Germanischer Lloyd Rudder scantlings

[RThompson]

Hi,

Re: GL 2009 Part 1 Section 14 A Rudder and Manoeuvring Arrangement.

I'm trying to work out the moments and forces acting on the stock of a semi spade rudder - I must be having a brain fade or something and I'm missing something in reading the rules.

In section 3 "Analysis" it gives a nice schematic of the bending moment and shear force etc of a semi spade rudder (along with other rudder types) , but then only gives equations for the bending moment etc for a spade rudder.

The maximum bending moment for the spade rudder :
Mb = CR(L20 + (L10(2*x1 + x2) / 3(x1 + x2))

Where
CR = rudder force
L10 = span or rudder
L20 = distance from top of rudder to bearing
x1, x2 are the top and bottom chord length of rudder

Are the rules implying that I work out the bending moment etc for a semi spade from first principles or am I missing something?
I could use that except it does not account for (spring) support provided by the horn...

Any illumination here would be muchly appreciated.
Cheers,

Rob

[MikeJohns]

Rob I suspect you are only looking at section2 of Chapter 3 1-part3. Have a look at section 1 chapter 3 'Hull structures' . Bottom of page 6 and on for several pages.

[RThompson]

Mike,
Thanks a lot for your reply.
For the record I am involved in a volunteer group restoring an old steam powered crane-ship; that is why I am thinking about ships rudder stocks (in particular semi spade rudders).

When you say " ...Have a look at section 1 chapter 3 'Hull structures' . Bottom of page 6 and on for several pages." is that in reference to the "Special Craft - yachts up to 24m" section of the rules?
That does seem to be quite clear about rudder stock diameter for semi spade rudders.

I had been looking at rules for sea going ships (the crane-ship is ~46m LOA):
I - Ship Technology / Part 1 - Seagoing Ships / Chapter 1 - Hull structure / Section 14 -Rudder and Manoeuvring Arrangements

i.e.: http://www.gl-group.com/infoServices...h/abschn14.pdf

This rule is clear about rudder force, torque and pressure etc. It gives BM and shear force diagrams for the rudder assembly, but equations for a spade rudder only. I could work out shear force at bearings for a semi-spade rudder from first principles, but I would think it was covered in the rule somehow (at least: the rudder stock shear force at top bearing should be less than a spade rudder of the same dimensions) .
I'll have another crack at it tonight. I can see a RTFM Doh! moment coming up.
My apologies if this post is incomprehensible..

Thanks again,
Rob

[MikeJohns]

Originally Posted by RThompson

............. I can see a RTFM Doh! moment coming up.
My apologies if this post is incomprehensible..

Only the RTFM doh ... I can't find that in my engineering dictionary, must have missed that lecture

If there is an upper and lower pintle and a rudder shaft then there is no bending moment on the shaft only the torque.



If there is no upper pintle then the rudder shaft is the top bearing. Then consider that the rudder is designed as a beam to resist both the torsion and bending. Depending how stiff the rudder is (and how stiff the connection) will determine the moment transferred into the shaft, usually it’s a 15% thickness foil and it can be made very rigid. The rudder shaft (which is bolted on) is then subject to a bending moment along it’s own span from its share of the rudder force ( low with your arrangement) and it is subject to the full torque , direct shear is not that significant compared with the torque shearing stress.

Often it’s helpful to look at other class rules as well and you might find Lloyds registers approach a little clearer for the same result.