Aluminium rudder stock - bearings choice

Discussion in 'Boat Design' started by Ugo, Dec 2, 2012.

  1. Ugo
    Joined: Dec 2012
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    Ugo Junior Member

    Dear All,
    I am an amateur boat builder, I completed my "spirit-of-freedom" project four years ago (for further details see www.spirit-of-freedom.it).

    Now I've come to the decision to replace the original spade rudder (no skeg, two bearings.).
    the new one will have the following specs:

    Design speed: 7.7knts (27'racer, LWL 8m, displ. 2 tons)
    Side Area: 0.39mq
    span: 1.27m
    Root chord: 0.35m
    middle-span chord: 0.33m
    tip chord: about 0.18m
    Bending arm (distance between blade centroid and bottom bearing centre) 0.62m
    Design load :3600N​

    As a mechanical Engineer, I've done all the maths by myself using ABS guide and ISO/CD 12215-8.
    I've come to a stock diameter of about 50mm using 6082 Aluminium alloy.
    pls see the picture attached.

    According to my calcs, the maximum angular deflection to be expected at bottom bearing neck is about 1.9deg.
    This is confirmed by FEA simulation I've done by Solidworks-Cosmos.

    Now I can't figure out if plain bearings (basically delrin bushings) will be OK, or if self-aligning bearings are needed.
    Is there any rule-of-thumb about maximum angular deflection allowed by plain bearings in order to avoid jamming?

    My preferred option would be to go for plain bushings for budget reasons.
    Any advice would be appreciated!

    With best regards,
    Ugo Mancuso
     

    Attached Files:

  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Why aluminium ? Its soft and prone to corrosion.

    Composite is cheap, strong and durable. Why use alloy ?

    Self aligning bearings ?

    Every boat Ive sailed for the past 30 years has had self aligning bearings.

    I cant imagine a spade rudder without them . Its nearly impossible to install a bearing that is aligned at rest, then aligned when the yacht is sailing and loaded up.

    its true that Self aligning bearings are expensive compared to bushings.

    What about the long term cost ?

    And be careful with plastic, delrin bearings. Delrin swells. Allow for this swelling when you machine them .

    Other plastics are available . Vesconite.

    http://www.vesconite.com/index.htm
     
  3. Ugo
    Joined: Dec 2012
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    Ugo Junior Member

    thanks michael
    the "old" rudder has delrin bushings (ok till now, no play or jammimg) and I still have a 100mm delrin diameter bar available - so I'd like to use it again, but I'll definitely check Vesconite.
    Composite is an interesting alternative, but I'm not able to build a composite stock and I don't want to buy a custom one...
    cheers
    Ugo
     
  4. sean9c
    Joined: Jan 2011
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    sean9c Senior Member

    Is a 50mm shaft in alum really strong enough? Did you calc load using your 7.7kt design speed? I'd think a 4k# 8mm boat would be capable of much higher speeds. Another thing I think about with sailboat rudders is that with tiller steering you can make big changes in the AoA instantly and have really large AoA's. With wheel steered sail and power boats it's hard to get those big changes as fast.
    I'd think aluminum would be an odd choice.
    A lot of spade rudder boats were built for a lot of years with simple bushings for rudder bearings. You just need to build in some clearance between shaft and bushing.
     
  5. Ugo
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    Ugo Junior Member

    well - the maximum theoretical speed is about 11.6 knots (at Froude n. of 1.32), and I've already sailed her at about 11knots in a 20knts gust, with full main and gennaker.

    The design speed used come from ISO/CD 12215-8, it is much lower (7.69knts) and it is calculated as follows:
    V = 2.7*(Kv*LWL)^0.5
    where Kv is a "Light sailing boat speed coefficient" depending on displacement and waterline Lenght (Kv = 1.03 for my boat).
    Another correction come from a "Design category factor for rudder", wich reduces rudder design side force for inshore and sheltered water boats (0.75 for my boat)
    Unfortunately I have no data about similar designs (25-28ft inshore racing sailboat, aluminium alloy stock), so I have to rely on my calcs... but any advice is really appreciated.

    my point about simple bushing is related to shaft angular deflection (alu is 3 times less stiff than SS). I have a 46mm diam. A304 shaft at the moment, and I have no problems with plastic plain bushings.
    my question is: will they get stuck under full load, when the alu stock will bend?

    thanks ;-)
    Ugo
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    6000 series aluminum underwater isn't a good idea. 304 isn't the best way to go for salt water, though not a bad choice for fresh. I'll still prefer 316 or 316L for a shaft, of course with an appropriate diameter. As to bearing, you have several routes, from very simple UHMWPE to a full capture assemble with ball bearings and thrust surfaces. On such a small blade, simple might be best with a standard port and top bushing, well braced.
     
