress relieving - basically ignore all numbers after the first.
The breaking strength of 6082-T6 is about the same as the strength at which ordinary mild structural steel (Grade 43 in old designation, or now called BS EN 10025:S275). This is about 275 N/mm^2. However the steel fails at 430 N/mm^2, so is rather stronger, and has a large deformation before failure, giving you lots of warning.
5086-H8 is weaker than 6082-T6.
Welding the 6000 series alloys ruins their strength, knocking T6 back to T4 or softer. After a year or two, the strength recovers as the alloy age hardens again. The 5000 series alloys can be welded without so much damage to their strength, but the strength does not recover with time. Welding steel, butt joints are as strong as the parent metal if properly done.
Regarding fatigue. If the stress is low enough, steel will never fatigue. But Aluminium will always eventually fatigue, whatever the stress.

All in all, I suggest you get a mechanical, structural or naval engineer to look at your specific problem. If you can't find one locally, PM me & I'll try & help remotely.

Regards

Richard.


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[firstascent2002]

Thanks very much for such a full reply. Hadn;t considered the electrolysis problems that having a stainless steel bolt throught an aluminum plate will create, but this would be minimised as the plate rotates up almost entirely out of the water...almost.

The reason I want to stay with sheet metal is this.

The rudder design hasa hefty wooded stock with approx 10 mm between the wooded cheeks. Only approx 12 inches of the rudder sits between the stocks. How can I find something that is strong enough to fit into this stock with out weighing the best part of 50KG. I really need to reduce weight astern as she dries out very down by the stern. I've already moved the batterys and water takk forward. The rudder seemed the next thing to try and lighten.

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[charles_reed]

Having tried the same (on a Seal 22) I would strongly advise against trying this.

Even if you use marine grade aluminium (which is difficult to get hold of and quite expensive) and have the blade anodised, it will soon succumb to galvanic corrosion.
You also need to appreciate that you need to increase the guage of the sheet between x2.5 and x3 to get the same strength.

Mine lasted about 3 years when I had a new blade, plasma cut in ms and the blade hot dipped galvanised at about 1/3 the price of the aluminium one.

Angus Dixon and John Baker knew what they were doing.

Having had a blade fracture (on a later boat) I know that many naval architects underestimate the forces exerted on rudder-blades.

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