Aluminium rudder stock calculation-help

[Wiktor Witwicki]

We intend to redesign our sailing 30 ft performance cruiser rudder stock.
Instead of original 316 ss we intend to use Aluminium grade 6082 or alternatively 7075 (both types recommended by the steering systems suppliers) hoping to get the weight savings and cost savings for the next boats to be build.
However for this type of material the yield strength expressed as 0,2% proof stress is noticeably lower than the tensile strength.
E.g. respectively 260 N/mm2 and 310 N/mm2 for 6082 (AlSi1MgMn) grade.
The original rudder stock calculation was based on the ABS ORY guidelines.
What shall be the approach for the calculation of the stock made of such material.

[Specmar.Aus]

Wiktor Witwicki

Aluminium rudder stock calculation-help

Hello Wiktor Witwicki

I can’t help much with the math’s you need. However I have made a few Rudder stocks from solid billet 6060 and my only advice is that you hard anodize the stock after you have machined all that is necessary. This will give you the best protection and from my experience a great surface to bond your blade to that is if you are building a composite Rudder blade.

Good luck,

Cheers Specmar.Aus

[D'ARTOIS]

Mr Witwicky,

Are you referring to the actual rudderstock? If you are, which companies do recommend aluminium rudderstocks?

For a long list of reasons, the change you want to apply will oblige you to increase the thickness of your rudderstock by factor 8 if you require the same specifications as from SSt. The specific gravity of Alu alloy is 2.7 - 2.8;
Stainless is 7.8 to 8 so the calculation is quite simple.

The general tendency in this forum is that the people here want to make easier and safer boats. I believe that many members of this forum will underline that.

On small applications, the use of aluminium is justified. With small applications I mean steeringsystems on day-sailers, small ones, where everything is in sight and can be seen, properly observed.

As high corrosive material such as aluminium certainly is, should not be used in any vital area, where it is not only out of sight; wear and tear cannot be observed, also other aspects then tensile strength only are showing up:

1. The low density of the aluminium, the low torsional resistance, the abolute low form-memory, the high galvanic corrosion sensitivity with almost all other materials, even FRP, FRE;
in short: to me it looks like a very bad idea.

2. Low form stability (already mentioned in 1.);

3. Becomes brittle under low temperature conditions

4. Due to its low galvanic corrosionresistance you can use neither bronze nor nylon as bearing, so what are you going to use?

5. Weightsaving and costsaving are in boatbuilding a contradictio in terminus;
save for that: the question what will be the actual effect on the performance of the particular boat: not measurable!
May be on a Volvo/Whitbread racer something might experienced but they will never use aluminium as/for a rudderstock; Secondly how many of them had failed their purpose by using non-qualified materials?

I hope sincerely that I have raised sufficient contra's to reconsider your intended change.


that is: opposing objectives

[sorenfdk]

D'ARTOIS:

I couldn't agree less!

Aluminium rudderstocks have been used for years with great success by some of the worlds best known yacht builders. Just to name a few: X-Yachts, Farr Pilot House Yachts, Hallberg-Rassy, Standfast, Etap, Beneteau, Wauquiez...

You name low density as one of the bad things about aluminium. What's bad about low density? Another bad thing you mention is low torsional resistance. Doesn't torsional resistance depend more on the geometry than on the material?


Wictor:

Of the two suggested alloys, I would go for 6082, which is fully sea water resistant. 7075 is not, so you would have to have it anodized, and the owners should remember to inspect the rudderstock once every year.

The aluminium rudderstocks should also be calculated using the ABS Guideline.

I'm sure an aluminium rudderstock will be lighter. Whether it's cheaper is quite another question...

I don't know who your supplier is, but I recommend to have a look at http://www.jefa.com, where you can learn more about aluminium (and stainless steel) rudderstocks. Jefa produce both types, so the information on their website should be quite unbiased. BTW: I'm not in any way connected with Jefa.

[mistral]

have you done a rough calculations of how much weight do you save??? Is it worth?? have you considered fatigue cycle problems??? corrosion due to galvanic cell between ball bearings and stock?? if you've dealed with all theese issues go on and let us know how it works.
I 've never experienced alluminium rudder stock, so i'll shout my mouth and i'll wait to see the results ;)

fair wind
Mistral

[D'ARTOIS]

Mr Soren,

The problem with a low density material is that it deforms much quicker than a high density material.
That Aluminium stocks are presently used in production boats is apparently common practice, I am not in the small boat business, however, you mention Standfast, well, I used to have two in a row and they didnot have alu rudderstocks.

On present designs, with modern rudder-configurations, with the limited support the suspension offers, I wouldn't think about it. You wouldn' t change the steering system of your car for an alu one, would you?

If you want to save weight and maintain a solid and strong configuration take Titanium 318 equal to Ti 6Al 4V - Ind Spec AMS 4928F; the former Warschau pact countries are saturated with it. The owners would like it too. Trade price something like DEM 17,50/kg

I sail regularly on the Atlantic and in the Med, and see what havoc seawater environmental conditions create on aluminium surfaces, when not sufficiently protected.

Then, in case of using Alu, why not Teflon-impregnate it - it kills of any chance with galvanic corrosion and the surface impregnation alters the general specs of Alu somewhat in the positive direction.

[sorenfdk]

The problem with a low density material is that it deforms much quicker than a high density material.
That's true, but if the rudder stock is dimensioned according to the ABS Guide or similar, then this shouldn't be a problem.

[ABoatGuy]

If you decide to use the aluminum stock, think about having any structural welds x-rayed. I don't have any idea what your configuration is, but heavy aluminum structures require a great deal of heat to get good penetration. This may also anneal the aluminum so the published physicals will be lower then expected locally so avoid welds at critical points.

I agree with those that discourage use of 7000 series aluminum in a marine enviornment. It is a great material with good physicals, but probably not for this application.

Good luck.

[Phil Locker]

Is a carbon stock not under consideration due to cost?
Light, strong....

[sorenfdk]

If you decide to use the aluminum stock, think about having any structural welds x-rayed. I don't have any idea what your configuration is, but heavy aluminum structures require a great deal of heat to get good penetration. This may also anneal the aluminum so the published physicals will be lower then expected locally so avoid welds at critical points.

We're talking about a rudder stock for a 30 footer - I don't think it qualifies as a heavy structure...

[Dutch Peter]

If you decide to use the aluminum stock, think about having any structural welds x-rayed.
Sorry boatguy,

Any welding on a rudderstock is most likely to be a fillet weld. Not a configuration that can be assessed with x-ray. But I do agree that some kind of inspection would be appropriate (depends on who's doing the welding, procedures, conditions etc. etc.) and recommend a Magnetic Particle Inspection.

[ABoatGuy]

Dutch,

I stand corrected. Thanks.