  7. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Ugo,

    Have you considered titanium? So long as its isolated from the aluminium there won't be any corrosion issues at all, and the strength is an order of magnitude higher than aluminium. Figure a cost of about $30/kilo for the tube, and you aren't looking at that much of a price premium as compared to stainless, and it permanently solves all corrosion issues in the foil.

    As for the bearing, in a boat this size I am with Par, go simple. The loads are light, and I don't see much of a justification to getting exotic with it. At the speeds you are talking about the rudder has to be pretty balanced, so the bearing loads should be pretty small.
     
  8. Ugo
    Joined: Dec 2012
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    Ugo Junior Member

    Dear Greg,
    yes, Titanium is quite an option!
    With Titanium (design stress >450N/mmq) a 40mm stock would be Ok as regards the tensile stress, and corrosion would not be a concern.
    Unfortunately it has a lower elastic modulus than Alu (45kN/mmq as stated in ISO/CD 12215-8, but up to 120kN/mmq are available to my knowledge), so the concern that the deflection of the stock at bottom bearing neck under the nominal force (3600N @ 0.6m) may impair the functioning of the rudder plain bushings is even worse: with Titanium a 50mm stock would deflect 3.1deg (against 1.9deg Alu, 0.7deg. A316), so it would be mandatory to go for self-aligning bearings.

    regards,
    Ugo
     
    Last edited: Dec 2, 2012
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Aluminum is a bad idea. Titanium has come down in price and it is very corrosion resistant.
     
  10. Paul B

    Paul B Previous Member

    If you design to the "design speed" you have calculated you would probably be OK in more than 95% of any sailing you would do. However, no working yacht designer would do this for a client. The normal way is to design for full stall at predicted maximum speeds.

    In the conditions where you will exceed your design limits you are in a situation where you really don't want to be handicapped by a bent and jammed rudder shaft.

    I think aluminum is a poor choice of material (corrosion, strength). So is Titanium (too flexible).

    For bearings the Delrin (acetal) is OK. It is not very hydroscopic, and pretty much inert with regard to oils and chemicals. It can flex/creep a bit, so should not bind.

    We used to use rods of this material to make "needle bearings" that rode inside the rudder tube with the shaft centered in between. This worked very well, and some have not even been replaced in more than 20 years now.

    Do not make the mistake of substituting nylon. It is very hydroscopic and will swell and bind.
     
  11. JSL
    Joined: Nov 2012
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    JSL Senior Member

    Take a look at Thorden bearings.
    6061 T6 not a good choice for a rudder stock.
     
  12. Ugo
    Joined: Dec 2012
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    Ugo Junior Member

    great advice, JSL - lot of info found at Thorden bearings website (engineering manual, bearings sizing software), I'll take a look.

    almost all of you discouraged me from using 6082 T6 (aka Anticorodal here in Italy).
    This is quite surprising to me, for several reasons:
    -my main source of info is JEFA rudders company, and 6082 alu alloy is widely used by these guys;
    - according to ISO scantlings guide 7xxx alu alloy like 7075 are to be used carefully due to poor corrosion resistance, but 6082 T6 can be used with no restriction;
    - 6082 T6 design stress is just 12% lower than A304/A316 design stress;
    - low E causes 3 times much bending than SS (304/316), but this can be dealt with self-aligning bearings, if needed.
    On the other hand, I'm aware that corrosion may be an issue as well as low surface hardness (hard coating/anodizing would help?)

    I'm not a pro and I don't have field experience (this is my second rudder, the first one I designed and built is still operating and it as a SS stock on delrin bushings) so I''l take in the highest consideration your advice about avoiding alu alloy.
    I'll check other options than Aluminium.

    My initial question still remains:;)
    how to decide if a 2° deflection at bottom bearing is too much for a simple plain bushing (assuming it is well designed, it has the right Thickness/clearance, and the right material has been used)?

    thanks and regards
    Ugo
     
  13. Frosty

    Frosty Previous Member

    I used a 10inch cutlass bearing the same as the a prop shaft uses. Its rubber absorbs vibration and chatter.

    Even though it is not water lubricated it has been faultless in operation.
     
  14. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Cutlass bearings are often used and work well. However, they may have too much slack for a racer where precise control is required.
     

  15. Frosty

    Frosty Previous Member


    Machine the shaft for the fit you want.

    They never wear out as they dont even rotate once.
     
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